Patent Publication Number: US-11050035-B2

Title: Light emitting diode and display device including the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation application based on pending application Ser. No. 15/806,665, filed Nov. 8, 2017, the entire contents of which is hereby incorporated by reference. 
     Korean Patent Application No. 10-2017-0020537, filed on Feb. 15, 2017, and entitled, “Light Emitting Diode and Display Device Including the Same,” is incorporated by reference herein in its entirety. 
    
    
     BACKGROUND 
     1. Field 
     One or more embodiments described herein relate to a light emitting diode and a display device including a light emitting diode. 
     2. Description of the Related Art 
     A display made from light emitting diodes has as a wide viewing angle, fast response speed, reduced thickness, and low power consumption, and thus is suitable for use in televisions, monitors, and mobile phones. However, light emitting diodes have low emission efficiency. As a result, a high driving voltage may be applied to obtain high luminance. The high driving voltage may shorten the lifespan of the light emitting diode and its host display. 
     SUMMARY 
     In accordance with one or more embodiments, a light emitting diode, comprising a first electrode overlapping a second electrode; an emission layer between the first and second electrodes; a first hole injection layer and a second hole injection layer between the first electrode and the emission layer; and a first hole transporting layer between the first hole injection layer and the second hole injection layer, wherein each of the first hole injection layer and the second hole injection layer includes an inorganic dipole material, at least one of the first hole injection layer or the second hole injection layer including an organic material. 
     The first hole injection layer may be adjacent to the first electrode, and the second hole injection layer may be between the first hole transporting layer and the emission layer. The light emitting diode may include a second hole transporting layer between the second hole injection layer and the emission layer. The inorganic dipole material may include at least one of CuI, AgI, AuI, ZnI 2 , NiI 2 , PdI 2 , PtI 2 , CoI 2 , RhI 2 , IrI 2 , FeI 2 , RuI 2 , IrI 2 , FeI 2 , RuI 2 , IrI 2 , OsI 2 , MnI 2 , TcI 2 , ReI 2 , CrI 3 , MoI 3 , WI 3 , VI 3 , NbI 3 , TaI 3 , TiI 4 , ZrI 4 , HfI 4 , SnI 2 , SnI 4 , GeI 2 , GeI 4 , CuBr, AgBr, AuBr, ZnBr 2 , PdBr 2 , PtBr 2 , CoBr 2 , RhBr 2 , IrBr 2 , FeBr 2 , RuBr 2 , OsBr 2 , MnBr 2 , TcBr 2 , ReBr 2 , CrBr 3 , MoBr 3 , WBr 3 , VBr 3 , NbBr 3 , TaBr 3 , TiBr 4 , ZrBr 4 , HfBr 4 , CuCl, AgCl, AuCl, ZnCl 2 , PdCl 2 , PtCl 2 , CoCl 2 , RhCl 2 , IrCl 2 , FeCl 2 , RuCl 2 , OsCl 2 , MnCl 2 , TcCl 2 , ReCl 2 , CrCl 3 , MoCl 3 , WCl 3 , VCl 3 , NbCl 3 , TaCl 3 , TiCl 4 , ZrCl 4 , HfCl 4 , CuF, AgF, AuF, ZnF 2 , PdF 2 , PtF 2 , CoF 2 , RhF2, IrF 2 , FeF 2 , RuF 2 , OsF 2 , MnF 2 , TcF 2 , ReF 2 , CrF 3 , MoF 3 , WF 3 , VF 3 , NbF 3 , TaF 3 , TiF 4 , ZrF 4 , HfF 4 , or BiI 3 . 
     Each of the first hole injection layer and the second hole injection layer may include an organic material and an inorganic dipole material, and the organic material of the first hole injection layer and the organic material of the second hole injection layer may be different from each other. The emission layer may emit white light by combining a plurality of layers. 
     The light emitting diode may include a charge generation layer between the plurality of layers and the charge generation layer includes the inorganic dipole material. The inorganic dipole material in the charge generation layer may be the same as inorganic dipole material in the first or second hole injection layer. 
     The light emitting diode may include an electron injection layer between the emission layer and the second electrode, and the electron injection layer includes at least one of a lanthanum element, a first element of an alkali metal, or a second element of a halogen. The electron injection layer may include at least one of: a) a dipole material having the lanthanum element and the second element having different polarities, b) a first compound made of the lanthanum element, the first element, and the second element and having a perovskite structure, c) a positive ion of the first element, or d) a monomolecular molecule including the lanthanum element. 
     In accordance with one or more other embodiments, a display device includes a substrate; a plurality of transistors on the substrate; and a first light emitting diode, a second light emitting diode, and a third light emitting diode respectively connected to the plurality of transistors, wherein each of the first light emitting diode, the second light emitting diode, and the third light emitting diode includes a first electrode, a second electrode overlapping the first electrode, an emission layer between the first electrode and the second electrode, and a hole transporting layer between the first electrode and the emission layer, and wherein: the first light emitting diode includes a first hole injection layer positioned between the first electrode and the hole transporting layer and a second hole injection layer between the hole transporting layer and the emission layer, each of the first hole injection layer and the second hole injection layer includes an inorganic dipole material, and at least one of the first hole injection layer or the second hole injection layer includes an organic material. 
     The substrate may include a red light emission region, a green light emission region, and a blue light emission region, and the first light emitting diode may be in the green light emission region. The second light emitting diode may be in the red light emission region, the second light emitting diode may include a third hole injection layer between the first electrode and the hole transporting layer and a fourth hole injection layer between the hole transporting layer and the emission layer, each of the third hole injection layer and the fourth hole injection layer may include an inorganic dipole material, and at least one of the third hole injection layer and the fourth hole injection layer may include an organic material. 
     The third hole injection layer may be connected to the first hole injection layer, and the fourth hole injection layer may be separated from the second hole injection layer. The third light emitting diode may be in the blue light emission region, and the third light emitting diode may include an auxiliary layer under an emission layer of the light emitting diode. The display device may include a green resonance auxiliary layer under the emission layer of the first light emitting diode, and a red resonance auxiliary layer under the emission layer of the second light emitting diode. 
     The inorganic dipole material may include at least one of CuI, AgI, AuI, ZnI 2 , NiI 2 , PdI 2 , PtI 2 , CoI 2 , RhI 2 , IrI 2 , FeI 2 , RuI 2 , IrI 2 , FeI 2 , RuI 2 , IrI 2 , OsI 2 , MnI 2 , TcI 2 , ReI 2 , CrI 3 , MoI 3 , WI 3 , VI 3 , NbI 3 , TaI 3 , TiI 4 , ZrI 4 , HfI 4 , SnI 2 , SnI 4 , GeI 2 , GeI 4 , CuBr, AgBr, AuBr, ZnBr 2 , PdBr 2 , PtBr 2 , CoBr 2 , RhBr 2 , IrBr 2 , FeBr 2 , RuBr 2 , OsBr 2 , MnBr 2 , TcBr 2 , ReBr 2 , CrBr 3 , MoBr 3 , WBr 3 , VBr 3 , NbBr 3 , TaBr 3 , TiBr 4 , ZrBr 4 , HfBr 4 , CuCl, AgCl, AuCl, ZnCl 2 , PdCl 2 , PtCl 2 , CoCl 2 , RhCl 2 , IrCl 2 , FeCl 2 , RuCl 2 , OsCl 2 , MnCl 2 , TcCl 2 , ReCl 2 , CrCl 3 , MoCl 3 , WCl 3 , VCl 3 , NbCl 3 , TaCl 3 , TiCl 4 , ZrCl 4 , HfCl 4 , CuF, AgF, AuF, ZnF 2 , PdF 2 , PtF 2 , CoF 2 , RhF2, IrF 2 , FeF 2 , RuF 2 , OsF 2 , MnF 2 , TcF 2 , ReF 2 , CrF 3 , MoF 3 , WF 3 , VF 3 , NbF 3 , TaF 3 , TiF 4 , ZrF 4 , HfF 4 , or BiI 3 . 
     Each of the first hole injection layer and the second hole injection layer may include the organic material and the inorganic dipole material, and the organic material of the first hole injection layer and the organic material of the second hole injection layer may be different from each other. The display device may include an electron injection layer between the emission layer and the second electrode, and the electron injection layer includes at least one of a lanthanum element, a first element of an alkali metal, or a second element of a halogen. 
     The electron injection layer may include at least one of: a) a dipole material having the lanthanum element and the second element having different polarities, b) a first compound made of the lanthanum element, the first element, and the second element and having a perovskite structure, c) a positive ion of the first element, and d) a monomolecular molecule including the lanthanum element. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features will become apparent to those of skill in the art by describing in detail exemplary embodiments with reference to the attached drawings in which: 
         FIG. 1  illustrates an embodiment of a light emitting diode; 
         FIG. 2  illustrates an embodiment of a perovskite structure; 
         FIG. 3  illustrates an embodiment of an electron injection layer; 
         FIG. 4  illustrates an embodiment of a display device; 
         FIG. 5  illustrates another embodiment of a light emitting diode; 
         FIG. 6  illustrates another embodiment of a light emitting diode; and 
         FIG. 7  illustrates another embodiment of a display device. 
     
