KR20090031145A - Organic light emitting device - Google Patents

Abstract

An organic light emitting device is provided to improve a driving voltage, light emitting efficiency, and lifetime by adding dopant to an electron injection layer. An organic light emitting device comprises a substrate(100), a first electrode(110), an organic functional layer(120), and a second electrode(130). The first electrode is positioned on the substrate. The organic functional layer is positioned on the first electrode, and includes a light emitting layer(125), an electron transport layer(126), and an electron injection layer(127). The second electrode is positioned on the organic functional layer. The electron injection layer includes organic material and dopant(128). A conducting band minimum level of the dopant is lower than a conducting band minimum level of the organic material. A valence band maximum level of the dopant is higher than a valence band maximum level of the organic material.

Description

Organic Light Emitting Device

The present invention relates to an organic light emitting display device, and more particularly, to an organic light emitting display device having an electron injection layer containing a dopant.

Recently, the importance of the flat panel display (FPD) has increased with the development of multimedia. In response, Liquid Crystal Display (LCD), Plasma Display Panel (PDP), Field Emission Display (FED), Organic Light Emitting Diode Display Device, etc. The same various displays are put to practical use.

Among them, the organic light emitting display device has a high response time with a response speed of 1 ms or less, low power consumption, and no self-emission, thus having no problem in viewing angle.

In a typical organic light emitting display device, a substrate, a first electrode is positioned on the substrate, and an organic layer including at least a light emitting layer is disposed on the first electrode. The second electrode is positioned on the organic layer. The organic layer may further include a hole injection layer (HIL), a hole transportation layer (HTL), an electron transport layer (ETL) and an electron injection layer (EIL). have.

The principle of operation of the organic light emitting device of this structure is as follows. When a voltage is applied between the first electrode and the second electrode, holes are injected from the first electrode and injected into the light emitting layer through the hole injection layer and the hole transport layer, and electrons are injected from the second electrode through the electron injection layer and the electron transport layer. It is injected into the light emitting layer. Holes and electrons injected into the emission layer recombine to form an exciton, and emit light by energy generated as the exciton transitions to the ground state.

For the high luminous efficiency of the organic light emitting device, the balance of holes and electrons injected into the light emitting layer should be well balanced. To this end, conventionally, a hole injection layer or a hole transport layer is formed between the first electrode and the light emitting layer, and an electron transport layer or an electron injection layer is formed between the light emitting layer and the second electrode to lower the energy barrier between the layers to move electrons and holes. Was made smooth.

However, in the conventional organic light emitting device, since the hole mobility is 10 times faster than the electron mobility, the amount of holes and electrons injected into the light emitting layer is changed. Therefore, there is a problem that the luminous efficiency of the organic light emitting device is lowered.

Accordingly, the present invention provides an organic light emitting device capable of improving luminous efficiency.

In order to achieve the above object, the organic light emitting display device according to an embodiment of the present invention is located on the substrate, the first electrode on the substrate, the first electrode, the light emitting layer, the electron transport layer and the electron injection layer And an organic functional layer including a second electrode positioned on the organic functional layer, wherein the electron injection layer includes an organic material and a dopant, and the lowest level of conduction band of the dopant may be lower than the lowest level of conduction band of the organic material. have.

The organic light emitting display device of the present invention has an advantage of improving luminous efficiency.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

<Example 1>

1 is a cross-sectional view illustrating an organic light emitting display device according to an embodiment of the present invention.

Referring to FIG. 1, in an organic light emitting display device according to an embodiment of the present invention, a substrate 100 and a first electrode 110 positioned on the substrate 100 are positioned and positioned on the first electrode 110. The organic functional layers 120 including the hole injection layer 121, the hole transport layer 122, the light emitting layer 125, the electron transport layer 126, and the electron injection layer 127 are positioned, and the organic functional layers 120 are formed. ) May include a second electrode 130 positioned on the substrate. The electron injection layer 127 adjacent to the second electrode 130 may further include a dopant 128.

First, the first electrode 110 is positioned on the substrate 100. At least one thin film transistor including a semiconductor layer, a gate electrode, a source electrode, and a drain electrode may be included between the substrate 100 and the first electrode 110.

The substrate 100 may be made of a material including insulating glass, plastic, or a conductive material.

