KR20160096379A - Highly efficient organic light-emitting diodes having heavily-doped dopants - Google Patents

Abstract

The present invention relates to an organic light-emitting diode comprising: an anode; a hole injection layer (HIL) formed on the anode; a hole transport layer (HTL) formed on the hole injection layer; an emitting layer (EML) formed on the hole transport layer and comprising 2-4 wt% of a fluorescent dopant and 96-98 wt% of a host; an exciton blocking layer (EBL) formed on the emitting layer; an electron transport layer (ETL) formed on the exciton blocking layer; and a cathode formed on the electron transport layer, wherein the highest occupied molecular orbital (HOMO) level of the dopant is higher than the HOMO level of the hole transport layer and lower than the HOMO level of the host, and the lowest unoccupied molecular orbital (LUMO) level of the electron transport layer is higher than the LUMO level of the hole transport layer and lower than the LUMO level of the host.

Description

BACKGROUND ART [0002] Highly efficient organic light-emitting diodes having heavily-doped dopants

The present invention relates to a high-efficiency organic light-emitting diode, and more particularly, to an organic light-emitting diode in which efficiency is greatly improved by directly injecting holes into a dopant in a light-emitting layer.

Organic light-emitting diodes (OLEDs) are semiconductor devices that change their electrical energy directly to light energy and generate self-luminescence. They have fast response time, low driving voltage, wide viewing angle, low power consumption, high luminous efficiency, (LCD), it has received great attention as a next generation display.

The operation principle of the organic light emitting diode will be briefly described. In the organic thin film (low molecular or polymer), electrons injected through a cathode and an anode are recombined with holes, Exciton falls to the ground state and a visible ray corresponding to the energy gap of the light emitting layer material is emitted. The material used for the light emitting layer can be changed into various colors such as red, green and blue, and a combination of these colors can emit light of a desired color.

In the organic light emitting diode as described above, according to the results of a large number of studies, it has been found that the dopant concentration in the light emitting layer must be low to reduce quenching of excitons due to dipole-dipole interaction, It is reported that the efficiency can be improved.

For example, an Alq ₃ (tris (8-hydroxyquinoline) aluminum) host and C545T (10- (2-benzothiazolyl) -2,3,6,7-tetrahydro-1,1,7,7-tetramethyl- , And 11H- (1) -benzopyropyrano (6,7-8-i, j) quinolizin-11-one dopant, light emission of only about 1 wt% And is known to have optimal current efficiency.

Disclosure of Invention Technical Problem [8] The present invention provides a fluorescent organic light emitting diode having improved efficiency even when an excessive amount of dopant is doped.

According to an aspect of the present invention, there is provided a fuel cell comprising: an anode; A hole injection layer (HIL) formed on the anode; A hole transport layer (HTL) formed on the hole injection layer; An emission layer (EML) formed on the hole transport layer and containing 2 to 4 wt% of a fluorescent dopant and 96 to 98 wt% of a host; An exciton blocking layer (EBL) formed on the light emitting layer; An electron transport layer (ETL) formed on the hole blocking layer; And a cathode formed on the electron transporting layer,

The highest occupied molecular orbital (HOMO) level of the dopant is higher than the HOMO level of the hole transport layer and lower than the HOMO level of the host,

Wherein the lowest unoccupied molecular orbital (LUMO) level of the electron transport layer is higher than the LUMO level of the hole transport layer and lower than the LUMO level of the host.

The positive electrode is made of indium tin oxide (ITO), and the hole transport layer is made of 4,4'-bis [N- (1-naphthyl) -N-phenylamino] biphenyl Wherein the light emitting layer comprises Alq ₃ and C545T as a host and a dopant respectively, and the electron transport layer 4,7-diphenyl-1,10-phenanthroline (Bphen), and the cathode is LiF / Al The organic light emitting diode according to claim 1,

Also, the light emitting layer includes 96 wt% Alq ₃ and 4 wt% C545T.