    
    
     DETAILED DESCRIPTION 
     Example embodiments are described with reference to the drawings; however, they may be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will convey exemplary implementations to those skilled in the art. The embodiments (or portions thereof) may be combined to form additional embodiments 
     In the drawings, the dimensions of layers and regions may be exaggerated for clarity of illustration. It will also be understood that when a layer or element is referred to as being “on” another layer or substrate, it can be directly on the other layer or substrate, or intervening layers may also be present. Further, it will be understood that when a layer is referred to as being “under” another layer, it can be directly under, and one or more intervening layers may also be present. In addition, it will also be understood that when a layer is referred to as being “between” two layers, it can be the only layer between the two layers, or one or more intervening layers may also be present. Like reference numerals refer to like elements throughout. 
     When an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the another element or be indirectly connected or coupled to the another element with one or more intervening elements interposed therebetween. In addition, when an element is referred to as “including” a component, this indicates that the element may further include another component instead of excluding another component unless there is different disclosure. 
       FIG. 1  illustrates an embodiment of a light emitting diode, and  FIG. 2  illustrates an embodiment of a perovskite structure. Referring to  FIG. 1 , the light emitting diode includes a first electrode  120  overlapping a second electrode  190 , an emission layer  150  between the first electrode  120  and the second electrode  190 , a first hole injection layer  130   a  between the first electrode  120  and the emission layer  150 , a first hole transporting layer  140   a  on the first hole injection layer  130   a , a second hole injection layer  130   b  between the first hole transporting layer  140   a  and the emission layer  150 , a second hole transporting layer  140   b  between the second hole injection layer  130   b  and the emission layer  150 , an electron transporting layer  160   a  between the emission layer  150  and the second electrode  190 , a buffer layer  160   b  between the emission layer  150  and the electron transporting layer  160   a , an electron injection layer  180  between the electron transporting layer  160   a  and the second electrode  190 , and a capping layer  200  on the second electrode  190 . 
     The first electrode  120  may be a reflecting electrode, e.g., an electrode including a material that reflects light emitted from the emission layer  150  for transmission to the second electrode  190 . For example, the material of the reflecting electrode may reflect incident light by about 70% or more to about 100% or less, or about 80% or more to about 100% or less. 
     The first electrode  120  may include silver (Ag), aluminum (Al), chromium (Cr), molybdenum (Mo), tungsten (W), titanium (Ti), gold (Au), palladium (Pd), or alloys thereof, which may be used as the reflection layer while having the anode function. In one embodiment, the first electrode  120  may have a triple layer structure of, for example, silver (Ag)/indium tin oxide (ITO)/silver (Ag) or indium tin oxide (ITO)/silver (Ag)/indium tin oxide (ITO). 
     The first electrode  120  may be formed, for example, by a sputtering method, a vapor phase deposition method, an ion beam deposition method, or an electron beam deposition method. 
     The first hole injection layer  130   a  facilitates injection of holes from the first electrode  120 . The first hole transporting layer  140   a  performs a function of smoothly transporting the holes from the first hole injection layer  130   a . The second hole injection layer  130   b  allows the holes from the first hole transporting layer  140   a  to be injected to the second hole transporting layer  140   b . The second hole transporting layer  140   b  may control a resonance distance while transmitting the holes injected from the second hole injection layer  130   b  to the emission layer  150 . 
     The first hole injection layer  130   a  is adjacent to the first electrode  120  on the first electrode  120  and includes an inorganic dipole material. The inorganic dipole material may be an inorganic ion compound of a first component and a second component of different polarities. The first component may be an element that becomes a positive ion when the inorganic ion compound is ionized. The second component may be an element that becomes a negative ion. The first component may be a transition metal, and the second component may include a halogen. 
     The inorganic dipole material may be, for example, at least one of CuI, AgI, AuI, ZnI 2 , NiI 2 , PdI 2 , PtI 2 , CoI 2 , RhI 2 , IrI 2 , FeI 2 , RuI 2 , IrI 2 , FeI 2 , RuI 2 , IrI 2 , OsI 2 , MnI 2 , TcI 2 , ReI 2 , CrI 3 , MoI 3 , WI 3 , VI 3 , NbI 3 , TaI 3 , TiI 4 , ZrI 4 , HfI 4 , SnI 2 , SnI 4 , GeI 2 , GeI 4 , CuBr, AgBr, AuBr, ZnBr 2 , PdBr 2 , PtBr 2 , CoBr 2 , RhBr 2 , IrBr 2 , FeBr 2 , RuBr 2 , OsBr 2 , MnBr 2 , TcBr 2 , ReBr 2 , CrBr 3 , MoBr 3 , WBr 3 , VBr 3 , NbBr 3 , TaBr 3 , TiBr 4 , ZrBr 4 , HfBr 4 , CuCl, AgCl, AuCl, ZnCl 2 , PdCl 2 , PtCl 2 , CoCl 2 , RhCl 2 , IrCl 2 , FeCl 2 , RuCl 2 , OsCl 2 , MnCl 2 , TcCl 2 , ReCl 2 , CrCl 3 , MoCl 3 , WCl 3 , VCl 3 , NbCl 3 , TaCl 3 , TiCl 4 , ZrCl 4 , HfCl 4 , CuF, AgF, AuF, ZnF 2 , PdF 2 , PtF 2 , CoF 2 , RhF2, IrF 2 , FeF 2 , RuF 2 , OsF 2 , MnF 2 , TcF 2 , ReF 2 , CrF 3 , MoF 3 , WF 3 , VF 3 , NbF 3 , TaF 3 , TiF 4 , ZrF 4 , HfF 4 , or BiI 3 . 
     The first component may have a predetermined work function, e.g., 4.3 eV or more. By disposing the first hole injection layer  130   a  with a strong dipole characteristic at an interface of the first electrode  120  of the inorganic material and the first hole transporting layer  140   a  of the organic material layer, the vacuum level of the organic material layer may shift so that a hole injection barrier decreases. The electric field increases at the interface to allow for tunneling of holes. 
     For example, when a dipole material of a metal or a non-metal having a work function of 4.3 eV or more (a high work function) and a halogen dissociates on the interface, the carrier injection barrier including a hole is decreased by the metal having a high work function. When the metal having a high work function oxidizes and captures electrons on the interface, holes are formed in the organic material layer by separation of the electrons and a p-doped interface region is formed. As a result, a hole injection layer is formed. Halogen ions dissociated from the dipole material prevent In and Sn ions in ITO of the first electrode  120  from moving to the organic material layer. Thus, it is possible to prevent crystallization of the organic material layer due to the In and Sn ions. In addition, pin-holes formed on the surface of ITO are covered to allow for a reduction in the size of any bulge that may form due to bonding of Ag and S in the atmosphere. 