The first electrode 110 may be an anode and may be a transparent electrode or a reflective electrode. When the first electrode 110 is a transparent electrode, the first electrode 110 may be any one of indium tin oxide (ITO), indium zinc oxide (IZO), or zinc oxide (ZnO). In addition, when the first electrode 110 is a reflective electrode, the first electrode 110 may be formed of aluminum (Al), silver (Ag), or nickel (Ni) under a layer made of any one of ITO, IZO, or ZnO. It may further include a reflective layer made of any one, and in addition, may include the reflective layer between two layers made of any one of ITO, IZO or ZnO.

The first electrode 110 may be formed using a sputtering method, an evaporation method, a vapor phase deposition method, or an electron beam deposition method.

Organic functional layers 120 are positioned on the first electrode 110. The organic functional layers 120 may include at least the light emitting layer 125. In addition, the organic functional layers 120 may further include any one or more selected from the group consisting of a hole injection layer 121, a hole transport layer 122, an electron transport layer 126, and an electron injection layer 127. .

 The hole injection layer 121 may play a role of smoothly injecting holes from the first electrode 110 to the light emitting layer 125, and may include CuPc (cupper phthalocyanine) and PEDOT (poly (3,4) -ethylenedioxythiophene). ), PANI (polyaniline) and NPD (N, N-dinaphthyl-N, N'-diphenyl benzidine) may be made of any one or more selected from the group consisting of, but not limited to.

The hole injection layer 121 may have a valence band level of -5.0 to -5.5 eV, and a conducting band level of -1.8 to -2.8 eV to facilitate movement of holes to the light emitting layer. have.

The hole injection layer 121 may be formed using an evaporation method or a spin coating method.

The hole injection layer 121 may have a thickness of about 5 nm to about 150 nm. Here, when the thickness of the hole injection layer 121 is 5 nm or more, there is an advantage that the hole injection characteristics may be prevented from being lowered. If the thickness is 150 nm or less, the hole injection layer 121 may be too thick to move the holes. There is an advantage that can be prevented from increasing the driving voltage to improve.

A buffer layer may be further included between the first electrode 110 and the hole injection layer 121. The buffer layer may include a compound including a hydroxyl group (-OH), a cyan group (-CN), or a halogen group having a strong electron affinity. For example, the buffer layer may include a compound having the following formula.

At least one of R1 to R6 may be a hydroxyl group (-OH), a cyan group (-CN) or a halogen group.

Since the buffer layer serves to stabilize the interface and includes a material having a strong electron affinity, the buffer layer attracts electrons present in the hole injection layer 121 into the buffer layer. Thus, a relatively large amount of holes are generated in the hole injection layer 121, these holes are transferred to the hole transport layer, the light emitting layer is supplied with a sufficient amount of holes. Therefore, the buffer layer has an effect of lowering the driving voltage by stabilizing the interface and generating holes in the hole injection layer.

The difference between the conduction band level and the valence band level of the buffer layer may be within ± 5 eV. Here, when the difference between the conduction band level and the valence band level of the buffer layer is -0.5 eV or more, hole trapping in which holes cannot move from the first electrode to the hole injection layer can be prevented from occurring.

The thickness of the buffer layer may be 10 to 100 kPa. If the thickness of the buffer layer is 10 GPa or more, the interface between the first electrode and the hole injection layer can be stabilized. If the thickness of the buffer layer is 100 GPa or less, holes can be easily moved from the first electrode to the hole injection layer.

The hole transport layer 122 serves to facilitate the transport of holes, NPD (N, N-dinaphthyl-N, N'-diphenyl benzidine), TPD (N, N'-bis- (3-methylphenyl)- N, N'-bis- (phenyl) -benzidine), s-TAD and MTDATA (4,4 ', 4 "-Tris (N-3-methylphenyl-N-phenyl-amino) -triphenylamine) It may be made of any one or more, but is not limited thereto.

The hole transport layer 122 may have a valence band level of -5.0 to -5.8 eV, and a conduction band level of -1.8 to -3.0 eV to facilitate movement of holes to the light emitting layer.

The hole transport layer 122 may be formed using an evaporation method or a spin coating method.

The hole transport layer 122 may have a thickness of about 5 nm to about 150 nm. In this case, when the thickness of the hole transport layer 122 is 5 nm or more, there is an advantage that the hole transporting property may be prevented from being lowered. In order to prevent the driving voltage from rising, there is an advantage.

