Patent Publication Number: US-10332939-B2

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

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from and the benefit of Korean Patent Application No. 10-2016-0057786, filed on May 11, 2016, which is hereby incorporated by reference for all purposes as if fully set forth herein. 
     BACKGROUND 
     Field 
     Exemplary embodiments relate to an organic light emitting diode having improved light-emitting efficiency and a light emitting device including the same. 
     Discussion of the Background 
     An organic light emitting diode may emit light when an exciton is stabilized, which may be formed by the combination of holes supplied from an anode and electrons supplied from a cathode in an organic emission layer. The organic light emitting diode has several merits, such as a wide viewing angle, a fast response speed, a thin thickness, and low power consumption, such that the organic light emitting diode is widely applied to various electrical and electronic devices, such as a television, a monitor, a mobile phone, etc. However, since the organic light emitting diode may have low emission efficiency, a high driving voltage may be required to obtain high luminance for the display or as a light, which may shorten a lifespan of elements therein. 
     The above information disclosed in this Background section is only for enhancement of understanding of the background of the inventive concept, and, therefore, it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art. 
     SUMMARY 
     Exemplary embodiments provide an organic light emitting diode having improved light-emitting efficiency and a light emitting device including the same. 
     Additional aspects will be set forth in the detailed description which follows, and, in part, will be apparent from the disclosure, or may be learned by practice of the inventive concept. 
     According to an exemplary embodiment, an organic light emitting diode includes a first electrode and a second electrode overlapping each other, an emission layer disposed between the first electrode and the second electrode, and a hole transport layer disposed between the first electrode and the emission layer, the hole transport layer having a refractive index in a range of 1.0 to 1.6, in which the organic light emitting diode has a microcavity structure between the first electrode and the second electrode. 
     The first electrode may include a reflecting electrode, and the second electrode may include a transflective electrode. 
     The microcavity structure may be a primary microcavity structure that satisfies Equation 1: 
     
       
         
           
             
               
                 
                   
                     2 
                     ⁢ 
                     
                       π 
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                       q 
                     
                   
                   = 
                   
                     
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                       j 
                       
                           
                       
                     
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                       ( 
                       
                         
                           
                             2 
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                               n 
                               
                                 j 
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                           λ 
                         
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                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   1 
                 
               
             
           
         
       
     
     In which, in Equation 1, n jλ  denotes a refractive index of a j th  layer for a wavelength λ, dj denotes a thickness of a j th  layer, δ j  denotes a magnitude of a phase variation generated when light passes through the j th  layer or is reflected by a reflection surface of the first electrode or the second electrode, and q is 0. 
     The hole transport layer may contact the first electrode. 
     A thickness of the hole transport layer may be in a range of 15 nm to 40 nm. 
     A refractive index of the hole transport layer may be in a range of 1.2 to 1.5. 
     The refractive index of the hole transport layer may be in a range of 1.2 to 1.45. 
     The hole transport layer may include a hole-injection layer disposed between the first electrode and the emission layer, and a hole-transporting layer disposed between the hole-injection layer and the emission layer, in which the refractive indices of the hole-injection layer and the hole transfer layer may each be in a range of 1.2 to 1.45. 
     The emission layer may include a red emission layer, a green emission layer, and a blue emission layer, the red emission layer, the green emission layer, and the blue emission layer may be horizontally disposed in a direction parallel with the first electrode, the hole transport layer may be disposed in common between the red emission layer and the first electrode, between the green emission layer and the first electrode, and between the blue emission layer and the first electrode, and a thickness of the hole transport layer may be the same at common regions. 
     The emission layer may include layers configured to emit light of different colors, the light of different colors being combined to emit white light. 
     The layers may have a dual or triple layer structure. 
     The hole transport layer may include an amine derivative and a perfluoro oligomer. 
     The hole transport layer may include poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) or poly(3,4-ethylenedioxythiophene) polystyrene sulfonate perfluorinated ionomer(PEDOT:PSS:PFI). 
     The organic light emitting diode may have a surface plasmon fraction of 20% or less. 
     The organic light emitting diode may further include a capping layer disposed on the second electrode. 
     According to an exemplary embodiment, a light emitting device includes a substrate, a driving thin film transistor disposed on the substrate, and an organic light emitting diode connected to the driving thin film transistor, the organic light emitting diode including a first electrode and a second electrode overlapping each other, an emission layer disposed between the first electrode and the second electrode, and a hole transport layer disposed between the first electrode and the emission layer and having a refractive index that is in a range of 1.0 to 1.6, in which the organic light emitting diode has a microcavity structure between the first electrode and the second electrode. 
     The first electrode may include a reflecting electrode, and the second electrode may include a transflective electrode. 
     The first electrode may be disposed between the substrate and the second electrode. 
     A thickness of the hole transport layer may be in a range of 15 nanometer (nm) to 40 nm. 
     The microcavity structure may be a primary microcavity structure that satisfies Equation 1: 
     
       
         
           
             
               
                 
                   
                     2 
                     ⁢ 
                     
                       π 
                       · 
                       q 
                     
                   
                   = 
                   
                     
                       ∑ 
                       j 
                       
                           
                       
                     
                     ⁢ 
                     
                       ( 
                       
                         
                           
                             2 
                             ⁢ 
                             π2 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               n 
                               
                                 j 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 λ 
                               
                             
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                               d 
                               j 
                             
                           
                           λ 
                         
                         + 
                         
                           δ 
                           j 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   1 
                 
               
             
           
         
       
     