     In the present exemplary embodiment, the first hole injection layer  130   a  may further include the organic material in a first hole transporting layer  140   a . For example, the first hole injection layer  130   a  may include NPD (N,N-dinaphthyl-N,N′-diphenyl benzidine), TPD (N,N′-bis-(3-methylphenyl)-N,N′-bis-(phenyl)-benzidine), s-TAD, or MTDATA (4,4′,4″-tris(N-3-methylphenyl-N-phenyl-amino)-triphenylamine). The first hole injection layer  130   a  may be in a state such that the inorganic dipole material is doped to the organic material. 
     The first hole transporting layer  140   a  may include the organic material. For example, the first hole transporting layer  140   a  may include NPD (N,N-dinaphthyl-N,N′-diphenyl benzidine), TPD (N,N′-bis-(3-methylphenyl)-N,N′-bis-(phenyl)-benzidine), s-TAD, or MTDATA (4,4′,4″-tris(N-3-methylphenyl-N-phenyl-amino)-triphenylamine). The thickness of the first hole transporting layer  140   a  may be greater than the thickness of the first hole injection layer  130   a.    
     The second hole injection layer  130   b  is on the first hole transporting layer  140   a . The second hole injection layer  130   b  includes an inorganic dipole material, such as previously described relative to the first hole injection layer  130   a . The second hole injection layer  130   b  may further include an organic material, such as previously described relative to the first hole injection layer  130   a . The second hole injection layer  130   b  may be made of the same or a different material from the first hole injection layer  130   a . When the second hole injection layer  130   b  and the first hole injection layer  130   a  are made of different materials, the organic material of the first hole injection layer  130   a  may be different from the organic material of the second hole injection layer  130   b.    
     The second hole transporting layer  140   b  is on the second hole injection layer  130   b  and may include organic material. For example, the second hole transporting layer  140   b  may include NPD (N,N-dinaphthyl-N,N′-diphenyl benzidine), TPD (N,N′-bis-(3-methylphenyl)-N,N′-bis-(phenyl)-benzidine), s-TAD, or MTDATA (4,4′,4″-tris(N-3-methylphenyl-N-phenyl-amino)-triphenylamine). The thickness of the second hole transporting layer  140   b  may be greater than the thickness of the second hole injection layer  130   b.    
     The emission layer  150  is on the second hole transporting layer  140   b  and includes a light emitting material displaying a particular color. For example, the emission layer  150  may display a primary color such as blue, green, or red, or a combination thereof. The thickness of the emission layer  150  may be in a predetermined range, e.g., 10 nm to 50 nm. The emission layer  150  includes a host and a dopant. The emission layer  150  may contain materials for emitting red, green, blue, and white light, and may be formed using, for example, a phosphorescent or fluorescent material. 
     When the emission layer  150  emits red light, the emission layer  150  includes a host material that includes CBP (carbazole biphenyl) or mCP (1,3-bis(carbazol-9-yl)), 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. 
     When the emission layer  150  emits green light, the emission layer  150  includes a host material including CBP or mCP. In this case, the emission layer  150  may include, for example, a phosphorescent material including a dopant material such as Ir(ppy)3 (fac-tris(2-phenylpyridine)iridium) or a fluorescent material including Alq3 (tris(8-hydroxyquinolino)aluminum). 
     When the emission layer  150  emits blue light, the emission layer  150  includes a host material including CBP or mCP. In this case, the emission layer  150  may include a phosphorescent material including a dopant that includes (4,6-F2ppy)2Irpic. In one embodiment, the emission layer  150  may include a host material having an anthracene group. In this case, emission layer  150  may include 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, or a PPV-based polymer. 
     Also, as above-described in the present exemplary embodiment, it is not essential for the emission layer  150  to be formed of the organic material. The emission layer  150  may include, for example, an inorganic material such as a quantum dot. 
     The electron transport layer  160   a  and the electron injection layer  180  are between the emission layer  150  and the second electrode  190 . The electron transport layer  160   a  is adjacent to the emission layer  150 , and the electron injection layer  180  is adjacent to the second electrode  190 . The buffer layer  160   b  may be between the emission layer  150  and the electron transporting layer  160   a . In one embodiment, the buffer layer  160   b  may be omitted. 
     The electron transporting layer  160   a  may include organic material. For example, electron transporting layer  160   a  may include Alq3 (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), or BAlq(8-hydroxyquinoline beryllium salt). 
     The electron transport layer  160   a  may transmit the electrons from the second electrode  190  to the emission layer  150 . Also, the electron transport layer  160   a  may prevent holes injected from the first electrode  120  from moving into the second electrode  190  through the emission layer  150 . For example, the electron transport layer  160  may perform the function of a hole blocking layer, and may help the holes and the electrons in the emission layer  150  to combine. 
     The electron injection layer  180  may improve electron injection from the second electrode  190  to the electron transport layer  160 . In the present exemplary embodiment, the thickness of the electron injection layer  180  may be in a predetermined range, e.g., about 2 Å to 25 Å taking into consideration a process margin. In the one embodiment, the electron injection layer  180  includes a lanthanum group element, a first element of an alkali metal, and/or a second element of a halogen. 
     The electron injection layer  180  may be formed by codepositing a first material including metal and a second material including metal halide. The first material and the second material are selected from materials that react with each other to cause a substitution reaction. For example, when the first material made of Yb of a lanthanum group element and the second material made of KI of a metal halide are codeposited, a substitution reaction such as Reaction Formula 1, Reaction Formula 2, or Reaction Formula 3 may be generated.
 
