WO2006033492A1

WO2006033492A1 - White organic light emitting device using three emissive layer

Info

Publication number: WO2006033492A1
Application number: PCT/KR2004/002438
Authority: WO
Inventors: Jong-Wook Park; Ho-Cheol Park
Original assignee: Doosan Corporation
Priority date: 2004-09-22
Filing date: 2004-09-22
Publication date: 2006-03-30
Also published as: JP2008513970A; JP4782791B2

Abstract

The present invention relates to white organic light emitting devices including a substrate, an anode, a hole injecting layer, a hole transporting layer, red, green, and blue emission layers, a white balancing layer, an electron transporting layer, and a cathode deposited on the substrate. In this structure, since it is possible to control the emission ratio of the three-color emission layers, excitons which do not contribute to blue light emission contribute to yellow or red light emission. Green light emission is enhanced by an exciton confinement effect. As a result, all three-color layers contribute to generation of white light with high efficiency. Furthermore, by using the white balancing layer, it is possible to control the emission ratio of the plurality of emission layers to extremely enhance efficiency of emission of white light.

Description

WHITE ORGANIC LIGHT EMITTING DEVICE USING THREE EMISSIVE

LAYER

BACKGROUND OF THE INVENTION (a) Field of the Invention

The present invention relates to organic light emitting devices (OLEDs), and more particularly to white OLEDs using three emission layers to make white light.

(b) Description of the Related Art

Organic light emitting devices (OLEDs) show improved brightness and driving voltage, good response speed, and possibility of multi-colors compared to inorganic light emitting devices, because the OLEDs utilize organic materials for the light emission layer and can self-emit. In addition, the OLEDs show good color reproduction when they are applied to a color display. The OLEDs are suitable for portable information communication displays due to their many advantages such as thinness, compactness, and light weight.

When a color display is embodied by using an OLED emitting white light, color filters are used for producing colors. In this case, several advantages can be expected such as lower inferior rate and relatively simple process in comparison to the independent-evaporation process, in which red, green, and blue (RGB) emission layers are successively deposited while a metal mask having minute patterns is moved delicately. A white OLED having a blue and a red light emission layers has a structure of FIG. 1. An anode 1 is deposited on a substrate, and a hole i transporting and blue emission layer 10, a hole blocking and red emission layer 11 , an electron transporting layer 12, and a cathode 9 are successively deposited on the anode 1. It is also possible that a hole transporting and blue emission layer 13, a red emission layer 14, a hole blocking layer 15, an electron transporting layer 16, and a cathode 9 are successively deposited on the anode 1 to obtain white light emission.

Both of the above-mentioned structures have similar driving principles, which will now be described.

When a driving voltage is applied between the anode 1 and the cathode 9, holes injected from the anode 1 are transferred to the red emission layers 11 and 14 through the hole transporting layers 10 and 13. At this time, the hole blocking layer 15 having the highest occupied molecular orbital

(HOMO) blocks the holes to not to reach the cathode 9.

Meanwhile, electrons are injected to the red emission layers 11 and 14 from the cathode 9 through the electron transporting layers 12 and 16, and excitons are generated when the carriers are recombined with the hole at the interface and in the bulk of the hole transporting layers 10 and 13 and the red emission layers 11 and 14. Light is emitted when the generated excitons transit to the equilibrium state. That is, the hole transporting layers 10 and 13 emit blue light and the red emission layers 11 and 14 emit red light, so that white light is produced by the two color lights.

In the OLED operating in the above-mentioned manner, it is difficult to control the purity degree of white light because distribution of the excitons is hard to control and rapidly varies depending on the applied voltage. In addition, it has a low efficiency of generating white light because the white light consists of only two colors, i.e., the blue emitted by the excitons generated in the hole transporting layers 10 and 13 and the red emitted by the excitons generated in the red emission layers 11 and 14. It is difficult to apply to a color display device even when using color filters, because it generates a white light including little green light. FIG. 2 shows an energy level diagram of a conventional white OLED having three emission layers. An anode 1 is deposited as a bottom layer. A hole transporting layer 18, a blue emission layer 19, a green emission layer

20, and a red emission layer 21 are successively deposited on the anode 1 , and then an electron transporting layer 22 and a cathode 9 are deposited thereon.

The OLED having such a structure generates a white light by the following principle.

