WO2011002109A1

WO2011002109A1 - Organic light emitting device

Info

Publication number: WO2011002109A1
Application number: PCT/KR2009/003539
Authority: WO
Inventors: Dae-Gyu Moon; Chang-Kyo Kim; Yu-Seok Seo; Tae Yong Kim
Original assignee: Soonchunhyang University Industry Academy Cooperation Foundation
Priority date: 2009-06-30
Filing date: 2009-06-30
Publication date: 2011-01-06

Abstract

An organic light emitting device is provided. The organic light emitting device includes a lower electrode disposed on a substrate, a first organic layer disposed on the lower electrode and configured to be used as a hole transport layer and an emitting layer, a second organic layer disposed on the first organic layer and configured to be used as a hole blocking layer and an electron transport layer, and an upper electrode disposed on the second organic layer. With manufacturing of an organic light emitting device having only two organic layers (first and second organic layers) between the upper electrode and the lower electrode, the structure of the organic light emitting device is simplified. Thus, the driving voltage of the organic light emitting device is reduced, and luminous efficiency is improved. Moreover, the deposition process is simplified to shorten the processing time by forming only two organic layers.

Description

ORGANIC LIGHT EMITTING DEVICE

The present disclosure relates to an organic light emitting device, and more particularly, to an organic light emitting device having excellent operation characteristics by being manufactured in a simple structure of two organic layers.

With the rapid development of information communication technologies, flat panel displays are attracting much attention for use in display devices. Representative examples of flat panel displays include liquid crystal displays, plasma display panels, and organic light emitting devices.

Organic light emitting devices may be implemented in high quality displays due to their wide viewing angle and quick response time. An organic light emitting device includes a substrate, a lower electrode on the substrate, an organic layer on the lower electrode, and an upper electrode on the organic layer. Here, the organic layer includes a hole injection layer, a hole transport layer, an emitting layer, a hole blocking layer, and an electron transport layer. An organic light emitting device having a multi-layer structure is formed by stacking a plurality of organic layers between the lower electrode and the upper electrode. Accordingly, a high driving voltage is required to drive the organic light emitting device having such a multi-layer structure. This may cause low luminous efficiency.

The present disclosure provides an organic light emitting device having a simple structure, by forming a first organic layer used as a hole transport layer and an emitting layer, and a second organic layer used as a hole blocking layer and an electron transport layer between a lower electrode and an upper electrode.

In accordance with an exemplary embodiment, an organic light emitting device includes: a lower electrode disposed on a substrate; a first organic layer disposed on the lower electrode and configured to be used as a hole transport layer and an emitting layer; a second organic layer disposed on the first organic layer and configured to be used as a hole blocking layer and an electron transport layer; and an upper electrode disposed on the second organic layer.

The lower electrode may be used as an anode configured to inject holes, and the upper electrode may be used as a cathode configured to inject electrons.

The first organic layer may allow a host thereof to be doped with a blue phosphorescent dopant.

The host of the first organic layer may hve a Highest Occupied Molecular Orbital (HOMO) level from approximately 5.0eV to approximately 6.5eV, and a Lowest Unoccupied Molecular Orbital (LUMO) level from approximately 1.5eV to approximately 2.3eV.

The host of the first organic layer may include any one of TAPC, TTA, CBP, CDBP and NPB.

The dopant of the first organic layer may include any one of firpic, fir6, fac-Ir(Pmb)3 and mer-Ir(pmb)3 that is a blue phosphorescent material.

The second organic layer may include a material having a HOMO level of approximately 6.5eV, and a LUMO level from approximately 1.8eV to approximately 2.0eV.

The second organic layer may include any one of BCP, TAZ and 3TPYMB.

As describe above, an organic light emitting device includes an organic layer including a first organic layer and a second organic layer between a lower electrode and an upper electrode. Here, the first organic layer is used as a hole transport layer and an emitting layer, and the second organic layer is used as a hole blocking layer and an electron transport layer. That is, by manufacturing an organic light emitting device having only two organic layers (first and second organic layers) between the upper electrode and the lower electrode, the structure of the organic light emitting device is simplified. Thus, the driving voltage of the organic light emitting device is reduced, and luminous efficiency is improved. Moreover, the deposition process is simplified to shorten the processing time by forming only two organic layers.

