Patent Publication Number: US-2023165132-A1

Title: Organic light emitting diode and organic light emitting device including thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority benefit under 35 U.S.C. § 119(a) to Korean Patent Application No. 10-2021-0160442, filed in the Republic of Korea on Nov. 19, 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 organic light emitting diode. 
     Discussion of the Related Art 
     As display devices have become larger, there exists a need for a flat display device which take up less space. Among the flat display devices, a display device using an organic light emitting diode (OLED) has come into the spotlight. 
     The OLED can be formed as a thin film having a thickness less than 2000 Å and can be used to 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 be used in a flexible or foldable display with ease. In addition, the OLED has advantages over liquid crystal display devices (LCD devices); for example, the OLED can be driven at a low voltage of 10 V or less 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. 
     Luminescent materials such as fluorescent and phosphorescent materials can be used in OLEDs. Of these, fluorescent materials from the related art have shown low luminous efficiency, because only the singlet excitons are involved in the luminescence process thereof. 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, many metal complexes from the prior art, which are used as phosphorescent materials may have a luminous lifespan that is too short for many commercial devices. In particular, blue phosphorescent materials, which have a longer triplet lifetime compared to blue fluorescent materials, have short luminous lifespan as the triplet energy level thereof increases. Accordingly, the blue phosphorescent materials with relatively high triplet energy level for inducing deep blue color light can have a disadvantage of reduced luminous lifespan. 
     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 can comprise a first electrode; a second electrode facing the first electrode; and an emissive layer disposed between the first and second electrodes and comprising at least one emitting material layer, wherein the at least one emitting material layer can comprise at least one of a first compound and/or a second compound, and wherein the first compound has the following structure of Formula 1 and the second compound has the following structure of Formula 7: 
     
       
         
         
             
             
         
       
     
     wherein, in Formula 1, 
     each of R 1  to R 9  is independently hydrogen, deuterium, tritium, halogen, unsubstituted or substituted C 1 -C 20  alkyl, unsubstituted or substituted C 1 -C 20  alkyl amino, an unsubstituted or substituted C 6 -C 30  aromatic group or an unsubstituted or substituted C 3 -C 30  hetero aromatic group, wherein two to four of R 1  to R 9  are a moiety having the following structure of Formula 2, 
     
       
         
         
             
             
         
       
     
     wherein, in Formula 2, 
     each of R 11  to R 18  is independently hydrogen, deuterium, tritium, halogen, unsubstituted or substituted C 1 -C 20  alkyl, unsubstituted or substituted C 1 -C 20  alkyl silyl, unsubstituted or substituted C 1 -C 20  alkyl amino, an unsubstituted or substituted C 6 -C 30  aromatic group or an unsubstituted or substituted C 3 -C 30  hetero aromatic group, or 
     adjacent two of R 11  to R 18  form an unsubstituted or substituted hetero aromatic ring having the following structure of Formula 3, 
     wherein at least adjacent two of R 11  to R 18  form an unsubstituted or substituted hetero aromatic ring having the following structure of Formula 3; and 
     asterisk indicates a linking position, 
     
       
         
         
             
             
         
       
     
     wherein, in Formula 3, 
     X is NR 25 , O or S; 
     each of R 21  to R 25  is independently hydrogen, deuterium, tritium, halogen, unsubstituted or substituted C 1 -C 20  alkyl, unsubstituted or substituted C 1 -C 20  alkyl silyl, unsubstituted or substituted C 1 -C 20  alkyl amino, an unsubstituted or substituted C 6 -C 30  aromatic group or an unsubstituted or substituted C 3 -C 30  hetero aromatic group; and 
     a dotted line indicates a fused portion, 
     
       
         
         
             
             
         
       
     
     wherein, in Formula 7, 
     each of R 31  to R 34  is independently hydrogen, deuterium, tritium, halogen, unsubstituted or substituted C 1 -C 20  alkyl, unsubstituted or substituted C 1 -C 20  alkyl silyl, unsubstituted or substituted C 1 -C 20  alkyl amino, an unsubstituted or substituted C 6 -C 30  aromatic group or an unsubstituted or substituted C 3 -C 30  hetero aromatic group, optionally, 
     two adjacent elements of R 31  to R 34  form an unsubstituted or substituted fused ring having boron and nitrogen; 
     each of R 35  to R 38  is independently deuterium, tritium, halogen, unsubstituted or substituted C 1 -C 20  alkyl, unsubstituted or substituted C 1 -C 20  alkyl silyl, unsubstituted or substituted C 1 -C 20  alkyl amino, an unsubstituted or substituted C 6 -C 30  aromatic group or an unsubstituted or substituted C 3 -C 30  hetero aromatic group, wherein each R 35  is identical to or different from each other when q is an integer of two or more, each R 36  is identical to or different from each other when r is an integer of two or more, each R 37  is identical to or different from each other when s is an integer of two or more and each R 38  is identical to or different from each other when t is an integer of two or more; 
     each of q and s is independently an integer of 0 to 5; 
     r is an integer of 0 to 3; and 
     t is an integer of 0 to 4. 
     An onset wavelength of the first compound can be less than a maximum absorbance wavelength of the second compound. As an example, the onset wavelength of the first compound can be between about 430 nm and about 440 nm. 
     The first compound can comprise an organic compound having the following structure of Formula 4: 
     
       
         
         
             
             
         
       
     
     wherein, in Formula 4, 
     each of R 1 , R 4 , R 5 , R 6  and R 7  is independently hydrogen, deuterium, protium, halogen, unsubstituted or substituted C 1 -C 20  alkyl, unsubstituted or substituted C 1 -C 20  alkyl amino, unsubstituted or substituted C 6 -C 30  aryl or unsubstituted or substituted C 3 -C 30  hetero aryl, wherein two of R 1 , R 4 , R 5 , R 6  and R 7  have the structure of Formula 2. 
     As an example, the moiety having the structure of Formula 2 can be selected from the following moieties: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     The second compound can comprise an organic compound having the following structure of Formula 8A to 8C: 
     
       
         
         
             
             
         
       
     
     wherein, in Formulae 8A to 8C, 
     each of R 31 , R 35  to R 38  and R 41  to R 44  is independently hydrogen, deuterium, tritium, halogen, unsubstituted or substituted C 1 -C 20  alkyl, unsubstituted or substituted C 1 -C 20  alkyl silyl, unsubstituted or substituted C 1 -C 20  alkyl amino, unsubstituted or substituted C 6 -C 30  aryl or unsubstituted or substituted C 3 -C 30  hetero aryl. 
     In a preferred embodiment, in Formula 1, each of R 4  to R 6  is independently hydrogen, deuterium, or tritium. Preferably, in Formula 1, at least two of R 2 , R 5  and R 8  is a moiety having the structure of Formula 2. In an embodiment, in Formula 1, R 4  and R 6  are each independently a moiety having the structure of Formula 2. In an embodiment, in Formula 1, R 2  and R 8  are each independently a moiety having the structure of Formula 2. In an embodiment, in Formula 1, R 2  and R 5  are each independently a moiety having the structure of Formula 2. In an embodiment, wherein in Formula 1, R 5  and R 8  are each independently a moiety having the structure of Formula 2. 
     In a preferred embodiment, in Formula 7, each of R 31  to R 38  is independently hydrogen, deuterium, tritium, halogen, or an unsubstituted or substituted C 1 -C 8  alkyl. In another embodiment, in Formula 7, each of R 31  to R 34  is independently hydrogen, deuterium, tritium, halogen, or an unsubstituted or substituted C 1 -C 8  alkyl. In another embodiment, in Formula 7, at least two of R 31  to R 38  is independently an unsubstituted or substituted C 1 -C 20  alkyl amino, an unsubstituted or substituted C 6 -C 30  aromatic group or an unsubstituted or substituted C 3 -C 30  hetero aromatic group. In another embodiment, in Formula 7, at least two of R 31  to R 38  is independently an unsubstituted or substituted carbazole. 
     In one aspect, the at least one emitting material layer can have a single-layered emitting material layer. The single-layered emitting material layer can further comprise a third compound. In this case, the single-layered emitting material layer can comprise the first compound of about 10 to about 40% by weight, the second compound of about 0.1 to about 5% by weight and the third compound of about 55 to about 85% by weight. 
     Alternatively, the at least one emitting material layer can comprise 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 comprises the first compound and the second emitting material layer comprises the second compound. The first emitting material layer can further comprise a third compound and the second emitting material layer can further comprise a fourth compound. 
     As an example, an excited triplet exciton energy level of the third compound and/or the fourth compound can be higher than an excited triplet exciton energy level of the first compound and the exited triplet exciton energy level of the first compound can be higher than an excited triplet exciton energy level of the second compound. 
     Alternatively, an excited singlet exciton energy level of the third compound and/or the fourth compound can be higher than an excited singlet exciton energy level of the first compound and the excited singlet exciton energy level of the first compound can be higher than an excited singlet exciton energy level of the second compound. 
     Alternatively, an excited singlet energy level of the fourth compound can be higher than an excited singlet energy level of the second compound. 
     Optionally, when the at least one emitting material layer comprises the first and second emitting material layers, the at least one emitting material layer can further comprises a third emitting material layer disposed oppositely to the second emitting material layer with respect to the first emitting material layer. 
     The third emitting material layer can comprise a fifth compound and a sixth compound, and the fifth compound can comprise the organic compound having the structure of Formula 7. 
     In one aspect, the emissive layer can comprise a first emitting part disposed between the first and second electrodes, a second emitting part disposed between the first emitting part and the second electrode and a charge generation layer disposed between the first and second emitting parts, and wherein at least one of the first emitting part and the second emitting part can comprise the at least one emitting material layer. 
     As an example, the first emitting part can comprise the at least one emitting material layer, and the second emitting part can emit at least one of red light and green light. 
     Alternatively, the emissive layer can further comprise a third emitting part disposed between the second emitting part and the second electrode and a second charge generation layer disposed between the second and third emitting part, and wherein at least one of the first emitting part and the third emitting part can comprise the at least one emitting material layer. 
     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. For instance, an organic light emitting display device, may comprise a substrate; and an OLED display comprising an array of light emitting pixels on the substrate, wherein each pixel comprises one or more individually addressable organic light emitting diodes 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 aspect of the present disclosure. 
         FIG.  3    is a schematic cross-sectional view illustrating an organic light emitting diode (OLED) in accordance with an aspect of the present disclosure. 
         FIG.  4    is a schematic graph illustrating that the luminous efficiency and color purity of an OLED can be improved by controlling an onset wavelength of the first compound and a maximum absorbance wavelength of the second compound in accordance with an aspect of the present disclosure. 
         FIG.  5    is a schematic graph illustrating that the luminous efficiency of an OLED is deteriorated in case an onset wavelength of the first compound does not have specific wavelength ranges. 
         FIG.  6    is a schematic graph illustrating that the luminous efficiency and color purity of an OLED is deteriorated in case an onset wavelength of the first compound is more than a maximum absorbance wavelength of the second compound. 
         FIG.  7    is a schematic diagram illustrating luminous mechanism by singlet and triplet energy levels among luminous material in an EML in accordance with an aspect of the present disclosure. 
         FIG.  8    is a schematic cross-sectional view illustrating an OLED in accordance with another aspect of the present disclosure. 
         FIG.  9    is a schematic diagram illustrating luminous mechanism by singlet and triplet energy levels among luminous materials in EMLs in accordance with another aspect of the present disclosure. 
         FIG.  10    is a schematic cross-sectional view illustrating an OLED in accordance with still another 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 aspect of the present disclosure. 
         FIG.  12    is a schematic cross-sectional view illustrating an OLED in accordance with still another aspect of the present disclosure. 
         FIG.  13    is a schematic cross-sectional view illustrating an organic light emitting display device in accordance with another aspect of the present disclosure. 
         FIG.  14    is a schematic cross-sectional view illustrating an OLED in accordance with still another 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 aspect of the present disclosure. 
         FIG.  16    is a schematic cross-sectional view illustrating an OLED in accordance with still another aspect of the present disclosure. 
         FIG.  17    is a schematic cross-sectional view illustrating an OLED in accordance with still another 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 a first compound and a second compound 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. 
       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 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, forms 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  (or over the substrate  110  when the buffer layer  122  is not present). In one 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 ). 
     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. 
     In addition, the organic light emitting display device  100  can include a color filter layer that includes dyes or pigments for transmitting specific wavelength light of light emitted from the OLED D. For example, the color filter layer can transmit light of specific wavelength such as red (R), green (G) and/or blue (B). Each of red, green, and blue color filter patterns can be disposed separately in each pixel region P. In this case, the organic light emitting display device  100  can implement full-color through the color filter layer. 
     For example, when the organic light emitting display device  100  is a bottom-emission type, the color filter layer can be disposed on the interlayer insulating layer  132  with corresponding to the OLED D. Alternatively, when the organic light emitting display device  100  is a top-emission type, the color filter layer can be disposed over the OLED D, for example, a second electrode  230 . 
     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  that is 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 a transparent conductive oxide (TCO). More particularly, 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. 
     In one 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 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) ( FIGS.  3 ,  8 ,  10  and  12   ). In one aspect, the emissive layer  220  can have single 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. For example, the second electrode  230  can include, but is not limited to, aluminum (Al), magnesium (Mg), calcium (Ca), silver (Ag), alloy thereof or combination thereof such as aluminum-magnesium alloy (Al—Mg). 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 aspect of the present disclosure. As illustrated in  FIG.  3   , the OLED D1 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 D1 can be disposed in the blue 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   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 aspect, the first electrode  210  can include, but is not limited to, ITO, IZO, ITZO, SnO, ZnO, ICO, 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, a second compound (Compound 2) FD and, optionally a third compound (Compound 3) H. For example, the first compound DF can be delayed fluorescent material, the second compound FD can be fluorescent material, and the third 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. 
     The first compound DF 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  between a singlet energy level S 1   DF  and a triplet energy level T 1   DF  ( FIG.  7   ). Accordingly, the excitons of singlet energy level S 1   DF  as well as the excitons of triplet energy level T 1   DF  in the first compound DF of the delayed fluorescent material can be transferred to an intermediate energy level state, i.e. ICT (intramolecular charge transfer) state (S 1   DF →ICT←T 1   DF ), and then the intermediate state excitons can be shifted to a ground state (ICT→S 0 ). 
     The delayed fluorescent material must has an energy level bandgap ΔE ST  ( FIG.  7   ) 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   DF  and the triplet energy level T 1   DF  so that exciton energy in both the singlet energy level S 1   DF  and the triplet energy level T 1   DF  can be transferred to the ICT state. The material having little energy level bandgap ΔE ST  between the singlet energy level S 1   DF  and the triplet energy level T 1   DF  can exhibit common fluorescence with Inter system Crossing (ISC) in which the excitons of singlet energy level S 1   DF  can be shifted to its ground state S 0   DF , as well as delayed fluorescence with Reverse Inter System Crossing (RISC) in which the excitons of triplet energy level T 1   DF  can be converted upwardly to the excitons of singlet energy level S 1   DF , and then the exciton of singlet energy level S 1   DF  transferred from the triplet energy level T 1   DF  can be transferred to the ground state S 0   DF . 
     The first compound DF can be delayed fluorescent material including a first moiety of an electron acceptor group with boron and oxygen atoms, and a second moiety of plural electron donor groups (EDGs). The first compound DF with delayed fluorescent property can have the following structure of Formula 1: 
     
