JP2003301172A - Luminescent material, organic electroluminescent element, and display device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a phosphorescent material which can emit a blue light at high efficiency, to provide an organic electroluminescent element containing the material in its luminescent layer, and a display device capable of highly efficient full color display by using the element. <P>SOLUTION: The luminescent material used for the organic electroluminescent element consisting of an anode 6, a hole injection layer 1, a hole transfer layer 2, a luminescent layer 3 and an electron transfer layer 4, alternatively an anode 6, a hole transfer layer 2, and a luminescent and electron transfer layer 4 comprises an organic metal complex the central metal of which comprises a group 6 heavy metal, and/or the electron configuration of the central metal comprises a closed shell structure represented by (Xe)4f<SP>0</SP>5d<SP>0</SP>6s<SP>0</SP>or (Xe)4f<SP>14</SP>5d<SP>0</SP>6s<SP>0</SP>. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light emitting material used in a display element for a display device such as a color display, and a self-luminous organic electroluminescent element using the same.
Furthermore, the present invention relates to a display device in which the light emitting elements are combined.

[0002]

2. Description of the Related Art In recent years, especially in multimedia-oriented products and the like, the importance of human-machine interfaces has been increasing. In order to operate the machine more comfortably and efficiently, it is necessary to retrieve a sufficient amount of information from the machine without error, concisely, and instantaneously. Therefore, research on various display devices including displays has been conducted. Has been done.

In addition, with the miniaturization of machines, demands for miniaturization and thinning of display devices are increasing every day. Under such circumstances, a liquid crystal display is used as an interface for many downsized products such as a laptop type information processing device, a small television receiver, a clock, and a calculator.

This liquid crystal display has been researched as a dedicated interface from small size to large capacity display device by taking advantage of the characteristic that liquid crystal is driven at low voltage and low power consumption.

However, since this liquid crystal display is of a light receiving type, a backlight is indispensable, and driving the backlight requires a power larger than that of the liquid crystal. Therefore, even though the storage battery and the like are built in, there is a problem in that there is a limitation in power supply, a short operating time, and other restrictions on use.

Further, since the liquid crystal display is a display based on the alignment state of the liquid crystal molecules, the contrast changes depending on the angle even within the viewing angle.
For this reason, the viewing angle is narrow and it is not suitable for large-scale displays such as large-scale displays. Moreover, since the alignment speed of liquid crystal molecules is slow, it is not suitable for displaying moving images.

On the other hand, in terms of a driving system, the active matrix system of the liquid crystal display shows a corresponding speed sufficient for handling a moving image, while it has a TFT (thin film transistor).
Since a drive circuit is used, it is difficult to increase the screen size due to pixel defects, and it is not a good idea from the viewpoint of cost reduction. In contrast to the active matrix method, the simple matrix method, which is another driving method, is low in cost and relatively easy to increase the screen size, but provides a response speed sufficient for handling moving images. I can't.

In contrast to such a liquid crystal element, a display element using a plasma display element which is a self-luminous element, an inorganic electroluminescent element, an organic electroluminescent element or the like has been studied.

Among them, the plasma display element uses plasma emission in gas for display, and is large in size,
Suitable for large capacity. However, it is not suitable for a mobile device because it has a problem in thinning and cost and requires a high voltage bias for driving.

As the inorganic electroluminescent device, a green light emitting display and the like have been commercialized. However, as with the plasma display device, several hundred V is required for AC bias driving.
I wasn't accepted by The. However, as a result of technological progress since then, we have succeeded in emitting the three primary colors of R, G, and B required for color displays today, but since an inorganic material is indispensable for its construction, the emission wavelength due to molecular design etc. It is impossible to control such as, and full colorization is expected to be difficult.

On the other hand, the organic electroluminescence device utilizes electroluminescence from an organic compound, and this phenomenon has already been discovered about 30 years ago. That is, 19
In the first half of the 1960s, it was observed that a peculiar luminescence phenomenon (due to the induction of luminescence) occurred when carriers were injected into the strongly fluorescent anthracene single crystal. Since then, organic electroluminescence devices have been the subject of research for a long period of time, but because of their low brightness, monochromatic color, and the use of single crystals, they are technically mainly used for carrier injection into organic materials. Emphasis was placed on the basic research stage.