The hole transport layer 122 may also serve to prevent the metal in the buffer layer from diffusing into the light emitting layer when the buffer layer is made of a metal compound.

The emission layer 125 may be formed of a material emitting red, green, and blue, and may be formed using phosphorescent or fluorescent materials.

When the light emitting layer 125 is red, it includes a host material including CBP (carbazole biphenyl) or mCP (1,3-bis (carbazol-9-yl), and PIQIr (acac) (bis (1-phenylisoquinoline) acetylacetonate iridium), PQIr (acac) (bis (1-phenylquinoline) acetylacetonate iridium), PQIr (tris (1-phenylquinoline) iridium) and PtOEP (octaethylporphyrin platinum) It may be made of a phosphor, and alternatively may be made of a fluorescent material including PBD: Eu (DBM) ₃ (Phen) or perylene, but is not limited thereto.

In this case, in the case of the light emitting layer 125 emitting red, the valence band level of the host material may be -5.0 to -6.5 eV, and the conduction band level may be -2.0 to -3.5 eV. And, the valence band level of the dopant material may be -4.0 to -6.0 eV, and the conduction band level may be -2.4 to -3.5 eV. The conduction band level of the host material may comprise the conduction band level of the dopant material.

When the light emitting layer 125 is green, the light emitting layer 125 may include a host material including CBP or mCP, and may be formed of a phosphor including a dopant material including Ir (ppy) 3 (fac tris (2-phenylpyridine) iridium). And, alternatively, it may be made of a fluorescent material including Alq3 (tris (8-hydroxyquinolino) aluminum), but is not limited thereto.

In this case, in the case of the light emitting layer 125 emitting green, the valence band level of the host material may be -5.0 to -6.5 eV, and the conduction band level may be -2.0 to -3.5 eV. And, the valence band level of the dopant material may be -4.5 to -6.0 eV, and the conduction band level may be -2.0 to -3.5 eV. The conduction band level of the host material may comprise the conduction band level of the dopant material.

When the light emitting layer 125 is blue, the phosphor may include a host material including CBP or mCP and a dopant material including (4,6-F ₂ ppy) ₂ Irpic. , spiro-DPVBi, spiro-6P, distilbenzene (DSB), distriarylene (DSA), PFO-based polymer and PPV-based polymer may be composed of a fluorescent material including any one selected from the group consisting of, but not limited to Do not.

In this case, in the case of the light emitting layer 125 emitting blue light, the valence band level of the host material may be 5.0 to 6.5 eV, and the conduction band level may be -2.0 to -3.5 eV. And, the valence band level of the dopant material may be -4.5 to -6.0 eV, and the conduction band level may be -2.0 to -3.5 eV. The conduction band level of the host material may comprise the conduction band level of the dopant material.

The electron transport layer 126 serves to facilitate the transport of electrons, made of one or more selected from the group consisting of Alq3 (tris (8-hydroxyquinolino) aluminum), PBD, TAZ, spiro-PBD, BAlq and SAlq. But it is not limited thereto.

The electron transport layer 126 may have a valence band level of -5.0 to -6.5 eV, and a conduction band level of -2.5 to -3.8 eV to facilitate movement of electrons to the light emitting layer.

The electron transport layer 126 may be formed using an evaporation method or a spin coating method.

The electron transport layer 126 may have a thickness of about 1 nm to about 50 nm. In this case, when the thickness of the electron transport layer 126 is 1 nm or more, there is an advantage that the electron transport characteristics may be prevented from being lowered. When the thickness of the electron transport layer 126 is 50 nm or less, the thickness of the electron transport layer 126 is too thick to improve the movement of electrons. In order to prevent the driving voltage from rising, there is an advantage.

The electron transport layer 126 may also prevent the holes injected from the first electrode from moving through the light emitting layer to the second electrode. In other words, it serves as a hole blocking layer to effectively bond holes and electrons in the light emitting layer.

The electron injection layer 127 serves to facilitate the injection of electrons, Alq3 (tris (8-hydroxyquinolino) aluminum), MTPD ((N, N'-diphenyl-N, N'-vis (3-methyl) phenyl) -1,1'-biphenyl-4,4'-diamine), PBD, TAZ, spiro-PBD, BAlq or SAlq may be used, but is not limited thereto.