       
         
         
             
             
         
       
     
     wherein, in Formula 1, 
     each of R 1  to R 9  is independently hydrogen, deuterium, tritium, halogen, unsubstituted or substituted C 1 -C 20  alkyl, unsubstituted or substituted C 1 -C 20  alkyl amino, an unsubstituted or substituted C 6 -C 30  aromatic group or an unsubstituted or substituted C 3 -C 30  hetero aromatic group, wherein two to four of R 1  to R 9  are a moiety having the following structure of Formula 2, 
     
       
         
         
             
             
         
       
     
     wherein, in Formula 2, 
     each of R 11  to R 18  is independently hydrogen, deuterium, tritium, halogen, unsubstituted or substituted C 1 -C 20  alkyl, unsubstituted or substituted C 1 -C 20  alkyl silyl, unsubstituted or substituted C 1 -C 20  alkyl amino, an unsubstituted or substituted C 6 -C 30  aromatic group or an unsubstituted or substituted C 3 -C 30  hetero aromatic group, or 
     adjacent two of R 11  to R 18  form an unsubstituted or substituted hetero aromatic ring having the following structure of Formula 3, 
     wherein at least adjacent two of R 11  to R 18  form an unsubstituted or substituted hetero aromatic ring having the following structure of Formula 3; and 
     asterisk indicates a linking position, 
     
       
         
         
             
             
         
       
     
     wherein, in Formula 3, 
     X is NR 25 , O or S; 
     each of R 21  to R 25  is independently hydrogen, deuterium, tritium, halogen, unsubstituted or substituted C 1 -C 20  alkyl, unsubstituted or substituted C 1 -C 20  alkyl silyl, unsubstituted or substituted C 1 -C 20  alkyl amino, an unsubstituted or substituted C 6 -C 30  aromatic group or an unsubstituted or substituted C 3 -C 30  hetero aromatic group; and a dotted line indicates a fused portion. 
     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, the C 3 -C 30  hetero aromatic ring, the C 6 -C 30  arylene and the C 3 -C 30  hetero arylene constituting R 1  to R 9  in Formula 1, R 11  to R 18  in Formula 2 and/or R 21  to R 25  in Formula 3 can be independently unsubstituted or substituted with at least one of deuterium, tritium, C 1 -C 20  alkyl, C 6 -C 30  aryl, C 3 -C 30  hetero aryl, C 6 -C 30  aryl amino and C 3 -C 30  hetero aryl amino. 
     As used herein, the term “hetero” in such as “a hetero aromatic group”, “hetero aryl”, “hetero aryl alkyl”, “hetero aryl oxy”, “hetero aryl amino” and “hetero arylene group” means that at least one carbon atom, for example 1-5 carbons atoms, constituting an aromatic group or ring is substituted with at least one hetero atom selected from the group consisting of N, O, S, P and combination thereof. 
     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. An aromatic group or aryl can be unsubstituted or substituted. As an example, the C 6 -C 30  aromatic group, which can constitute R 1  to R 9  in Formula 1, R 11  to R 18  in Formula 2 and/or R 21  to R 24  in Formula 3, 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 and C 6 -C 30  aryl amino. As an example, the C 6 -C 30  aromatic group and/or the C 6 -C 30  aryl group, which can constitute R 1  to R 9  in Formula 1, R 11  to R 18  in Formula 2 and/or R 21  to R 24  in Formula 3, 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. A hetero aromatic group can be unsubstituted or substituted. As an example, the C 3 -C 30  hetero aromatic group, which can be constitute R 1  to R 9  in Formula 1, R 11  to R 18  in Formula 2 and/or R 21  to R 25  in Formula 3, 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 and C 3 -C 30  hetero aryl amino. 
     As an example, the C 3 -C 30  hetero aryl group, which can constitute R 1  to R 9  in Formula 1, R 11  to R 18  in Formula 2 and/or R 21  to R 25  in Formula 3, 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, difluoro-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 addition, the C 6 -C 20  aromatic ring and the C 3 -C 20  hetero aromatic ring formed by two adjacent elements among R 11  to R 18  in Formula 2, but is not limited to, a benzene ring, a naphthalene ring, an indene ring, a phenanthrene ring, an indene ring, a fluorene ring, a pyridine ring, a pyrimidine ring, a triazine ring, a quinoline ring, an indole ring, a benzo-furan ring, a benzo-thiophene ring, a dibenzo-furan ring, a dibenzo-thiophene ring and/or combination thereof, each of which can be unsubstituted or substituted with at least one of deuterium, tritium, C 1 -C 20  alkyl, C 6 -C 30  aryl, C 3 -C 30  hetero aryl, C 6 -C 30  aryl amino and C 3 -C 30  hetero aryl amino. 
     For example, each of the C 6 -C 20  aromatic group, the C 3 -C 30  hetero aromatic group, fused aromatic ring and the fused hetero aromatic ring constituting each of R 1  to R 9  in Formula 1, R 11  to R 18  in Formula 2 and/or R 21  to R 25  in Formula 3 can be unsubstituted or substituted with at least one of C 1 -C 10  alkyl (ex. C 1 -C 5  alkyl such as tert-butyl), C 6 -C 30  aryl (ex. C 6 -C 15  aryl such as phenyl), C 3 -C 30  hetero aryl (ex. C 3 -C 15  hetero aryl such as pyridyl) and/or C 6 -C 30  aryl amino (ex. C 6 -C 15  aryl as diphenyl amino). 
     In Formula 1, the fused hetero aromatic ring with boron and oxygen atoms acts as an electron acceptor group moiety, and the fused hetero aromatic ring with at least one nitrogen atom having the structure of Formula 2 acts as an electron donor group (EDG) moiety. Accordingly, the organic compound having the structure of Formula 1 can have delayed fluorescent property. 
     Since the electron donor group moiety having the structure of Formula 2 includes 5-membered ring with a nitrogen atom between side benzene rings, the moiety shows improved thermal stability as the bond strength between the electron donor group moiety and the electron acceptor group moiety maximizes. The first compound DF having the delayed fluorescent property has excellent luminous efficiency, so that exciton energy can be transferred efficiently from the first compound DF to the second compound FD, so that the EML  240  can realized hyper-fluorescence. 
     The first compound DF having the structure of Formula 1 includes the first moiety of the fused ring with boron and oxygen atoms as a nuclear atom as the electron acceptor group, and plural (e.g. two to four, two or three, or two) second moieties each of which has the structure of Formula 2 as the electron donor group. Since the electron acceptor group and the electron donor group each of which includes plural fused rings are bulky, steric hindrance in those moieties can be induced. In addition, since plural bulky electron donor moieties are arranged adjacently within the molecule, steric hindrance among those electron donor moieties can be induced. As such, the delayed fluorescent property of the first compound DF becomes strong. 
     The first compound DF has a molecular conformation where the plural electron donor moieties are arranged adjacently outward of the electron acceptor moiety of the central fused hetero ring with boron and oxygen atoms. While a part of the HOMO (Highest Occupied Molecular Orbital) function overlaps with a part of the LUMO (Lowest Unoccupied Molecular Orbital) function in the molecule of the first compound DF, it is possible to minimize the HOMO to extend toward the central electron donor moiety. Accordingly, the first compound DF having the structure of Formula 1 can implement high intra-molecular charge mobility efficiency and realize very high quantum efficiency. 
     The first compound DF inducing plural electron donor moieties is designed to maximize its molecular steric hindrance and to overlap partially between the HOMO function and the LUMO function. As the intra-molecular charge mobility efficiency of the first compound DF, the first compound has reinforced delayed fluorescent property. The energy level bandgap ΔE ST  between excited singlet energy level S 1   DF  and excited triplet energy level T 1   DF  are very narrow ( FIG.  7   ), RISC can be performed rapidly as spin-orbital coupling (SOC) becomes strong. 
     The first compound DF having the Formulae 1 to 3 has delayed florescent property, as well as appropriate singlet and triplet energy levels, HOMO and LUMO energy levels and excellent luminous properties for transferring exciton energies efficiently to the second compound FD. 
     As an example, the first compound DF can have two electron donor group moieties each of which independently has the structure of Formula 2. In one aspect, two electron donor group moieties can be linked to each of the benzene ring that is formed by fusing boron atom and oxygen atom in the fused hetero ring constituting the electron acceptor group moiety in the first compound DF. In another aspect, two electron donor group moieties can be linked to one of the benzene ring that is formed by fusing boron atom and oxygen atom in the fused hetero ring constituting the electron acceptor group moiety, and another benzene ring that is formed by fusing two oxygen atoms in the first compound DF, respectively. In still another aspect, two electron donor group moieties can be linked to the benzene ring formed by fusing two oxygen atoms constituting the electron acceptor group moiety in the first compound DF. As an example, the first compound DF can have the following structure of Formula 4: 
     
       
         
         
             
             
         