However, in 1987 Eastman K
Since Odak's Tang et al. announced an organic thin film electroluminescent device having a laminated structure having an amorphous light emitting layer capable of low voltage driving and high brightness light emission, the light emission and stability of the three primary colors of R, G, and B in each direction. The increase in brightness, the laminated structure, the manufacturing method, and the research have been actively conducted in various fields, and have reached the present day.

An organic electroluminescent element using an organic luminescent material is one in which an organic electroluminescent layer containing an organic luminescent material is sandwiched between an anode and a cathode, at least one of which has a light transmitting property. Light is emitted by applying a voltage.

1 to 3 show examples of conventional organic electroluminescent elements (organic EL elements), respectively.

FIG. 1 shows an anode 6 made of translucent ITO (Indium Tin Oxide) on a substrate 10 made of translucent glass.
And an organic layer 15 composed of the hole transport layer 2 and the electron transport layer 4.
The organic electroluminescent element 1A having a single hetero structure has a laminated structure in which a and a cathode 7 are sequentially laminated, and the laminated structure is sealed by a protective layer 14. In this case, application of a DC voltage from the power source 20 causes emission 5 of a predetermined wavelength from the interface between the hole transport layer 2 and the electron transport layer 4.

Further, in FIG. 2, a transparent anode 6 and, if necessary, a hole injection layer 1, a hole transport layer 2, a light emitting layer 3 and an electron transport layer 4 are provided on a transparent substrate 10. Organic layer 15b consisting of
And a cathode 7 are sequentially laminated, which is a double hetero-structure organic electroluminescent element 1B in which this laminated structure is sealed by a protective layer 14. In this case, by applying a DC voltage 20 between the anode 6 and the cathode 7, the holes injected from the anode 6 pass through the hole transport layer 2,
Further, the electrons injected from the cathode 7 pass through the electron transport layer 4,
Each reaches the light emitting layer 3. As a result, electron / hole recombination occurs in the light emitting layer 3 to generate singlet excitons, and the singlet excitons generate light emission 5 having a predetermined wavelength.

FIG. 3 is a structural example of a flat display 21 using the above organic electroluminescent device. As shown in the figure, in the case of a full-color display, for example, an organic layer 15 capable of emitting three primary colors of red (R), green (G) and blue (B).
(15a, 15b) are arranged between the cathode 7 and the anode 6. The cathode 7 and the anode 6 can be provided in a stripe shape intersecting each other, and are selected by the luminance signal circuit 24 and the control circuit 25 with a built-in shift register, and a signal voltage is applied to each of them, whereby the selected cathode is selected. The organic layer at the position (pixel) where 7 and the anode 6 intersect is configured to emit light.

In the case of FIG. 1, the electron transport layer contains a light emitting material, and the two-layer structure of the electron transport layer 4 and the hole transport layer 2 which also serve as the light emitting layer has a two-layer structure by CWTang, SAVanSlyke and CH Chen. Appl. Phys. 65-9,3610-3616 (1989), patent application (Japanese Patent Laid-Open No. 63-264692)
Has also been done.

Here, in order to realize a full color display, stable, high color purity R, G and B light emitting elements are indispensable. Therefore, even in the field of organic electroluminescence devices, NTSC (National Television System C
The research and development of the element of the light emitting material having the chromaticity of ommittee) standard or sRGB (Standard RGB) has been actively conducted.

Up to now, an organic material having a fluorescent property has been exclusively used as a light emitting material, but in recent years, Forr has been used.
Est et al. announced a highly efficient organic electroluminescent device using a phosphorescent material composed of Pt or Ir complex, and research and development have become active.

[0021]

However, all of the complexes such as Pt, Ir, and Ru that have been published so far are luminescent materials of blue-green to red, and are indispensable blue for realizing a full color display. There are no reports on the element of material.