     In which, in Equation 1, n jλ  denotes a refractive index of a j th  layer for a wavelength λ, dj denotes a thickness of a j th  layer, δ j  denotes a magnitude of a phase variation generated when light passes through the j th  layer or is reflected by a reflection surface of the first electrode or the second electrode, and q is 0. 
     According to the exemplary embodiments, light-emitting efficiency of an organic light emitting diode and a light emitting device including the same may be improved by adjusting a refractive index and a thickness of a hole transport layer. 
     The foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the claimed subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawings will be provided by the office upon request and payment of the necessary fee. 
       The accompanying drawings, which are included to provide a further understanding of the inventive concept, and are incorporated in and constitute a part of this specification, illustrate exemplary embodiments of the inventive concept, and, together with the description, serve to explain principles of the inventive concept. 
         FIG. 1  is a cross-sectional view illustrating an organic light emitting diode according to an exemplary example embodiment of the present disclosure. 
         FIG. 2  is a graph illustrating radiance according to a refractive index and a thickness of a hole transport layer. 
         FIG. 3  is a graph illustrating a surface plasmon fraction according to a refractive index and a thickness of a hole transport layer. 
         FIG. 4  is a graph illustrating an anti-node thickness and a surface plasmon fraction according to a refractive index of a hole transport layer. 
         FIG. 5  is a graph illustrating relative average contributions in four modes according to a thickness of a hole transport layer in a comparative embodiment. 
         FIG. 6  is a graph illustrating relative average contributions in four modes according to a thickness of a hole transport layer according to an exemplary embodiment of the present disclosure. 
         FIG. 7  is a graph illustrating radiance depending on thicknesses of a hole transport layer and an electron-transporting layer in a comparative embodiment. 
         FIG. 8  is a graph illustrating radiance according to thicknesses of a hole transport layer and an electron-transporting layer according to an exemplary embodiment of the present disclosure. 
         FIG. 9  is a cross-sectional view illustrating an organic light emitting diode including an emission layer including a plurality of layers according to an exemplary embodiment of the present disclosure. 
         FIG. 10  is a cross-sectional view illustrating a light emitting device according to an exemplary embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS 
     In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of various exemplary embodiments. It is apparent, however, that various exemplary embodiments may be practiced without these specific details or with one or more equivalent arrangements. In other instances, well-known structures and devices are shown in block diagram form in order to avoid unnecessarily obscuring various exemplary embodiments. 
     In the accompanying figures, the size and relative sizes of layers, films, panels, regions, etc., may be exaggerated for clarity and descriptive purposes. Also, like reference numerals denote like elements. 
     When an element or layer is referred to as being “on,” “connected to,” or “coupled to” another element or layer, it may be directly on, connected to, or coupled to the other element or layer or intervening elements or layers may be present. When, however, an element or layer is referred to as being “directly on,” “directly connected to,” or “directly coupled to” another element or layer, there are no intervening elements or layers present. For the purposes of this disclosure, “at least one of X, Y, and Z” and “at least one selected from the group consisting of X, Y, and Z” may be construed as X only, Y only, Z only, or any combination of two or more of X, Y, and Z, such as, for instance, XYZ, XYY, YZ, and ZZ. Like numbers refer to like elements throughout. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     Although the terms first, second, etc. may be used herein to describe various elements, components, regions, layers, and/or sections, these elements, components, regions, layers, and/or sections should not be limited by these terms. These terms are used to distinguish one element, component, region, layer, and/or section from another element, component, region, layer, and/or section. Thus, a first element, component, region, layer, and/or section discussed below could be termed a second element, component, region, layer, and/or section without departing from the teachings of the present disclosure. 
     Spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper,” and the like, may be used herein for descriptive purposes, and, thereby, to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the drawings. Spatially relative terms are intended to encompass different orientations of an apparatus in use, operation, and/or manufacture in addition to the orientation depicted in the drawings. For example, if the apparatus in the drawings is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the exemplary term “below” can encompass both an orientation of above and below. Furthermore, the apparatus may be otherwise oriented (e.g., rotated 90 degrees or at other orientations), and, as such, the spatially relative descriptors used herein interpreted accordingly. 
     The terminology used herein is for the purpose of describing particular embodiments and is not intended to be limiting. As used herein, the singular forms, “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. Moreover, the terms “comprises,” “comprising,” “includes,” and/or “including,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, components, and/or groups thereof, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     Various exemplary embodiments are described herein with reference to sectional illustrations that are schematic illustrations of idealized exemplary embodiments and/or intermediate structures. As such, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be expected. Thus, exemplary embodiments disclosed herein should not be construed as limited to the particular illustrated shapes of regions, but are to include deviations in shapes that result from, for instance, manufacturing. For example, an implanted region illustrated as a rectangle will, typically, have rounded or curved features and/or a gradient of implant concentration at its edges rather than a binary change from implanted to non-implanted region. Likewise, a buried region formed by implantation may result in some implantation in the region between the buried region and the surface through which the implantation takes place. Thus, the regions illustrated in the drawings are schematic in nature and their shapes are not intended to illustrate the actual shape of a region of a device and are not intended to be limiting. 
     Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure is a part. Terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense, unless expressly so defined herein. 
       FIG. 1  is a cross-sectional view illustrating an organic light emitting diode according to an exemplary embodiment of the present disclosure. 
     Referring to  FIG. 1 , the organic light emitting diode includes a first electrode  120  and a second electrode  190  disposed overlapping each other, emission layers  150 R,  150 G, and  150 B disposed between the first electrode  120  and the second electrode  190 , a hole transport layer  130  disposed between the first electrode  120  and the emission layers  150 R,  150 G, and  150 B, a buffer layer  160  disposed between the emission layers  150 R,  150 G, and  150 B and the second electrode  190 , an electron-transporting layer  170  disposed on the buffer layer  160 , an electron-injection layer  180  disposed between the electron-transporting layer  170  and the second electrode  190 , and a capping layer  200  disposed on the second electrode  190 . 
     In the present exemplary embodiment, the first electrode  120  may be a reflecting electrode, and the second electrode  190  may be a transflective electrode. 
     As used herein, the reflecting electrode may be an electrode including a material having a light-reflecting property, which may reflect light emitted from the emission layers  150 R,  150 G, and  150 B to the second electrode  190 . For example, such light-reflecting property may include reflecting incident light in a range of about 70% to 100%, or about 80% to about 100%. 
     The first electrode  120  may contain silver (Ag), aluminum (Al), chromium (Cr), molybdenum (Mo), tungsten (W), titanium (Ti), gold (Au), palladium (Pd), or an alloy thereof, and may have a triple-layer structure of silver (Ag)/indium tin oxide (ITO)/silver (Ag) or indium tin oxide (ITO)/silver (Ag)/indium tin oxide (ITO). 
     As used herein, the transflective electrode may be an electrode including a material having a transflective property, which may transmit a portion of light incident upon the second electrode  190  and reflect the remaining portion of the light to the first electrode  120 . For example, such transflective property may include reflecting incident light in a range of about 0.1% to 70%, or about 30% to about 50%. 
     The second electrode  190  may include silver (Ag), magnesium (Mg), aluminum (Al), chromium (Cr), molybdenum (Mo), tungsten (W), titanium (Ti), gold (Au), palladium (Pd), or an alloy thereof. 
     The hole transport layer  130  may be disposed to correspond to an auxiliary layer disposed between the first electrode  120  and the emission layers  150 R,  150 G, and  150 B. Specifically, the hole transport layer  130  may be disposed in common between the red emission layer  150 R and the first electrode  120 , between the green emission layer  150 G and the first electrode  120 , and between the blue emission layer  150 B and the first electrode  120 , and a thickness of the hole transport layer  130  may be the same at common regions thereof. 
     In the present exemplary embodiment, the hole transport layer  130  may have a refractive index in a range of 1.0 to 1.6, or in a range of 1.2 to 1.5. A wavelength band of light having such refractive index range may include a blue wavelength region, e.g., about 450 nm. 
     The hole transport layer  130  may contact the first electrode  120 , and a thickness thereof may be in a range of about 15 to 40 nm. When the thickness of the hole transport layer  130  is in this range, a surface plasmon fraction, which will be described later, may be equal to or smaller than 50%. The surface plasmon may be a material disposed on a surface of a metal, which includes electrons that vibrate. As used herein, a surface plasmon fraction may refer to a degree to which excitons of the organic light emitting diode disappear by a surface plasmon mode to be described later. 
     The hole transport layer  130  may include at least one of the hole-injection layer and the hole-transporting layer. The hole injection layer may facilitate injection of holes from the first electrode  120 , and the hole-transporting layer may smoothly transport the holes transmitted from the hole injection layer. In this case, refractive indices of the hole-injection layer and the hole-transporting layer may each be in a range of 1.0 to 1.6, or in a range of 1.2 to 1.5, or in a range of 1.2 to 1.45. The hole transport layer  130  may have a dual-layered structure, in which the hole transport layer is disposed on the hole injection layer, or a single layer structure including mixed materials forming the hole injection layer and the hole-transporting layer. 
     The hole transport layer  130  may include perfluoro oligomer, or a material forming the hole-injection layer or the hole-transporting layer that is well known in the art. The perfluoro oligomer may reduce a refractive index of the hole transport layer  130 . 
     The perfluoro oligomer may be a compound that includes a repeating unit represented as Chemical Formula A and a repeating unit represented as Chemical Formula B. 
     In the present exemplary embodiment, the perfluoro oligomer may have a molecular weight of about 1500 or less in order to perform vacuum evaporation. 
     