       
     
     wherein, in Formula 4, 
     each of R 1 , R 4 , R 5 , R 6  and R 7  is independently hydrogen, deuterium, protium, halogen, unsubstituted or substituted C 1 -C 20  alkyl, unsubstituted or substituted C 1 -C 20  alkyl amino, unsubstituted or substituted C 6 -C 30  aryl or unsubstituted or substituted C 3 -C 30  hetero aryl, wherein two of R 1 , R 4 , R 5 , R 6  and R 7  have the structure of Formula 2. 
     As an example, each of the C 6 -C 30  aryl and the C 3 -C 30  hetero aryl of R 1 , R 4 , R 5 , R 6  and R 7  in Formula 4 can be independently unsubstituted or substituted with at least one of deuterium, tritium, C 1 -C 20  alkyl, C 6 -C 30  aryl, C 3 -C 30  hetero aryl, C 6 -C 30  aryl amino and C 3 -C 30  hetero aryl amino. 
     Adjacent at least two of R 11  to R 18  in Formula 2 can form the fused hetero aromatic ring having the structure of Formula 3. For example, adjacent at least two of R 11  to R 18  in Formula 2 can form an unsubstituted or substituted indene ring, an unsubstituted or substituted indole ring, an unsubstituted or substituted benzo-furan ring or an unsubstituted or substituted benzo-thiophene ring. Accordingly, the hetero aromatic moiety having the structure of Formula 2 acting as an electron donor group moiety can include, but is not limited to, an unsubstituted or substituted indeno-carbazolyl moiety, an unsubstituted or substituted indolo-carbazolyl moiety, an unsubstituted or substituted benzofuro-carbazolyl moiety and an unsubstituted or substituted benzothieno-carbazolyl moiety. As an example, the electron donor group moiety having the structure of Formula 2 can be selected from, but is not limited to, the following moieties of Formula 5: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     More particularly, the first compound DF can be selected from, but is not limited to, organic compounds having the following structure of Formula 6: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     The first compound DF of the delayed fluorescent material has little energy bandgap ΔE ST  between the excited singlet energy level SP and the excited triplet energy level T 1   DF  of equal to or less than about 0.3 eV ( FIG.  7   ) and shows excellent quantum efficiency because the excited triplet exciton energy of the first compound DF is converted to the excited singlet exciton thereof by RISC. However, The first compound DF has a distorted chemical conformation due to the binding structure between the electron donor group moiety and the electron acceptor group moiety. Since the first compound DF utilizes triplet excitons, addition charge transfer transition (CT transition) is induced in the first compound DF. The first compound DF having the structure of Formulae 1 to 6 has a limit in terms of color purity owing to wide full-width at half maximum (FWHM) caused by the CT luminous mechanism. 
     When the EML  240  includes only the first compound DF as an emitter, the triplet exciton energy of the first compound DF cannot contribute efficiently to the light emission, and the luminous lifespan of the OLED D1 can be reduced owing to quenching mechanisms such as TTA (triplet-triplet annihilation) and/or TPA (triplet-polaron annihilation). 
     The EML  240  includes the second compound FD of the fluorescent material in order to maximize the luminous properties of the first compound DF of the delayed fluorescent material and to implement hyper-fluorescence. As described above, the first compound DF of the delayed fluorescent material can utilize both the singlet exciton energy and the triplet exciton energy. When the EML  240  includes the second compound FD of the fluorescent material having proper energy levels comparted to the first compound DF of the delayed fluorescent material, the second compound FD can absorb exciton energies released from the first compound DF, and then the second compound FD can generate 100% singlet excitons utilizing the absorbed exciton energies with maximizing its luminous efficiency. 
     The singlet exciton energy of the first compound DF, which includes the singlet exciton energy of the first compound DF converted from its own triplet exciton energy and initial singlet exciton energy of the first compound DF in the EML  240 , is transferred to the second 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 second compound FD. Organic material having an absorbance spectrum widely overlapped with a photoluminescence spectrum of the first compound DF can be used as the second compound FD so that the exciton energy generated at the first compound DF can be efficiently transferred to the second compound FD. Since the ultimately emitting second compound FD has narrow FWHM and excellent luminous lifespan, the color purity emitted from the OLED D1 and the luminous lifespan of the OLED D1 can be enhanced. 
     The second compound FD in the EML  240  can be blue fluorescent material. For example, the second compound FD induced into the EML  240  can be boron-based fluorescent material with equal to or less than about 35 nm of FWHM. As an example, the second compound FD of the boron-based fluorescent material can have the following structure of Formula 7: 
     
       
         
         
             
             
         
       
     
     wherein, in Formula 7, 
     each of R 31  to R 34  is independently hydrogen, deuterium, tritium, halogen, unsubstituted or substituted C 1 -C 20  alkyl, unsubstituted or substituted C 1 -C 20  alkyl silyl, unsubstituted or substituted C 1 -C 20  alkyl amino, an unsubstituted or substituted C 6 -C 30  aromatic group or an unsubstituted or substituted C 3 -C 30  hetero aromatic group, 
     optionally, 
     two adjacent elements of R 31  to R 34  form an unsubstituted or substituted fused ring having boron and nitrogen; 
     each of R 35  to R 38  is independently deuterium, tritium, halogen, unsubstituted or substituted C 1 -C 20  alkyl, unsubstituted or substituted C 1 -C 20  alkyl silyl, unsubstituted or substituted C 1 -C 20  alkyl amino, an unsubstituted or substituted C 6 -C 30  aromatic group or an unsubstituted or substituted C 3 -C 30  hetero aromatic group, wherein each R 35  is identical to or different from each other when q is an integer of two or more, each R 36  is identical to or different from each other when r is an integer of two or more, each R 37  is identical to or different from each other when s is an integer of two or more and each R 38  is identical to or different from each other when t is an integer of two or more; 
     each of q and s is independently an integer of 0 to 5; 
     r is an integer of 0 to 3; and 
     t is an integer of 0 to 4. 
     For example, each of the C 6 -C 30  aromatic group, the C 3 -C 30  hetero aromatic group and the fused ring having boron and nitrogen constituting R 31  to R 38  can be independently unsubstituted or substituted with at least one of deuterium, tritium, C 1 -C 20  alkyl, C 6 -C 30  aryl, C 3 -C 30  hetero aryl C 6 -C 30  aryl amino and/or C 3 -C 30  hetero aryl amino. 
     Similar to Formulae 1 to 3, the C 6 -C 30  aromatic group constituting each of R 31  to R 38  in Formula 7 can independently include, but is not limited to, C 6 -C 30  aryl, C 7 -C 30  aryl alkyl, C 6 -C 30  aryl oxy and C 6 -C 30  aryl amino. The C 3 -C 30  hetero aromatic group constituting each of R 31  to R 38  in Formula 7 can independently include, 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 and C 3 -C 30  hetero aryl amino. 
     The second compound FD of the boron-based compound having the structure of Formula 7 has excellent luminous properties. Since the second compound FD of the boron-based compound having the structure of Formula 7 includes wide plate-like molecular conformation, the second compound FD can accept efficiently exciton energies released from the first compound DF and maximize the luminous efficiency in the EML  240 . 
     In one aspect, R 31  to R 34  in Formula 7 can be bound to each other. Alternatively, R 32  and R 33  in Formula 7 can form the fused ring with boron and nitrogen atoms. As an example, the second compound FD can include a boron-based organic compound having the structure of Formulae 8A to 8C. 
     
       
         
         
             
             
         
       
     