Exemplary embodiments can be understood in more detail from the following description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a cross-sectional view of an organic light emitting device in accordance with an exemplary embodiment;

FIG. 2 is a graph illustrating voltage (V)-current density (mA/cm²) of a related-art organic light emitting device;

FIG. 3 is a graph illustrating voltage (V)-current density (mA/cm²) of an organic light emitting device in accordance with an exemplary embodiment;

FIG. 4 is a graph illustrating voltage (V)-luminance (cd/m²) of a related-art organic light emitting device;

FIG. 5 is a graph illustrating voltage (V)-luminance (cd/m²) of an organic light emitting device in accordance with an exemplary embodiment;

FIG. 6 is a graph illustrating current density (mA/cm²)-current efficiency (cd/A) of a related-art organic light emitting device; and

FIG. 7 is a graph illustrating current density (mA/cm²)-current efficiency (cd/A) of an organic light emitting device in accordance with an exemplary embodiment.

Hereinafter, specific embodiments will be described in detail with reference to the accompanying drawings. The present invention may, however, 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 fully convey the scope of the present invention to those skilled in the art. Like reference numerals refer to like elements throughout.

FIG. 1 is a cross-sectional view of an organic light emitting device in accordance with an exemplary embodiment.

Referring to FIG. 1, the organic light emitting device includes a substrate 100, a lower electrode 200 on the substrate 100, an organic layer 300 on the lower electrode 200, an upper electrode 400 on the organic layer 300. The organic layer 300 includes a first organic layer 310 and a second organic layer 320. The organic light emitting device is a blue phosphorescent organic light emitting device.

The substrate 100 may use any one of glass substrates and plastic substrates (for example, PE, PES, and PEN), which have a transmittance of approximately 80 %. In an exemplary embodiment, a glass substrate is used as the substrate 100. The lower electrode 200 as an anode is disposed on the substrate 100. Here, the lower electrode 200 uses a material having a high work function of, for example, approximately 4.5eV or more to smoothly transfer holes to the first organic layer 310. The lower electrode 200 may be formed by depositing transparent conductive materials such as Indium Tin Oxide (ITO), Indium Zinc Oxide (IZO), Zinc Oxide (ZnO), and In2O3 through a sputtering process. Without being limited thereto, however, various vapor deposition processes other than the sputtering process may be used to form the lower electrode 200 in accordance with the transparent conductive materials. In an exemplary embodiment, ITO having a work function from approximately 4.5eV to approximately 5.1eV is deposited on the substrate 100 to form the lower electrode 200.

The organic layer 300 includes a first organic layer 310 on the lower electrode 200, and a second organic layer 320 on the first organic layer 320. Here, the first and second