Also, the organic light emitting diode has a current efficiency of 12.5 cd / A or more at a luminance of 100 to 15000 cd / m ² .

In the organic light emitting diode according to the present invention, the dopant doped at a high concentration in the light emitting layer directly injects holes from the hole transport layer (HTL) into the dopant to promote the injection of holes into the light emitting layer, and traps electrons from the electron transport layer (ETL) The efficiency is greatly improved as compared with the conventional fluorescent organic light emitting diode, and the display device and the organic light emitting diode are improved in efficiency even though the quenching is increased due to the increase of the dopant concentration. A lighting device, and the like.

1 is a schematic view of a cross-sectional structure of an organic light emitting diode according to the present invention.
2 is an energy level diagram for an example of an organic light emitting diode according to the present invention.
FIG. 3 is a schematic view showing a cross-sectional structure of the organic light emitting diode manufactured in Examples and Comparative Examples of the present invention.
FIG. 4 is a graph showing the current density-voltage measurement results of the organic light emitting diode manufactured in Examples, Reference Examples, and Comparative Examples of the present invention.
FIG. 5 is a schematic diagram showing a hole-only device (HOD) (ITO / HATCN (30 nm) / NPB (60 nm) / Alq3: C545T (60 nm, X weight%) / MoO3 ) Of the organic light emitting diodes manufactured in the examples, reference examples and comparative examples of the present invention, in which [ITO / Alq3: C545T (120 nm, X weight%) / Bphen (25 nm) / LiF / Density-voltage measurement results.
FIG. 6 is a graph showing voltage-luminance measurement results of organic light emitting diodes manufactured in Examples, Reference Examples, and Comparative Examples of the present invention.
FIG. 7 is a graph showing the luminous efficacy-luminance measurement results of the organic light-emitting diode manufactured in Examples, Reference Examples and Comparative Examples of the present application.
FIG. 8 is a graph showing the power efficiency-luminance measurement results of the organic light emitting diode manufactured in Examples, Reference Examples, and Comparative Examples of the present invention.

In the following description of the present invention, a detailed description of known functions and configurations incorporated herein will be omitted when it may make the subject matter of the present invention rather unclear.

Embodiments in accordance with the concepts of the present invention can make various changes and have various forms, so that specific embodiments are illustrated in the drawings and described in detail in this specification or application. It should be understood, however, that it is not intended to limit the embodiments according to the concepts of the present invention to the particular forms of disclosure, but includes all modifications, equivalents, and alternatives falling within the spirit and scope of the present invention.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The singular expressions include plural expressions unless the context clearly dictates otherwise. In this specification, the terms "comprises ",or" having ", or the like, specify that there is a stated feature, number, step, operation, , Steps, operations, components, parts, or combinations thereof, as a matter of principle.

Hereinafter, the present invention will be described in detail.

A high efficiency organic light emitting diode according to the present invention includes: an anode; A hole injection layer (HIL) formed on the anode; A hole transport layer (HTL) formed on the hole injection layer; An emission layer (EML) formed on the hole transport layer and containing 2 to 4 wt% of a fluorescent dopant and 96 to 98 wt% of a host; An exciton blocking layer (EBL) formed on the light emitting layer; An electron transport layer (ETL) formed on the hole blocking layer; And a cathode formed on the electron transport layer.

Here, the highest occupied molecular orbital (HOMO) level of the dopant is higher than the HOMO level of the hole transport layer, lower than the HOMO level of the host, and the lowest occupied molecular orbital (Lowest Unoccupied Molecular Orbital , LUMO) level is higher than the LUMO level of the hole transport layer and lower than the LUMO level of the host.

