JPH11354282A - Organic light-emitting device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To efficiently obtain a luminescent color that dopant of a luminescent layer originally has without generating a wavelength shift or lowering of quantum efficiency by providing at least an organic electric charge transport layer and an organic luminescent layer between an anode and a cathode and providing a blocking layer made of compound which does not act with the organic luminescent layer each other on a layer-to-layer boundary face. SOLUTION: Inorganic compound utilizing organic compound or a tunnel effect is used for a blocking layer and thickness of 0.1 to 20 nm is desirable. It is desirable that a positive hole transport layer 11, an electron transport layer 12 of 0.1 to 20 nm in thickness, a luminescent layer 13 in which a fluorescent material is doped in a host material, an electron transport layer 14 of only the host material and a cathode are constituted to be successively laminated on an anode such as an ITO. The electronic transport layer 12 made of tris(8-hydroxyquinoline) aluminum is the blocking layer and prevents a mutual action of the fluorescent material and a material of the positive hole transport layer 11. It is desirable that the fluorescent material has a phenoxazone skeleton and the positive hole transport layer 11 is constituted of a triphenylamine derivative.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an organic light emitting device used as a light emitting display or a backlight for a liquid crystal display.

[0002]

2. Description of the Related Art Electroluminescent (EL) panels have high visibility, excellent display capability, and high-speed response. In recent years, reports have been made on organic light-emitting devices using an organic compound as a constituent material (for example, see the related paper “Applied Physics Letters”, Vol.
1, 913, 1987 (Applied Physics Letters, 51,
1987, P.913.)).

[0003] This report describes an organic light emitting device having a structure in which an organic light emitting layer and a charge transport layer are laminated. A tris (8-quinolinol) aluminum complex (hereinafter Alq) has been developed as a luminescent material, and is an excellent luminescent material having both high luminous efficiency and electron transport. "Journal of Applied Physics," Vol. 65, p. 36, 1989 (Journal of Applied Physics, 6
5,1989, p.3610.), A device was prepared in which Alq for forming an organic light emitting layer was doped with a coumarin derivative or a fluorescent dye such as DCM1, and it was found that the emission color was changed by appropriate selection of the dye.

Further, it has been clarified that the luminous efficiency is increased as compared with the undoped one. The element configuration at this time is such that a hole transport layer and a doped layer using Alq as a host material are laminated, and these dopants exhibit functions without causing interaction with Alq or an adjacent hole transport material. Nothing else. Furthermore, a scheme in which a doped layer is provided in an Alq layer is described, but this method obtains the exciton diffusion distance in Alq by sandwiching the doped layer extremely thinly.

The conditions required for a fluorescent dye as a guest material include a large overlap between the absorption spectrum of the fluorescent dye and the emission spectrum of the host material (eg, Alq), a high fluorescence quantum yield, Good stability. There are still few luminescent materials that satisfy all of these conditions, and there are currently very few red luminescent materials.

[0006]

Generally, a fluorescent dye used as a guest material for doping has a complicated structure having an aromatic ring and various functional groups. In organic solid-state devices, many of such organic compounds are
It is easy to react with an adjacent host material or another element constituting material, and the stable dopant found by Tang et al. Is rare. The excited singlet state in fluorescence emission is susceptible to heat and vibration, so it can easily follow a path other than fluorescence irradiation, resulting in a decrease in quantum yield, or a shift in wavelength due to the formation of complexes or aggregates. Problem arises.

[0007] Phenoxazone 9 described in JP-A-7-272854 has been found to be promising as a red material having a maximum emission wavelength of 630 nm. However, in a system in which TPD is stacked as a hole transport layer and Alq is stacked as a host material of the electron transporting light emitting layer, a wavelength shift occurs due to the interaction between TPD constituting the hole transport layer and phenoxazone 9 as a dopant. It has been reported that a problem of becoming orange (maximum wavelength 600 nm) occurs [Sugiura, Inaba, IEICE Technical Report, OME94-4.
4, 1 (1994)].

[0008]

Therefore, in order to sufficiently bring out the luminescent characteristics originally required for the dye, we have proposed a method of sandwiching a material which does not easily react between a dye which is a luminescent center and a material which is liable to react. This has led to the solution of the above-mentioned problem by inhibiting the reaction induced due to the structure and properties possessed by.

Specifically, the first invention (Claim 1)
A blocking layer comprising a compound that does not interact with the organic light emitting layer at the interface between the organic light emitting layer and the organic charge transport layer, having at least an organic charge transport layer and an organic light emitting layer between the positive electrode and the negative electrode; It is characterized by comprising.