organic layers

310 and 320 are formed through a thermal evaporation method. In this case, the first organic layer 310 is used as a hole transport layer transporting holes injected from the lower electrode 200 and an emitting layer emitting light. The first organic layer 310 is manufactured by doping a host with a light emitting material to smoothly deliver holes injected from the lower electrode 200 to the first organic layer 310. In this case, the host of the first organic layer 310 may be a material having a Highest Occupied Molecular Orbital (HOMO) level that can decrease a hole injection energy barrier with respect to the lower electrode 200. Additionally, the host may be formed of a material having a high Lowest Unoccupied Molecular Orbital (LUMO) level so that electrons delivered to the first organic layer 310 may not move to the lower electrode 200. For example, a material having a HOMO level from approximately 5.0eV to about 6.5eV and a LUMO level from approximately 1.5eV to approximately 2.3eV may be used as a host of the first organic layer 310. When the HOMO level of the host is deviated from the range from approximately 5.0eV to approximately 6.5eV, holes of the lower electrode 200 may not be smoothly delivered to the first organic layer 310. When the LUMO level of the host is deviated from the range from approximately 1.5eV to approximately 2.3eV, electrons delivered from the upper electrode 400 to the first organic layer 310 may move to the lower electrode 200. Thus, low recombination probability between electrons and holes may result in low luminous efficiency. The organic layer 310 may use any one of TAPC (1-Bis-(4-bis(4-methyl-phenyl)-amino-phenyl)-cyclohexane), TTA (4,4',4"-Trismethyl-triphenylamine), CBP(4,4'-N,N'-dicarbazole-biphyenyl), CDBP(4,4'-bis(9-carbazolyl)-2,2'-dimethyl-biphenyl) and NPB(N,N'-bis(α-naphthyl)-N,N'-diphenyl-4,4'-diamine). In an exemplary embodiment, TAPC is used as the first organic layer 310. Here, the HOMO level of TAPC is approximately 5.3eV, and the LUMO level of TAPC is approximately 1.8eV. Since the work function of the lower electrode 200 is similar to the HOMO level of the first organic layer 310, the hole injection energy barrier is low. Accordingly, holes of the lower electrode 200 are smoothly delivered to the first organic layer 310. Since TAPC has a high LUMO level of approximately 1.8eV, electrons from the upper electrode 400 to the first organic layer 310 may be prevented from moving to the lower electrode 200. Thus, electrons delivered to the first organic layer 310 may be trapped in the first organic layer 310, thereby increasing the recombination probability between electrons and holes. In an exemplary embodiment, a blue colored organic light emittingd device is manufactured by doping a host formed of TAPC with a blue phosphorescent material, firpic (Iridium(III) bis(4,6-difluorophenyl-pyridinato-N,C2) picolinate). Without being limited thereto, however, dopant may used any one of fir6 (iridium(III) bis(4',6'-difluorophenylpyridinato)tetrakis(1-pyrazoyl)borate), fac-Ir(Pmb)3 (fac-Tris(1-phenyl- 3-methylbenzimidazolin-2-ylidene -C,C2")iridium(III)) and mer-Ir(pmb)3 (mer-Tris(1-phenyl- 3-methylbenzimidazolin-2-ylidene -C,C2")iridium(III)). In this case, firpic is doped on the TAPC host by approximately 1 % to approximately 10 %. Accordingly, if holes and electrons are delivered to the first organic layer 310, the holes and the electrons are recombined with each other to excite TAPC. Being a dopant, firpic absorbs most of an energy generated from the excitation of TAPC to emit blue light. Here, electrons and holes are recombined with each other at an interface between the first organic layer 310 and the second organic layer 320 to generate an exciton and emit light. Since the triplet energy of host, TAPC is greater than the triplet energy of the dopant, firpic, the triplet energy of the host does not escape to other layers except the first organic layer 310.

The second organic layer 320 in accordance with the embodiment is used as a hole transport layer for delivering electrons injected from the upper electrode 400 to the first organic layer 310. The second organic layer 320 is used as a hole blocking layer for blocking holes delivered from the first organic layer 310 to the second organic layer 320. For this, the second organic layer 320 uses a material capable of smoothly delivering holes injected from the upper electrode 400 to the second organic layer 320. In this case, the second organic layer 320 may include a material having a LUMO level that can reduce a hole injection energy barrier with respect to the upper electrode 400. Also, the second organic layer 320 may include a material having a high HOMO material level so that holes delivered to the first organic layer 310 may not move to the second organic layer 320. In this case, the HOMO level of the second organic layer 320 may be higher than the HOMO level of the first organic layer 310. Accordingly, a material having a LUMO level from approximately 2.0eV to approximately 3.5eV and a HOMO level of approximately 6.5eV or more is used for the second organic layer 320. For example, when the LUMO level value of the second organic layer 320 deviates from a range from approximately 2.0eV to approximately 3.5eV, electrons of the lower electrode 400 are difficult to inject into the second organic layer 320. When the HOMO level value of the second organic layer 320 is less than approximately 6.5eV, holes delivered to the first organic layer 310 are moved to the second organic layer 320, causing a low recombination probability between the electrons and the holes in the first organic layer 310. This may causes deterioration of luminous efficiency. In an exemplary embodiment, BCP (2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline) satisfying the LUMO and HOMO level ranges is used to form the second organic layer 320. Without being limited thereto, however, second organic layer 320 may use any of TAZ (3-phenyl-4-(1'naphthyl)-5-phenyl-1,2,4-triazole) and 3TPYMB (Tris(2,4,6-trimethyl-3-(pyridin-3-yl)phenyl)borane). Here, the HOMO level of BCP is approximately 6.5eV, and the LUMO level of BCP is approximately 3.0eV. Electrons injected from the upper electrode 400 are smoothly delivered to the second organic layer 320. Since BCP has a high HOMO level of approximately 6.5eV, holes delivered to the first organic layer 310 can be prevented from being delivered to the second organic layer 320. Thus, holes delivered to the first organic layer 310 can be trapped in the first organic layer 310 to increase the recombination probability between the electrons and the holes in the first organic layer 310.