2KI+Yb→YbI 2 +2K + +2 e − or YbI 2 +2K  Reaction Formula 1
 
3KI+Yb→YbI 3 +3K + +3 e − or YbI 3 +3K  Reaction Formula 2
 
3KI+Yb→KYbI 3 +2K + +2 e − or KYbI 3 +2K  Reaction Formula 3
 
     In the present exemplary embodiment, the first material and the second material may include metals having similar standard electrode potentials. For example, when the first material and the second material respectively include any one among a Group 1 element, a Group 2 element, and the lanthanum group element, a spontaneous reaction may be caused depending on strong reactivity, for example, through an experimental example discussed below. 
     The following Examples and Comparative Examples are provided in order to highlight characteristics of one or more embodiments, but it will be understood that the Examples and Comparative Examples are not to be construed as limiting the scope of the embodiments, nor are the Comparative Examples to be construed as being outside the scope of the embodiments. Further, it will be understood that the embodiments are not limited to the particular details described in the Examples and Comparative Examples. 
     Experimental Example 
     The first material is made of the lanthanum group such as Yb, Eu, or Sm. The second material is made of the metal iodine such as KI, RbI, or CsI. The first and second materials are combined and codeposited through the experimental example, and the first material and the second material react to form a layer. 
     As the layer becomes transparent, conductivity increases. However, when the first material made of Ag and the second material made of the metal iodine such as KI, RbI, or CsI are combined and codeposited to form the layer, the layer is not transparent and the conductivity is not increased. Also, when the first material made of the lanthanum group such as Yb, Eu, or Sm and the second material made of CuI are codeposited to form the layer, the layer is not transparent and conductivity is not increased. Accordingly, the metals in the first material and the second material are materials having high reactivity to induce the spontaneous reaction. 
     Among the halide compounds, an iodine compound has low electron affinity of the iodine itself and low electronegativity. Thus, it is relatively easy for the iodine compound to be dissociated to form an iodine vacancy or be combined with other reactive metals to generate a new compound. Accordingly, electron injection characteristics may be improved by compounds generated by a substitution reaction of the first material made of the metal and the metal iodine including the iodine. 
     Also, since iodine has a small refractive index difference with organic material compared to fluorine, such an embodiment may be suitable for optical design. Further, since iodine has a low thermal evaporation temperature compared to the material such as fluorine, process characteristics may be improved. In addition, when fluorine is pyrolyzed, gas is emitted such that a vacuum degree may decrease. However, even when heat is applied to iodine, there is no problem of the vacuum degree decreasing by a remaining solid. 
     In this example, the experiment is executed for the iodine compound among the halide compound. However, a result similar to that of metal iodine appears when using the metal halide such as KCl, KBr, RbCl, RbBr, CsCl, or CsBr as the second material. 
     A conduction mechanism will be described below. 
     In the present exemplary embodiment, the metal in the first material and the metal in the second material may be substituted with each other. In this case, the valence electron number of the metal in the first material may be greater than or equal to the valence electron number of the metal in the second material. When the valence electron number of the metal in the first material is greater than the valence electron number of the metal in the second material, the conductivity due to the free electrons that are additionally generated may be improved. 
     Also, when the halogen elements in the second material are moved in the first material to form a new material, the free electrons are formed depending on a halogen vacancy, such that conductivity may be improved. Further, conductivity may be improved by the remaining metal ions that participate in the reaction. 
     In the present exemplary embodiment, the electron injection layer  180  may include a first compound made of the metal of the first material and a dipole material made of the halogen in the second material and/or the metal of the first material and the metal halide of the second material. In this case, the dipole material may include at least one of a compound including the lanthanum group element as a bivalent element or a compound including the lanthanum group element as a trivalent element. 
     In the present exemplary embodiment, the metal of the first material may be the lanthanum group element having a work function of about 2.7 eV or less. As the lanthanum group element, ytterbium (Yb) having a work function of 2.6 eV, samarium (Sm) having a work function of 2.7 eV, or europium (Eu) having a work function of 2.5 eV may be used. 
     Among the lanthanum group elements, ytterbium (Yb), samarium (Sm), and europium (Eu) have low ionization energy and a low ion radius. Thus, they may be easily reacted with the alkali halide material and may be easily diffused in the alkali halide material. Accordingly, ytterbium (Yb), samarium (Sm), and europium (Eu) are easily reacted with KI, RbI, or CsI such that a reactant of a perovskite structure, and the alkali metal or the ion, remain, thereby smoothly lowering an injection barrier. 
     The first compound may have a perovskite structure. The perovskite structure may be made of the lanthanum group element, the first element of the alkali metal, and the second element of the halogen. Referring to the above-described Reaction Formulas 1, 2, and 3, the electron injection layer  180  may include a positive ion of the first element and a free electron as well as the described dipole material and the first compound having the perovskite structure. 
     In the present exemplary embodiment, the electron injection layer  180  may be formed by codepositing the first material made of the lanthanum group metal and the second material made of the alkali metal halide. Content of the second material in an entire content including the first material and the second material may be in a predetermined range, e.g., from about 1 vol % to about 20 vol %. If it is described based on the electron injection layer  180  as a final product, among the entire material having the lanthanum group element, the first element, and the second element, a sum of the material having the first element and the material having the second element may have a predetermined range, e.g., from about 1 vol % to about 20 vol %. 
     Since more of the first material having the lanthanum group element is present than the second material, the electron injection layer  180  may include a monomolecular molecule including the lanthanum group element. 
     The first material may be, for example, Yb, Eu, or Sm. The second material may be, for example, KI, RbI, or CsI. When RbI and Yb are codeposited, a chemical reaction is generated such that at least one among YbI 2 , YbI 3 , and RbYbI 3  may be generated. Here, RbYbI 3 , as shown in  FIG. 2 , may have the perovskite structure. In this way, even when a chemical reaction is generated, the remaining material may exist as RbI and Yb. 
     The electron injection layer  180  according to the present exemplary embodiment may be a single layer structure in which the first material and the second material are codeposited. If more of the first material is distributed than the second material in the electron injection layer  180 , conductivity relatively increases and transmittance may decrease compared with the opposite case. 
     Accordingly, in the present exemplary embodiment, the volume at which the first material and the second material are distributed may be optimized (or may achieve a predetermined level) by considering the sheet resistance and transmittance for operating the light emitting diode. 
       FIG. 3  illustrates an embodiment of an electron injection layer having a free electron. Referring to  FIG. 3 , one layer may be formed using ytterbium (Yb) in the first material and RbI in the second material. Ytterbium (Yb) and RbI are reacted to form the conductor, e.g., Rb and Yb are substituted with each other. As a result, a free electron may form somewhere in the reactant. The free electron may be formed by an iodine vacancy generated depending on the YbI 2  or YbI 3  compound. In this way, because of the free electron formed by RbI (that is one kind of the metal halide) and/or the free electron formed by the iodine vacancy, and the metal ion, the electron injection layer  180  according to the present exemplary embodiment may have conductivity which allows the electron injection speed to be very fast. Although RbI is described as the metal halide in  FIG. 3 , another material (e.g., KI or CsI) may be equally applied. 
     Next, an increase in lifespan due to a decrease in driving voltage when the content of the first material having the lanthanum group element and the second material having the codeposited alkali metal halide is in the range of 1 vol % to 20 vol % will be described with reference to Table 1. 
     In Table 1, Exemplary Embodiment 1 represents forming the electron injection layer by codepositing ytterbium (Yb) and RbI with a volume ratio of 9:1. Exemplary Embodiment 2 represents forming the electron injection layer by codepositing ytterbium (Yb) and KI with a volume ratio of 9:1. Table 1 is a result of evaluating white/red/green/blue light efficiency depending on time for about 240 h at 85° C. for Exemplary Embodiment 1 and Exemplary Embodiment 2. 
     Referring to Table 1, an increasing width of the driving voltage according to Exemplary Embodiments 1 and 2 is not large, but the driving voltage is mainly reduced. 
     In this way, in the exemplary embodiment, power efficiency is improved by 17% to 26% due to the driving voltage reduction. Accordingly, lifespan may be improved. A condition corresponding to 85° C. may be an environment similar to when a car is exposed to strong sunlight. Accordingly, when the light emitting diode according to an exemplary embodiment is applied to a car, there is an effect that the driving voltage is maintained and the lifespan is improved at the high temperature. 
     