[0010] The blocking layer may have a thickness of 0.1-20.
In this case, the thickness is set to be nm.

According to a second aspect of the present invention, there is provided an organic light emitting device having at least a hole transport layer and an organic light emitting layer between a positive electrode and a negative electrode, wherein the organic light emitting layer is doped with a fluorescent substance. An electron transport layer is formed between the hole transport layer and the organic light emitting layer with a thickness of 0.1 to 20 nm.

The fluorescent substance may have a phenoxazone skeleton, and the fluorescent substance may be 9-diethylamino-5-H-benzo [a] phenoxazine-5-
It is turned on.

Further, the hole transport layer is constituted by at least a triphenylamine derivative.

Further, the hole transport layer is preferably made of at least a compound represented by the following general formula (1) (wherein R1, R2, R3, R4, and R5 may be the same or different, and R1, R2, and R3 are hydrogen atoms, lower alkyls) And R4 and R5 represent a hydrogen atom, a lower alkyl group, a lower alkoxy group, or a chlorine atom).

[0015] Further, the hole transport layer contains at least N, N'-bis (4'-diphenylamino-4-biphenylyl) -N, N'-diphenylbenzidine.

Further, the hole transporting layer is preferably made of at least a compound represented by the general formula (Chemical Formula 2) (wherein R1 and R2 may be the same or different and each represents a hydrogen atom, a lower alkyl group, a lower alkoxy group, a substituted or unsubstituted aryl group). R3 represents a hydrogen atom, a lower alkyl group, a lower alkoxy group, or a chlorine atom).

Further, the hole transport layer contains at least N, N'-diphenyl-N, N'-bis (3-methylphenyl) -1,1'-biphenyl-4,4'-diamine. Things.

[0018]

Embodiments of the present invention will be described below.

Tang et al. Improved the characteristics of an organic light emitting device by forming a layered structure in which organic layers were separated from each other in function. In general, the organic layer was composed of a layer made of a hole transporting material and a layer made of an electron transporting material. Of at least two layers.

The hole transporting material is a material that easily forms a cation radical state, and the electron transporting material is a material that easily forms an anion radical state.

Therefore, the dye tends to react with either one of the dyes depending on its own structure and characteristics. Therefore, a dye that easily reacts with the cation is doped into the electron transport layer, and a dye that easily reacts with the anion is doped into the hole transport layer.

Further, a blocking layer that inhibits the reaction is provided at the interface between each charge transport layer and the light emitting layer. When the blocking layer is composed of an organic compound, it is preferable to sandwich an electron transporting material in the former case and a hole transporting material in the latter case.

The blocking layer may be an inorganic compound, and can exhibit a function of inhibiting a reaction while tunneling holes or electrons by utilizing a tunnel effect.

The thickness of the blocking layer is sufficient if it is sufficient to prevent the interaction between the dye and the hole transporting material, and is preferably 0.1 to 20 nm, more preferably 0.5 to 1 nm.
0 nm is preferable, and especially 0.5 to 3 nm is preferable.
The blocking layer does not need to be a uniform film. For example, it is considered that an extremely thin film having a thickness of, for example, 0.5 to 3 nm can form an island structure. However, since the dye to be doped has a low concentration of about 1% by weight or less,
It can be considered that the existence probability of the dye is extremely small with respect to the area occupied by the island-shaped blocking layer. Therefore, it can be said that a blocking material is necessarily interposed between the dye and the charge transporting layer that reacts with the dye.

Next, a more detailed description will be given with reference to the drawings. 1 and 2 show the case where the electron transport layer is doped with a dye.
FIG. 3 and FIG. 4 are schematic cross-sectional views when the dye is doped into the hole transport layer.

In FIG. 1, after the hole transport layer 11 is formed, the electron transport layer 12 is formed thin, and the light emitting layer 13 in which a dye and a host material are co-evaporated is provided. When the desired film thickness is reached, the deposition of the dye is stopped, and the electron transport layer 14 is formed using only the host material.
Is formed.

In FIG. 2, after the hole transport layer 11 is formed, the electron transport layer 12 is formed thin, and then the light emitting layer 20 which is also used as an electron transport layer in which a dye and a host material are co-deposited is provided. The thickness of the electron transport layer 12 adjacent to the hole transport layer 11 may be a thickness sufficient to prevent the interaction between the dye and the hole transport material, and is preferably 0.1 to 20 nm, more preferably 0.5 to 20 nm. It is preferably from 10 to 10 nm, particularly preferably from 0.5 to 3 nm.