When a voltage is applied between the anode 1 and the cathode 9, holes and electrons are respectively injected from the anode 1 and the cathode 9. The injected holes and electrons respectively pass through the hole transporting layer 18 and the electron transporting layer 22 and generate excitons in the three light emitting layers 19, 20, and 21. The generated excitons are arbitrarily distributed to the three light emitting layers 19, 20, and

21 , and transit to the equilibrium state to emit light. However, the OLED operating in the above-mentioned manner also has a difficulty in adjusting the purity degree of white light because it is not easy to control the amount of excitons that contribute to the RGB emission, and has a low emission efficiency. Moreover, there is a structural limitation that the blue emission layer needs to be located closer to the anode 1 than the green emission layer 20 and the red emission layer 21 in view of the energy level. SUMMARY OF THE INVENTION

Technical subjects of the present invention are to solve the disadvantages and difficulties of the above-mentioned prior art and to provide a white organic light emitting device evenly emitting red, green, and blue light so as to generate a white light in high efficiency.

To solve such subjects, the present invention provides an OLED having a white balancing layer and an exciton confinement structure.

In detail, the present invention provides an organic light emitting device comprising: an anode; a hole injecting layer formed on the anode; a hole transporting layer formed on the hole injecting layer; a plurality of light emission layers formed on the hole transporting layer; at least one organic material layer formed between two predetermined layers of the plurality of light emission layers and having an electron blocking effect; an electron transporting layer formed on the light emission layers; and a cathode formed on the electron transporting layer.

At least one of the plurality of light emission layers preferably has a low energy level compared with two layers adjacent to both its sides to form an exciton confinement structure.

A green emission layer, a red emission layer, and a blue emission layer are preferably included in the plurality of light emission layers, and the green emission layer is located between the hole transporting layer and the red emission layer to form an exciton confinement structure. A green emission layer, a red emission layer, and a blue emission layer are preferably included in the plurality of light emission layers and at least one organic material layer having an electron blocking effect is formed between the red emission layer and the blue emission layer. At least one of the hole transporting layer, the plurality of light emission layers, and the electron transporting layer may include a dopant of 0.1 to 5.0wt.%. The dopant includes at least one of coumarin 6, rubrene, A- (dicyanomethylene)-2-methyl-6-(P- dimethylaminostyryl)-4H-pyran DCM), A- (dicyanomethylene)-2-t-butyl-6-(1 ,1 ,7,7,-tetramethyljulolidyl-9-enyl)- 4H-pyran (DCJTB), perylene, quinacridone, DCM2, 2,3,7,8,12,13,17,18-octaethyl-21 H, 23H- porphine platinum (PtOEP), and iridium(IH)bis[(4,6-difluoropheny)- pyridinato-N,C2']picolinate (Firpic), wherein PtOEP and Firpic are phosphorescent dopants. The green emission layer, the red emission layer, and the blue emission layer may have thicknesses within the range of 100A to 500A, respectively. The green emission layer includes aluminum tris (8- hydroxyquinoline) as a main material, the red emission layer includes at least one of [N,N'-bis(naphthalene-1-yl)phenyl]-N,N'-bis(phenyl)benzidine (NPB) and 4,4'-bis(carbazol-9-yl)biphenyl (CBP) which is phosphorescent host as a main material, and the blue emission layer includes at least one of 4,4'- bis(2,2-diphenyl-ethen-1-yl)-diphenyl (DPVBi), 4,4"-bis (2,2-diphenylvinyl-1- yl)-p-terphenylene (DPVTP), and spiro-DPVBi as a main material.

The hole injecting layer may have a thickness of 400A to 1500A, and the hole transporting layer may have a thickness of 100A to 500A. The hole injecting layer and the hole transporting layer may include at least one of 4,4',4"-tris[N-3-methylphenyl-N-phenyl-amino]-triphenylamine (m- MTDATA) and N,N'-bis(naphthalene-1-yl)-N,N'-bis(phenyl)benzidine (NPB) as a main material. The electron transporting layer may have a thickness within 100A to

1 ,000A. At least one of the organic material layers having the electron blocking effect may have a thickness within 1OA to 30 A. At least one of the organic material layers having the electron blocking effect includes [N₁N'- bis(naphthalene-1-yl)phenyl]-N,N'-bis(phenyl)benzidium (NPB)(=α-NPD) as a main material.

The anode may have a thickness within 1.000A to 2,000 A and is made of at least one of indium-tin-oxide(ITO), SnO₂,and ZnO.