     wherein, in Formulae 8A to 8C, each of R 31 , R 35  to R 38  and R 41  to R 44  is independently hydrogen, deuterium, tritium, halogen, unsubstituted or substituted C 1 -C 20  alkyl, unsubstituted or substituted C 1 -C 20  alkyl silyl, unsubstituted or substituted C 1 -C 20  alkyl amino, unsubstituted or substituted C 6 -C 30  aryl or unsubstituted or substituted C 3 -C 30  hetero aryl. 
     As an example, each of the C 6 -C 30  aryl and the C 3 -C 30  hetero aryl of R 31 , R 35  to R 38  and R 41  to R 44  can be independently unsubstituted or substituted with at least one of deuterium, tritium, C 1 -C 20  alkyl, C 6 -C 30  aryl, C 3 -C 30  hetero aryl C 6 -C 30  aryl amino and/or C 3 -C 30  hetero aryl amino. 
     More particularly, the second compound FD of the boron-based organic compound can be selected from, but is not limited to, organic compound having the following structure of Formula 9: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     The third 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 compound DF and/or the second compound FD. As an example, when the EML  240  includes the third compound H of the host, the first compound DF can be a first dopant and the second compound FD can be a second dopant. 
     In an aspect, the third 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)dibenzo[b,d]thiophene (DCzDBT), 3′,5′-Di(carbazol-9-yl)-[1,1′-biphenyl]-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-(triphenylene-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′-bicarbazole and combination thereof. 
     In an aspect, when the EML  240  includes the first compound DF, the second compound FD and the third compound H, the contents of the third compound H in the EML  240  can be larger than the contents of the first compound DF in the EML  240 , and the contents of the first compound DF in the EML  240  can be larger than the contents of the second compound FD in the EML  240 . When the contents of the first compound DF is larger than the contents of the second compound FD, exciton energy can be effectively transferred from the first compound DF to the second compound FD via FRET mechanism. For example, the contents of the third compound H in the EML  240  can be about 55 wt % to about 85 wt %, the contents of the first compound DF in the EML  240  can be about 10 wt % to about 40 wt %, for example, about 10 wt % to about 30 wt %, and the contents of the second 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. 
     It may be necessary to control an photoluminescence wavelength and an absorbance wavelength between the first compound DF and the second compound FD in order to improve the luminous efficiency and color purity of the OLED D  1 .  FIG.  4    is a schematic graph illustrating that the luminous efficiency and color purity of an OLED can be improved by controlling an onset wavelength of the first compound and a maximum absorbance wavelength of the second compound in accordance with an aspect of the present disclosure. 
     As illustrated in  FIG.  4   , when the degree of overlap between the photoluminescence (PL) spectrum PL DF  of the first compound DF and the absorbance spectrum Abs FD  of the second compound FD becomes large, transfer efficiency of exciton energies from the first compound DF to the second compound FD can be improved. As an example, the distance between the maximum photoluminescence wavelength λ PL.max   DF  of the first compound DF and the maximum absorbance wavelength λ Abs.max   FD  of the second compound FD can be equal to or less than about 30 nm, for example, about 20 nm. The maximum PL wavelength λ PL.max   DF  of the first compound DF can be between about 460 nm and about 480 nm, for example, about 470 nm and about 480 nm. 
     In one aspect, an onset wavelength λ onset   DF  of the first compound DF can be between about 430 nm and about 440 nm. As used herein, the term “onset wavelength” indicates a wavelength value at the point wherein the extrapolation line and an X-axis (wavelength) intersect in a linear region of a short wavelength region in the PL spectrum of the organic compound. More specifically, the onset wavelength can be defined as a wavelength corresponding to a shorter wavelength among two wavelengths having an emission intensity corresponding to 1/10 of the maximum value in the PL spectrum. The onset wavelength λ onset   DF  of the first compound DF can be identical to or shorter than the maximum absorbance wavelength λ Abs.max   FD  of the second compound FD. As an example, the maximum absorbance wavelength λ Abs.max   FD  of the second compound FD can be equal to or longer than about 440 nm, for example, between about 440 nm and about 470 nm or between about 450 nm and about 460 nm. 
     When the onset wavelength λ onset   DF  of the first compound DF is between about 430 nm and about 440 nm and is equal to or shorter than the maximum absorbance wavelength λ Abs.max   FD  of the second compound FD, both the initial singlet exciton energy and the singlet exciton energy converted by RISC mechanism of the first compound DF can be transferred efficiently to the second compound FD. 
     Since the first compound DF includes plural electron donor group moieties, the first compound DF shows very large steric hindrance. Therefore, controlling the HOMO and LUMO in the molecule of the first compound DF can maximize its intramolecular charge mobility efficiency, and therefore, the conversion to singlet state from the triplet state in the first compound DF can be occurred rapidly. Accordingly, the triplex exciton generated in the first compound DF can be converted upwardly its own singlet exciton by RISC mechanism without transferring to the second compound FD. The singlet exciton energies generated in the first compound DF is transferred to the second compound FD via FRET mechanism, which is performed with great rapidity. 
     As such, as the triplet exciton generated in the first compound DF is converted upwardly its own singlet exciton, the converted singlet exciton of the first compound DF can be transferred rapidly to the singlet exciton of the second compound FD. Accordingly, the exciton energy can be efficiently to the second compound FD from the first compound DF, and thus, the luminous efficiency of the OLED D1 can be maximized. 
     On the contrary, as illustrated in  FIG.  5   , when the onset wavelength λ onset   DF  of the first compound DF is less than 430 nm, the first compound DF can show lower delayed fluorescent property and/or the second compound H as the host transferring exciton energies to the first compound DF must have very high excited triplet energy level T 1   H . In this case, the triplet exciton generated in the first compound DF is not converted its own singlet exciton by RISC and is transferred to the triplet exciton of the second compound FD. Since the triplex exciton transferred to the second compound FD is quenched without involving the luminous process, the luminous efficiency of the OLED can be deteriorated. 
     In addition, when the onset wavelength λ onset   DF  of the first compound is longer than 440 nm, the maximum PL wavelength λPL.max DF  of the first compound DF is excessively spaced apart from the maximum absorbance wavelength λ Abs.max   FD  of the second compound FD. As the degree of overlap between the PL spectrum PL DF  of the first compound DF and the absorbance spectrum Abs FD  of the second compound FD decreases, the exciton energy transfer efficiency from the first compound DF to the second compound FD decreases. As the exciton not transferred to the second compound FD remains in the first compound FD, the luminous efficiency of the OLED D1 is reduced because the excitons remained in the first compound DF is quenched as non-emission. In addition, the color purity of the OLED D1 can be deteriorated as the first compound DF and the second compound FD emit light simultaneously. 
     Similarly, as illustrated in  FIG.  6   , when the onset wavelength λ onset   DF  of the first compound DF is longer than the maximum absorbance wavelength λ Abs.max   FD  of the second compound FD, as the degree of overlap between the PL spectrum PL DF  of the first compound DF and the absorbance spectrum Abs FD  of the second compound FD decreases, the exciton energy transfer efficiency from the first compound DF to the second compound FD decreases. As the exciton not transferred to the second compound FD remains in the first compound FD, the luminous efficiency of the OLED D1 is reduced because the excitons remained in the first compound DF is quenched as non-emission. In addition, the color purity of the OLED D1 can be deteriorated as the first compound DF and the second compound FD emit light simultaneously. 
     In other words, when the onset wavelength λ onset   DF  of the first compound DF is beyond 440 nm and/or the onset wavelength λ onset   DF  of the first compound DF is longer than the maximum absorbance wavelength λ Abs.max   FD  of the second compound FD, a part of the excitons in the state of excited singlet energy level S 1   DF  of the first compound DF is converted to the excited triplet energy level T 1   DF  by Inter System Crossing (ISC). The excitons at the triplet energy level T 1   DF  in the first compound is not converted upwardly its excited singlet energy level S 1   DF  by RISC and thus, triplet excitons remained at the excited triplet energy level T 1   DF  are generated. As such triplet excitons interact with peripheral triplet excitons or polarons, they are quenched by TTA and/or TPA. 
     In addition, HOMO energy levels and/or LUMO energy levels among the third compound H of the host, the first compound DF of the delayed fluorescent material and the second compound FD of the fluorescent material in the EML  240  should 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 third compound H of the host, the first compound DF of the delayed fluorescent material and the second compound FD of the fluorescent material should be adjusted. 
     For example, the HOMO energy level HOMO H  of the third compound H of the host can be deeper than the HOMO energy level HOMO DF  of the first compound DF of the delayed fluorescent material, and the LUMO energy level LUMO H  of the third compound H can be shallower than the LUMO energy level LUMO DF  of the first compound DF. In other words, the energy level bandgap between the HOMO energy level HOMO H  and the LUMO energy level LUMO H  of the third compound H can be wider than the energy level bandgap between the HOMO energy level HOMO DF  and the LUMO energy level LUMO DF  of the first compound. DF. 
     As an example, an energy level bandgap (|HOMO H −HOMO DF |) between the HOMO energy level (HOMO H ) of the third compound H and the HOMO energy level (HOMO DF ) of the first compound DF, or an energy level bandgap (|LUMO H −LUMO DF |) between the LUMO energy level (LUMO H ) of the third compound H and the LUMO energy level (LUMO DF ) of the first compound DF 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 third compound H to the first compound DF and thereby enhancing the ultimate luminous efficiency in the OLED D1. 
     In addition, the energy level bandgap (|HOMO DF −HOMO FD |) between the HOMO energy level HOMO DF  of the first compound DF and the HOMO energy level HOMO FD  of the second compound can be less than about 0.3 eV, for example, equal to or less than about 0.2 eV. In this case, the holes injected into the EML  240  can be transferred rapidly to the first compound DF. Accordingly, the first compound DF can implement 100% of internal quantum efficiency utilizing both the initial singlet exciton energy and the singlet exciton energy converted from the triplet exciton energy by RISC mechanism and the first compound DF can transfer the exciton energy efficiently to the second compound FD. 
     In another aspect, the LUMO energy level LUMO DF  of the first compound DF can be identical to or shallower than the LUMO energy level LUMO FD  of the second compound FD. As an example, the energy level bandgap between the LUMO energy level LUMO DF  of the first compound DF and the LUMO energy level LUMO FD  of the second compound can be equal to or less than about 0.5 eV, for example, about 0.2 eV. In this case, the electrons injected into the EML  240  can be transferred rapidly to the first compound DF. 
     On the contrary, when the energy level bandgap (|HOMO DF −HOMO FD |) between the HOMO energy level HOMO DF  of the first compound DF and the HOMO energy level HOMO FD  of the second compound FD is equal to or more than 0.3 eV, the holes injected into the EML  240  are not transferred to the first compound DF from the third compound H of the host, but are trapped in the second compound FD. The holes trapped at the second compound FD are directly recombined to form excitons with emission. Since the triplet exciton energy of the first compound DF is quenched without contributing to the light emission, the luminous efficiency of the EML  240  is reduced. 
     In addition, when the LUMO energy level LUMO DF  of the first compound DF is deeper than the LUMO energy level LUMO FD  of the second compound FD, an exciplex between the holes trapped in the second compound FD and the electrons transferred to the first compound FD is formed. As the triplet exciton energy of the first compound DF is quenched with non-emission, the luminous efficiency in the EML  240  can be deteriorated. In addition, as the energy bandgap between the LUMO energy level and the HOMO energy level forming the exciplex are excessively narrow, light with longer wavelengths are emitted. As the first compound DF as well as the second compound FD emits light simultaneously, the EML  240  emits light with deteriorated color purity owing to wider FWHM. 
     As an example, the first compound DF can have, but is not limited to, the HOMO energy level HOMO DF  between about −5.5 eV and about −5.7 eV and the LUMO energy level LUMO DF  between about −2.5 eV and about 2.8 eV. The second compound FD can have, but is not limited to, the HOMO energy level HOMO FD  between about −5.3 eV and about −5.6 eV and the LUMO energy level LUMO FD  between about −2.7 eV and about −2.9 eV. 
     The energy level bandgap between the HOMO energy level HOMO DF  and the LUMO energy level LUMO DF  of the first compound DF can be wider than the energy level bandgap between the HOMO energy level HOMO FD  and the LUMO energy level LUMO FD  of the second compound FD. In one aspect, the energy level bandgap between the HOMO energy level HOMO DF  and the LUMO energy level LUMO DF  of the first compound DF can be between about 2.6 eV and about 3.1 eV, for example, about 2.7 eV and about 3.0 eV. The energy level bandgap between the HOMO energy level HOMO FD  and the LUMO energy level LUMO FD  of the second compound FD can be between about 2.4 eV and about 2.9 eV, for example, about 2.5 eV and about 2.8 eV. In this case, the exciton energies generated in the first compound DF can be transferred efficiently to the second compound FD in which enough light emissions is occurred. 
     In case of adjusting the photoluminescence wavelength ranges of the first compound DF and absorbance wavelength ranges of the second compound FD, and HOMO and LUMO energy levels between those compounds, excitons can be recombined in the first compound DF of the delayed fluorescent material, and therefore, 100% of internal quantum efficiency can be realized using RISC mechanism. The excited singlet exciton energy generated in the first compound DF via RISC is transferred to the second compound FD of the fluorescent material by FRET, and the efficient light emission in the second compound FD can be occurred. Accordingly, the OLED D1 having excellent color purity can be realized. 
     Now, we will describe the luminous mechanism in the EML  240 .  FIG.  7    is a schematic diagram illustrating luminous mechanism by singlet and triplet energy levels among luminous materials in an EML in accordance with one aspect of the present disclosure. As schematically illustrated in  FIG.  7   , the singlet energy level S 1   H  of the third compound H, which can be the host in the EML  240 , is higher than the singlet energy level S 1   DF  of the first compound DF having the delayed fluorescent property. In addition, the triplet energy level T 1   H  of the third compound H can be higher than the triplet energy level T 1   DF  of the first compound DF. As an example, the triplet energy level T 1   H  of the third compound H can be higher than the triplet energy level T 1   DF  of the first compound DF 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 third compound H is not high enough than the triplet energy level T 1   DF  and/or the singlet energy level S 1   DF  of the first compound DF, the excitons at the triplet energy level T 1   DF  of the first compound DF can be reversely transferred to the triplet energy level T 1   H  of the third compound H. In this case, the triplet exciton reversely transferred to the third 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 having the delayed fluorescent property cannot contribute to luminescence. As an example, the first compound DF having the delayed fluorescent property can have the energy level bandgap ΔE ST  between the singlet energy level S 1   DF  and the triplet energy level T 1   DF  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 in the first compound DF of the delayed fluorescent material that is converted to ICT complex by RISC in the EML  240 , should be efficiently transferred to the second compound FD of the fluorescent material so as to implement OLED D1 having high luminous efficiency and high color purity. To this end, the singlet energy level S 1   DF  of the first compound DF of the delayed fluorescent material is higher than the singlet energy level S 1   FD  of the second compound FD of the fluorescent material. Optionally, the triplet energy level T 1   DF  of the first compound DF can be higher than the triplet energy level T 1   FD  of the second compound FD. 
     Since the second compound FD can utilize both the singlet exciton energy and the triplet exciton energy of the first compound DF, the luminous efficiency of the OLED D1 can be maximized. In addition, the luminous lifespan of the OLED D1 can be greatly improved owing to minimizing quenching phenomena such as TTA and/or TPA. 
     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 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 D1. 