Therefore, in a display device using an organic electroluminescent element, full-color display using a phosphorescent material capable of emitting blue light with high efficiency has not been realized.

Therefore, the present invention provides a phosphorescent material capable of emitting blue light with high efficiency, and an organic electroluminescent device containing this material in a light emitting layer, and a display device capable of high efficiency full color display by using the same. It is intended to provide.

[0024]

That is, the light emitting material of the present invention for achieving such an object has the following characteristics (1) or (2), and further has the following (3). ~ (6)
Is a desirable feature.

(1) The light emitting material is an organometallic complex, and the central metal thereof is a heavy metal having a sixth period in the periodic table. (2) The electron arrangement of the central metal is (Xe) 4f ⁰ 5d ⁰
The closed shell structure is 6s ⁰ or (Xe) 4f ¹⁴ 5d ⁰ 6s ⁰ . (3) LUMO (the lowest unoccupied molecular orbital) is composed of the unoccupied d orbital of the central metal and the unoccupied π ^* orbital of the ligand. (4) The central metal is La ³⁺ , Ce ⁴⁺ , Hf ⁴⁺ , T
a ⁵⁺ , W ⁶⁺ , Re ⁷⁺ , Hg ²⁺ , Tl ³⁺ , Pb ⁴⁺ or Bi
^{When 5+} , the main ligand is bound to the central metal by two covalent bonds per ligand. (5) The central metal is La ³⁺ , Ce ⁴⁺ , Hf ⁴⁺ , Au ⁺
Alternatively, in the case of Hg ²⁺ , the main ligand is bonded to the central metal by one covalent bond per ligand. (6) The molecular structure of the ligand is a molecule represented by the following chemical formula (1) in the case of the above (4), or a derivative thereof in the following chemical formula (2) in the case of the above (5).

[0026]

Embedded image Chemical formula (1): Chemical formula (2):

Among the above characteristics, (1) and / or (2) are indispensable for realizing blue phosphorescence with high efficiency.

For phosphorescence, S ⁰ , which is a ground singlet state,
The excited singlet state S ¹ and the excited triplet state T ¹ are involved. And spin forbidden S ¹ → T
The transitions of ¹ and T ¹ → S ⁰ must occur efficiently. To solve this spin forbidden, a large spin-orbit coupling is needed. And, the spin-orbit interaction is proportional to the fourth power of the atomic number, and thus the more advantageous the heavy metal is, the more advantageous it is. This is the feature of (1) above.

Further, in order to obtain blue light emission, the energy gap of HOMO (highest occupied molecular orbital) -LUMO (lowest unoccupied molecular orbital) of the light emitting molecule needs to be 2.7 eV or more. Organometallic complexes, which have been reported as phosphorescent materials, have central metals such as Ir ³⁺ and Pt ²⁺ ,
As for the electronic state, the 5d orbit has an open shell structure. In this case, the d-orbital splits due to ligand field splitting, and the d-d transition between the occupied d-orbital and the empty d-orbital causes HO
The MO-LUMO gap is limited. Since the energy of ligand field splitting cannot be expected to be higher than 2.5 eV, it is very difficult to secure the energy gap of HOMO-LUMO required for blue light emission in a complex having an open shell electronic state. Is.

Therefore, in the light emitting material of the present invention, the electron arrangement of the central metal is (Xe) 4f ⁰ 5d ⁰ 6s ⁰ or (X
e) The closed shell structure is 4f ¹⁴ 5d ⁰ 6s ⁰ . This is the feature of (2) above. In this case, the energy level of HOMO depends on the ligand and LUMO consists of unoccupied 5d orbitals of the central metal. Therefore, the energy gap required for blue light emission can be secured.

More desirably, the unoccupied π ^* orbital of the ligand contributes to LUMO in order to increase the transition probability. This is the feature of (3) above.