For example, the HOMO level (5.5 eV) of the dopant (C545T) is higher than the HOMO level (5.4 eV) of the hole transport layer (NPB) and higher than that of the host (Alq ₃ ), as in the organic light emitting diode system shown in FIG. The LUMO level (3.2 eV) of the electron transport layer (Bphen) is lower than the LUMO level (3.3 eV) of the dopant (C545T) and the LUMO level (3.0 eV) of the host (Alq ₃ ) Lt; / RTI >

When the above energy level relationship is satisfied, holes are injected directly from the hole transporting layer into the dopant to promote injection of holes into the light emitting layer, and electrons are trapped by the dopant to suppress electron injection into the light emitting layer. As a result, It is possible to achieve an optimized charge balance so as to cancel the efficiency reduction due to extinction even when doped at a high concentration, thereby realizing an organic light emitting diode having an improved efficiency.

The anode is connected to one of a source electrode and a drain electrode of the thin film transistor to receive a drive current applied from the thin film transistor and is made of a known electrode material used for an organic light emitting diode And may be a transparent metal oxide electrode such as indium tin oxide (ITO), indium zinc oxide (IZO), zinc oxide (ZnO), or the like.

The hole injection layer formed on the anode moves the holes injected from the anode in a direction in which the light emitting layer is positioned, and assists injection of holes.

Such a hole injecting layer may include a hole injecting material and a metal oxide or organic p-type dopant.

The hole injection material is a material that facilitates injection of holes from the anode. The most important requirement is that the anode and adjacent organic layers have high bonding and interfacial adhesion. This ultimately improves the power efficiency of the device and increases the lifetime of the device. The hole injecting material in contact with the anode should have a small difference between the work function of the anode and the energy level of the HOMO level so as to facilitate the injection of holes from the anode. In order to increase the external quantum efficiency, absorption in the visible light region should not be possible. Of the hole injecting materials, CuPc is excellent in hole injection property and excellent in thermal stability, but has the problem that absorption occurs in blue and red visible light regions. In order to overcome this problem, starburst-type amines such as 4,4 ', 4'-tris [N- (2-naphthyl) -N-phenylamino] triphenylamine (2-TNATA) And a low HOMO energy level to enable low voltage operation. However, in the present invention, it has an excellent hole mobility as compared with 2T-NATA and has a very low energy band gap and is an n-type hole injecting material and has a characteristic of injecting holes through LUMO 1,4,5,8,9 , And 11-hexaazatriphenylene-hexacarbonitrile (HAT-CN) are preferred.

The metal oxide may be an oxide containing a transition metal. Examples of the transition metal include molybdenum, tungsten, vanadium, rhenium, ruthenium, chromium, manganese, nickel, iridium, APC (silver-palladium-copper alloy), combinations thereof, and the like. Examples of the organic p-type dopant include tetrafluoro-tetracyanoquinodimethane (F4-TCNQ) or tris [1,2-bis (trifluoromethyl) ethane-1,2-dithiolene [Mo (tfd) 3] and the like.

Next, a hole transporting layer for transporting the holes injected from the anode to the light emitting layer is provided on the hole injecting layer. The hole transporting layer material may be a known material for the hole transporting layer, and the 4,4'-bis [ N-phenylamino] biphenyl (NPB), 4,4'-bis [N- (3-methylphenyl) Bis (4- (N, N-di-p-tolylamino) phenyl) cyclohexane (TAPC), 4 "-tris [(3- methylphenyl) phenylamino] triphenylamine (MTDATA) (TCTA), 9,9 '- [1,1', 8-tetramethyl-4- (9H-carbazol-9- (CBP), 9,9 '- (1,3-phenylene) bish-9H-carbazole (mCP), or 2,2' , 2 '' - (1,3,5-benzenetriyl) tris- [1-phenyl-1H-benzimidazole] (TPBi), and the like.

A light emitting layer is provided on the hole transporting layer. The light emitting layer is injected from the cathode and injected from the anode through the electron transport layer, and the holes passing through the hole transport layer are recombined to generate an exciton, and the resulting exciton exits from the excited state to the base state And can be formed using a variety of well-known phosphors and dopants. Particularly in the case of a dopant, a known fluorescent dopant and a known phosphorescent dopant can be used.