     The HTL  260  is disposed between the HIL  250  and the EML  240 . In one 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, 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 aspect, the ETL  270  can include, but is not limited to, any one of oxadiazole-based compounds, triazole-based compounds, phenanthroline-based compounds, benzoxazole-based compounds, benzothiazole-based compounds, benzimidazole-based compounds, triazine-based 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-N1,08)-(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-phenanthroline (BCP), 3-(4-Biphenyl)-4-phenyl 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), 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 D1. In one 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 D1 can have short lifespan and reduced luminous efficiency. In order to prevent these phenomena, the OLED D1 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 D1 of one aspect includes the EBL  265  between the HTL  260  and the EML  240  so as to control and prevent electron transfers. In one 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-3-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 D1 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 aspect, the HBL  275  can comprise, but is not limited to, any one of oxadiazole-based compounds, triazole-based compounds, phenanthroline-based compounds, benzoxazole-based compounds, benzothiazole-based compounds, benzimidazole-based compounds, and triazine-based 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), DPEPO, 9-(6-(9H-carbazol-9-yl)pyridine-3-yl)-9H-3,9′-bicarbazole and combination thereof. 
     In the above aspect, the first compound DF having the delayed fluorescent material and the second compound FD having the fluorescent material are included within the same EML. Unlike that aspect, the first compound and the second compound are included in separate EMLs. 
       FIG.  8    is a schematic cross-sectional view illustrating an OLED in accordance with another aspect of the present disclosure.  FIG.  9    is a schematic diagram illustrating luminous mechanism by singlet and triplet energy levels among luminous materials in EMLs in accordance with another aspect of the present disclosure. 
     As illustrated in  FIG.  8   , the OLED D2 includes first and second electrodes  210  and  230  facing each other and an emissive layer  220 A having single emitting part disposed between the first and second electrodes  210  and  230 . 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 D2 can be disposed in the blue pixel region. 
     In one aspect, the emissive layer  220 A includes an EML  240 A. Also, the emissive layer  220 A can include at least one of an HTL  260  disposed between the first electrode  210  and the EML  240 A and an ETL  270  disposed between the second electrode  230  and the EML  240 A. Also, the emissive layer  220 A can further comprise 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 A can further comprise an EBL  265  disposed between the HTL  260  and the EML  240 A and/or an HBL  275  disposed between the EML  240 A and the ETL  270 . The configuration of the first and second electrodes  210  and  230  as well as other layers except the EML  240 A in the emissive layer  220 A can be substantially identical to the corresponding electrodes and layers in the OLED D1. 
     The EML  240 A includes a first EML (EML1, lower EML, first layer)  242  disposed between the EBL  265  and the HBL  275  and a second EML (EML2, upper EML, second layer)  244  disposed between the EML1  242  and the HBL  275 . Alternatively, the EML2  244  can be disposed between the EBL  265  and the EML1  242 . 
     One of the EML1  242  and the EML2  244  includes the first compound (first dopant) DF of the delayed fluorescent material, and the other of the EML1  242  and the EML2  244  includes the second compound (second dopant) FD of the fluorescent material. Also, each of the EML1  242  and the EML2  244  includes a third compound (Compound 3) H1 of a first host and a fourth compound (Compound 4) H2 of a second host. As an example, the EML1  242  can include the first compound DF and the third compound H1, and the EML2  244  can include the second compound FD and the fourth compound H2. 
     The first compound DF in the EML1  242  can include any delayed fluorescent material having the structure of Formulae 1 to 6. The triplet exciton energy of the first compound DF having delayed fluorescent property can be converted upwardly to its own singlet exciton energy via RISC mechanism. While the first compound DF has high internal quantum efficiency, but it has poor color purity owing to its wide FWHM. 
     The EML2  244  includes the second compound FD of the florescent material. The second compound FD includes any organic compound having the structure of Formulae 7 to 9. While the second compound FD of the fluorescent material having the structure of Formulae 7 to 9 has an advantage in terms of color purity due to its narrow FWHM (e.g., equal to or less than about 35 nm). 
     In this aspect, the singlet exciton energy as well as the triplet exciton energy of the first compound DF having the delayed fluorescent property in the EML1  242  can be transferred to the second compound FD in the EML2  244  disposed adjacently to the EML1  242  by FRET mechanism, and the ultimate light emission occurs in the second compound FD within the EML2  244 . 
     In other words, the triplet exciton energy of the first compound DF is converted upwardly to its own singlet exciton energy in the EML1  242  by RISC mechanism. Then, both the initial singlet exciton energy and the converted singlet exciton energy of the first compound DF is transferred to the singlet exciton energy of the second compound FD in the EML2  244 . The second compound FD in the EML2  244  can emit light using the triplet exciton energy as well as the singlet exciton energy. 
     As the singlet exciton energy generated in the first compound DF in the EML1  242  is efficiently transferred to the second compound FD in the EML2  244 , the OLED D2 can implement hyper fluorescence. In this case, while the first compound DF having the delayed fluorescent property only acts as transferring exciton energy to the second compound FD, substantial light emission is occurred in the EML2  244  including the second compound FD. The quantum efficiency and the color purity with narrow FWHM of the OLED D2 can be enhanced. 
     Each of the EML1  242  and the EML2  244  includes the third compound H1 and the fourth compound H2, respectively. The third compound H1 can be identical to or different from the fourth compound H2. For example, each of the third compound H1 and the fourth compound H2 can include, but is not limited to, the third compound H, as described above. 
     As described above, the onset wavelength λ onset   DF  of the first compound DF can be equal to or shorter than the maximum absorbance wavelength λ Abs.max   FD  of the second compound FD, for example, can be between about 430 nm and about 440 nm. In addition, the first and second compounds DF and FD can have the HOMO and LUMO energy levels as described above. 
     Also, an energy level bandgap (|HOMO H −HOMO DF |) between the HOMO energy levels (HOMO H1  and HOMO H2 ) of the third and fourth compounds H1 and H2 and the HOMO energy level (HOMO DF ) of the first compound DF, or an energy level bandgap (|LUMO H −LUMO DF |) between the LUMO energy levels (LUMO H1  and LUMO H2 ) of the third and fourth compounds H1 and H2 and the LUMO energy level (LUMO DF ) of the first compound DF can be equal to or less than about 0.5 eV. When the HOMO or LUMO energy level bandgap between the third and fourth compounds H1 and H2 and the first compound DF does not satisfy that condition, the exciton energy in the first compound DF can be quenched as a non-radiative recombination, or exciton energies may not be transferred efficiently to the first compound DF and/or the second compound FD from the third and fourth compounds H1 and H2, thus the internal quantum efficiency in the OLED D2 can be reduced. 
     Also, each of the exciton energies generated in each of the third compound H1 in the EML1  242  and the fourth compound H2 in the EML2  244  should be transferred primarily to the first compound DF of the delayed florescent material and then to the second compound FD of the fluorescent material in order to realize efficient light emission. As illustrated in  FIG.  9   , each of the singlet energy levels S 1   H1  and S 1   H2  of the third and fourth compounds H1 and H2 is higher than the singlet energy level S 1   DF  of the first compound DF having the delayed fluorescent property. Also, each of the triplet energy levels T 1   H1  and T 1   H2  of the third and fourth compounds H1 and H2 can be higher than the triplet energy level T 1   DF  of the first compound DF. For example, the triplet energy levels T 1   H1  and T 1   H2  of the third and fourth compound H1 and H2 can be higher than the triplet energy level T 1   DF  of the first compound DF 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 fourth compound H2 of the second host is higher than the singlet energy level S 1   FD  of the second compound FD of the fluorescent material. Optionally, the triplet energy level T 1   H2  of the fourth compound H2 can be higher than the triplet energy level T 1   FD  of the second compound FD. In this case, the singlet exciton energy generated at the fourth compound H2 can be transferred to the singlet energy of the second compound FD. 
     In addition, the singlet exciton energy, which is generated in the first compound DF having the delayed fluorescent property that is converted to ICT complex by RISC in the EML1  242 , should be efficiently transferred to the second compound FD of the fluorescent material in the EML2  244 . To this end, the singlet energy level S 1   DF  of the first compound DF of the delayed fluorescent material in the EML1  242  is higher than the singlet energy level S 1   FD  of the second compound FD of the fluorescent material in the EML2  244 . Optionally, the triplet energy level T 1   DF  of the first compound DF in the EML1  242  can be higher than the triplet energy level T 1   FD  of the second compound FD in the EML2  244 . 
     Each of the contents of the third and fourth compounds H1 and H2 in the EML1  242  and the EML2  244  can be larger than or identical to each of the contents of the first and second compounds DF and FD in the same layer, respectively. Also, the contents of the first compound DF in the EML1  242  can be larger than the contents of the second compound FD in the EML2  244 . In this case, exciton energy is efficiently transferred from the first compound DF to the second compound FD via FRET mechanism. As an example, the EML1  242  can include the first compound DF between about 1 wt. % and about 50 wt. %, for example, about 10 wt. % and about 40 wt. % such as about 20 wt. % and about 40 wt. %. The EML2  244  can include the second compound FD between about 1 wt % and about 10 wt. %, for example, about 1 wt. % and 5 wt. %. 
     In one aspect, when the EML2  244  is disposed adjacently to the HBL  275 , the fourth compound H2 in the EML2  244  can be the same material as the HBL  275 . In this case, the EML2  244  can have a hole blocking function as well as an emission function. In other words, the EML2  244  can act as a buffer layer for blocking holes. In one aspect, the HBL  275  can be omitted where the EML2  244  can be a hole blocking layer as well as an emitting material layer. 
     In another aspect, when the EML2  244  is disposed adjacently to the EBL  265 , the fourth compound H2 in the EML2  244  can be the same as the EBL  265 . In this case, the EML2  244  can have an electron blocking function as well as an emission function. In other words, the EML2  244  can act as a buffer layer for blocking electrons. In one aspect, the EBL  265  can be omitted where the EML2  244  can be an electron blocking layer as well as an emitting material layer. 
     An OLED having a triple-layered EML will be explained.  FIG.  10    is a schematic cross-sectional view illustrating an OLED in accordance with still another 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 aspect of the present disclosure. 
     As illustrated in  FIG.  10   , the OLED D3 includes first and second electrodes  210  and  230  facing each other and an emissive layer  220 B disposed between the first and second electrodes  210  and  230 . 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 D3 can be disposed in the blue pixel region. 
     In one aspect, the emissive layer  220 B having single emitting part includes a triple-layered EML  240 B. The emissive layer  220 B can include at least one of an HTL  260  disposed between the first electrode  210  and the EML  240 B and an ETL  270  disposed between the second electrode  230  and the EML  240 B. Also, the emissive layer  220 B 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 B can further include an EBL  265  disposed between the HTL  260  and the EML  240 B and/or an HBL  275  disposed between the EML  240 B and the ETL  270 . The configurations of the first and second electrodes  210  and  230  as well as other layers except the EML  240 B in the emissive layer  220 B is substantially identical to the corresponding electrodes and layers in the OLEDs D1 and D2. 
     The EML  240 B includes a first EML (EML1, middle EML, first layer)  242 , a second EML (EML2, lower EML, second layer)  244  and a third EML (EML3, upper EML, third layer)  246 . The EML1  242  is disposed between the EBL  265  and the HBL  275 , the EML2  244  is disposed between the EBL  265  and the EML1  242  and the EML3  246  is disposed between the EML1  242  and the HBL  275 . 
     The EML1  242  includes the first compound (first dopant) DF of the delayed fluorescent material. Each of the EML2  244  and the EML3  246  includes the second compound (second dopant) FD1 and a fifth compound (Compound 5, third dopant) FD2 each of which is the fluorescent material, respectively. Also, each of the EML1  242 , the EML2  244  and the EML3  246  includes the third compound H1 of the first host, the fourth compound H2 of the second host and a sixth compound (Compound 6) H3 of a third host, respectively. 
     In accordance with this aspect, both the singlet energy as well as the triplet energy of the first compound DF of the delayed fluorescent material in the EML1  242  can be transferred to the second and fifth compounds FD1 and FD2 of the fluorescent materials each of which is included in the EML2  244  and EML3  246  disposed adjacently to the EML1  242  by FRET mechanism. Accordingly, the ultimate emission occurs in the second and fifth compounds FD1 and FD2 in the EML2  244  and the EML3  246 . 
     In other words, the triplet exciton energy of the first compound DF having the delayed fluorescent property in the EML1  242  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 compound DF is transferred to the singlet exciton energy of the second and fifth compounds FD1 and FD2 in the EML2  244  and the EML3  246  because the first compound DF has the singlet energy level S 1   DF  higher than each of the singlet energy levels S 1   FD1  and S 1   FD2  of the second and fifth compounds FD1 and FD2. The singlet exciton energy of the first compound DF in the EML1  242  is transferred to the second and fifth compounds FD1 and FD2 in the EML2  244  and the EML3  246  which are disposed adjacently to the EML1  242  by FRET mechanism. 
     Both the second and fifth compounds FD1 and FD2 in the EML2  244  and EML3  246  can emit light using the singlet exciton energy as well as the triplet exciton energy derived from the first compound DF. Each of the second and fifth compounds FD1 and FD2 has narrow FWHM (e.g., equal to or less than about 35 nm) compared to the first compound DF. The quantum efficiency and color purity owing to narrow FWHM of the OLED D3 can be enhanced. The ultimate emission occurs in the EML2  244  and the EML3  246  each of which includes the second compound FD1 and the fifth compound FD2, respectively. 
     The first compound DF of the delayed fluorescent material includes any organic compound having the structure of Formulae 1 to 6. Each of the second and fifth compounds FD1 and FD2 of the fluorescent material includes independently any organic compound having the structure of Formulae 7 to 9. The third compound H1, the fourth compound H2 and the sixth compound H3 can be identical to or different from each other. For example, each of the third compound H1, the fourth compound H2 and the sixth compound H3 can independently include, but is not limited to, the third compound H as described above. 
     Similar to the first and second aspects, the onset wavelength λ onset   DF  of the first compound DF can identical to or shorter than each of the maximum absorbance wavelengths λ Abs.max   FD  of the second and fifth compound FD1 and FD2, for example, can be between about 430 nm and about 440 nm. In addition, the first compound DF1 and the second and fifth compounds FD1 and FD2 can have the HOMO and LUMO energy levels as described above. 
     Also, an energy level bandgap (|HOMO H −HOMO D |) between the HOMO energy levels (HOMO H1 , HOMO H2  and HOMO H3 ) of the third, fourth and sixth compounds H1, H2 and H3 and the HOMO energy level (HOMO DF ) of the first compound DF, or an energy level bandgap (|LUMO H −LUMO DF |) between the LUMO energy levels (LUMO H1 , LUMO H2  and LUMO H3 ) of the third, fourth and sixth compounds H1, H2 and H3 and the LUMO energy level (LUMO DF ) of the first compound DF 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 third, fourth and sixth compounds H1, H2 and H3 of the first to third hosts is higher than the singlet energy level S 1   DF  of the first compound DF having the delayed fluorescent property. Also, each of the triplet energy levels T 1   H1 , T 1   H2  and T 1   H 3 of the third, fourth and sixth compounds H1, H2 and H3 can be higher than the triplet energy level T 1   DF  of the first compound DF. 
     In addition, the singlet exciton energy, which is generated in the first compound DF having the delayed fluorescent property that is converted to ICT complex by RISC in the EML1  242 , should be efficiently transferred to each of second and fifth compounds FD1 and FD2 of the fluorescent material in the EML2  244  and the EML3  246 . To this end, the triplet energy level S 1   DF  of the first compound DF of the delayed fluorescent material in the EML1  242  is higher than each of the singlet energy levels S 1   DF  and S 1   FD2  of the second and fifth compounds FD1 and FD2 of the fluorescent material in the EML2  244  and the EML3  246 . Optionally, the triplet energy level T 1   DF  of the first compound DF in the EML1  242  can be higher than each of the triplet energy levels T 1   FD1  and T 1   FD2  of the second and fifth compounds FD1 and FD2 in the EML2  244  and the EML3  246 . 