The above (4) and (5) are the valence of the central metal and
This is due to a request from the coordination number. For example, W⁶⁺
Is stable in 6-coordination, so it has a bidentate ligand
Is bound to the central metal through two covalent bonds per ligand.
Must be matched. Hf ⁴⁺If so, 8-coordination is stable
Therefore, if it has a bidentate ligand, one ligand
One covalent bond and one coordination bond form the central metal
Must be matched.

The specific molecular structure of the ligand is shown in (3) above.
The size of the conjugated system may be adjusted so that In addition, it is necessary to consider the difficulty of synthesis.

By using the molecules shown above in the light emitting layer, a highly efficient organic electroluminescent device can be produced.

The layer structure may be appropriately designed in consideration of efficiency, reliability and the like.

The present invention also relates to a display device using such an organic electroluminescent device as a blue light emitting device.

In such a display device, as described above, since the organic electroluminescent element which emits light with high efficiency is used as the blue light emitting element, the efficiency is high by combining with other red light emitting element and green light emitting element. Full color display is possible.

The constitutional examples of the organic electroluminescent element of the present invention and the display device using the same will be described below in detail with reference to the drawings.

The organic electroluminescent device and the display device using the same may have the same structure as that shown in FIG. 1 or 2.

The light emitting device shown in FIG. 1 comprises a substrate 10 and the constituent layers of the organic electroluminescent device 1A provided on the substrate 10. That is, on the substrate 10, the lower electrode 6,
The organic layers 2 and 4 and the upper electrode 7 are sequentially laminated, and the emitted light 5 is extracted from the substrate 10 side or the upper electrode 7 side. In addition, in this figure, 1 is placed on the substrate 10.
Although the configuration in which the organic electroluminescent elements 1A for pixels are provided is shown, the display device 21 includes a plurality of pixels similarly to that shown in FIG. 3, and the plurality of organic electroluminescent elements are arrayed in each pixel. It has been done.

Next, the detailed structure of each part constituting the display device 21 will be described with reference to the substrate 10, the lower electrode 6 and the upper electrode 7,
The organic layers 2 and 4 will be described in this order.

The substrate 10 is made of glass, silicon, a plastic substrate, or a TFT substrate having a TFT (Thin Film Transistor) formed thereon. In particular, when the display device 21 is of a transmissive type in which the emitted light 5 is extracted from the substrate 10 side, the substrate 10 is made of a light transmissive material.

The lower electrode 6 formed on the substrate 10
Is used as an anode or a cathode. still,
In the drawings, the case where the lower electrode 6 is an anode is illustrated as a representative.

It is assumed that the lower electrode 6 is patterned into a shape suitable for the driving method of the display device 21. For example, when the drive system of the display device 21 is a simple matrix system, the lower electrode 6 is formed in a stripe shape, for example. Further, when the driving system of the display device 21 is an active matrix system in which a TFT is provided for each pixel, the lower electrode 6 is formed in a pattern corresponding to each of a plurality of arranged pixels, and is similarly provided in each pixel. It is assumed that the TFTs are connected to each other through contact holes (not shown) formed in an interlayer insulating film covering these TFTs.

On the other hand, the upper electrode 7 provided on the lower electrode 6 via the organic layer is used as a cathode when the lower electrode 6 is an anode, and is used as an anode when the lower electrode 6 is a cathode. To be In the drawing, the case where the upper electrode 7 is a cathode is shown.

When the display device 21 is of a simple matrix type, the upper electrode 7 is formed in a stripe shape that intersects with the stripe of the lower electrode 6, for example, as shown in FIG. The part thus formed becomes an organic electroluminescence device. When the display device 21 is of the active matrix type, the upper electrode 7 is
It is formed as a solid film formed so as to cover one surface of the substrate 10, and is used as an electrode common to each pixel. When the active matrix method is adopted as the driving method of the display device 21, it is desirable to adopt a top emission type in which the emitted light 5 is taken out from the upper electrode 7 side in order to secure the aperture ratio of the organic electroluminescent element. .