The cathode may have a thickness within 500A to 5,00OA and is made of at least one of Li, LiF, Mg, Al, Al-Li, Ca, Mg-In, and Mg-Ag. The cathode may have a double-layered structure including a LiF layer having a thickness within 5 A to 20 A and an Al layer having a thickness within 1 ,000A to 2,00OA.

A green emission layer, a red emission layer, and a blue emission layer may be included in the plurality of light emission layers, and further comprised is an emission reinforcing layer for enhancing emission efficiency between the blue emission layer and the electron transporting layer, the emission reinforcing layer including a blue emission layer, and at least one of a n-type layer and a p-type layer.

The present invention provides an organic light emitting device comprising: an anode; a hole injecting layer formed on the anode; a hole transporting layer formed on the hole injecting layer; a plurality of light emission layers formed on the hole transporting layer; an electron transporting layer formed on the light emission layers; and a cathode formed on the electron transporting layer, wherein at least one of the plurality of light emission layers has a low energy level compared with two layers adjacent to both its sides to form an exciton blocking structure. The device may further comprises at least one organic material layer having an electron blocking effect and interposed between two predetermined layers of the plurality of light emission layers.

BRIEF DESCRIPTION OF THE DRAWINGS

The above objects and other advantages of the present invention will become more apparent by describing preferred embodiments thereof in detail with reference to the accompanying drawings, in which:

FIG. 1 is a diagram showing the energy level structure of a conventional white OLED having only a blue and a red emission layer.

FIG. 2 is a diagram showing the energy level structure of a conventional white OLED having three emission layers.

FIG. 3A is a cross-sectional view showing a layer structure of a white OLED according to an embodiment of the present invention. FIG. 3B is a diagram showing an energy level structure of a white

OLED according to an embodiment of the present invention.

FIG. 4 is a diagram showing an energy level structure of a white OLED according to another embodiment of the present invention.

FIG. 5 is a diagram showing an energy level structure of a white OLED according to still another embodiment of the present invention.

FIGs. 6 to 8 show light emission spectra of white OLEDs according to the several embodiments of the present invention.

FIGs. 9 and 10 show light emission spectra, color coordinates, and efficiency characteristics of white OLEDs of two comparative embodiments. Detailed Description of the Preferred Embodiments

The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. These embodiments are intended only as illustrative examples and the invention is not to be limited thereto.

FIG. 3A is a cross sectional view showing a layer structure of a white OLED according to an embodiment of the present invention, and FIG. 3B is a diagram showing an energy level structure of a white OLED according to an embodiment of the present invention. As shown in FIG. 3A, according to one embodiment of the invention, the white OLED includes an insulating substrate 100 and an anode 1 , a hole injecting layer 2, a hole transporting layer 3, a green emission layer 4, a red emission layer 5, a white balancing layer 6, a blue emission layer 7, an electron transporting layer 8, and a cathode 9 successively deposited on the insulating substrate 100.

In this structure, it is possible to control the emission ratio of R, G, and B by controlling the thickness of the white balancing layer 6 interposed between the blue emission layer 7 and red emission layer 5. The white balancing layer 6 effectively blocks movement of electrons which are introduced through the cathode 9, the electron transporting layer 8, and the blue emission layer 7 to control the emission ratio of R, G, and B.

Since the green emission layer 4 has a lower LUMO (lowest unoccupied molecular orbital) than those of the hole transporting layer 3 and red emission layer 5 respectively adjacent to both its sides, an exciton confinement structure 17 is formed, so that the green emission efficiency is enhanced. In the embodiment, at least one or more of the hole transporting layer 3, the emission layers 4, 5, and 7, and the electron transporting layer 8 include dopants enabling the layers 4, 5, and 7 to emit light through the hole- electron combination. Coumarin 6, rubrene, 4-(dicyanomethylene)-2-methyl- 6-(P-Dimethylaminostyryl)-4H-pyran (DCM), or 4-(dicyanomethylene)-2-t- butyl-6-(1 ,1 ,7,7,-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) may be applied as the dopants. For generating blue light, perylene, quinacridone, or (2-metyle-6-(2-(2,3,6,7-tetrahydro-1 H, 5H-benzo quinolizin-9-yl)ethenyl)-4H- pyran-4-ylidene)propane-dinitrile (DCM2), etc., may be applied as the dopants.

As a phosphorescent dopant, 2,3,7,8,12, 13,17,18-octaethyl-21 H or 23H-porphine platinum (PtOEP) may be used for red light, and iridium(πi)bis[(4,6-di-fluoropheny)-pyridinato-N,C2¹]picolinate (Firpic), etc. may be used for blue light. The amount of dopant is 1~20 wt.% of that of the host materials of the hole transporting layer 3, the emission layers 4, 5, and 7, and the electron transporting layer 8. The above described dopants have the following chemical formulas.