     In addition, exciton energy transferred to each of the second and fifth compounds FD1 and FD2 from the first compound DF should not be transferred to each of the fourth and sixth compounds H2 and H3 in order to realize efficient luminescence. To this end, each of the singlet energy levels S 1   H2  and S 1   H3  of the fourth and sixth compounds H2 and H3, 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 FD1 and FD2 of the fluorescent material, respectively. Optionally, each of the triplet energy levels T 1   H2  and T 1   H3  of the fifth and sixth compounds H2 and H3 is higher than each of the triplet energy levels T 1   FD1  and T 1   FD2  of the third and sixth compounds FD1 and FD2, respectively. 
     As an example, the EML1  242  can include the first compounds DF between about 1 wt % and about 50 wt %, for example, about 10 wt % and about 40 wt % or about 20 wt % and about 40 wt %. Each of the EML2  244  and the EML3  246  can include the second and fifth compounds FD1 and FD2 between about 1 wt % and about 10 wt %, for example, about 1 wt % and 5 wt %. 
     In one aspect, when the EML2  244  is disposed adjacently to the EBL  265 , the fourth compound H2 in the EML2  244  can be the same material as the EBL  265 . In this case, the EML2  244  can have an electron blocking function as well as an emission function. In other words, the EML2  244  can act as a buffer layer for blocking electrons. In one aspect, the EBL  265  can be omitted where the EML2  244  can be an electron blocking layer as well as an emitting material layer. 
     When the EML3  246  is disposed adjacently to the HBL  275 , the sixth compound H3 in the EML3  246  can be the same material as the HBL  275 . In this case, the EML3  246  can have a hole blocking function as well as an emission function. In other words, the EML3  246  can act as a buffer layer for blocking holes. In one aspect, the HBL  275  can be omitted where the EML3  246  can be a hole blocking layer as well as an emitting material layer. 
     In still another aspect, the fourth compound H2 in the EML2  244  can be the same material as the EBL  265  and the sixth compound H3 in the EML3  246  can be the same material as the HBL  275 . In this aspect, the EML2  244  can have an electron blocking function as well as an emission function, and the EML3  246  can have a hole blocking function as well as an emission function. In other words, each of the EML2  244  and the EML3  246  can act as a buffer layer for blocking electrons or hole, respectively. In one aspect, the EBL  265  and the HBL  275  can be omitted where the EML2  244  can be an electron blocking layer as well as an emitting material layer and the EML3  246  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 aspect of the present disclosure. 
     As illustrated in  FIG.  12   , the OLED D4 includes first and second electrodes  210  and  230  facing each other and an emissive layer  220 C with two emitting parts disposed between the first and second electrodes  210  and  230 . 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 D4 can be disposed in the blue pixel region. The first electrode  210  can be an anode and the second electrode  230  can be a cathode. 
     The emissive layer  220 C includes a first emitting part  320  that includes a first EML (lower EML, EML1)  340  and a second emitting part  420  that includes a second EML (upper EML, EML2)  440 . Also, the emissive layer  220 C can further include a charge generation layer (CGL)  380  disposed between the first emitting part  320  and the second emitting part  420 . 
     The CGL  380  is disposed between the first and second emitting parts  320  and  420  so that the first emitting part  320 , the CGL  380  and the second emitting part  420  are sequentially disposed on the first electrode  210 . In other words, the first emitting part  320  is disposed between the first electrode  210  and the CGL  380  and the second emitting part  420  is disposed between the second electrode  230  and the CGL  380 . 
     The first emitting part  320  includes the EML1  340 . The first emitting part  320  can further includes at least one of an HIL  350  disposed between the first electrode  210  and the EML1  340 , a first HTL (HTL1)  360  disposed between the HIL  350  and the EML1  340  and a first ETL (ETL1)  370  disposed between the EML1  340  and the CGL  380 . Alternatively, the first emitting part  320  can further include a first EBL (EBL1)  365  disposed between the HTL1  360  and the EML1  340  and/or a first HBL (HBL1)  375  disposed between the EML1  340  and the ETL1  370 . 
     The second emitting part  420  includes the EML2  440 . The second emitting part  420  can further include at least one of a second HTL (HTL2)  460  disposed between the CGL  380  and the EML2  440 , a second ETL (ETL2)  470  disposed between the EML2  440  and the second electrode  230  and an EIL  480  disposed between the ETL2  470  and the second electrode  230 . Alternatively, the second emitting part  420  can further include a second EBL (EBL2)  465  disposed between the HTL2  460  and the EML2  440  and/or a second HBL (HBL2)  475  disposed between the EML2  440  and the ETL2  470 . 
     The CGL  380  is disposed between the first emitting part  320  and the second emitting part  420 . The first emitting part  320  and the second emitting part  420  are connected via the CGL  380 . The CGL  380  can be a PN-junction CGL that junctions an N-type CGL (N-CGL)  382  with a P-type CGL (P-CGL)  384 . 
     The N-CGL  382  is disposed between the ETL1  370  and the HTL2  460  and the P-CGL  384  is disposed between the N-CGL  382  and the HTL2  460 . The N-CGL  382  transports electrons to the EML1  340  of the first emitting part  320  and the P-CGL  384  transport holes to the EML2  440  of the second emitting part  420 . 
     In this aspect, each of the EML1  340  and the EML2  440  can be a blue emitting material layer. For example, at least one of the EML1  340  and the EML2  440  can include the first compound DF of the delayed fluorescent material, the second compound FD of the fluorescent material, and optionally the third compound H of the host. 
     As an example, when the EML1  340  and/or the EML2  440  includes the first to third compounds DF, FD and H, the contents of the third compound H in the EML1  340  and/or the EML2  440  can be larger than or equal to the contents of the first compound DF, and the contents of the first compound DF can be larger than the contents of the second compound FD. In this case, exciton energy can be transferred efficiently from the first compound DF to the second compound FD. 
     In one aspect, the EML2  440  can include the first and second compounds DF and FD, and optionally the third compound H as the same as the EML1  340 . Alternatively, the EML2  440  can include another compound that is different from at least one of the first compound DF and the second compound FD in the EML1  340 , and thus the EML2  440  can emit light different from the light emitted from the EML1  340  or can have different luminous efficiency different from the luminous efficiency of the EML1  340 . 
     In  FIG.  12   , each of the EML1  340  and the EML2  440  has a single-layered structure. Alternatively, each of the EML1  340  and the EML2  440 , each of which can include the first to third compounds DF, FD and H, can have a double-layered structure ( FIG.  8   ) or a triple-layered structure ( FIG.  10   ), respectively. 
     In the OLED D4, the singlet exciton energy of the first compound DF of the delayed fluorescent material is transferred to the second compound FD of fluorescent material, and the ultimate emission is occurred at the second compound FD. Accordingly, the luminous efficiency and color purity of the OLED D4 can be improved. Particularly, at least the EML1  340  includes the first compound DF having the structure of Formulae 1 to 6 and the second compound FD having the structure of Formulae 7 to 9, and thus the luminous efficiency and color purity of the OLED D4 can be further enhanced. Moreover, since the OLED D4 has a double stack structure of a blue emitting material layer, the color sense of the OLED D4 can be further improved and the luminous efficiency of the OLED D4 can be further optimized. 
       FIG.  13    is a schematic cross-sectional view illustrating an organic light emitting display device in accordance with another aspect of the present disclosure. As illustrated in  FIG.  13   , an organic light emitting display device  500  includes a substrate  510  that defines first to third pixel regions P 1 , P 2  and P 3 , a thin film transistor Tr disposed over the substrate  510  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 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  510  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  512  is disposed over the substrate  510  and the thin film transistor Tr is disposed over the buffer layer  512 . The buffer layer  512  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  550  is disposed over the thin film transistor Tr. The passivation layer  550  has a flat top surface and includes a drain contact hole  552  that exposes a drain electrode of the thin film transistor Tr. 
     The OLED D is disposed over the passivation layer  550 , and includes a first electrode  610  that is connected to the drain electrode of the thin film transistor Tr, and an emissive layer  620  and a second electrode  630  each of which is disposed sequentially on the first electrode  610 . 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  610  is separately formed for each of the first to third pixel regions P 1 , P 2  and P 3 , and the second electrode  630  corresponds to the first to third pixel regions P 1 , P 2  and P 3  and is formed integrally. 
     The first electrode  610  can be one of an anode and a cathode, and the second electrode  630  can be the other of the anode and the cathode. In addition, one of the first electrode  610  and the second electrode  630  can be a transmissive (or semi-transmissive) electrode and the other of the first electrode  610  and the second electrode  630  can be a reflective electrode. 
     For example, the first electrode  610  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  630  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  610  can include any one of ITO, IZO, ITZO, SnO, ZnO, ICO and AZO, and the second electrode  630  can include Al, Mg, Ca, Ag, alloy thereof (e.g. Mg—Ag) or combination thereof. 
     When the organic light emitting display device  500  is a bottom-emission type, the first electrode  610  can have a single-layered structure of a transparent conductive oxide layer. Alternatively, when the organic light emitting display device  500  is a top-emission type, a reflective electrode or a reflective layer can be disposed under the first electrode  610 . 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  610  can have a triple-layered structure of ITO/Ag/ITO or ITO/APC/ITO. Also, the second electrode  630  is thin so as to have light-transmissive (or semi-transmissive) property. 
     A bank layer  560  is disposed on the passivation layer  550  in order to cover edges of the first electrode  610 . The bank layer  560  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  610 . 
     An emissive layer  620  is disposed on the first electrode  610 . In one aspect, the emissive layer  620  can have a single-layered structure of an EML. Alternatively, the emissive layer  620  can include at least one of an HIL, an HTL, and an EBL disposed sequentially between the first electrode  610  and the EML and/or an HBL, an ETL and an EIL disposed sequentially between the EML and the second electrode  630 . 
     In one aspect, the EML of the emissive layer  630  in the first pixel region P 1  of the blue pixel region can include the first compound DF of the delayed fluorescent material having the structure of Formulae 1 to 6, the second compound FD of the fluorescent material having the structure of Formula 7 to 9, and optionally the third compound H. 
     An encapsulation film  570  is disposed over the second electrode  630  in order to prevent outer moisture from penetrating into the OLED D. The encapsulation film  570  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  500  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  500  is a bottom-emission type, the polarizer can be disposed under the substrate  510 . Alternatively, when the organic light emitting display device  500  is a top emission type, the polarizer can be disposed over the encapsulation film  570 . 
       FIG.  14    is a schematic cross-sectional view illustrating an OLED in accordance with still another aspect of the present disclosure. As illustrated in  FIG.  14   , the OLED D5 includes a first electrode  610 , a second electrode  630  facing the first electrode  610  and an emissive layer  620  disposed between the first and second electrodes  610  and  630 . 
     The first electrode  610  can be an anode and the second electrode  630  can be a cathode. As an example, the first electrode  610  can be a reflective electrode and the second electrode  630  can be a transmissive (or semi-transmissive) electrode. 
     The emissive layer  620  includes an EML  640 . The emissive layer  620  can include at least one of an HTL  660  disposed between the first electrode  610  and the EML  640  and an ETL  670  disposed between the EML  640  and the second electrode  630 . Also, the emissive layer  620  can further include at least one of an HIL  650  disposed between the first electrode  610  and the HTL  660  and an EIL  680  disposed between the ETL  670  and the second electrode  630 . In addition, the emissive layer  620  can further include at least one of an EBL  665  disposed between the HTL  660  and the EML  640  and an HBL  675  disposed between the EML  640  and the ETL  670 . 
     In addition, the emissive layer  620  can further include an auxiliary hole transport layer (auxiliary HTL)  662  disposed between the HTL  660  and the EBL  665 . The auxiliary HTL  662  can include a first auxiliary HTL  662   a  located in the first pixel region P 1 , a second auxiliary HTL  662   b  located in the second pixel region P 2  and a third auxiliary HTL  662   c  located in the third pixel region P 3 . 
     The first auxiliary HTL  662   a  has a first thickness, the second auxiliary HTL  662   b  has a second thickness and the third auxiliary HTL  662   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 D5 has a micro-cavity structure. 
     Owing to the first to third auxiliary HTLs  662   a ,  662   b  and  662   c  having different thickness to each other, the distance between the first electrode  610  and the second electrode  630  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  610  and the second electrode  630  in the second pixel region P 2  emitting light in the second wavelength range (green light), which is longer than the first wavelength range. In addition, the distance between the first electrode  610  and the second electrode  630  in the second pixel region P 2  is smaller than the distance between the first electrode  610  and the second electrode  630  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 luminous efficiency of the OLED D5 is improved. 
     In  FIG.  14   , the first auxiliary HTL  662   a  is located in the first pixel region P 1 . Alternatively, the OLED D5 can implement the micro-cavity structure without the first auxiliary HTL  662   a . In addition, a capping layer can be disposed over the second electrode  630  in order to improve out-coupling of the light emitted from the OLED D5. 
     The EML  640  includes a first EML (EML1)  642  located in the first pixel region P 1 , a second EML (EML2)  644  located in the second pixel region P 2  and a third EML (EML3)  646  located in the third pixel region P 3 . Each of the EML1  642 , the EML2  644  and the EML3  646  can be a blue EML, a green EML and a red EML, respectively. 
     In one aspect, the EML1  642  located in the first pixel region P 1  can include the first compound of the delayed fluorescent material having the structure of Formulae 1 to 6, the second compound FD of the fluorescent material having the structure of Formulae 7 to 9, and optionally the third compound H of the host. The EML1  642  can have a single-layered structure, a double-layered structure ( FIG.  8   ) or a triple-layered structure ( FIG.  10   ). 
     In the EML1  642 , the contents of the third compound H can be larger than or equal to the contents of the first compound DF, and the contents of the first compound DF can be larger than the contents of the second compound FD. In this case, exciton energy can be transferred efficiently from the first compound DF to the second compound FD. 
     The EML2  644  located in the second pixel region P 2  can include a host and a green dopant and the EML3  646  located in the third pixel region P 3  can include a host and a red dopant. For example, the host in the EML2  644  and the EML3  646  can include the third compound H, and each of the green dopant and the red dopant can include independently at least one of green or red phosphorescent material, green or red fluorescent material and green or red delayed fluorescent material. 
     The OLED D5 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  500  ( FIG.  13   ) can implement a full-color image. 
     The organic light emitting display device  500  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  500  is a bottom-emission type, the color filter layer can be disposed between the OLED D and the substrate  510 . Alternatively, when the organic light emitting display device  500  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 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 blue pigment, the second color filter pattern  1024  can include at least one of green dye or green pigment and the third color filter pattern  1026  can include at least one of red dye or red 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  that is 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 of the delayed fluorescent material having the structure of Formulae 1 to 6, the second compound FD of the boron-based fluorescent material having the structure of Formulae 7 to 9, and optionally the third compound H of the host. 
     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  with the top-emission type structure. 