Here, the lower electrode 6 (or the upper electrode 7)
As the anode material constituting the, it is preferable that the work function is as large as possible, for example, nickel, gold, platinum, palladium, selenium, rhodium, ruthenium, iridium, rhenium, tungsten, molybdenum, chromium, tantalum, niobium, or these Alloys, oxides, tin oxide, ITO (Indium Tin Oxide), zinc oxide, titanium oxide and the like are preferable.

On the other hand, the cathode material forming the upper electrode 7 (or the lower electrode 6) preferably has a work function as small as possible, and for example, magnesium, calcium, indium, lithium, aluminum, silver and alloys thereof are preferable. .

However, for the electrode on the side from which the emitted light 5 generated in this organic electroluminescent layer is taken out, a material having light transmittance is appropriately selected from the above-mentioned materials and used. A material that transmits more than 30% of light in the wavelength region of the emitted light 5 of the light emitting element is preferably used.

The substrate, the electrode material, and the stacking order may be appropriately selected depending on whether the light emission is taken out from the substrate side, the upper surface side, or both of them.

The hole injection material of the hole injection layer 1 is P
A conductive polymer such as PV (polyphenylene vinylene), a material such as phthalocyanine copper, and a starburst type amine can be used as a single layer or a laminate.

As the hole transport material used for the hole transport layer 2, known materials such as triphenylamine multimers or derivatives thereof can be used.

Further, for the light emitting layer 3 (or the light emitting material which can be mixed with the electron transporting layer 4), for example, a W complex represented by the chemical formula (3) described later may be used. This is T at an appropriate concentration
A wide-gap host material such as AZ (triazole derivative) may be doped.

Alq is used as the electron transport material of the electron transport layer 4.
₃ (tris (8-quinolinol) aluminum), TA
Known materials such as Z and OXD (oxadiazole derivative) can be used. The electron-transporting layer may be doped with the light-emitting material of the present invention to form an electron-transporting light-emitting layer.

A buffer layer composed of an alkali metal oxide such as lithium oxide, cesium oxide or strontium fluoride, an alkali metal fluoride, an alkaline earth oxide or an alkaline earth fluoride between the electron transporting light emitting layer and the cathode. Is more preferable because the efficiency of electron injection is increased.

A well-known method such as vacuum deposition or spin coating can be used for producing a multilayer film of the above-mentioned materials.

When the light emitting device of the present invention is driven, in order to eliminate the influence of moisture and oxygen in the atmosphere and enhance the safety, it is sealed in advance with, for example, magnesium fluoride or SiNx. Alternatively, it is desirable that the surrounding space be purged with a dry inert gas or be evacuated.

[0058]

EXAMPLES The present invention will be specifically described below based on examples.

Example 1 A 2-TNATA (4, 4 ', 4', 4 ', hole-injecting layer was formed on a glass substrate on which ITO having a film thickness of 190 nm was formed.
35 nm of 4 ″ -tris (N, N- (2-naphthyl) phenylamino) triphenylamine), and α-NPB (4,4′-bis [N- (1-naphthyl)) as a hole transport layer.
-N-phenylamino] biphenyl) is 30 nm, a light emitting layer is a layer in which the W complex represented by the following chemical formula (3) is doped in 8 wt% TAZ is 25 nm, an electron transport layer is Alq ₃ is 25 nm, and a buffer layer is Lithium oxide having a thickness of 0.5 nm and aluminum serving as a cathode having a thickness of 200 nm were sequentially formed by vacuum vapor deposition to produce an organic electroluminescence device.

[0060]

Embedded image Chemical formula (3):

The characteristics of the organic electroluminescent device thus produced were measured. The EL spectrum had a peak at 460 nm, and the coordinates on the CIE chromaticity coordinates were (0.14, 0.11). The quantum efficiency of light emission was as high as 6%.

Comparative Example 1 An organic electroluminescent device was produced in the same manner as in Example 1 except that the Zn complex represented by the following chemical formula (4) was doped as the light emitting layer.

[0063]

Embedded image Chemical formula (4):

The characteristics of the organic electroluminescent device thus produced were measured. The EL spectrum had a peak at 460 nm, and the coordinates on the CIE chromaticity coordinates were (0.14, 0.18). The quantum efficiency of light emission was as low as 1%.