       
         
         
             
             
         
       
     
     In Chemical Formula A and Chemical Formula B, n, m, x, and y are natural numbers, and may be determined such that the molecular weight of the perfluoro oligomer is about 1500 or less. 
     The material constituting the hole-injection layer or the hole-transporting layer may include an aromatic amine derivative that may be represented as one of Chemical Formulae 1 to 5. 
     
       
         
         
             
             
         
       
     
     In Chemical Formulae 1 to 5, each of Ar 1  to Ar 24  may refer to a substituted or unsubstituted ring-forming C6 to C50 aryl group, or a substituted or unsubstituted ring-forming Z5 to Z50 heteroaryl group. As used herein, “Zn”, where n is a natural number, denotes the number of atoms forming the ring. 
     Each of L 1  to L 9  may refer to a substituted or unsubstituted ring-forming C6 to C50 arylene group or a substituted or unsubstituted ring-forming Z5 to Z50 heteroarylene group. As used herein, “Zn”, where n is a natural number, may denote the number of atoms forming the ring. 
     Possible substituents of Ar 1  to Ar 24  and L 1  to L 9  are a C1 to C15 straight or branched alkyl group, a ring-forming C3 to C15 cycloalkyl group, a trialkylsilyl group having a C1 to C15 straight or branched alkyl group, a triaryl silyl group having a ring-forming C6 to C14 aryl group, an alkyl aryl silyl group having a C1 to C15 straight or branched alkyl group and a ring-forming C6 to C14 aryl group, a ring-forming C6 to C50 aryl group, a ring-forming Z5 to Z50 heteroaryl group, a halogen atom, or a cyano group. As used herein, “Zn”, where n is a natural number, may denote the number of atoms forming the ring. A plurality of substituents that are adjacently disposed may be combined to form a ring-forming saturated or unsaturated divalent group. 
     At least one of Ar 1  to Ar 24  may be a substituent represented by any one of Chemical Formulae 6 and 7. 
     