PtOEP

Host materials for forming the emission layers 4, 5, and 7 are as follows. The host material of the green emission layer 4 is aluminum tris(8- hydroxyquinoline). The host material of the red and/or yellow emission layer 5 is one of [N,N'-bis(naphthalene-1-yl)phenyl]-N,N'-bis(phenyl)benzidine (NPB) and 4,4'-bis(carbazol-9-yl)biphenyl (CBP), which are phosphorescent hosts. The host material of the blue emission layer 7 is 4,4'-bis(2,2-diphenyl-ethen-1 - yl)-diphenyl (DPVBi).

4,4"-bis (2,2-diphenylvinyl-1-yl)-p-terphenylene (DPVTP), spiro-DPVBi, etc. which are low molecular materials may also be used as the host material of the blue emission layer 7,.

DPVBi

SpUo-DPVBi

To enhance efficiency and tune the color, the emission layers 4, 5, and 7 are formed by adding the above dopant materials.

It is preferable that the emission layers 4, 5, and 7 have thicknesses within the range of 100 A to 200A in accordance with the degree of contribution to generation of the white light, and the range of the thickness may be changed depending on characteristics of the materials used.

Materials suitable for the hole injecting layer 2 and the hole transporting layer 3 have triphenyl amine radicals having hole-transporting characteristics. For example, 4,4',4"-tris[N-3-methylphenyl-N-phenyl-amino]- triphenylamine (m-MTDATA), N,N'-bis(naphthalene-1-yl)-N,N'- bis(phenyl)benzidine (NPB), etc. may be used as the hole injecting layer 2 and the hole transporting layer 3. The above materials have the following chemical formulas.

It is preferable that the hole injecting layer 2 formed on the anode 1 has a thickness within the range of 400A to 1 ,50OA, and that the hole transporting layer 3 has a thickness within the range of 100A to 500A.

In the embodiment, the OLED includes the insulating substrate 100 which is generally used for typical OLEDs. Substantially, it is preferable that the substrate is a glass substrate or a transparent plastic substrate having good transparency, a very flat surface, ease of handling, and a good waterproof characteristic.

The anode 1 is formed of a material having outstanding transparency and conductivity such as indium-tin-oxide (ITO), SnO₂, ZnO, etc., to have a thickness within the range of 1 ,000 A to 2,000 A.

The cathode 9 is formed of a metal such as Li, Mg, Al, Al-Li, Ca, Mg-In, Mg-Ag, etc., to have a thickness within the range of 1 ,000 A to 2,000 A.

Here, the cathode 9 is preferably formed of a LiF layer having high reactivity, a low work function, and a thickness within the range of 5 A to 20 A, and an Al layer deposited on the LiF layer and having a high work function and a thickness within the range of 5,00 A to 5,00OA. Such a double-layered structure is preferable for stability and efficiency of the device.

The electron transporting layer 8 is formed of an electron transporting material such as tris(8-quinolinolate)-aluminum (Alq3) to have a thickness within the range of 100 A to 1 ,00OA.

The white balancing layer 6 is formed of [N,N'-bis(naphthalene-1 - yl)phenyl]-N,N'-bis(phenyl)benzidium (NPB)(=α-NPD) to have a thickness within the range of 10A to 3OA. FIG. 3B is a diagram showing the energy level structure of a white

OLED according to an embodiment of the present invention. The white balancing layer 6 having an electron blocking effect is interposed between the red emission layer 5 and the blue emission layer 7, and the green emission layer 4 having low LUMO level is interposed between the hole transporting layer 3 and the red emission layer 5 to form a quantum well that is the exciton confinement structure 17.

In such a structure, desirable thickness of the white balancing layer 6 is within the range of 10 A to 3OA depending on the contribution degree to generating white light. By adjusting the thickness of the white balancing layer 6, it is possible to control the amount of electrons which reach the red emission layer 5 and the green emission layer 4 and the amount of electrons which accumulate at the interface of the white balancing layer 6 by the energy wall.

To form the exciton confinement structure 17, the green emission layer 4 must be interposed between the hole transporting layer 3 and the red emission layer 5 to form a well-shaped energy level structure.