       
         
           
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Driving voltage (V) 
               
            
           
           
               
               
               
               
               
            
               
                 Example (time) 
                 W 
                 R 
                 G 
                 B 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Exemplary Embodiment 1 (0 h) 
                 2.66 
                 2.5 
                 1.67 
                 1.56 
               
               
                 Exemplary Embodiment 1 120 h 
                 2.65 
                 2.01 
                 1.48 
                 1.47 
               
               
                 Exemplary Embodiment 1 240 h 
                 2.52 
                 2.27 
                 1.68 
                 1.5 
               
               
                 Exemplary Embodiment 2 (0 h) 
                 2.46 
                 2.26 
                 1.71 
                 1.64 
               
               
                 Exemplary Embodiment 2 120 h 
                 2.29 
                 2.06 
                 1.45 
                 1.42 
               
               
                 Exemplary Embodiment 2 240 h 
                 2.55 
                 2.05 
                 1.63 
                 1.46 
               
               
                   
               
            
           
         
       
     
     Hereinafter, light efficiency and reliability of the element depending on volume ratio of the first material having the lanthanum group element and the second material having the alkali metal halide will be described. 
     Table 2 is a case in which Comparative Example 1-1 forms the electron injection layer with ytterbium (Yb). Exemplary Embodiment 1-1 to Exemplary Embodiment 1-6 form the electron injection layer by codepositing ytterbium (Yb) and KI in the light emitting diode including the electron transport layer including Liq and a negative electrode including AgMg. In Exemplary Embodiment 1-1 to Exemplary Embodiment 1-6, the content of K in the electron injection layer is 1 vol %, 3 vol %, 5 vol %, 10 vol %, 15 vol %, and 20 vol %, respectively. Numbers of Table 2 are values corresponding to an average of efficiencies of 30 panels for each condition. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 R effi- 
                 G effi- 
                 B effi- 
                 W effi- 
               
               
                   
                 ciency 
                 ciency 
                 ciency 
                 ciency 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Comparative Example 1-1 
                 37.4 
                 53.8 
                 4.973 
                 29.9 
               
               
                 Exemplary Embodiment 1-1 
                 38.3 
                 56.3 
                 5.131 
                 31.2 
               
               
                 Exemplary Embodiment 1-2 
                 38.6 
                 56.8 
                 5.179 
                 31.2 
               
               
                 Exemplary Embodiment 1-3 
                 39.1 
                 57.1 
                 5.218 
                 31.2 
               
               
                 Exemplary Embodiment 1-4 
                 38.9 
                 59.0 
                 5.360 
                 31.7 
               
               
                 Exemplary Embodiment 1-5 
                 39.6 
                 59.0 
                 5.467 
                 31.9 
               
               
                 Exemplary Embodiment 1-6 
                 38.3 
                 59.1 
                 5.492 
                 32.0 
               
               
                   
               
            
           
         
       
     
     Table 3 shows, for the light emitting diode including the electron transport layer including Liq and the negative electrode formed of AgYb, the case where Comparative Example 1-1 forms the electron injection layer of ytterbium (Yb). Exemplary Embodiment 2-1 to Exemplary Embodiment 2-6 form the electron injection layer by codepositing ytterbium (Yb) and KI. In Exemplary Embodiment 2-1 to Exemplary Embodiment 2-6, the content of Kl in the electron injection layer is 1 vol %, 3 vol %, 5 vol %, 10 vol %, 15 vol %, and 20 vol %, respectively. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 R effi- 
                 G effi- 
                 B effi- 
                 W effi- 
               
               
                   
                 ciency 
                 ciency 
                 ciency 
                 ciency 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Comparative Example 1-1 
                 37.4 
                 53.8 
                 4.973 
                 29.9 
               
               
                 Exemplary Embodiment 2-1 
                 37.7 
                 55.2 
                 5.096 
                 31.2 
               
               
                 Exemplary Embodiment 2-2 
                 38.0 
                 56.5 
                 5.230 
                 31.4 
               
               
                 Exemplary Embodiment 2-3 
                 38.6 
                 57.1 
                 5.142 
                 31.2 
               
               
                 Exemplary Embodiment 2-4 
                 38.9 
                 57.7 
                 5.291 
                 31.2 
               
               
                 Exemplary Embodiment 2-5 
                 38.7 
                 58.9 
                 5.564 
                 31.7 
               
               
                 Exemplary Embodiment 2-6 
                 37.4 
                 60.3 
                 5.469 
                 32.1 
               
               
                   
               
            
           
         
       
     
     Referring to Table 2 and Table 3, compared with Comparative Example 1-1, in the case of Exemplary Embodiment 1-1 to Exemplary Embodiment 1-6, and Exemplary Embodiment 2-1 to Exemplary Embodiment 2-6, there is an effect that the white light efficiency is increased by about 4% to 7%. 
     Table 4 shows a result of estimating the reliability of the display panel including the light emitting diode depending on the volume ratio of the two codeposited materials when codepositing ytterbium (Yb) and RbI to form the electron injection layer and codepositing ytterbium (Yb) and KI to form the electron injection layer. 
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 4 
               
               
                   
               
               
                 Electron 
                 Volume 
                   
                 Electron 
                 Volume 
                   
               
               
                 injection 
                 ratio 
                   
                 injection 
                 ratio 
               
               
                 layer 
                 (vol %) 
                 Reliability 
                 layer 
                 (vol %) 
                 Reliability 
               
               
                   
               
             
            
               
                 Yb:RbI 
                 9:1 
                 Good 
                 Yb:KI 
                 9:1 
                 Good 
               
               
                 Yb:RbI 
                 8:2 
                 Good 
                 Yb:KI 
                 8:2 
                 Good 
               
               
                 Yb:RbI 
                 7:3 
                 Bad 
                 Yb:KI 
                 7:3 
                 Bad 
               
               
                 Yb:RbI 
                 5:5 
                 Bad 
                 Yb:KI 
                 5:5 
                 Bad 
               
               
                 Yb:RbI 
                 3:7 
                 Bad 
                 Yb:KI 
                 3:7 
                 Bad 
               
               
                 Yb:RbI 
                 1:9 
                 Bad 
                 Yb:KI 
                 1:9 
                 Bad 
               
               
                 RbI alone 
                 — 
                 Bad 
                 KI alone 
                 — 
                 Bad 
               
               
                   
               
            
           
         
       
     