       
         
         
             
             
         
       
     
     In Chemical Formula 6, X may be an oxygen atom, a sulfur atom, or N—Ra, Ra may be a C1 to C15 straight or branched alkyl group, a ring-forming C3 to C15 cycloalkyl group, a ring-forming C6 to C50 aryl group, or a ring-forming Z5 to Z50 heteroaryl group, and L 10  may be a single-bonded, substituted or unsubstituted ring-forming C6 to C50 arylene group, or a substituted or unsubstituted ring-forming Z5 to Z50 heteroarylene group. As used herein, “Zn”, where n is a natural number, may denote the number of atoms forming the ring. 
     In Chemical Formula 7, L 11  may be a substituted or unsubstituted ring-forming C6 to C50 arylene group or a substituted or unsubstituted ring-forming Z5 to Z50 heteroarylene group. As used herein, “Zn”, where n is a natural number, may denote the number of atoms forming the ring. 
     In Chemical Formulae 6 and 7, each of R 1  to R 4  may be a C1 to C15 straight or branched alkyl group, a ring-forming C3 to C15 cycloalkyl group, a trialkylsilyl group having a C1 to C15 straight or branched alkyl group, a triaryl silyl group having a ring-forming C6 to C14 aryl group, an alkyl aryl silyl group having a C1 to C15 straight or branched alkyl group and a ring-forming C6 to C14 aryl group, a ring-forming C6 to C14 aryl group, a ring-forming Z5 to Z50 heteroaryl group, a halogen atom, or a cyano group. As used herein, “Zn”, where n is a natural number, may denote the number of atoms forming the ring. Further, a plurality of R 1  to R 4  that are adjacently disposed may be combined to form a ring. In addition, a, c, and d may each be an integer in a range of 0 to 4, and b may be an integer in a range of 0 to 3. 
     A compound represented as Chemical Formula 1 may be a compound represented as Chemical Formula 8. 
     
       
         
         
             
             
         
       
     
     In Chemical Formula 8, Cz may be a substituted or unsubstituted carbazolyl group. 
     L 12  may be a substituted or unsubstituted ring-forming C6 to C50 arylene group, or a substituted or unsubstituted ring-forming Z5 to Z50 heteroarylene group. Each of Ar 25  and Ar 26  indicates a substituted or unsubstituted ring-forming C6 to C50 aryl group, or a substituted or unsubstituted ring-forming Z5 to Z50 heteroaryl group. As used herein, “Zn”, where n is a natural number, may denote the number of atoms forming the ring. 
     A compound represented as Chemical Formula 8 may be a compound represented as Chemical Formula 9. 
     
       
         
         
             
             
         
       
     
     In Chemical Formula 9, each of R 5  and R 6  may be a C1 to C15 straight or branched alkyl group, a ring-forming C3 to C15 cycloalkyl group, a trialkylsilyl group having a C1 to C15 straight or branched alkyl group, a triaryl silyl group having a ring-forming C6 to C14 aryl group, an alkyl aryl silyl group having a C1 to C15 straight or branched alkyl group and a ring-forming C6 to C14 aryl group, a ring-forming C6 to C14 aryl group, a ring-forming Z5 to Z50 heteroaryl group, a halogen atom, or a cyano group. As used herein, “Zn”, where n is a natural number, may denote the number of atoms forming the ring. Further, a plurality of R 5  and R 6  that are adjacently disposed may be combined to form a ring. In addition, e and f may each be an integer in a range of 0 to 4. L 12 , Ar 25 , and Ar 26  respectively correspond to L 12 , Ar 25 , and Ar 26  in Chemical Formula 8. 
     A compound represented as Chemical Formula 9 may be a compound represented as Chemical Formula 10. 
     
       
         
         
             
             
         
       
     
     In Chemical Formula 10, each of R 7  and R 8  indicates a C1 to C15 straight or branched alkyl group, a ring-forming C3 to C15 cycloalkyl group, a trialkylsilyl group having a C1 to C15 straight or branched alkyl group, a triaryl silyl group having a ring-forming C6 to C14 aryl group, an alkyl aryl silyl group having a C1 to C15 straight or branched alkyl group and a ring-forming C6 to C14 aryl group, a ring-forming C6 to C14 aryl group, a ring-forming Z5 to Z50 heteroaryl group, a halogen atom, or a cyano group. As used herein, “Zn”, where n is natural number, may denote the number of atoms forming the ring. Further, a plurality of R 5  and R 6  that are adjacently disposed may be combined to form a ring. In addition, g and h may each be an integer in a range of 0 to 4. R 5 , R 6 , e, f, Ar 25 , and Ar 26  respectively correspond to R 5 , R 6 , e, f, Ar 25 , and Ar 26  in Chemical Formula 9. 
     The hole transport layer  130  may alternatively include poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) or PEDOT:PSS:PFI. Herein, PFI may be a perfluorinated ionomer, which is Nafion® developed by DuPont Company. 
     Hereinabove, the material of the hole transport layer  130  has been described. However, when the hole transport layer  130  includes a hole-injection layer and a hole-transporting layer, each of the hole-injection layer and the hole-transporting layer may include the aforementioned material of the hole transport layer  130 . 
     The emission layers  150 R,  150 G, and  150 B disposed on the hole transport layer  130  include a red emission layer  150 R, a green emission layer  150 G, and a blue emission layer  150 B, which are horizontally disposed in a direction parallel with an upper surface of the first electrode  120 . A pixel definition layer  25  may be disposed between adjacent emission layers  150 R,  150 G, and  150 B. 
     In the present exemplary embodiment, an auxiliary layer BIL may be disposed below the blue emission layer  150 B to increase efficiency of the blue emission layer  150 B. The auxiliary layer BIL may increase the efficiency of the blue emission layer  150 B by adjusting a hole charge balance. The auxiliary layer BIL may include a compound represented by Chemical Formula 11. 
     
       
         
         
             
             
         
       
     
     In Chemical Formula 11, each of A1, A2, and A3 may be hydrogen, an alkyl group, an aryl group, carbazolyl group, dibenzothiophenyl group, dibenzofuryl group (DBF), or biphenyl group, and each of a, b, and c may be in a range of 0 to 4. 
     Examples of the compound represented as Chemical Formula 11 may include compounds represented as Chemical Formulae 11-1, 11-2, 11-3, 11-4, 11-5, and 11-6. 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     According to an exemplary embodiment, the auxiliary layer BIL may include a compound represented as Chemical Formula 12. 
     
       
         
         
             
             
         
       
     
     In Chemical Formula 12, each of a, b, and c may be in a range of 0 to 3, X may be one of O, N, and S, and a plurality of Xs may be the same as or different from each other. 
     Examples of the compound represented as Chemical Formula 12 may include compounds represented as Chemical Formulae 12-1 to 12-5. 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     According to an exemplary embodiment, the auxiliary layer BIL may include a compound represented as Chemical Formula 13. 
     