As illustrated, the white OLED according to an embodiment of the present invention has the successively disposed structure as follows: the anode; the hole injecting layer; the hole transporting layer; the green emission layer; the red emission layer; the white balancing layer; the blue emission layer; the electron transporting layer; and the cathode, wherein the red emission layer can be replaced by a yellow emission layer.

Here, it is possible to exchange the positions of the red or yellow emission layer 5 and the white balancing layer 6 among the successively accumulated organic layers, depending on the arrangement of energy level or the used host materials. For example, as shown in Figure 4, the device can be formed in the order of the anode, the hole injecting layer, the hole transporting layer, the green emission layer, the white balancing layer, the red emission layer, the blue emission layer, the electron transporting layer, and the cathode.

To achieve high efficiency of white OLED, as shown in Figure 5, the device may further includes an emission reinforcing layer 23 between the blue emission layer 7 and the electron transporting layer 8 to enhance emission efficiency of the blue emission layer 7. The emission reinforcing layer 23 has a multi-layered structure such as a blue emission layer/n-type layer, a blue emission layer/p-type layer, and a blue emission layer/n-type layer/p-type layer.

In the white OLED of the embodiment of the present invention, since the excitons generated during the device operation are controlled to distribute properly in accordance with each contribution degree of the R, G, and B emission layers 5, 4, and 7 to generating white light by the white balancing layer 6. Accordingly, the maximum emission efficiency of each color can be obtained without serious reliance upon the structure of the R, G, and B layers such as concentration of the dopants and thickness of the emission layers.

Referring to FIG. 1 , a conventional white OLED generates white light by inducing light emission at the interface and on the bulk of the hole transporting layers 10 and 13 and the emission layers 11 and 14 through hole blocking effect of the hole blocking layer 10 and 15. However, in the present invention, the white balancing layer 6 controls the electron distribution through the electron blocking effect to enables the unused excitons in the blue emission layer 7 to contribute to the red emission layer 5 and the green emission layer 4. In case of using the hole blocking layer (HBL) material as FIG. 1 , some structural limitations are unavoidable to satisfy the emission mechanism in the aspect of energy level. For example, the blue emission layers 10 and 13 must be disposed closer to the anode 1 than the red emission layers 11 , 14, and 21. However, there are no such limitations in the white OLED according to the embodiment of the present invention.

Furthermore, by doping the hole transporting layer 5 having considerable influence on the organic device's life, it is possible to solve the problems such as reduction of the device's life and degradation generated when the hole transporting layer 5 is exposed to charge carriers by energy transfer between the host and guest.

The exciton blocking structure 17 of the present invention is proposed to solve a typically known problem in the field of OLEDs that the green light emission is softly generated when the green emission layer 4 is deposed in the front of the anode 1. This exciton blocking structure 17 prevents such a problem to enable generating white light comprising balanced three balanced kinds of light (R, G, and B).

The exciton blocking effect may be obtained by the exciton blocking structure 17 that is a quantum well in which the excitons introduced to the green emission layer 4 are thrown and confined. Accordingly, the lower contribution of the green emission can be solved, which is a problem that is not only typical with an HBL using emission device but is typical to a white OLED that obtains white light by three emission layers. Moreover, since the surplus excitons unused for blue and red emission participate in only green emission through the above mentioned quantum well structure, it is benificial to prevent contribution without light emission to the anode 1 which is a cause of life reduction of the device.

Hereinafter, some preferred embodiments of the present invention in which thickness and material of layers are concerned will be described in more detail.

The below embodiments are provided to help the understanding of the present invention, and the present invention is not intended to be limited to the embodiments.

The below embodiments will be illustrated on the basis of reference numbers of Figure 3.

ITO (indium tin oxide) is deposited on a glass substrate 100 to form an anode 1 having a thickness of 1 ,80OA. On the ITO coated substrate 100, m- MTDTA is deposited by vacuum deposition to form a hole injecting layer 2 having a thickness of 400A.

A hole transporting layer 3 having a thickness of 100 A is formed by vacuum deposition of NPB on the hole injecting layer 2.

A green emission layer 4 having a thickness of 180A is formed by vacuum deposition of AIq₃ and coumarin 6 on the hole transporting layer 3. In this step, both materials are deposited at the same time, and the green emission layer 4 has 99wt.% of Alq3 and 1.0wt.% of coumarin 6.

Successively, a 120A red emission layer 5 is formed by vacuum deposition of NPB and DCM. NPB and DCM are deposited at the same time in the similar manner with the deposition of the green emission layer 4. Here, the red emission layer 5 has 96wt.% of NPB and the 4.0wt.% of DCM. On the red emission layer 5, a white balancing layer 6 capable of controlling the exciton distribution is formed by NPB vacuum deposition. The white balancing layer 6 has a thickness of 17 A.