For example, known hosts include Alq3, CBP (4,4'-N, N'-dicarbazole-biphenyl), PVK (poly (n-vinylcarbazole) 2-yl) anthracene (ADN), TPBI (1,3,5-tris (N-phenylbenzimidazole-2-yl) benzene) But are not limited to, TBADN (3-tert-butyl-9,10-di (naphth-2-yl) anthracene), E3 and DSA (distyrylarylene).

As the red dopant, PtOEP, Ir (piq) 3, Btp2Ir (acac), DCJTB, or the like may be used, but the present invention is not limited thereto. As the known green dopant, C545T, Ir (ppy) 3 (ppy = phenylpyridine), Ir (ppy) 2 (acac), Ir (mpyp) 3 and the like can be used. Further, as known blue dopants, there can be used at least one compound selected from the group consisting of F2Irpic, (F2ppy) 2Ir (tmd), Ir (dfppz) 3, ter- fluorene, 4,4'- DPAVBi), 2,5,8,11-tetra-t-butyl perylene (TBP), and the like, but the present invention is not limited thereto.

In the present invention, the dopant is contained at a high concentration of 2 to 4% by weight based on the total weight of the material forming the light emitting layer, and holes are injected directly from the hole transport layer (HTL) into the dopant to promote injection of holes into the light emitting layer, It is possible to trap the electrons injected from the transport layer (ETL) to suppress the injection of electrons, thereby achieving the optimum charge balance in the luminescent region. Therefore, despite the increase in quenching due to the increase in the dopant concentration, The efficiency can be greatly improved.

An electron transport layer is provided on the light emitting layer and serves to move electrons injected from the cathode into a direction in which the light emitting layer is positioned. The electron transporting material constituting the electron transport layer includes 4,7-diphenyl-1,10- Phenanthroline (Bphen), ((2,2 ', 2 "- (benzene-1,3,5-triyl) -tris (1-phenyl-1H-benzimidazole : TPBI), 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), bis (2- Methyl-8-quinolinolato) -4- (phenylphenolato) aluminum (Balq), 1,3-bis [2- (2 Bipyridine-6-yl) -1, 3, 4-oxadiazo-5-yl] , 3,4-oxadiazo-5-yl] benzene: Bpy-OXD), 6,6'-bis [5- (biphenyl- -2,6-bis [5- (biphenyl-4-yl) -1,3,4-oxadiazo-2-yl] -2,2'-bipyridyl: BP-OXD -Bpy), 3- (4-biphenyl) -4- (phenyl-5-tert-butyl 4- (4-biphenyl) -4-phenyl-5-tert-butylphenyl-1,2,4-triazole TAZ, 4- (naphthalen- (Naphthalen-1-yl) -3,5-diphenyl-4H-1,2,4-triazole: NTAZ), 2,9- Bis (naphthalen-2-yl) -4,7-diphenyl-1,10-phenanthroline: 2,7- NBphen), tris (2,4,6-trimethyl-3- (pyridin-3-yl) phenyl) borane (Tris ), Phenyl-dipyrenylphosphine oxide (POPy2), 3,3 ', 5,5'-tetra [(m-pyridyl) (3-pyridyl) -phen-3-yl] benzene (1, 5,5'-tetra [(m-pyridyl) -phen-3-yl] biphenyl: BP4mPy) , 3,5-tri [(3-pyridyl) -phen-3-yl] benzene: TmPyPB), 1,3- bis [10-hydroxybenzo [h] quinolinato) beryllium: Bepq2 (5-hydroxybenzo [h] quinolinato) ), Bis (10- Dibenzo [h] quinolinato), beryllium (Bebq2), diphenylbis (4- (pyridin-3-yl) phenyl) pyridin-3-yl) phenyl) silane DPPS and 1,3,5-tri (p-pyrid-3-yl- phenyl) benzene: TpPyPB), but the present invention is not limited thereto.