The embodiments and examples of the present invention described above can be variously modified based on the technical idea of the present invention.

[0066]

As described above, the present invention provides a molecular design of a phosphorescent light emitting material which realizes a highly efficient blue light emitting device which has hitherto been impossible, and realizes it. By using this phosphorescent material as a light emitting material, it is possible to realize a highly efficient blue light emitting element which has been impossible up to now, and a highly efficient full color display device.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view of a main part of an organic electroluminescent device.

FIG. 2 is a schematic cross-sectional view of a main part of another organic electroluminescent device of the same.

FIG. 3 is a configuration diagram of a full-color flat display using the same organic electroluminescent element.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Hole injection layer, 2 ... Hole transport layer, 3 ... Light emitting layer (light emitting material), 4 ... Electron transport layer, 5 ... Emitting light, 6 ... Transparent electrode (anode), 7 ... Cathode, 10 ... Substrate, 14 ... Protective film, 15a,
15b ... organic layer, 20 ... power supply

Claims (21)

[Claims] 1. A luminescent material comprising an organometallic complex whose central metal is a heavy metal having a sixth period in the periodic table. 2. The electron configuration of the central metal is (Xe) 4f ⁰ 5
A luminescent material comprising an organometallic complex having d ⁰ 6s ⁰ or (Xe) 4f ¹⁴ 5d ⁰ 6s ⁰ . 3. The central metal is La ³⁺ , Ce ⁴⁺ , H
f ⁴⁺ , Ta ⁵⁺ , W ⁶⁺ , Re ⁷⁺ , Hg ²⁺ , Tl ³⁺ , Pb ⁴⁺
Alternatively, the luminescent material according to claim 1 or 2, which is Bi ⁵⁺ . 4. The luminescent material according to claim 3, wherein the main ligand is bound to the central metal by two covalent bonds per ligand. 5. The light emitting material according to claim 4, wherein the ligand comprises the following chemical formula (1) or a derivative thereof. Chemical formula (1): 6. The central metal is La ³⁺ , Ce ⁴⁺ , H
The light emitting material according to claim 1 or 2, which is f ⁴⁺ , Au ⁺ or Hg ²⁺ . 7. The luminescent material according to claim 6, wherein the main ligand is bound to the central metal by one covalent bond and one coordinate bond per ligand. 8. The light emitting material according to claim 7, wherein the ligand comprises the following chemical formula (2) or a derivative thereof. Embedded image Chemical formula (2): 9. An organic electroluminescence device comprising at least a hole transport layer and a light emitting layer sandwiched between an anode and a cathode, wherein the light emitting layer contains the light emitting material according to claim 1. A characteristic organic electroluminescent device. 10. The organic electroluminescent device according to claim 9, wherein the light emitting layer contains the light emitting material according to any one of claims 3 to 8. 11. The organic electroluminescent device according to claim 9, wherein an electron transport layer is provided between the cathode and the light emitting layer. 12. The organic electroluminescent device according to claim 9, wherein at least one of the anode and the cathode is made of a light transmissive material. 13. The organic electroluminescent device according to claim 12, wherein the cathode is made of a light transmissive material. 14. The organic electroluminescent device according to claim 12, wherein the cathode is made of an alloy of magnesium and silver. 15. A display device comprising the organic electroluminescent element according to claim 9 arranged in a plurality of pixels. 16. The display device according to claim 15, wherein the organic electroluminescent element has a light emitting layer containing the light emitting material according to any one of claims 3 to 8. 17. The display device according to claim 15, wherein the organic electroluminescent element is provided as a blue light emitting element in a part of the pixels of the plurality of pixels. 18. The display device according to claim 15, wherein an electron transport layer is provided between the cathode and the light emitting layer. 19. The display device according to claim 15, wherein at least one of the anode and the cathode is made of a light transmissive material. 20. The display device according to claim 19, wherein the cathode is made of a light transmissive material. 21. The display device according to claim 19, wherein the cathode is made of an alloy of magnesium and silver.