       
         
         
             
             
         
       
     
     In Chemical Formula 13, A1 may be hydrogen, an alkyl group, an aryl group, carbazolyl group, dibenzothiophenyl group, or dibenzofuryl group (DBF), each of L1 and L2 may represented by Chemical Formula 14, and the DBF connected with L1 and L2 may be replaced with carbazolyl group or dibenzothiophenyl group. 
     
       
         
         
             
             
         
       
     
     In Chemical Formula 14, n may be an integer in a range of 0 to 3. 
     In the organic light emitting diode according to the present exemplary embodiment, a red resonance auxiliary layer  150 R′ may be disposed under the red emission layer  150 R, and a green resonance auxiliary layer  150 G′ may be disposed under the green emission layer  150 G. The red resonance auxiliary layer  150 R′ and the green resonance auxiliary layer  150 G′ may control a resonance distance for each color. In contrast, a separate resonance auxiliary layer may not be disposed between the blue emission layer  150 B/auxiliary layer BIL and the hole transport layer  130 . 
     Each of the emission layers  150 R,  150 G, and  150 B includes a host and a dopant. Materials forming the emission layers  150 R,  150 G, and  150 B are not particularly restricted. 
     The red emission layer  150 R includes a host material that includes CBP (carbazole biphenyl) or mCP (1,3-bis(carbazol-9-yl)benzene, and may be formed of a phosphorescent material including at least one of PIQIr(acac) (bis(1-phenylisoquinoline)acetylacetonate iridium), PQIr(acac) (bis(1-phenylquinoline)acetylacetonate iridium), PQIr (tris(1-phenylquinoline)iridium), and PtOEP (platinum octaethylporphyrin), or a fluorescent material including PBD:Eu(DBM) 3 (phen) or perylene. 
     The green emission layer  150 G includes a host material including CBP or mCP, and may be formed of a phosphorescent material including a dopant material including Ir(ppy) 3  (fac-tris(2-phenylpyridine)iridium) or a fluorescent material including Alq 3 (tris(8-hydroxyquinolino)aluminum), but is not limited thereto. 
     The blue emission layer  150 B includes a host material including CBP or mCP, and may be made of a phosphorescent material including a dopant that includes (4,6-F 2 ppy) 2 Irpic. Alternatively, the blue emission layer  150 B may include the host material having an anthracene group, and may be made of a fluorescent material including the dopant including a diamine group or at least one of spiro-DPVBi, spiro-6P, distyrylbenzene (DSB), distyrylarylene (DSA), a PFO-based polymer, and a PPV-based polymer, but is not limited thereto. 
     The buffer layer  160 , the electron-transporting layer  170 , and the electron-injection layer  180  may be disposed on the emission layers  150 R,  150 G, and  150 B. The buffer layer  160  may serve as a hole-blocking layer, but may alternatively be omitted. 
     The electron-transporting layer  170  may include an organic material. For example, the electron-transporting layer  170  may include at least one of Alq 3  (tris(8-hydroxyquinolino)aluminum), PBD (2-[4-biphenyl-5-[4-tert-butylphenyl]]-1,3,4-oxadiazole), TAZ (1,2,4-triazole), spiro-PBD (spiro-2-[4-biphenyl-5-[4-tert-butylphenyl]]-1,3,4-oxadiazole), and BAlq (8-hydroxyquinoline aluminum salt), however is not limited thereto. The electron-transporting layer  170  may include a metal. 
     The electron injection layer  180  may improve electron injection from the second electrode  190  to the electron-transporting layer  170 . In the present exemplary embodiment, the electron-injection layer  180  may include a metal having a work function of about 4.0 eV or less. For example, the metal may include at least one of Li, Na, K, Rb, Cs, Ca, Sr, Ba, Ce, Sm, Eu, Gd, La, Nd, Tb, Lu, Yb, and Mg. 
     The capping layer  200  disposed on the second electrode  190  may induce a change in resonance intensity and resonance phase in cooperation with the second electrode  190 . The capping layer  200  may include an inorganic material or an organic material having a refractive index that is in a range of about 1.80 to 1.90. The capping layer  200  may include Alq3. 
     According to the present exemplary embodiment, the organic light emitting diode may have a microcavity structure between the first electrode  120  and the second electrode  190  in order to improve light-emitting efficiency. The microcavity structure may satisfy Equation 1. 
     
       
         
           
             
               
                 
                   
                     2 
                     ⁢ 
                     
                       π 
                       · 
                       q 
                     
                   
                   = 
                   
                     
                       ∑ 
                       j 
                       
                           
                       
                     
                     ⁢ 
                     
                       ( 
                       
                         
                           
                             2 
                             ⁢ 
                             π2 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               n 
                               
                                 j 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 λ 
                               
                             
                             ⁢ 
                             
                               d 
                               j 
                             
                           
                           λ 
                         
                         + 
                         
                           δ 
                           j 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Eq 
                     . 
                     
                         
                     
                     ⁢ 
                     1 
                   
                   ] 
                 
               
             
           
         
       
     