     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 green color conversion film, a red color conversion film and a blue color conversion film each of which can convert the white light emitted from the OLED D into green light, red light and blue 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 aspect of the present disclosure. As illustrated in  FIG.  16   , the OLED D6 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, EML1)  1240 , a second emitting part  1320  comprising a second EML (middle EML, EML2)  1340  and a third emitting part  1420  comprising a third EML (upper EML, EML3)  1440 . In addition, the emissive layer  1120  can further includes a first charge generation layer (CGL1)  1280  disposed between the first emitting part  1220  and the second emitting part  1320  and a second charge generation layer (CGL2)  1380  disposed between the second emitting part  1320  and the third emitting part  1420 . Accordingly, the first emitting part  1220 , the CGL1  1280 , the second emitting part  1320 , the CGL2  1380  and the third emitting part  1420  are disposed sequentially over 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 EML1  1240 , a first HTL (HTL1)  1260  disposed between the EML1  1240  and the HIL  1250  and a first ETL (ETL1)  1270  disposed between the EML1  1240  and the CGL1  1280 . Alternatively, the first emitting part  1220  can further include at least one of a first EBL (EBL1)  1265  disposed between the HTL1  1260  and the EML1  1240  and a first HBL (HBL1)  1275  disposed between the EML1  1240  and the ETL1  1270 . 
     The second emitting part  1320  can further include at least one of a second HTL (HTL2)  1360  disposed between the CGL1  1280  and the EML2  1340  and a second ETL (ETL2) 1370 disposed between the EML2  1340  and the CGL2  1380 . Alternatively, the second emitting part  1320  can further include a second EBL (EBL2)  1365  disposed between the HTL2  1360  and the EML2  1340  and/or a second HBL (HBL2)  1375  disposed between the EML2  1340  and the ETL2  1370 . 
     The third emitting part  1420  can further include at least one of a third HTL (HTL3)  1460  disposed between the CGL2  1380  and the EML3  1440 , a third ETL (ETL3)  1470  disposed between the EML3  1440  and the second electrode  1130  and an EIL  1480  disposed between the ETL3  1470  and the second electrode  1130 . Alternatively, the third emitting part  1420  can further comprise a third EBL (EBL3)  1465  disposed between the HTL3  1460  and the EML3  1440  and/or a third HBL (HBL3)  1475  disposed between the EML3  1440  and the ETL3  1470 . 
     The CGL1  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 CGL1  1280 . The CGL1  1280  can be a PN-junction CGL that junctions a first N-type CGL (N-CGL1)  1282  with a first P-type CGL (P-CGL1)  1284 . 
     The N-CGL1  1282  is disposed between the ETL1  1270  and the HTL2  1360  and the P-CGL1  1284  is disposed between the N-CGL1  1282  and the HTL2  1360 . The N-CGL1  1282  transports electrons to the EML1  1240  of the first emitting part  1220  and the P-CGL1  1284  transport holes to the EML2  1340  of the second emitting part  1320 . 
     The CGL2  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 CGL2  1380 . The CGL2  1380  can be a PN-junction CGL that junctions a second N-type CGL (N-CGL2)  1382  with a second P-type CGL (P-CGL2)  1384 . 
     The N-CGL2  1382  is disposed between the ETL2  1370  and the HTL3  1460  and the P-CGL2  1384  is disposed between the N-CGL2  1382  and the HTL3  1460 . The N-CGL2  1382  transports electrons to the EML2  1340  of the second emitting part  1320  and the P-CGL2  1384  transport holes to the EML3  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 EML1  1240  can be a blue EML, the EML2  1340  can be a green EML and the EML3  1440  can be a red EML. Alternatively, the EML1  1240  can be a red EML, the EML2  1340  can be a green EML and the EML3  1440  can be a blue EML. Hereinafter, the OLED D6 where the EML1  1240  is a blue EML, the EML2  1340  is a green EML and the EML3  1440  is a red EML will be described in more detail. 
     The EML1  1240  can include the first compound DF of the delayed fluorescent material having the structure of Formulae 1 to 6, the second compound FD of the fluorescent material having the structure of Formulae 7 to 9, and optionally, the third compound H of the host. The EML1  1240  including the first to third compounds DF, FD and H can have a single-layered structure, a double-layered structure ( FIG.  8   ) or a triple-layered structure ( FIG.  10   ). 
     In the EML1  1240 , the contents of the third compound H can be equal to or larger than the contents of the first compound DF and the contents of the first compound DF can be larger than the contents of the second compound FD. When the contents of the first compound DF is larger than the contents of the second compound FD, exciton energy from the first compound DF to the second compound FD can be transferred sufficiently. 
     The EML2  1340  can include a host and a green dopant and the EML3  1440  can include a host a red dopant. As an example, the host can include the third compound H, and each of the green and red dopants can include at least one of green and red phosphorescent material, green and red fluorescent material and green and red delayed fluorescent material, respectively, in each of the EML2  1340  and the EML3  1440 . 
     The OLED D6 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 aspect of the present disclosure. As illustrated in  FIG.  17   , the OLED D7 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 EML1 (lower EML)  1540 , a second emitting part  1620  comprising an EML2 (middle EML)  1640  and a third emitting part  1720  comprising an EML3 (upper EML)  1740 . In addition, the emissive layer  1120 A can further include a CGL1  1580  disposed between the first emitting part  1520  and the second emitting part  1620  and a CGL2  1680  disposed between the second emitting part  1620  and the third emitting part  1720 . Accordingly, the first emitting part  1520 , the CGL1  1580 , the second emitting part  1620 , the CGL2  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 EML1  1540 , an HTL1  1560  disposed between the EML1  1540  and the HIL  1550  and an ETL1  1570  disposed between the EML1  1540  and the CGL1  1580 . Alternatively, the first emitting part  1520  can further comprise an EBL1  1565  disposed between the HTL1  1560  and the EML1  1540  and/or an HBL1  1575  disposed between the EML1  1540  and the ETL1  1570 . 
     The EML2  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 HTL2  1660  disposed between the CGL1  1580  and the EML2  1640 , an ETL2  1670  disposed between the EML2  1640  and the CGL2  1680 . Alternatively, the second emitting part  1620  can further comprise at least one of an EBL2  1665  disposed between the HTL2  1660  and the EML2  1640  and an HBL2  1675  disposed between the EML2  1640  and the ETL2  1670 . 
     The third emitting part  1720  can further include at least one of an HTL3  1760  disposed between the CGL2  1680  and the EML3  1740 , an ETL3  1770  disposed between the EML3  1740  and the second electrode  1130  and an EIL  1780  disposed between the ETL3  1770  and the second electrode  1130 . Alternatively, the third emitting part  1720  can further include an EBL3  1765  disposed between the HTL3  1760  and the EML3  1740  and/or an HBL3  1775  disposed between the EML3  1740  and the ETL3  1770 . 
     The CGL1  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 CGL1  1580 . The CGL1  1580  can be a PN-junction CGL that junctions an N-CGL1  1582  with a P-CGL1  1584 . The N-CGL1  1582  is disposed between the ETL1  1570  and the HTL2  1660  and the P-CGL1  1584  is disposed between the N-CGL1  1582  and the HTL2  1560 . 
     The CGL2  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 CGL2  1680 . The CGL2  1680  can be a PN-junction CGL that junctions an N-CGL2  1682  with a P-CGL2  1684 . The N-CGL2  1682  is disposed between the ETL2  1570  and the HTL3  1760  and the P-CGL2  1684  is disposed between the N-CGL2  1682  and the HTL3  1760 . 
     In this aspect, each of the EML1  1540  and the EML3  1740  can be a blue EML. In an aspect, each of the EML1  1540  and the EML3  1740  can include the first compound DF of the delayed fluorescent material having the structure of Formulae 1 to 6, the second compound FD of the fluorescent material having the structure of Formulae 7 to 9, and optionally, the third compound H. 
     In one aspect, the EML3  1740  can include the first and second compounds DF and FD, and optionally the third compound H as the same as the EML1  1540 . Alternatively, the EML3  1740  can include another compound that is different from at least one of the first compound DF and the second compound FD in the EML1  1540 , and thus the EML3  1740  can emit light different from the light emitted from the EML1  1540  or can have different luminous efficiency different from the luminous efficiency of the EML1  1540 . 
     As an example, each of the EML1  1540  and the EML3  1740  includes the first to third compounds DF, FD and H, the contents of the third compound H can be equal to or larger than the contents of the first compound DF and the contents of the first compound DF can be larger than the contents of the second compound FD in each of the EML1  1540  and the EML3  1740 . In this case, energy from the first compound DF to the second compound FD can be transferred sufficiently. 
     One of the middle lower EML  1642  and the middle upper EML  1644  of the EML2  1640  can be a green EML and the other of the middle lower EML  1642  and the middle upper EML  1644  of the EML2  1640  can be a red EML. The green EML and the red EML are sequentially disposed to form the EML2  1640 . 
     As an example, the middle lower EML  1642  of the green EML can include the host and the green dopant and the middle upper EML  1644  can include the host and the red dopant. As an example, the host in the middle lower EML  1642  and the middle upper EML  1644  can include the third compound H, and each of the green and red dopants can include at least one of the green and red phosphorescent material, the green and red fluorescent material and the green and red delayed fluorescent material, respectively. 
     The OLED D7 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.  16   ) can implement a full-color image. 
     In  FIG.  17   , the OLED D7 has a three-stack structure including the first to three emitting parts  1520 ,  1620  and  1720  which includes the EML1  1540  and the EML3  1740  as a blue EML. Alternatively, the OLED D7 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 EML1  1540  and the EML3  1740  as a blue EML is omitted. 
     Example 1 (Ex. 1): Fabrication of OLED 
     An OLED in which an EML includes 2,8-di(9H-carbazol yl)dibenzo[b,d]thiophene (DCzDBT) as a host and the Compound 1-1 of Formula 6 (HOMO: −5.58 eV, LUMO: −2.6 eV, maximum photoluminescence wavelength (PL λmax): 472 nm, onset wavelength: 433 nm) as the first compound DF was fabricated. The ITO substrate was washed by UV-Ozone treatment 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, 7 nm); an HTL (NPB, 45 nm); an EBL (TAPC, 10 nm), an EML (DCzDBT (70 wt %), Compound 1-1 (30 wt %), 30 nm); an HBL (B3PYMPM, 10 nm); an ETL (TPBi, 25 nm), an EIL (LiF); and a cathode (Al). 
     After the emissive layer and the cathode were deposited, the OLED was transferred from the deposition chamber to a dry box in order to form a film, and then the OLED was encapsulated with UV-cured epoxy and water getter. The materials applied in the emissive layer are indicated below: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     Example 2 (Ex. 2): Fabrication of OLED 
     An OLED was fabricated using the same materials as Example 1, except that DczDBT (69 wt %) as the host, the Compound 1-1 (30 wt %) as the first compound and Compound 2-20 (HOMO: −5.4 eV, LUMO: −2.8 eV, maximum absorbance wavelength: 457 nm, 1 wt %) of Formula 9 as the second compound FD were applied in the EML. 
     Example 3 (Ex. 3): Fabrication of OLED 
     An OLED was fabricated using the same materials as Example 2, except that Compound 2-21 (HOMO: −5.5 eV, LUMO: −2.8 eV, maximum absorbance wavelength (Abs λmax): 459 nm) of Formula 9 as the second compound FD in the EML was used instead of the Compound 2-20. 
     Example 4 (Ex. 4): Fabrication of OLED 
     An OLED was fabricated using the same materials as Example 2, except that Compound 2-36 (HOMO: −5.4 eV, LUMO: −2.8 eV, Abs λmax: 457 nm) of Formula 9 as the second compound FD in the EML was used instead of the Compound 2-20. 
     Example 5 (Ex. 5): Fabrication of OLED 
     An OLED was fabricated using the same materials as Example 1, except that Compound 1-5 (HOMO: −5.58 eV, LUMO: −2.6 eV, PL λmax: 470 nm, onset wavelength: 435 nm) of Formula 6 as the first compound DF in the EML was used instead of the Compound 1-1. 
     Example 6 (Ex. 6): Fabrication of OLED 
     An OLED was fabricated using the same materials as Example 5, except DczDBT (69 wt %) as the host, the Compound 1-5 (30 wt %) as the first compound and Compound 2-20 (1 wt %) of Formula 9 (as the second compound FD were applied in the EML. 
     Example 7 (Ex. 7): Fabrication of OLED 
     An OLED was fabricated using the same materials as Example 6, except that Compound 2-21 of Formula 9 as the second compound FD in the EML was used instead of the Compound 2-20. 
     Example 8 (Ex. 8): Fabrication of OLED 
     An OLED was fabricated using the same materials as Example 6, except that Compound 2-36 of Formula 9 as the second compound FD in the EML was used instead of the Compound 2-20. 
     Example 9 (Ex. 9): Fabrication of OLED 
     An OLED was fabricated using the same materials as Example 1, except that Compound 1-7 (HOMO: −5.6 eV, LUMO: −2.7 eV, PL λmax: 472 nm, onset wavelength: 434 nm) of Formula 6 as the first compound DF in the EML was used instead of the Compound 1-1. 
     Example 10 (Ex. 10): Fabrication of OLED 
     An OLED was fabricated using the same materials as Example 9, except DczDBT (69 wt %) as the host, the Compound 1-7 (30 wt %) as the first compound and Compound 2-20 (1 wt %) of Formula 9 as the second compound FD were applied in the EML. 
     Example 11 (Ex. 11): Fabrication of OLED 
     An OLED was fabricated using the same materials as Example 10, except that Compound 2-21 of Formula 9 as the second compound FD in the EML was used instead of the Compound 2-20. 
     Example 12 (Ex. 12): Fabrication of OLED 
     An OLED was fabricated using the same materials as Example 10, except that Compound 2-36 of Formula 9 as the second compound FD in the EML was used instead of the Compound 2-20. 
     Example 13 (Ex. 13): Fabrication of OLED 
     An OLED was fabricated using the same materials as Example 1, except that Compound 1-16 (HOMO: −5.6 eV, LUMO: −2.6 eV, PL λmax: 473 nm, onset wavelength: 434 nm) of Formula 6 as the first compound DF in the EML was used instead of the Compound 1-1. 
     Example 14 (Ex. 14): Fabrication of OLED 
     An OLED was fabricated using the same materials as Example 13, except DczDBT (69 wt %) as the host, the Compound 1-16 (30 wt %) as the first compound and Compound 2-20 (1 wt %) of Formula 9 as the second compound FD were applied in the EML. 
     Example 15 (Ex. 15): Fabrication of OLED 
     An OLED was fabricated using the same materials as Example 14, except that Compound 2-21 of Formula 9 as the second compound FD in the EML was used instead of the Compound 2-20. 
     Example 16 (Ex. 16): Fabrication of OLED 
     An OLED was fabricated using the same materials as Example 14, except that Compound 2-36 of Formula 9 as the second compound FD in the EML was used instead of the Compound 2-20. 
     Comparative Example 1 (Com. 1): Fabrication of OLED 
     An OLED was fabricated using the same materials as Example 1, except that the following Ref. 1 Compound (HOMO: −5.5 eV, LUMO: −2.7 eV, PL λmax: 487 nm, onset wavelength: 449 nm) as the first compound DF in the EML was used instead of the Compound 1-1. 
     Comparative Example 2 (Com. 2): Fabrication of OLED 
     An OLED was fabricated using the same materials as Comparative Example 1, except DczDBT (69 wt %) as the host, the Ref 1 Compound (30 wt %) as the first compound and Compound 2-20 (1 wt %) of Formula 9 as the second compound FD were applied in the EML. 
     Comparative Example 3 (Com. 3): Fabrication of OLED 
     An OLED was fabricated using the same materials as Example 1, except that the following Ref. 2 Compound (HOMO: −5.6 eV, LUMO: −2.6 eV, PL λmax: 460 nm, onset wavelength: 421 nm) as the first compound DF in the EML was used instead of the Compound 1-1. 
     Comparative Example 4 (Com. 4): Fabrication of OLED 
     An OLED was fabricated using the same materials as Comparative Example 3, except DczDBT (69 wt %) as the host, the Ref 2 Compound (30 wt %) as the first compound and Compound 2-20 (1 wt %) of Formula 9 as the second compound FD were applied in the EML. 
     Comparative Example 5 (Com. 5): Fabrication of OLED 
     An OLED was fabricated using the same materials as Example 1, except that the following Ref. 3 Compound (HOMO: −5.6 eV, LUMO: −2.7 eV, PL λmax: 462 nm, onset wavelength: 426 nm) as the first compound DF in the EML was used instead of the Compound 1-1. 
     Comparative Example 6 (Com. 6): Fabrication of OLED 
     An OLED was fabricated using the same materials as Comparative Example 6, except DczDBT (69 wt %) as the host, the Ref 3 Compound (30 wt %) as the first compound and Compound 2-20 (1 wt %) of Formula 9 as the second compound FD were applied in the EML. 
     Comparative Example 7 (Com. 7): Fabrication of OLED 
     An OLED was fabricated using the same materials as Example 1, except that the following Ref. 4 Compound (HOMO: −5.7 eV, LUMO: −2.7 eV, PL λmax: 458 nm, onset wavelength: 422 nm) as the first compound DF in the EML was used instead of the Compound 1-1. 
     Comparative Example 8 (Com. 8): Fabrication of OLED 
     An OLED was fabricated using the same materials as Comparative Example 6, except DczDBT (69 wt %) as the host, the Ref 3 Compound (30 wt %) as the first compound and Compound 2-20 (1 wt %) of Formula 9 as the second compound FD were applied in the EML. 
     [Reference Compounds] 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     Experimental Example 1: Measurement of Luminous Properties of OLED 
     Each of the OLED fabricated in Ex. 1-16 and Ref 1-8 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, driving voltage (V), current efficiency (cd/A), external quantum efficiency (EQE, %) and maximum electroluminescence (EL λmax, nm) at 8.6 mA/cm 2  current density the OLEDs were measured. The measurement results for the OLEDs are shown in the following tables 1 and 2. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Luminous Properties of OLED 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Sample 
                 DF 
                 FD 
                 V 
                 cd/A 
                 EQE 
                 CIEy 
                 EL λmax 
                 λonsetDF 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 Ex. 
                 1 
                 1-1 
                 — 
                 3.49 
                 33.1 
                 18.2 
                 0.271 
                 478 
                 433 
               