A negative electrode is provided on the electron transporting layer. The cathodes are commonly connected to the power supply voltage and inject electrons into the electron transport layer. The cathode may be formed of a metal having a low work function as a metal such as magnesium, calcium, sodium, potassium, titanium, indium, yttrium, lithium, gadolinium, aluminum, silver, have. Further, it may be formed of a multilayer structure material such as LiF / Al or LiO2 / Al.

In the organic light emitting diode according to the present invention as described above, the dopant heavily doped in the light emitting layer injects holes directly from the hole transport layer (HTL) to the dopant to promote injection of holes into the light emitting layer, It is possible to achieve an optimum charge balance in the luminescent region by trapping electrons by suppressing electron injection so that the efficiency is greatly improved as compared with the conventional fluorescent organic light emitting diode despite the increase of quenching due to the increase of the dopant concentration And can be usefully used for a display device, a lighting device, and the like.

Hereinafter, the present invention will be described in detail on the basis of embodiments. The presented embodiments are illustrative and are not intended to limit the scope of the invention.

< Example  1>

An ITO (Indium Tin Oxide) glass substrate was used as an anode. A hole injection layer (HIL) composed of a layer of 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HAT-CN) having a thickness of 60 nm was formed on the ITO glass substrate, the thickness of 30nm as the hole transport layer (HTL) N, N -bis- ( 1-naphthyl) - N, N '-diphenyl-1,1'-biphenyl-4,4'diamine and form a (NPB) layer, the hole A light emitting layer (EML) having a thickness of 30 nm, which contains 98% by weight of Alq ₃ as a host and 2% by weight of C545T as a dopant, is formed on the upper part of the transport layer and a 25 nm thick 4,7- Phenyl-1,10-phenanthroline 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (Bphen) layer was formed on the electron transport layer. Finally, LiF 1 nm) / Al was formed, thereby completing the organic light emitting diode having the schematic sectional structure shown in Fig.

< Example  2>

An organic light emitting diode was completed in the same manner as in Example 1 except that the light emitting layer contained 96 wt% of Alq ₃ and 4 wt% of C545T as a dopant.

< Reference example >

The organic light emitting diode was completed in the same manner as in Example 1 except that the light emitting layer was made of Alq ₃ 100 wt%.

< Comparative Example  1>

An organic light emitting diode was completed in the same manner as in Example 1 except that the light emitting layer contained 99 wt% of Alq ₃ and 1 wt% of C545T as a dopant.

< Comparative Example  2>

An organic light emitting diode was completed in the same manner as in Example 1 except that the light emitting layer contained 94 wt% of Alq ₃ and 6 wt% of C545T as a dopant.

< Experimental Example > Example , Reference example  And In the comparative example  Evaluation of device characteristics for fabricated organic light emitting diodes

The organic light emitting diodes manufactured in each of the examples, the reference example and the comparative example were measured for current density-voltage relationship, voltage-luminance (luminance) The relationship, the luminous efficiency-luminance relationship, and the power efficiency-luminance relationship were measured. The results are shown in FIGS. 4, 6, 7 and 8, respectively.

4, the device of Comparative Example 1 doped with 1 wt% of C545T exhibited the highest current density, followed by Example 2 (doping with 4 wt% C545T), Example 1 (doping with 2 wt% C545T) And Comparative Example 2 (6 wt% C545T doping).

That is, the current density did not decrease or increase with increasing dopant concentration. This result shows that the electrons are trapped in the dopant and the charge injection is obstructed as described above with reference to FIG. 2, while the holes are directly injected into the dopant This is because charge injection is facilitated and the tendency of injection of electrons and holes differs according to the doping concentration.

(30 nm) / NPB (60 nm) / Alq ₃ : C545T (60 nm, X%) were used to determine the relationship between the current density and the doping concentration. ) / MoO ₃ (10 nm) / Al] and electron only device (EOD) [ITO / Alq ₃ : C545T (120 nm, X%) / Bphen (25 nm) / LiF / Al] And the results are shown in FIG. 5.