     In Equation 1, n jλ  denotes a refractive index of a j th  layer for a wavelength λ, dj denotes a thickness of a j th  layer, δ j  denotes a magnitude of a phase variation generated when light passes through the j th  layer or is reflected by a reflection surface included in the first electrode  120  or the second electrode  190 , and q is an integer, such as 0, 1, 2, etc. 
     In the present exemplary embodiment, in the microcavity structure, a resonance distance of light emitted from the emission layers  150 R,  150 G, and  150 B may be a distance between the first electrode  120  and the second electrode  190 . However, when the first electrode  120  has a triple-layer structure including indium tin oxide (ITO)/silver (Ag)/indium tin oxide (ITO), the resonance distance may be a distance between the second electrode  190  and a layer formed of silver (Ag) included in the first electrode  120 . Accordingly, a refractive index and thickness condition of a layer formed of indium tin oxide (ITO) disposed at an uppermost end of the first electrode  120  should be considered in Equation 1. 
     In Equation 1, the hole transport layer  130  may be formed to include a material having a high refractive index, by reducing a physical thickness of the hole transport layer  130  while refractive indices and thicknesses of other layers are fixed. Herein, a standard high refractive index may be a refractive index of about 1.7 or more. In this case, the physical thickness of the hole transport layer  130  may be reduced to improve a yield and facilitate mass production. 
     However, when a hole transport layer  130  having the high refractive index is applied to an organic light emitting diode having a primary microcavity structure, light-emitting efficiency is reduced by 60% as compared to a secondary microcavity structure. Herein, in the case of the primary microcavity structure, the value of q in Equation 1 is 0, and in the case of the secondary microcavity structure, the value of q in Equation 1 is 1. 
     The reason for reduction of light-emitting efficiency in an organic light emitting diode having the primary microcavity structure, in which the hole transport layer thereof has the high refractive index, is because, when the refractive index of the hole transport layer is increased, the thickness of the hole transport layer is generally reduced, and thus, a quenching phenomenon of which excitons spread to a surface plasmon of an electrode may occur. 
     The organic light emitting diode according to the present exemplary embodiment described above may reduce the quenching phenomenon of the excitons by using a hole transport layer having a relatively low refractive index to improve the light-emitting efficiency. The quenching phenomenon of the excitons may refer to a state where exitons are not discharged in a form of light energy but disappear by the surface plasmon of the electrode. 
     Hereinafter, effects of an improved light-emitting efficiency according to an exemplary embodiment of the present disclosure will be described. 
       FIG. 2  is a graph illustrating radiance according to a refractive index and a thickness of a hole transport layer.  FIG. 3  is a graph illustrating a surface plasmon fraction according to a refractive index and a thickness of a hole transport layer. 
     Referring to  FIG. 2 , a connecting line schematically connecting a peak of contour lines along the y-axis direction may be a thickness and a refractive index of the hole transport layer that satisfies a primary resonance condition. In this case, when the refractive index is changed from 2 to 1 based on the connecting line, radiance emitted toward a front of the organic light emitting diode is increased by about two times. Herein, the front of the organic light emitting diode may be a direction from the first electrode  120  toward the second electrode  190  of the organic light emitting diode of  FIG. 1 . 
     Referring to  FIG. 3 , in the surface plasmon fraction according to the thickness and the refractive index of the hole transport layer, a surface plasmon fraction is sharply reduced as the refractive index of the hole transport layer is reduced. Accordingly, the hole transport layer having a thin thickness with the primary microcavity structure may have an increased current efficiency by the sharply reduced quenching of the excitons formed in the emission layer. 
       FIG. 4  is a graph illustrating an anti-node thickness and a surface plasmon fraction according to a refractive index of a hole transport layer. 
     In  FIG. 4 , the anti-node thickness may be a thickness for emitting light with a maximum amount toward the front direction according to the refractive index of the hole transport layer. The front direction may be a direction from the first electrode  120  toward the second electrode  190  of the organic light emitting diode of  FIG. 1 . Herein, the hole transport layer may include a hole-injection layer and a hole-transporting layer, and it is assumed that the hole-injection layer and the hole-transporting layer have the same refractive index. 
     Referring to  FIG. 4 , a large inflection point of the surface plasmon fraction is found when the refractive index of the hole transport layer is in a range of about 1.7 to 1.8, and the surface plasmon fraction is further reduced when the refractive index is in a range of about 1.5 to 1.7. Further, a large inflection point of the surface plasmon fraction is found when the refractive index is in a range of about 1.4 to 1.5, and a surface plasmon fraction of similar magnitude is found when the refractive index is in a range of about 1.0 to 1.4. 
     Accordingly, light-emitting efficiency towards the front direction may be increased by setting the refractive index of the hole transport layer in a range of about 1.0 to 1.6 and the thickness of the hole transport layer in a range of 15 nm to 40 nm. 
     The refractive index of the hole transport layer may be in a range of about 1.2 to 1.5, or 1.2 to 1.45, in order to improve the light-emitting efficiency towards the front direction. When the refractive index of the hole transport layer is in a range of about 1.2 to 1.45, the surface plasmon fraction is equal to or smaller than 20%, and in this case, the anti-node thickness is in a range of about 25 nm to 30 nm. 