In the next step, a blue emission layer 7 is formed on the white balancing layer 6 by vacuum deposition of DPVBi, and an electron transporting layer 8 is formed by vacuum deposition of AIq₃ on the blue emission layer 7. The blue emission layer 7 has a thickness of 180A and the electron transporting layer 8 has a thickness of 120 A.

Next, a LiF layer which is an electron injecting layer is deposited on the electron transporting layer 8 to have thickness of 10A, then an Al layer is deposited on the LiF layer to have a thickness of 2,000 A to form a cathode 9.

In the resultant OLED, the electron blocking effect can be expected by forming the white balancing layer 6 between the red emission layer 5 and the blue emission layer 7. Due to this effect, one portion of electrons contributes to light emission at the DPVBi doped blue emission layer 7, another portion of electrons contributes to emission of orange-red light at the DCM doped red emission layer 5, and the other portion contributes to emission of the green light at the coumarin-doped green emission layer 4.

FIG. 6 shows a light emission spectrum, color coordinates, and efficiency characteristics of a white OLED according to the first embodiment of the present invention.

In the emission spectrum of FIG. 6, it can easily find that each R, G, and B lights evenly contributes to generating white light and respectively has peaks near 455 ran, 510 ran, and 590 ran. At this time, the emitted white light has a color coordinate position of (0.33, 0.38) and has the emission efficiency of 6.1cd/A, 2.31 m/W. Embodiment 2>

When the second embodiment is compared with the first embodiment, the second embodiment has the following two peculiar features that are different from the first embodiment. NPB and rubrene are simultaneously deposited under a vacuum to form the red emission layer 5 that includes 97wt.% of NPB and 3wt.% of rubrene. The white balancing layer 6 has a thickness of 19 A to tune the purity degree of color.

As a result, in the second embodiment, one portion of the excitons contributes to yellow-orange emission in rubrene of the red emission layer 5, while the other portion of the excitons contributes to blue and green emission to tune the purity degree of color by controlling thickness variation of the white balancing layer 6.

FIG. 7 shows the light emission spectrum, color coordinates, and efficiency characteristics of the second embodiment of white OLEDs according to the present invention.

Referring to the emission spectrum of FIG.7, all three colors of R, G, and B contribute to white light emission, and each of the colors respectively has the maximum wavelength peak of 455 nm, 511 nm, and 560 run. In this case, the emitted white light has a color coordinate position of (0.28, 0.36) and has an emission efficiency of 6.8cd/A, 2.51 m/W.

Embodiment 3>

The second and third embodiments are similar in the procedure except that, in the third embodiment, the thickness of the white balancing layer 6 is formed to have thickness of 17 A. As mentioned in the above, the white balancing layer 6 controls the distribution of electrons which effects light emission. That is, the white balancing layer 6 controls the distribution of excitons. For example, if the thickness of the white balancing layer 6 formed between the red emission layer 5 and the blue emission layer 7 is reduced, contribution of the blue emission layer 7 to white light emission is also reduced due to the reduction of electron blocking effect, while contribution of the green emission layer 4 and the red emission layer 5 is increased. Since the contributions of the green and yellow are increased, the color coordinate position of the white light moves toward the place of the long wavelength.

FIG. 8 shows a light emission spectrum, chromaticity coordinates, and efficiency characteristics of a white OLED according to a third embodiment of the present invention.

The emission spectrum of FIG. 8 thoroughly supports the above illustration, and in this case, the emitted white light has a color coordinate position of (0.31 , 0.40) and has an emission efficiency of 7.1 cd/A, 2.31 m/W.

To compare with the first to third embodiments, an OLED without the exciton blocking structure is prepared and its characteristics are illustrated hereinafter.

ITO (indium tin oxide) having a thickness of 1 ,80OA is deposited on a glass substrate to form an anode. On the ITO, m-MTDTA is deposited by vacuum deposition to form a hole injecting layer of 400 A. A hole transporting layer of 100A is formed by vacuum deposition of NPB on the hole injecting layer. A green emission layer of 140 A is prepared by vacuum deposition of

NPB and rubrene on the hole transporting layer. In this step, both materials are deposited at the same time, and the green emission layer has 97wt.% of

NPB and 3.0wt.% of rubrene. Successively, a white balancing layer for controlling the distribution of excitons is formed to have thickness of 17 A on the blue emission layer, and a red emission layer of 120A is prepared by vacuum deposition of AIq₃ and coumarin at the same time. Here, the red emission layer has 99wt,% Alq3 and 1.0wt.% coumarin. In the next step, a blue emission layer of 180A is formed on the red emission layer by DPVBi vacuum deposition.