From FIG. 5, it can be seen that, in the EOD, the dopant acts as a trap site as the doping concentration increases, and the electron injection is gradually suppressed.

On the other hand, in the case of HOD, the current density tended to increase with increasing doping concentration, except for HOD doped with 6 wt% C545T under a low operating voltage. This shows that the dopant lowers the hole injection barrier, This is a result of smooth operation. However, in the case of HOD doped with 6 wt% of C545T, the light emitting layer made of only an electron transfer material has a mechanism of blocking holes. Therefore, the current density value increases rapidly as the operating voltage increases, and then gradually decreases.

From the above results, it can be seen that the device of Comparative Example 1 exhibited the highest current density due to the relatively less suppression of electron injection compared to other devices and improved hole injection, and the device of Example 2 had a higher doping concentration The device of Comparative Example 2 exhibits the largest electron trap due to the increase of the doping concentration and the highest hole blocking effect by the light emitting layer material and the lowest current density I can understand.

According to FIG. 6 showing the luminous efficiency-voltage measurement results of the respective devices, each device shows a trend similar to the current density-voltage relationship except for the device of the second embodiment. In the case of the device of Example 2, the device exhibited the highest recombination efficiency and exhibited the highest luminance.

7 showing the results of the luminous efficiency-luminance measurement of each device, the device of Example 2 exhibited a current efficiency of 100 cd / m ² to 12.5 cd / A, a current efficiency of 1000 cd / m ² to 11.9 cd / (About 12.5 cd / m ² ) at a wide luminance range (100 to 15000 cd / m ² ) without roll-off phenomenon as well as the highest efficiency compared to other devices .

Thus, the device of Example 2 exhibits high efficiency because the fluorescence extinction is greatly increased with the increase of the dopant concentration, but the optimum charge balance is achieved by promoting the hole injection and suppressing the electron injection by increasing the dopant concentration. In addition, a large recombination region is formed according to the increase of the doping concentration, and high efficiency can be maintained in a wide luminance range.

8 showing the power efficiency-luminance measurement results of the respective devices showed the highest power efficiency of the device of Example 2 as shown in FIG. 7, which was 6.25 lm / W at 1000 cd / m ² .

Claims (5)

Anode;
A hole injection layer (HIL) formed on the anode;
A hole transport layer (HTL) formed on the hole injection layer;
An emission layer (EML) formed on the hole transport layer and containing 2 to 4 wt% of a fluorescent dopant and 96 to 98 wt% of a host;
An exciton blocking layer (EBL) formed on the light emitting layer;
An electron transport layer (ETL) formed on the hole blocking layer; And
And a cathode formed on the electron transporting layer,
The highest occupied molecular orbital (HOMO) level of the dopant is higher than the HOMO level of the hole transport layer and lower than the HOMO level of the host,
Wherein the lowest unoccupied molecular orbital (LUMO) level of the electron transport layer is higher than the LUMO level of the hole transport layer and lower than the LUMO level of the host. The method according to claim 1,
The anode is made of indium tin oxide (ITO)
Wherein the hole transport layer is made of 4,4'-bis [N- (1-naphthyl) -N-phenylamino] biphenyl (NPB)
The light emitting layer includes Alq ₃ and C545T as a host and a dopant, respectively,
The electron transport layer 4,7-diphenyl-1,10-phenanthroline (Bphen)
Lt; RTI ID = 0.0 &gt; LiF / Al. &Lt; / RTI &gt; 3. The method of claim 2,
Wherein the light emitting layer comprises 96 wt% Alq ₃ and 4 wt% C545T. The organic light emitting diode according to claim 3, wherein the organic light emitting diode has a current efficiency of 12.5 cd / A or more at a luminance of 100 to 15000 cd / m ² . A display device comprising the organic light emitting diode according to any one of claims 1 to 4.

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