The electron injection layer 127 may further include a dopant 128, and the dopant may include rubrene and DCM (4- (dicyanomethylene) -2-methyl- represented by Chemical Formulas 1 to 3 below. 6- (4-dimethylaminostyryl) -4H-pyran (4- (dicyanomethylene) -2-methyl-6- (4-dimethylaminostyryl) -4H-pyran)) and naphthacene.

The valence band peak level of rubrene in the dopant 128 material may be -5.4 eV, and the conduction band minimum level may be -3.2 eV. In addition, the valence band highest level of DCM may be -3.2 eV and the conduction band lowest level may be -5.4 eV. The valence band peak level of naphthacene may be −5.3 eV, and the conduction band minimum level may be −3.0.

In addition, the electron injection layer 127 may have a valence band level of -5.0 to -6.5 eV, and a conduction band level of -2.0 to -3.0 eV to facilitate movement of electrons to the light emitting layer.

Here, the dopant 128 materials may be higher than the highest valence band level of the electron injection layer 127 and lower than the lowest level of conduction band. That is, the electron injection layer 127 including the dopant 128 may serve to lower the conduction band level of the organic material of the electron injection layer 127.

Accordingly, the dopant 128 in the electron injection layer 127 may facilitate hopping of electrons injected from the second electrode into the light emitting layer, thereby improving light emission efficiency by balancing holes and electrons injected into the light emitting layer. There is an advantage to that.

The highest valence band level of the electron injection layer 127 may be lower than or equal to the highest valence band level of the electron transport layer 126.

On the contrary, when the electron transport layer 126 is not formed, the valence band maximum level of the electron injection layer 127 may be lower than or equal to the peak level of the emission layer 125. The conduction band minimum level may be lower than or equal to the conduction band minimum level of the light emitting layer 125.

The electron injection layer 127 may have a thickness of about 1 nm to about 50 nm. In this case, when the thickness of the electron injection layer 127 is 1 nm or more, there is an advantage that the electron injection characteristics may be prevented from being lowered. There is an advantage that the driving voltage can be prevented from rising.

Here, in the embodiment of the present invention, the thickness of the electron injection layer 127 including the dopant may be any one of organic functional layers other than the electron injection layer 127, that is, the hole injection layer 121 and the hole transport layer 122. It may be thinner than the light emitting layer 125 or the electron transport layer 126.

That is, since the hole injected from the first electrode 110 has a mobility 10 times faster than the electron injected from the second electrode 130, when the thickness of the electron injection layer 127 is thinner than other layers, the second Since the electron injected from the electrode 130 has a small thickness of the electron injection layer 127, the electrons can move quickly, so that the hole and the charge can be balanced. Therefore, the formation of excitons in the light emitting layer 125 may be uniform, thereby improving luminous efficiency.

The second electrode 130 may be a cathode electrode, and may be made of magnesium (Mg), calcium (Ca), aluminum (Al), silver (Ag), or an alloy thereof having a low work function. Here, the second electrode 130 may be formed to a thickness thin enough to transmit light when the organic light emitting device is a front or double-side light emitting structure, and when the organic light emitting device is a bottom light emitting structure, It can be formed thick enough to reflect.

Hereinafter, referring to FIG. 2, unlike the above-described embodiment, an active matrix type organic light emitting display device including a thin film transistor on a substrate will be described.

2 is a cross-sectional view of an organic light emitting display device according to another embodiment of the present invention.

2, an organic light emitting display device according to another embodiment of the present invention will be described.

The buffer layer 205 is positioned on the substrate 200. The buffer layer 205 is formed to protect the thin film transistor formed in a subsequent process from impurities such as alkali ions flowing out of the substrate 200, and is selectively selected using silicon oxide (SiO ₂ ), silicon nitride (SiNx), or the like. It can be formed as.

The semiconductor layer 210 is positioned on the buffer layer 205. The semiconductor layer 210 may include amorphous silicon or polycrystalline silicon obtained by crystallizing the same. Although not illustrated, the semiconductor layer 210 may include a channel region, a source region, and a drain region, and the source region and the drain region may be doped with P-type or N-type impurities.

The gate insulating layer 215 is positioned on the substrate 200 including the semiconductor layer 210. The gate insulating layer 215 may be selectively formed using silicon oxide (SiO ₂ ), silicon nitride (SiNx), or the like.