               
                 Ex. 
                 2 
                 1-1 
                 2-23 
                 3.6 
                 28.2 
                 24.1 
                 0.159 
                 470 
                 433 
               
               
                 Ex. 
                 3 
                 1-1 
                 2-24 
                 3.52 
                 30.5 
                 24.6 
                 0.183 
                 474 
                 433 
               
               
                 Ex. 
                 4 
                 1-1 
                 2-39 
                 3.94 
                 34.2 
                 24.7 
                 0.197 
                 470 
                 433 
               
               
                 Ex. 
                 5 
                 1-5 
                 — 
                 3.36 
                 28.2 
                 14.6 
                 0.295 
                 480 
                 435 
               
               
                 Ex. 
                 6 
                 1-5 
                 2-23 
                 3.63 
                 26.6 
                 23.8 
                 0.153 
                 472 
                 435 
               
               
                 Ex. 
                 7 
                 1-5 
                 2-24 
                 3.48 
                 27.7 
                 21.5 
                 0.179 
                 470 
                 435 
               
               
                 Ex. 
                 8 
                 1-5 
                 2-39 
                 3.2 
                 27.5 
                 20.6 
                 0.192 
                 472 
                 435 
               
               
                 Ex. 
                 9 
                 1-7 
                 — 
                 3.32 
                 26.7 
                 15.1 
                 0.264 
                 478 
                 434 
               
               
                 Ex. 
                 10 
                 1-7 
                 2-23 
                 3.6 
                 24.5 
                 22.7 
                 0.146 
                 470 
                 434 
               
               
                 Ex. 
                 11 
                 1-7 
                 2-24 
                 3.4 
                 22.4 
                 19 
                 0.193 
                 472 
                 434 
               
               
                 Ex. 
                 12 
                 1-7 
                 2-39 
                 3.8 
                 35.8 
                 22.6 
                 0.232 
                 472 
                 434 
               
               
                 Ex. 
                 13 
                  1-16 
                 — 
                 3.4 
                 26.9 
                 14.3 
                 0.285 
                 478 
                 434 
               
               
                 Ex. 
                 14 
                  1-16 
                 2-23 
                 3.62 
                 25.3 
                 24.2 
                 0.145 
                 472 
                 434 
               
               
                 Ex. 
                 15 
                  1-16 
                 2-24 
                 3.38 
                 34.4 
                 21.6 
                 0.237 
                 472 
                 434 
               
               
                 Ex. 
                 16 
                  1-16 
                 2-39 
                 3.8 
                 35.9 
                 20.8 
                 0.26 
                 472 
                 434 
               
               
                   
               
               
                 DF: First Compound; FD: Second Compound; λonsetDF: Onset wavelength of First Compound 
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Luminous Properties of OLED 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Sample 
                 DF 
                 FD 
                 v 
                 cd/A 
                 EQE 
                 CIEy 
                 EL λmax 
                 λonsetDF 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Com. 1 
                 Ref. 1 
                 — 
                 3.46 
                 43 
                 20.1 
                 0.354 
                 490 
                 449 
               
               
                 Com. 2 
                 Ref. 1 
                 2-23 
                 3.58 
                 29.4 
                 21.2 
                 0.201 
                 472 
                 449 
               
               
                 Com. 3 
                 Ref. 2 
                 — 
                 3.56 
                 13.2 
                 7.5 
                 0.251 
                 474 
                 421 
               
               
                 Com. 4 
                 Ref. 2 
                 2-23 
                 3.6 
                 21.9 
                 14.5 
                 0.229 
                 474 
                 421 
               
               
                 Com. 5 
                 Ref. 3 
                 — 
                 3.25 
                 15.5 
                 9 
                 0.248 
                 474 
                 426 
               
               
                 Com. 6 
                 Ref. 3 
                 2-23 
                 3.31 
                 18.9 
                 16.2 
                 0.168 
                 474 
                 426 
               
               
                 Com. 7 
                 Ref. 4 
                 — 
                 3.94 
                 14.6 
                 8.9 
                 0.226 
                 470 
                 422 
               
               
                 Com. 8 
                 Ref. 4 
                 2-23 
                 4.18 
                 11.3 
                 10.9 
                 0.165 
                 472 
                 422 
               
               
                   
               
               
                 DF: First Compound; FD: Second Compound; λonsetDF: Onset wavelength of First Compound 
               
            
           
         
       
     
     As indicated in Tables 1 and 2, compared to the OLEDs fabricated in Ex. 1, 5, 9 and 13 in which the first compound as the sole dopant was applied into the EML, the OLEDs fabricated in Ex. 2-4, 6-8, 10-12 and 14-16 in which the first compound having plural electron donor moieties and onset wavelength between 430 nm and 440 nm and the second compound were applied into the EML exhibited luminous efficiency with great improvement and emitted deep blue light. On the other hand, compared to the OLEDs fabricated in Com. Com. 1, 3, 5 and 7 in which the first compound as the sole dopant was applied into the EML, the OLEDs fabricated in Com. 2, 4, 6 and 8 in which the first compound having only one electron donor moiety and onset wavelength less than 430 nm or more than 440 nm and the second compound were applied into the EML showed luminous efficiency with a little improvement or with greatly reduced. 
     More particularly, compared to the OLEDs fabricated in Com. 2, 4, 6 and 8 in which the first compound having only one electron donor moiety and the second compound were applied into the EML, the OLEDs fabricated in Ex. 2-4, 6-8, 10-12 and 14-16 in which the first compound having plural electron donor moieties and the second compound were applied into the EML reduced their driving voltages by maximally 23.4%, and improved their current efficiency and power efficiency by maximally 217.7% and 174.4%, respectively. 
     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.