The upper electrode 400 on the second organic layer 320 serves to inject electrons into the second organic layer 320. Here, the upper electrode 400 may include a material having a low work function of, for example, approximately 4.0eV to smoothly inject electrons into the second organic layer 320. The upper electrode 400 may be formed of any of Li, Ca, LiF-Ca, LiF-Al, Al, Mg, Ag, Ca-Ag, and Mg-Ag through a thermal evaporation process. In an exemplary embodiment, the upper electrode 400 is formed by depositing LiF-Al on the second organic layer 320. Without being limited thereto, however, various vapor deposition processes other than the sputtering process may be used to form the upper electrode 400 in accordance with the materials.

FIG. 2 is a graph illustrating voltage (V)-current density (mA/cm²) of a related-art organic light emitting device, and FIG. 3 is a graph illustrating voltage (V)-current density (mA/cm²) of an organic light emitting device in accordance with an exemplary embodiment. FIG. 4 is a graph illustrating voltage (V)-luminance (cd/m²) of a related-art organic light emitting device, and FIG. 5 is a graph illustrating voltage (V)-luminance (cd/m²) of an organic light emitting device in accordance with an exemplary embodiment. FIG. 6 is a graph illustrating current density (mA/cm2)-current efficiency (cd/A) of a related-art organic light emitting device. and FIG. 7 is a graph illustrating current density (mA/cm²)-current efficiency (cd/A) of an organic light emitting device in accordance with an exemplary embodiment.

Although a related-art organic light emitting device is not shown, an organic light emitting device having a multi-layer including a substrate, a lower electrode on the substrate, an organic layer formed on the lower electrode and including a hole injection layer, a hole transport layer, an emitting layer, a hole blocking layer and an electron transport layer, and an upper electrode on the organic layer is manufactured. In this case, the lower electrode may include ITO. For example, a hole injection layer is formed on the lower electrode by depositing 2T-NATA(4,4',4"-trs[2-naphthyl(phenyl)amino]triphenylamine) on the lower electrode, and then a hole transport layer is formed on the hole injection layer by depositing NPB(N,N'-bis(α-naphthyl)-N,N'-diphenyl-4,4'-diamine). Then, firpic (being a blue phosphorescent dopant) is doped on mcp(N,N-dicarbazolyl-3,5-benzene), a host by approximately 10 %, and is thereby deposited on the hole transport layer to form an emitting layer. Then, BCP is deposited on the emitting layer to form a hole blocking layer. Alq₃(tris(8-hydroxy-quinoline) aluminum) is deposited on the hole blocking layer to form an electron layer. LiF-Al is deposited on the electron transport layer to form an upper electrode.

As describe in FIG. 1, an organic light emitting device in accordance with an exemplary embodiment includes an organic layer 300 including a first organic layer 310 and a second organic layer 320 between the lower electrode 200 and the upper electrode 400. In an exemplary embodiment, the first organic layer 310 is formed by doping firpic being a blue phosphorescent dopant on TAPC being a host. The second organic layer 320 is formed by depositing BCP on the first organic layer 310.