       FIG. 5  is a graph illustrating relative average contributions in four modes according to a thickness of a hole transport layer in a comparative embodiment.  FIG. 6  is a graph illustrating relative average contributions in four modes according to a thickness of a hole transport layer in an exemplary embodiment of the present disclosure. 
     In  FIG. 5  and  FIG. 6 , the relative average contributions show where excitons generated in an emission layer are moved to and how they are changed. 
     In  FIG. 5  and  FIG. 6 , the four modes include a surface plasmon mode, a wave-guided mode, an absorption loss mode, and an out-coupled mode. In the surface plasmon mode, the excitons are moved to a surface plasmon of an electrode, thereby generating the quenching phenomenon. In the wave-guided mode, excitons are changed to photons, but are confined within the organic light emitting diode. In the absorption loss mode, excitons are changed to photons, but disappear by being absorbed by an electrode, an emission layer, and the like. In the out-coupled mode, excitons leak to the front of the organic light emitting diode. 
     In the comparative embodiment illustrated with reference to  FIG. 5 , the hole transport layer is formed by using an amine derivative having a refractive index of about 1.7 or more. In the exemplary embodiment illustrated with reference to  FIG. 6 , the hole transport layer having a refractive index of about 1.4 is formed by using a perfluoro oligomer, PEDOT:PSS, or PEDOT:PSS:PFI, described above. 
     In  FIG. 5  and  FIG. 6 , when a line is drawn based on a thickness of the hole transport layer in the y-axis direction, lengths of regions occupied in each mode may indicate a fraction of each mode. For example, a first length “d” shown in  FIG. 5  indicates that the fraction of the surface plasmon mode is 30% for a hole transport layer having a 30 nm thickness. 
     Referring to  FIG. 5  and  FIG. 6 , in the comparative embodiment of  FIG. 5 , the fraction of the surface plasmon mode is in a range of about 20% to 60% when the thickness of the hole transport layer is in a range of 15 nm to 40 nm. Referring to  FIG. 6 , however, the fraction of the surface plasmon mode is in a range of about 12% to 44% when the thickness of the hole transport layer is in a range of 15 nm to 50 nm. A fraction of the out-coupled mode for the same thickness range is in a range of about 10% to 20% in the comparative embodiment and about 20% to 30% in the exemplary embodiment. Accordingly, by using the hole transport layer having a relatively low refractive index according to the exemplary embodiment, the light-emitting efficiency is increased. 
       FIG. 7  is a graph illustrating radiance according to thicknesses of a hole transport layer and an electron-transporting layer in a comparative embodiment.  FIG. 8  is a graph illustrating radiance according to thicknesses of a hole transport layer and an electron-transporting layer according to an exemplary embodiment of the present disclosure. 
     In the comparative embodiment illustrated with reference to  FIG. 7 , the hole transport layer includes an amine derivative having a refractive index of about 1.7 or more. In the exemplary embodiment illustrated with reference to  FIG. 8 , the hole transport layer having a refractive index of about 1.4 is formed by using a perfluoro oligomer, PEDOT:PSS, or PEDOT:PSS:PFI, described above. 
     In  FIG. 7  and  FIG. 8 , a left contour line may be a radiance in the primary microcavity structure, and a right contour line may be a radiance in the secondary microcavity structure. 
     Referring to  FIG. 7 , the highest radiance in the secondary microcavity structure is about 6[W*m −2 *sr −1 ], and the highest radiance in the primary microcavity structure is about 3[W*m −2 *sr −1 ]. A difference therebetween is about 3[W*m −2 *sr −1 ]. Referring to  FIG. 8 , the highest radiance in the secondary microcavity structure is about 7.5[W*m −2 *sr −1 ], and the highest radiance in the primary microcavity structure is about 7[W*m −2 *sr −1 ]. A difference therebetween about 0.5[W*m −2 *sr −1 ]. Accordingly, when the hole transport layer has a relatively low refractive index, a degree by which the light-emitting efficiency is reduced as compared to the secondary microcavity structure is reduced, and the light-emitting efficiency itself is increased. Further, there is a radiance increasing effect of about 1.5[W*m −2 *sr −1 ], when the hole transport layer according the exemplary embodiment is applied to the secondary microcavity structure, as compared to the comparative embodiment. 
     Efficiency and a driving voltage of the comparative embodiment and the exemplary embodiment described above will be described with reference to Table 1. A detailed structure of the comparative embodiment will be described with reference to  FIG. 1 . 
     The organic light emitting diode according to the comparative embodiment includes: the first electrode  120  of about 200 Å and the second electrode  190  of about 200 Å disposed to overlap each other; the red emission layer  150 R of about 450 Å, the green emission layer  150 G of about 400 Å, and the blue emission layer  150 B of about 160 Å disposed between the first electrode  120  and the second electrode  190 ; the red resonance auxiliary layer  150 R′ of about 260 Å, the green resonance auxiliary layer  150 G′ of about 70 Å, and the auxiliary layer BIL disposed below the red emission layer  150 R, the green emission layer  150 G, and the blue emission layer  150 B, respectively; the hole transport layer  130  of about 150 Å disposed between the first electrode  120  and the emission layers  150 R,  150 G, and  150 B; the buffer layer  160  of about 50 Å disposed between the emission layers  150 R,  150 G, and  150 B and the second electrode  190 ; the electron-transporting layer  170  of about 180 Å disposed on the buffer layer  160 ; the electron-injection layer  180  of about 13 Å disposed between the electron-transporting layer  170  and the second electrode  190 ; and the capping layer  200  of about 650 Å disposed on the second electrode  190 . A structure of the exemplary embodiment is substantially the same as that of the comparative embodiment, except that the refractive index of the hole transport layer  130  is lower than that of the comparative embodiment, and the thickness thereof is 230 Å. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 CE(cd/A) 
                 Driving voltage (V) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Comparative Embodiment 
                 Red 
                 38 
                 4.0 
               