     Referring to Table 4, in the case that the volume ratio of ytterbium (Yb) and RbI is 9:1 and 8:2, the display panel is normally operated. If the content of RbI is over 20 vol %, a phenomenon such as a pattern stain is generated such a defect rate of the display panel may be high. 
     In the ratio of Yb and KI, as KI increases, more of the perovskite structure is formed in the reactor. Because the perovskite very sensitively reacts to a magnetic field generated in a magnetic jig of an opening aligning device, a pattern stain such as one formed by the magnetic jig is generated in the surface during a deposition process. In the case of the perovskite structure, because the perovskite structure has a high dielectric constant of about 10 times or more that of a binary compound, the perovskite structure may sensitively react to the magnetic field change. 
     As above-described, in the light emitting diode according to the present embodiment, the content of the first material having the lanthanum group element and the second material having the codeposited alkali metal halide is in the range of 1 vol % to 20 vol % in the perspective of reliability and light efficiency. 
     Referring again to  FIG. 1 , the second electrode  190  is on the electron injection layer  180 . The second electrode  190  may be a transflective electrode, e.g., an electrode including a material having a transflective characteristic transmitting part of light incident to the second electrode  190  and reflecting a remaining part of the light to the first electrode  120 . The transflective characteristic may correspond to the case where reflectivity for the incident light is in a predetermined range, e.g., about 0.1% or more to about 70% or less, or about 30% or more to about 50% or less. 
     The second electrode  190  may include silver (Ag), magnesium (Mg), aluminum (Al), chromium (Cr), molybdenum (Mo), tungsten (W), titanium (Ti), gold (Au), palladium (Pd), ytterbium (Yb), or alloys thereof. Above, it is described that the second electrode  190  is the transflective electrode as one example to explain a resonance structure. However, a non-resonance structure may also be applied according to another exemplary embodiment. In this case, the second electrode  190  may be a transparent conductive electrode such as ITO or IZO. 
     When the above-described second electrode  190  is formed of the alloy, an alloy ratio may be controlled by the temperature of a deposition source, the atmosphere, and/or the vacuum degree. The allow ratio may therefore be set to an appropriate or predetermined ratio. In the present exemplary embodiment, the second electrode  190  may have a predetermined thickness, e.g., about 50 Å to about 150 Å. If the thickness of the second electrode  190  is less than 50 Å, it may be difficult to obtain the sheet resistance. If the thickness is greater than 150 Å, reflectance is increased and a wide angle distribution (WAD) may not result. Consequently, a color change may be when viewed from the side. 
     In the present exemplary embodiment, the second electrode  190  may be formed of AgMg or AgYb. When applying the AgMg or AgYb electrode, the electron injection barrier decreases as the content of Mg or Yb increases. As a result, many electrons may be injected. When an electron-hole balance in the whole device is changed, degradation is generated in a way that will sharply reduce lifespan and efficiency. By considering the electron-hole balance, the content of Mg or Yb in the second electrode  190  is in a predetermined range, e.g., about 10 vol % to about 30 vol %. 
     When comparing the AgMg electrode and the AgYb electrode, because of high ignitability in the case of Mg, AgYb may be used to address environmental and safety concerns. Also, because Yb is in the electron injecting layer, AgYb may be used to improve process and cost concerns, compared with AgMg electrode manufacturing. This is because the number of materials for AgYb electrode manufacturing is less. 
     The capping layer  200  is on the second electrode  190 , may include organic material or inorganic material, and may perform the function of protecting the second electrode  190  or guiding a change of resonance intensity and resonance phase along with the second electrode  190 . 
       FIG. 4  illustrates a cross-sectional view of an embodiment of a light emitting diode display including an emission layer corresponding to the embodiment of  FIG. 1 . In  FIG. 4 , a light emitting diode respectively corresponding to the red pixel, the green pixel, and the blue pixel is on a substrate  23 . 
     Referring to  FIG. 4 , the emission layer  150  of  FIG. 1  includes a red emission layer  150 R, a green emission layer  150 G, and a blue emission layer  150 B. These layers are disposed horizontally in a direction parallel to the first electrode  120 . The first hole injection layer  130   a  and the hole transporting layer  140  are commonly positioned 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 . The thickness of the first hole injection layer  130   a  may be substantially the same. The thickness of the hole transporting layer  140  may be substantially the same on the commonly-positioned part. The description related to the material in the first hole injection layer  130   a  and the hole transporting layer  140  may be as described in  FIG. 1 . 
     A pixel definition layer  25  may be between the red emission layer  150 R, the green emission layer  150 G, and the blue emission layer  150 B adjacent to each other. In the green light emission region, the second hole injection layer  130   b  is on the hole transporting layer  140 . The description in  FIG. 1  may be applied to the description related to the material in the second hole injection layer  130   b . The second hole injection layer  130   b  may be between the adjacent pixel definition layers  25 . 
     In the present exemplary embodiment, an auxiliary layer BIL may be under 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 controlling hole charge balance. The auxiliary layer BIL may include a compound represented by Chemical Formula 1. 
     
       
         
         
             
             
         
       
     
     In Chemical Formula 1, A1, A2, and A3 may be an alkyl group, an aryl group, carbazole, dibenzothiophene, dibenzofuran (DBF), and biphenyl, respectively. Also, a, b, and c are independently positive numbers of zero to four. 
     The following Chemical Formulas 1-1, 1-2, 1-3, 1-4, 1-5, and 1-6 may be examples of the compounds represented by Chemical Formula 1. 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     In another exemplary embodiment, the assistant layer B-L may include a compound represented by Chemical Formula 2. 
     
       
         
         
             
             
         
       
     
     In Chemical Formula 2, a, b, and c may independently be 0 to 3, X may be selected from O, N, and S, and each X may be the same as or different from each other. 
     As an example of the compound represented by Chemical Formula 2, Chemical Formulas 2-1, 2-2, 2-3, 2-4, and 2-5 may be included. 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     In another exemplary embodiment, the auxiliary layer (BIL) may include a compound represented by Chemical Formula 3. 
     
       
         
         
             
             
         
       
     
     In Chemical Formula 3, A1 may be an alkyl group, an aryl group, carbazole, dibenzothiophene, or dibenzofuran (DBF), L1 and L2 may be 
                         
(wherein n is 0 to 3), and DBF connected to L1 and L2 may be replaced by carbazole or dibenzothiophene.
 
     In the light emitting diode according to the present exemplary embodiment, a red resonance auxiliary layer  150 R′ may be under the red emission layer  150 R. A green resonance auxiliary layer  150 G′ may be under the green emission layer  150 G. The red resonance auxiliary layer  150 R′ and the green resonance auxiliary layer  150 G′ are added to control a resonance distance for each color. In one embodiment, a separate resonance auxiliary layer between the blue emission layer  150 B and the auxiliary layer BIL, and the hole transport layer  140 , may not be formed under the blue emission layer  150 B and the auxiliary layer BIL. The green resonance auxiliary layer  150 G′ may correspond to the second hole transporting layer  140   b  described in  FIG. 1 . 
     The electron injection layer  180  and the second electrode  190  are commonly positioned between the red emission layer  150 R and the capping layer  200 , between the green emission layer  150 G and the capping layer  200 , and between the blue emission layer  150 B and the capping layer  200 . The thickness of the electron injection layer  180  and the second electrode  190  may be substantially the same in the commonly positioned portion. The electron transporting layer  160   a  and the buffer layer  160   b  of  FIG. 1  are not in  FIG. 4 . However, they may be applied to the exemplary embodiment of  FIG. 4 . 
     The thin film transistor, the element including the interlayer insulating layer, and the insulating layer may be between the substrate  23  and the first electrode  120  that are shown in  FIG. 4 . 
       FIG. 5  illustrates a cross-sectional view of another embodiment of a light emitting diode.  4 . The exemplary embodiment of  FIG. 5  may be the same as the light emitting diode described in  FIG. 4 , except for the following differences. 
     Referring to  FIG. 5 , the light emitting diode in the red light emission region further includes a third hole injection layer  130   c  and a fourth hole injection layer  130   d  that are formed by respectively extending the first hole injection layer  130   a  and the second hole injection layer  130   b  in the green light emission region. The description related to the first hole injection layer  130   a  and the second hole injection layer  130   b  may be applied to the description related to the third hole injection layer  130   c  and the fourth hole injection layer  130   d  as it is. 
     A driving voltage and light efficiency in the blue light emitting diode will be described with reference to Table 5. The driving voltage and the light efficiency in the green light emitting diode will be described with reference to Table 6. 
     In Table 5, Comparative Example 1 represents the blue light emitting diode configured of the first electrode made of ITO/Ag/ITO and having a 100 Å thickness, the hole injection layer of two kinds of organic materials are mixed and having a 100 Å thickness, the hole transporting layer including one kind of organic material and having a 1080 Å thickness, the emission layer of a 190 Å thickness, the electron transporting layer of which two kinds of organic materials are mixed and has a 310 Å thickness, the electron injection layer made of Yb and having a 13 Å thickness, the second electrode made of AgMg and having a 90 Å thickness, and the capping layer including the organic material. 
     Experimental Example 1 has the same configuration as most of Comparative Example 1, but represents the blue light emitting diode including the hole injection layer in which the organic material is doped with 6 vol % CuI. Experimental Example 2 has the same configuration as most of Comparative Example 1, but represents the blue light emitting diode including the hole injection layer in which the organic material is doped with 4 vol % CuI and the electron injection layer in which Yb is doped with 3 vol % KI. Experimental Example 3 has the same configuration as most of Comparative Example 1, but represents the blue light emitting diode including the hole injection layer in which the organic material is doped with 6 vol % CuI and the electron injection layer in which Yb is doped with 3 vol % KI. Experimental Example 4 has the same configuration as most of Comparative Example 1, but represents the blue light emitting diode including the hole injection layer in which the organic material is doped with 6 vol % CuI and the electron injection layer in which Yb is doped with 6 vol % KI. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 5 
               