The gate electrode 220 is positioned on a gate insulating layer 215 corresponding to a predetermined region of the semiconductor layer 210, that is, a channel region. The gate electrode 220 includes aluminum (Al), aluminum alloy (Al alloy), titanium (Ti), silver (Ag), molybdenum (Mo), molybdenum alloy (Mo alloy), tungsten (W), tungsten silicide (WSi _2). ), But may not be limited thereto.

An interlayer insulating layer 225 is disposed on the substrate 200 including the gate electrode 220. The interlayer insulating film 225 may be an organic film or an inorganic film, or may be a composite film thereof. have. When the interlayer insulating film 225 is an inorganic film, it may include silicon oxide (SiO ₂ ), silicon nitride (SiNx), or SOG (silicate on glass), and in the case of an organic film, acrylic resin, polyimide resin, or benzocyclobutene It may include (benzocyclobutene, BCB) resin, but is not limited thereto.

Contact holes 230a and 230b may be disposed to penetrate the interlayer insulating layer 225 and the gate insulating layer 215 to expose a portion of the semiconductor layer 210.

The source electrode 235a and the drain electrode 235b electrically connected to the semiconductor layer 210 through the contact holes 230a and 230b are positioned on the substrate 200 including the contact holes 230a and 230b. do. The source and drain electrodes 235a and 235b may include a low resistance material to reduce wiring resistance, and may include molybdenum (Mo), molybdenum tungsten (MoW), titanium (Ti), aluminum (Al), or aluminum alloy (Al). alloy). In this case, a multilayer structure of titanium / aluminum / titanium (Ti / Al / Ti), molybdenum / aluminum / molybdenum (Mo / Al / Mo) or molybdenum tungsten / aluminum / molly tungsten (MoW / Al / MoW) may be used. Can be.

The planarization layer 240 is positioned on the source electrode 235a and the drain electrode 235b. The planarization layer 240 may include organic materials such as benzocyclobutene (BCB) -based resin, acrylic resin, or polyimide resin, but is not limited thereto.

The first electrode 250 is electrically connected to the drain electrode 235b through the via hole 245 formed in the planarization layer 240. The first electrode 250 may be an anode and may include a transparent conductive layer such as indium tin oxide (ITO) or indium zinc oxide (IZO). Alternatively, the first electrode 250 may have a stacked structure further including a reflective film under the transparent conductive layer, such as ITO / Ag / ITO or ITO / Ag.

The bank layer 255 exposing a portion of the first electrode 250 is disposed on the substrate 200 on which the first electrode 250 is formed. The bank layer 250 may include an organic material such as benzocyclobutene (BCB) -based resin, acrylic resin, or polyimide resin, but is not limited thereto.

The organic functional layer 260 is positioned on the first electrode 250 exposed by the bank layer 255. The organic functional layer 260 may be formed of an organic material, and the organic functional layer 260 may include at least a light emitting layer, and may further include an electron injection layer, an electron transport layer, a hole transport layer, or a hole injection layer on or below the light emitting layer. have.

The organic functional layer 260 may include a dopant in the electron injection layer as in the above-described embodiment. Since the description of the organic functional layer 260 has been described with the above-described embodiments, a detailed description thereof will be omitted.

The second electrode 270 is positioned on the substrate 200 including the organic functional layer 260. The second electrode 270 may be a cathode for supplying electrons to the organic functional layer 260, and may include magnesium (Mg), silver (Ag), calcium (Ca), aluminum (Al), or an alloy thereof. can do.

Hereinafter, an experimental example according to an organic light emitting display device according to an embodiment of the present invention. However, the following experimental examples are only preferable experimental examples of the present invention, and the present invention is not limited by the following experimental examples.