Hereinafter, electrical characteristics will be compared between a related-art organic light emitting device and an organic light emitting device in accordance with an exemplary embodiment with reference to FIGS. 2 to 7.

Referring to FIGS. 2 and 3, while the related-art organic light emitting device represented a current density of approximately 30mA/cm² at approximately 16V, the organic light emitting device in accordance with an exemplary embodiment represented a current density of approximately 160mA/cm² at approximately 16V. That is, the organic light emitting device in accordance with an exemplary embodiment represents a higher current density than the related-art organic light emitting device under same voltage conditions. Moreover, while the related-art organic light emitting device shows a driving voltage of approximately 7.2V, the organic light emitting device in accordance with an exemplary embodiment shows a driving voltage of approximately 5.4V. That is, the organic light emitting device in accordance with an exemplary embodiment has a lower driving voltage than the related-art organic light emitting device. This is because the organic light emitting device in accordance with an exemplary embodiment has a simpler structure than the related-art organic light emitting device. Referring to FIGS. 4 and 5, while the related-art organic light emitting device represented a luminance of approximately 2,000cd/m² at approximately 16V, the organic light emitting device in accordance with an exemplary embodiment represented a luminance of approximately 5,600cd/m² at approximately 16V. That is, the organic light emitting device in accordance with an exemplary embodiment represents a higher luminance than the related-art organic light emitting device at the same voltage condition. Referring to FIGS. 6 and 7, while the related-art organic light emitting device represented a maximum current efficiency of approximately 7.06cd/A at approximately 30mA/m², the organic light emitting device in accordance with an exemplary embodiment represented a maximum current efficiency of approximately 11.74cd/A at approximately 0.2015mA/cm². That is, the organic light emitting device in accordance with an exemplary embodiment represents a higher maximum current efficiency than the related-art organic light emitting device at the same voltage condition.

Thus, driving voltage can be reduced, and current efficiency can be improved by forming the organic layer 300 having a simple structure of the first and second

organic layers

310 and 320 in an exemplary embodiment. Moreover, the deposition processes can be simplified to shorten the total processing time by forming the organic layer 300 including only two

organic layers

310 and 320.

Although an organic light emitting device has been described with reference to the specific embodiments, it is not limited thereto. Therefore, it will be readily understood by those skilled in the art that various modifications and changes can be made thereto without departing from the spirit and scope of the present invention defined by the appended claims.

Claims

An organic light emitting device comprising:

a lower electrode disposed on a substrate;

a first organic layer disposed on the lower electrode and configured to be used as a hole transport layer and an emitting layer;

a second organic layer disposed on the first organic layer and configured to be used as a hole blocking layer and an electron transport layer; and

an upper electrode disposed on the second organic layer.
The organic light emitting device of claim 1, wherein the lower electrode is used as an anode configured to inject holes, and the upper electrode is used as a cathode configured to inject electrons.
The organic light emitting device of claim 1, wherein the first organic layer allows a host thereof to be doped with a blue phosphorescent dopant.
The organic light emitting device of claim 3, wherein the host of the first organic layer comprises a material having a Highest Occupied Molecular Orbital (HOMO) level from approximately 5.0eV to approximately 6.5eV, and a Lowest Unoccupied Molecular Orbital (LUMO) level from approximately 1.5eV to approximately 2.3eV.
The organic light emitting device of claim 4, wherein the host of the first organic layer comprise any one of TAPC, TTA, CBP, CDBP and NPB.
The organic light emitting device of claim 3, wherein the dopant of the first organic layer comprises any one of firpic, fir6, fac-Ir(Pmb)3 and mer-Ir(pmb)3 that is a blue phosphorescent material.
The organic light emitting device of claim 1, wherein the second organic layer comprises a material having a HOMO level of approximately 6.5eV, and a LUMO level from approximately 1.8eV to approximately 2.0eV.
The organic light emitting device of claim 7, wherein the second organic layer comprises any one of BCP, TAZ and 3TPYMB.