Next, an electron transporting layer of 150A is formed on the blue emission layer, and a LiF electron injecting layer of 10 A and a Al electrode of

2,000 A are formed by vacuum deposition to form an OLED. FIG. 9 shows a light emission spectrum, color coordinates, and efficiency characteristics of a white OLED according to the first comparative embodiment of the present invention.

Referring to the spectrum of FIG. 9, we can find that it is impossible to obtain the white light in the first comparative embodiment without the exciton blocking effect, since the peak of the blue region is remarkably reduced and thus only the peak of the yellow-green region contributes to the light emission.

Accordingly, the emitted light has a color coordinate position of (0.37, 0.54) and is a shallow yellow light.

<Comparative Embodiment 2> To compare with the preferred embodiments 1 to 3, an OLED without the white balancing layer is produced and its characteristics are illustrated hereinafter.

ITO (indium tin oxide) having a thickness of 1 ,800 A is deposited on a glass substrate to form an anode. On the ITO, m-MTDTA is deposited by vacuum deposition to form a hole injecting layer of 370A. A hole transporting layer of 100A is formed by vacuum deposition of NPB on the hole injecting layer. On the hole injecting layer, a green emission layer and a red emission layer are deposited without the white balancing layer. The green emission layer has 97wt.% of NPB and 3.0wt.% of rubrene, and the red emission layer is prepared by DPVBi vacuum deposition to have a thickness of 200A.

Next, an electron transporting layer of 160 A is deposited and a LiF electron injecting layer is successively deposited on the electron transporting layer by LiF vacuum deposition to have thickness of 10A. On the electron injecting layer, an Al electrode of 200 A is deposited by Al vacuum deposition.

Figure 10 shows the light emission spectrum, color coordinates, and efficiency characteristics of the second comparative embodiment of white

OLEDs according to the present invention. Referring to Figure 10, all excitons are distributed over the green emission layer of rubrene, making the yellow light emission. That is, because there is no white balancing layer capable of making the blue light emission in accordance with the electron blocking effect, the blue light emission cannot be expected. Accordingly, the emitted light has a color coordinate position of (0.46, 0.47). Meanwhile, characteristics of brightness efficiency, maximum brightness, quantum efficiency, and color coordinates of the first, second, and third embodiments and the first and second comparative embodiments according to the present invention are charted in Table 1.

Referring to Table 1 , it can be seen that the OLEDs of the first to third embodiments with the exciton blocking structure and the white balancing 0 layer are more suitable than the OLEDs of the first and second comparative experiments without either the exciton blocking structure or the white balancing layer in view of contribution of three-color emission to the white light emission.

The OLEDs of the present invention include the white balancing layer 5 that enables controlling the exciton distribution, and include the exciton blocking structure that enables generating white light including a balanced amount of RGB. As a result, according to the present invention, the purity degree of color of emitted light is easily adjusted and light emitting efficiency of the device can be extremely enhanced by using the maximum emission efficiency of the plurality of emission layers. Since white light containing all three colors is generated, a color display can be manufactured by using color filters. These features differentiate the present invention from the conventional OLEDs that are fabricated by considering the purity degree of color more than the maximum emission efficiency of the plural emission layers or to generate a white light containing only two colors. The present invention should not be considered to be limited to the particular examples described above, but rather should be understood to cover all aspects of the invention as fairly set out in the attached claims. Various modifications, equivalent processes, as well as numerous structures to which the present invention may be applicable will be readily apparent to those of skill in the art to which the present invention is directed upon review of the instant specification.

Claims

What is claimed is:

1. An organic light emitting device comprising: an anode; a hole injecting layer formed on the anode; a hole transporting layer formed on the hole injecting layer; a plurality of light emission layers formed on the hole transporting layer; at least one organic material layer formed between two predetermined layers of the plurality of light emission layers and having an electron blocking effect; an electron transporting layer formed on the light emission layers; and a cathode formed on the electron transporting layer.

2. The device of claim 1 , wherein at least one of the plurality of light emission layers has a low energy level compared with two layers adjacent to both its sides to form an exciton confinement structure.