               
                   
                   
               
               
                   
                 Driving voltage (V) 
                 B efficiency (cd/A) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 Comparative Example 1 
                 4.0 
                 180.5 
               
               
                 Experimental Example 1 
                 3.7 
                 183.3 
               
               
                 Experimental Example 2 
                 3.6 
                 187.9 
               
               
                 Experimental Example 3 
                 3.6 
                 185.7 
               
               
                 Experimental Example 4 
                 3.5 
                 198.6 
               
               
                   
               
            
           
         
       
     
     In Table 6, Comparative Example 2 represents the green light emitting diode configured of the first electrode made of ITO/Ag/ITO and having a 100 Å thickness, the first hole injection layer of which two kinds of organic materials are mixed and has a 100 Å thickness, the first hole transporting layer including one kind of organic material and having the 1075 Å thickness, the second hole injection layer of which two kinds of organic materials are mixed and has the 100 Å thickness, the second hole transporting layer including one kind of organic material and having the 200 Å thickness, the emission layer of the 380 Å thickness, the electron transporting layer of which two kinds of organic materials are mixed and having the 310 Å thickness, the electron injection layer made of Yb and having the 13 Å thickness, the second electrode made of AgMg and having the 100 Å thickness, and the capping layer including the organic material. 
     Experimental Example 5 has the same configuration as most of Comparative Example 2, but represents the green light emitting diode including the first hole injection layer in which the organic material is doped with 4 vol % CuI. Experimental Example 6 has the same configuration as most of Comparative Example 2, but represents the green light emitting diode including the first hole injection layer of which the organic material is doped with 6 vol % CuI. Experimental Example 7 has the same configuration as most of Comparative Example 2, but represents the green light emitting diode including the first hole injection layer of which the organic material is doped with 8 vol % CuI. 
     Experimental Example 8 has the same configuration as most of Comparative Example 2, but represents the green light emitting diode including the second hole injection layer of which the organic material is doped with 2 vol % CuI. Experimental Example 9 has the same configuration as most of Comparative Example 2, but represents the green light emitting diode including the second hole injection layer in which the organic material is doped with 4 vol % CuI. Experimental Example 10 has the same configuration as most of Comparative Example 2, but represents the green light emitting diode including the second hole injection layer in which the organic material is doped with 6 vol % CuI. Experimental Example 11 has the same configuration as most of Comparative Example 2, but represents the green light emitting diode including the second hole injection layer in which the organic material is doped with 8 vol % CuI. 
     Experimental Example 12 has the same configuration as most of Comparative Example 2, but represents the green light emitting diode including the first hole injection layer in which the organic material is doped with 2 vol % CuI and the electron injection layer in which Yb is doped with 10 vol % KI. Experimental Example 13 has the same configuration as most of Comparative Example 2, but represents the green light emitting diode including the first hole injection layer in which the organic material is doped with 4 vol % CuI and the electron injection layer in which Yb is doped with 10 vol % KI. 
     Experimental Example 14 has the same configuration as most of Comparative Example 2, but represents the green light emitting diode including the first hole injection layer in which the organic material is doped with 6 vol % CuI and the electron injection layer in which Yb is doped with 10 vol % KI. Experimental Example 15 has the same configuration as most of Comparative Example 2, but represents the green light emitting diode including the first hole injection layer in which the organic material is doped with 8 vol % CuI and the electron injection layer in which Yb is doped with 10 vol % KI. 
     Experimental Example 16 has the same configuration as most of Comparative Example 2, but represents the green light emitting diode including the second hole injection layer in which the organic material is doped with 2 vol % CuI and the electron injection layer in which Yb is doped with 10 vol % KI. Experimental Example 17 has the same configuration as most of Comparative Example 2, but represents the green light emitting diode including the second hole injection layer in which the organic material is doped with 4 vol % CuI and the electron injection layer in which Yb is doped with 10 vol % KI. Experimental Example 18 has the same configuration as most of Comparative Example 2, but represents the green light emitting diode including the second hole injection layer in which the organic material is doped with 6 vol % CuI and the electron injection layer in which Yb is doped with 10 vol % KI. Experimental Example 19 has the same configuration as most of Comparative Example 2, but represents the green light emitting diode including the second hole injection layer in which the organic material is doped with 8 vol % CuI and the electron injection layer in which Yb is doped with 10 vol % KI. 
     Experimental Example 20 has the same configuration as most of Comparative Example 2, but represents the green light emitting diode including the first hole injection layer made of CuI and the electron injection layer in which Yb is doped with 10 vol % KI. Experimental Example 21 has the same configuration as most of Comparative Example 2, but represents the green light emitting diode including the second hole injection layer made of CuI and the electron injection layer in which Yb is doped with 10 vol % KI. Experimental Example 22 has the same configuration as most of Comparative Example 2, but represents the green light emitting diode including the first hole injection layer made of CuI, the second hole injection layer made of CuI, and the electron injection layer in which Yb is doped with 10 vol % KI. 
     Experimental Example 23 has the same configuration as most of Comparative Example 2, but represents the green light emitting diode including the first hole injection layer in which the organic material is doped with 6 vol % CuI, the second hole injection layer in which the organic material is doped with 6 vol % CuI, and the electron injection layer in which Yb is doped with 10 vol % KI. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 6 
               
               
                   
                   
               
               
                   
                 Driving voltage (V) 
                 G efficiency (cd/A) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 Comparative Example 2 
                 4.2 
                 133.4 
               
               
                 Experimental Example 5 
                 3.8 
                 145.7 
               
               
                 Experimental Example 6 
                 3.8 
                 142.5 
               
               
                 Experimental Example 7 
                 3.8 
                 142.2 
               
               
                 Experimental Example 8 
                 3.8 
                 142.4 
               
               
                 Experimental Example 9 
                 3.7 
                 143.1 
               
               
                 Experimental Example 10 
                 3.7 
                 146.2 
               
               
                 Experimental Example 11 
                 3.7 
                 146.7 
               
               
                 Experimental Example 12 
                 3.9 
                 147.1 
               
               
                 Experimental Example 13 
                 3.9 
                 148.8 
               
               
                 Experimental Example 14 
                 3.9 
                 149 
               
               
                 Experimental Example 15 
                 3.8 
                 149.5 
               
               
                 Experimental Example 16 
                 3.7 
                 145.7 
               
               
                 Experimental Example 17 
                 3.6 
                 145.9 
               
               
                 Experimental Example 18 
                 3.6 
                 150.8 
               
               
                 Experimental Example 19 
                 3.6 
                 149.5 
               
               
                 Experimental Example 20 
                 3.9 
                 148.9 
               
               
                 Experimental Example 21 
                 3.6 
                 146.8 
               
               
                 Experimental Example 22 
                 3.7 
                 147.8 
               
               
                 Experimental Example 23 
                 3.7 
                 149.8 
               
               
                   