               
                   
                 Green 
                 125 
                 4.0 
               
               
                   
                 Blue 
                 6.1 
                 3.1 
               
               
                   
                 White 
                 40 
                 — 
               
               
                 Exemplary Embodiment 
                 Red 
                 45.2 
                 4.0 
               
               
                   
                 Green 
                 150 
                 4.0 
               
               
                   
                 Blue 
                 8.2 
                 3.1 
               
               
                   
                 White 
                 51.8 
                 — 
               
               
                   
               
            
           
         
       
     
     Referring to Table 1, red light-emitting efficiency, green light-emitting efficiency, and blue light-emitting efficiency are respectively increased by 10% or more, 20% or more, and 30% or more as compared with the comparative embodiment, when the same driving voltage are applied to the exemplary embodiment and the comparative embodiment. Further, white power consumption is reduced by 20% in the exemplary embodiment as compared with in the comparative embodiment. 
       FIG. 9  is a cross-sectional view illustrating an organic light emitting diode including an emission layer including a plurality of layers according to an exemplary embodiment of the present disclosure. 
     The configurations of the organic light emitting diode illustrated with reference to  FIG. 9  are substantially the same as those of the organic light emitting diode illustrated with reference to  FIG. 1 . As such, differences elements or configuration therebetween will be described, and the elements previously described with reference to  FIG. 1  may be applied to the exemplary embodiment illustrated with reference to  FIG. 9 . 
     Referring to  FIG. 9 , the buffer layer  160  of  FIG. 1  may be omitted. The organic light emitting diode according to the present exemplary embodiment of  FIG. 9  includes an emission layer  150  emitting white light, by combining light of different colors from a plurality of layers  150   a ,  150   b , and  150   c .  FIG. 9  shows that the emission layer  150  includes three layers  150 ,  150   b , and  150   c , however, the number of layers emitting light of different colors may be varied. 
     Three layers of the emission layer  150  may respectively represent blue, yellow, and blue colors. Alternatively, the emission layer  150  may include two layers respectively representing blue and yellow colors. Although not shown, at least one charge generating layer may be disposed between adjacent layers  150   a ,  150   b , and  150   c  of  FIG. 9 . 
     The hole transport layer  130  in  FIG. 9  may be substantially the same as the hole transport layer  130  of  FIG. 1 , and thus, repeated description thereof will be omitted. 
       FIG. 10  is a cross-sectional view illustrating a light emitting device according to an exemplary embodiment of the present disclosure. 
     Referring to  FIG. 10 , the light emitting device diode display according to the present exemplary embodiment includes a substrate  23 , a driving thin film transistor  30 , a first electrode  120 , a light-emitting diode layer  100 , and a second electrode  190 . The first electrode  120  may be an anode and the second electrode  190  may be a cathode, or the first electrode  120  may be a cathode and the second electrode  190  may be an anode. 
     A substrate buffer layer  26  may be disposed on the substrate  23 . The substrate buffer layer  26  may prevent penetration of impurities and planarize the surface thereof. Alternatively, the substrate buffer layer  26  may be omitted according to a type and process conditions of the substrate  23 . 
     A driving semiconductor layer  37  is disposed on the substrate buffer layer  26 . The driving semiconductor layer  37  may be formed of a material including a polysilicon. Further, the driving semiconductor layer  37  includes a channel region  35  not doped with an impurity, and a source region  34  and a drain region  36  doped with an impurity may be disposed at opposing sides of the channel region  35 . The doped ion materials may be P-type impurities such as boron (B), and B 2 H 6  may be generally used. The impurities may depend on the type of the driving thin film transistor. 
     A gate insulating layer  27  is disposed on the driving semiconductor layer  37 . A gate wire including a driving gate electrode  33  is disposed on the gate insulating layer  27 . The driving gate electrode  33  is formed to overlap at least a portion of the driving semiconductor layer  37 , particularly, the channel region  35 . 
     An interlayer insulating layer  28  covering the driving gate electrode  33  is disposed on the gate insulating layer  27 . A first contact hole  22   a  and a second contact hole  22   b  that expose the source region  34  and the drain region  36  of the driving semiconductor layer  37  are formed in the gate insulating layer  27  and the interlayer insulating layer  28 . A data wire including a driving source electrode  73  and a driving drain electrode  75  may be disposed on the interlayer insulating layer  28 . The driving source electrode  73  and the driving drain electrode  75  are connected to the source region  34  and the drain region  36  of the driving semiconductor layer  37  through the first contact hole  22   a  and the second contact hole  22   b  formed in the interlayer insulating layer  28  and the gate insulating layer  27 , respectively. 
     As described above, the driving thin film transistor  30  including the driving semiconductor layer  37 , the driving gate electrode  33 , the driving source electrode  73 , and the driving drain electrode  75  is formed. The configuration of the driving thin film transistor  30  is not limited to the aforementioned example, and may be variously modified as generally known in the art. 
     In addition, a planarization layer  24  covering the data wire is formed on the interlayer insulating layer  28 . The planarization layer  24  may remove a step and planarize the surface thereof to increase emission efficiency of the organic light emitting diode to be formed thereon. Further, the planarization layer  24  has a third contact hole  22   c  to electrically connect the driving drain electrode  75  and the first electrode  120  that will be described later. One of the planarization layer  24  and the interlayer insulating layer  28  may be omitted. 
     A first electrode  120  of the organic light emitting diode LD is disposed on the planarization layer  24 . A pixel definition layer  25  is disposed on the planarization layer  24  and the first electrode  120 . The pixel definition layer  25  has an opening overlapping a part of the first electrode  120 . The light-emitting diode layer  100  may be disposed for each opening formed in the pixel definition layer  25 . 
     The light-emitting diode layer  100  is disposed on the first electrode  120 . The light-emitting diode layer  100  corresponds to the hole transport layer  130 , the emission layers  150 R,  150 G and  150 B, the buffer layer  160 , the electron-transporting layer  170 , and the electron injection layer  180  of the organic light emitting diode described with reference to  FIG. 1 . 
     A second electrode  190  and a capping layer  200  are disposed on the light-emitting diode layer  100 . The capping layer  200  may cover and protect the second electrode  190 . The capping layer  200  may induce a change in resonance intensity and resonance phase in cooperation with the second electrode  190 . The capping layer  200  may be formed of an inorganic material or an organic material having a refractive index in a range of about 1.80 to 1.90. 
     In the present exemplary embodiment, all the descriptions with reference to  FIG. 1  are applicable to the organic light emitting diode LD. 
     A thin film encapsulation layer  300  is disposed on the capping layer  200 . The thin film encapsulation layer  300  encapsulates the organic light emitting diode LD and the driving circuit unit formed on the substrate  23  from the outside for protection. 
     The thin film encapsulation layer  300  includes encapsulation organic layers  300   a  and  300   c  and encapsulation inorganic layers  300   b  and  300   d  that are alternately stacked with each other. In  FIG. 10 , the thin film encapsulation layer  300  is configured by alternately depositing two encapsulation organic layers  300   a  and  300   c  and two encapsulation inorganic layers  300   b  and  300   d  one by one, but is not limited thereto. 
     Although the aforementioned organic light emitting diode has been described to be applied to the light emitting device with reference to  FIG. 10 , the organic light emitting diode according to the present exemplary embodiment is applicable to a lighting device as well as a display device. 
     Although certain exemplary embodiments and implementations have been described herein, other embodiments and modifications will be apparent from this description. Accordingly, the inventive concept is not limited to such exemplary embodiments, but rather to the broader scope of the presented claims and various obvious modifications and equivalent arrangements.