3. The device of claim 2, wherein a green emission layer, a red emission layer, and a blue emission layer are included in the plurality of light emission layers, and the green emission layer is located between the hole transporting layer and the red emission layer to form an exciton confinement structure.

4. The device of claim 1 , wherein a green emission layer, a red emission layer, and a blue emission layer are included in the plurality of light emission layers and at least one organic material layer having an electron blocking effect is formed between the red emission layer and the blue emission layer.

5. The device of claim 4, wherein at least one of the hole transporting layer, the plurality of light emission layers, and the electron transporting layer includes a dopant at 0.1 to 5.0wt.%.

6. The device of claim 5, wherein the dopant includes at least one of coumarin 6, rubrene, 4-(dicyanomethylene)-2-methyl-6-(P- dimethylaminostyryl)-4H-pyran (DCM), 4-(dicyanomethylene)-2-t-butyl-6- (1 ,1 ,7,7,-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB), perylene, quinacridone, DCM2,2,3,7,8,12,13,17,18-octaethyl-21 H, 23H- porphine platinum (PtOEP), and iridium(III)bis[(4,6-difluoropheny)-pyridinato- N,C2']picolinate (Firpic), wherein PtOEP and Firpic are phosphorescent dopants.

7. The device of claim 3, wherein the green emission layer, the red emission layer, and the blue emission layer have thicknesses within the range of 100 A to 500A.

8. The device of claim 7, wherein the green emission layer includes aluminum tris (8-hydroxyquinoline) as a main material, the red emission layer includes at least one of [N,N'-bis(naphthalene-1-yl)phenyl]-N,N'- bis(phenyl)benzidine (NPB) and 4,4'-bis(carbazol-9-yl)biphenyl (CBP) which are phosphorescent hosts as a main material, and the blue emission layer includes at least one of 4,4'-bis(2,2-diphenyl-ethen-1-yl)-diphenyl (DPVBi), 4,4"-bis (2,2-diphenylvinyl-1-yl)-p-terphenylene (DPVTP), and spiro-DPVBi as a main material.

9. The device of claim 3, wherein the hole injecting layer has a thickness of 400 A to 1 ,500 A, and the hole transporting layer has a thickness of 100A to 500A.

10. The device of claim 9, wherein the hole injecting layer and the hole transporting layer include at least one of 4,4',4"-tris[N-3-methylphenyl-N- phenyl-amino]-triphenylamine (m-MTDATA) and N,N'-bis(naphthalene-1-yl)- N,N'-bis(phenyl)benzidine (NPB) as a main material.

11. The device of claim 3, wherein the electron transporting layer has a thickness of 100 A to 1 ,00OA.

12. The device of claim 3, wherein at least one of the organic material layers having the electron blocking effect has a thickness of 10A to 3OA.

13. The device of claim 12, wherein at least one of the organic material layers having the electron blocking effect includes [N₁N¹- bis(naphthalene-1-yl)phenyl]-N,N'-bis(phenyl)benzidium (NPB)(=α-NPD) as a main material.

14. The device of claim 3, wherein the anode has a thickness within 1 ,00OA to 2,000 A and is made of at least one of indium-tin-oxide (ITO),

SnO₂, and ZnO.

15. The device of claim 3, wherein the cathode has a thickness of 500 A to 5,000 A, and is made of at least one of Li, LiF, Mg, Al, Al-Li, Ca, Mg- In, and Mg-Ag.

16. The device of claim 15, wherein the cathode has a double- layered structure including a LiF layer having a thickness within 5 A to 2OA, and an Al layer having a thickness of 1 ,000A to 2,00OA.

17. The device of claim 1 , wherein a green emission layer, a red emission layer, and a blue emission layer are included in the plurality of light emission layers, and they further comprise an emission reinforcing layer for enhancing emission efficiency between the blue emission layer and the electron transporting layer, the emission reinforcing layer including a blue emission layer and at least one of a n-type layer and a p-type layer.

18. An organic light emitting device comprising: an anode; a hole injecting layer formed on the anode; a hole transporting layer formed on the hole injecting layer; a plurality of light emission layers formed on the hole transporting layer; an electron transporting layer formed on the light emission layers; and a cathode formed on the electron transporting layer, wherein at least one of the plurality of light emission layers has a low energy level compared with two layers adjacent to both its sides to form an exciton blocking structure.

19. The device of claim 18, further comprising at least one organic material layer having an electron blocking effect and being interposed between two predetermined layers of the plurality of light emission layers.