Experimental Example 1

Indium tin oxide (ITO) was formed as a first electrode on a substrate to a thickness of 130 nm, and NPD (4,4'-bis [N- (1-naphthyl) -N-phenyl was formed as a hole injection layer on the first electrode. -amino] biphenyl) was formed to a thickness of 40 nm, and NPD (4,4'-bis [N- (1-naphthyl) -N-phenyl-amino] biphenyl) was formed to a thickness of 30 nm as the hole transport layer. Blue light emitting layer by mixing a concentration of 2 wt% perylene (Perylene) with a dopant in DPVBi (4,4'-bis (2,2'-diphenyl vinyl) -1,1'-biphenyl) as a host on the hole transport layer Was formed to a thickness of 25nm, Alq ₃ (8-hydroxyquinoline aluminum) was formed to a thickness of 20nm as the electron transport layer. Next, dopants were rubbed with 1 part by weight relative to 100 parts by weight of Alq ₃ (8-hydroxyquinoline aluminum) to form an electron injection layer having a thickness of 10 nm. Subsequently, an Al film was formed to a thickness of 150 nm with a second electrode, and an Al film was formed to a thickness of 1000 kPa on the MgAg film.

Experimental Example 2

Except for doping with 3 parts by weight of rubrene, it was formed under the same process conditions as in Experimental Example 1.

Experimental Example 3

Except that doped with 5 parts by weight of rubrene, it was formed in the same manner as the process conditions of Experimental Example 1.

Experimental Example 4

Except that doped with 7 parts by weight of rubrene, it was formed under the same process conditions as in Experimental Example 1.

Comparative Example

Except that the electron injection layer was formed of Alq ₃ (8-hydroxyquinoline aluminum) was formed in the same manner as the process conditions of Experimental Example 1.

The driving voltage, the luminous efficiency and the lifetime of the organic light emitting diodes manufactured according to Experimental Examples 1, 2, 3, 4 and Comparative Examples were measured and the results are shown in Table 1.

Comparative example Experimental Example 1 Experimental Example 2 Experimental Example 3 Experimental Example 4 Driving voltage (V) 6 5.9 5.6 5.4 5.7 Luminous Efficiency (cd / A) 5 5.3 5.8 6.0 5.6 Life (hour) 6000 6500 7000 8000 7000

As shown in Table 1, the organic light emitting device according to an embodiment of the present invention includes a dopant in the electron injection layer, compared to the organic light emitting device having an electron injection layer made of a conventional organic material, the driving voltage, It can be seen that the luminous efficiency and lifespan are further improved.

That is, by including the dopant in the electron injection layer, it is possible to improve the mobility of the electrons to maintain the charge balance between the hole and the electrons to increase the efficiency of the organic light emitting device.

Although the embodiments of the present invention have been described above with reference to the accompanying drawings, the above-described technical configuration of the present invention may be embodied in other specific forms by those skilled in the art to which the present invention pertains without changing its technical spirit or essential features. It will be appreciated that it may be practiced. Therefore, the embodiments described above are to be understood as illustrative and not restrictive in all aspects. In addition, the scope of the present invention is shown by the claims below, rather than the above detailed description. Also, it is to be construed that all changes or modifications derived from the meaning and scope of the claims and their equivalent concepts are included in the scope of the present invention.

1 is a cross-sectional view of an organic light emitting display device according to an embodiment of the present invention.

2 is a cross-sectional view of an organic light emitting display device according to an embodiment of the present invention.

Claims (9)

Board; A first electrode on the substrate; An organic functional layer on the first electrode, the organic functional layer including an emission layer, an electron transport layer, and an electron injection layer; And A second electrode on the organic functional layer, The electron injection layer includes an organic material and a dopant, The lowest conduction band level of the dopant is lower than the lowest conduction band level of the organic material. The method of claim 1, The dopant is an organic electroluminescent device comprising any one or more selected from the group consisting of rubrene, DCM and naphtacene. The method of claim 1, The thickness of the electron injection layer is an organic light emitting device thinner than any one of the organic layer. The method of claim 1, The organic light emitting device further comprises a hole transport layer or a hole injection layer between the first electrode and the light emitting layer. The method of claim 1, The valence band highest level of the dopant is higher than the valence band maximum level of the organic material. The method of claim 1, And a thin film transistor including a semiconductor layer, a gate electrode, a source electrode, and a drain electrode on the substrate. The method of claim 1, The valence band highest level of the electron injection layer is lower than or equal to the valence band maximum level of the electron transport layer. The method of claim 1, And the highest valence band of the electron injection layer is lower than or equal to the highest valence band of the light emitting layer, and the lowest conduction band of the electron injection layer is lower than or equal to the lowest conduction band of the light emitting layer. The method of claim 1, The organic light emitting device further comprising a buffer layer on the first electrode.

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