               
            
           
         
       
     
     Referring to Table 5 and Table 6, compared with Comparative Examples 1 and 2, in the case of Experimental Example 1 to Experimental Example 23, the driving voltage decreases, and the blue light efficiency or the green light efficiency increases. For the red light emitting diode, experimental data is not attached, however an experimental result of the same tendency as the green light emitting diode may be obtained. 
       FIG. 6  illustrates a cross-sectional view of another embodiment of a light emitting diode having a tandem structure.  FIG. 6  has the same configuration as most of the light emitting diode described in  FIG. 1 , except for the following differences. 
     Referring to  FIG. 6 , the light emitting diode according to the present exemplary embodiment includes the first electrode  120 , the second electrode  190 , and the emission layer  150  emitting white light or blue light by combining a plurality of layers  150 Y,  150 B, and  250 B between the first electrode  120  and the second electrode  190 . The plurality of layers may have a structure in which two or three layers are deposited, and the emission layer  150  of three layers is shown in  FIG. 6 . 
     The light generated from the emission layer  150  may realize the desired color while passing through a color convention layer on the second electrode  190 . In one embodiment, a plurality of color conversion layers may be formed to correspond to respective pixel areas. The color conversion layers may be, for example, red, green, and blue color filters including a dye or a pigment, and may include nanoparticles having different sizes, e.g., quantum dots. When the emission layer  150  emits blue light, the blue color filter may be omitted or the transmission layer may be formed at a portion corresponding to the blue pixel area instead of the color conversion layer. As a result, blue light generated from the emission layer  150  may be transmitted as it is. Like the case where layers  150 Y,  150 B, and  250 B are combined to emit blue light, a light emitting diode structure may be formed based on a combination of the emission layer emitting blue light as a single layer and the color conversion layer. 
     The three layers of the emission layer  150  may respectively represent blue, yellow, and blue. Two layers of the emission layer may respectively represent blue and yellow. Also, n-type charge generation layers  171   a  and  271   a  and  p -type charge generation layers  171   b  and  271   b  may be between layers adjacent to each other, among layers  150 Y,  150 B, and  250 B of  FIG. 6 . The n-type charge generation layers  171   a  and  271   a  may include an organic material doped with a lanthanum element. The p-type charge generation layers  171   b  and  271   b  may include inorganic dipole material. The inorganic dipole material may be applied with the contents related to the inorganic dipole material including the first hole injection layer  130   a  or the second hole injection layer  130   b  described with reference to  FIG. 1 . The inorganic dipole material in the p-type charge generation layers  171   b  and  271   b  may be the same as the inorganic dipole material in the first hole injection layer  130   a  or the second hole injection layer  130   b.    
     Referring to  FIG. 6 , the first hole injection layer  130   a  and the second hole injection layer  130   b  are respectively under and on a first hole transporting layer  141 . The first emission layer  150 Y is on the second hole injection layer  130   b . A first electron transporting layer  161  is on the first emission layer  150 Y. The charge generation layers  171   a  and  171   b  are on the first electron transporting layer  161 . A second hole transporting layer  142  is on the charge generation layers  171   a  and  171   b . A blue light emission auxiliary layer BIL is between the second hole transporting layer  142  and the second emission layer  150 B. A second electron transporting layer  162  is on the second emission layer  150 B. 
     The charge generation layers  271   a  and  271   b  are on the second electron transporting layer  162 . A third hole transporting layer  143  is on the charge generation layers  271   a  and  271   b . A blue light emission auxiliary layer BIL is between the third hole transporting layer  143  and the third emission layer  250 B. A third electron transporting layer  163  is on the third emission layer  250 B. The capping layer  200  is on the second electrode  190 . 
       FIG. 7  illustrates a cross-sectional view of another embodiment of a display device which includes a substrate  23 , a driving 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 on the substrate  23  and may prevent penetration of impure elements and planarize the surface. In one embodiment, the substrate buffer layer  26  may be omitted, for example, according to the type and/or process conditions of the substrate  23 . 
     A driving semiconductor layer  37  is on the substrate buffer layer  26  and may be formed, for example, of a material including a polysilicon. 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 and formed at respective sides of the channel region  35 . The doped ion materials may be p-type impurities, e.g., boron (B) or B 2 H 6 . The impurities depend, for example, on the type of the thin film transistor. 
     A gate insulating layer  27  is on the driving semiconductor layer  37 . A gate wire including a driving gate electrode  33  is on the gate insulating layer  27 . The driving gate electrode  33  overlaps at least a portion of the driving semiconductor layer  37 , and, for example, the channel region  35 . 
     An interlayer insulating layer  28  covering the gate electrode  33  is on the gate insulating layer  27 . A first contact hole  22   a  and a second contact hole  22   b  respectively expose the source region  34  and the drain region  36  of the driving semiconductor layer  37  and 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 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. 
     The driving thin film transistor  30  is formed to include the driving semiconductor layer  37 , the driving gate electrode  33 , the driving source electrode  73 , and the driving drain electrode  75 . The driving thin film transistor  30  may have a different configuration in another embodiment. 
     A planarization layer  24  covering the data wire is on the interlayer insulating layer  28 . The planarization layer  24  serves to remove and planarize a step in order to increase emission efficiency of the light emitting diode to be formed thereon. 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 is described later. In one embodiment, the planarization layer  24  or the interlayer insulating layer  28 , or both, may be omitted. 
     The first electrode  120  of the light emitting diode LD is on the planarization layer  24 . The pixel definition layer  25  is 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 . In this case, the light emitting diode layer  100  may be positioned for each opening formed in the pixel definition layer  25 . 
     The light emitting diode layer  100  is on the first electrode  120  and corresponds to the first hole injection layer  130   a , the first hole transporting layer  140   a , the second hole injection layer  130   b , the second hole transporting layer  140   b , the emission layer  150 , the electron transporting layer  160 , and the electron injection layer  180  in the light emitting diode described in  FIG. 1 . Other features of the light emitting diode of  FIGS. 4 and 6  may be included the display device of the present embodiment. 
     In  FIG. 7 , the light emitting diode layer  100  is only positioned in the opening of the pixel definition layer  25 . However, as shown in  FIG. 4 , partial layers configuring the light emitting diode layer  100  may also be on the upper surface of the pixel definition layer  25 , like the second electrode  190 . 
     A second electrode  190  and a capping layer  200  are on the light emitting diode layer  100 . 
     A thin film encapsulation layer  300  is on the capping layer  200  and encapsulates the light emitting diode LD formed on the substrate  23  and a driving circuit to protect them from the outside. The thin film encapsulation layer  300  includes organic encapsulation layers  300   a  and  300   c  and inorganic encapsulation layers  300   b  and  300   d  that are alternately deposited one-by-one. In  FIG. 7 , the thin film encapsulation layer  300  is configured by alternately depositing two organic encapsulation layers  300   a  and  300   c  and two inorganic encapsulation layers  300   b  and  300   d  one-by-one. These layers may be deposited in a different manner in another embodiment. 
     Example embodiments have been disclosed herein, and although specific terms are employed, they are used and are to be interpreted in a generic and descriptive sense only and not for purpose of limitation. In some instances, as would be apparent to one of ordinary skill in the art as of the filing of the present application, features, characteristics, and/or elements described in connection with a particular embodiment may be used singly or in combination with features, characteristics, and/or elements described in connection with other embodiments unless otherwise indicated. Accordingly, various changes in form and details may be made without departing from the spirit and scope of the embodiments set forth in the claims.