If the thickness exceeds 20 nm, the hole transport layer 1
Since excitons generated near the interface between the electron transport layer 1 and the electron transport layer 12 cannot reach the fluorescent dye in the light emitting layer 20,
It is not possible to obtain the fluorescent emission of the dye, and at the same time, the emission of Alq gradually increases, so that the color purity is reduced.

The light emitting layer may have either of the structures shown in FIGS. 1 and 2. However, since the dye in the electron transport layer can be considered as a trap site for electrons, as shown in FIG. The layer 14 preferably separates functions, and the thickness of the light emitting layer 13 is particularly preferably 10 to 30 nm.

FIG. 3 shows that after forming the hole transport layer 21, a light emitting layer 22 in which a dye and a host material are co-deposited is provided, and the hole transport layer 23 is formed thin again, and then the electron transport layer 24 is formed. .

FIG. 4 shows that after forming a light emitting layer 40 also serving as a hole transport layer in which a dye and a host material are co-deposited, forming a thin hole transport layer 23 and then providing an electron transport layer 24. The thickness of the hole transport layer 23 adjacent to the electron transport layer 24 may be a thickness sufficient to prevent the interaction between the dye and the electron transport material, and is preferably 0.1 to 20 nm, more preferably 0.5 to 20 nm. It is preferably from 10 to 10 nm, particularly preferably from 0.5 to 3 nm.

The light-emitting layer may have any of the structures shown in FIGS. 3 and 4. However, since the dye in the hole transport layer can be considered as a hole trap site, as shown in FIG. The layer 21 preferably has a function separation. Therefore, the thickness of the light emitting layer 22 is 10 to 30 n.
m is particularly preferred.

Next, for the hole transport layer in the present invention, a derivative having triphenylamine as a basic skeleton is preferable as a constituent material. For example, JP-A-7-126
No. 615, a tetraphenylbenzidine compound,
Triphenylamine trimer and benzidine dimer are exemplified. Also, various triphenyldiamine derivatives described in JP-A-8-48656 or JP-A-7-659.
No. 58, the MTPD (commonly known as TPD) may be used. Particularly, a triphenylamine tetramer described in Japanese Patent Application No. 9-341238 is preferable.

As a constituent material of the electron transport layer, tris (8-hydroxyquinoline) aluminum is preferable.
Other examples include metal complexes such as tris (4-methyl-8-hydroxyquinoline) aluminum. The electron transport layer preferably has a thickness of 10 to 1000 nm.

As for the organic layers of the hole transporting layer, the light emitting layer and the electron transporting layer, it is desirable to form a homogeneous film in an amorphous state, and it is preferable to form the film by a vacuum evaporation method. Furthermore, by forming each layer continuously in a vacuum, by preventing impurities from adhering to the interface between each layer, it is possible to improve characteristics such as lower operating voltage, higher efficiency, and longer life. it can.

In forming each of these layers by a vacuum deposition method, when one layer contains a plurality of compounds,
It is preferable to perform co-evaporation by individually controlling the temperature of each boat containing the compound, but it is also possible to vapor-deposit a mixture in advance. Further, as other film forming methods, a solution coating method, a Langmuir-Blodgett (LB) method, or the like can be used. In the solution coating method, each compound may be dispersed in a matrix material such as a polymer.

The organic light-emitting device can emit surface light by making at least one electrode transparent or translucent. Usually, an ITO (indium tin oxide) film is often used for an anode serving as a hole injection electrode. Other examples include tin oxide, Ni, Au, Pt, and Pd. For the purpose of improving the transparency or reducing the resistivity of the ITO film, a film forming method such as sputtering, electron beam evaporation, or ion plating is employed. The film thickness is determined from the required sheet resistance value and visible light transmittance. However, since the driving current density is relatively high in the organic light emitting device, it is necessary to reduce the sheet resistance value.
It is often used with a thickness of 0 nm or more.

The cathode as an electron injection electrode has Tang.
In many cases, an alloy of a metal having a low work function and a low electron injection barrier, such as an MgAg alloy or an AlLi alloy, and a metal having a relatively large work function and being stable is used. Further, a metal having a low work function may be formed on the organic layer side, and a metal having a large work function may be thickly laminated for the purpose of protecting the metal having a low work function, such as Li / Al or LiF / Al.
Such a laminated electrode can be used. For forming these cathodes, a vapor deposition method or a sputtering method is preferable.

The substrate may be any substrate as long as it can support the organic light-emitting element, which is formed by laminating the above-mentioned thin films, and may be any material that is transparent or translucent so as to extract the luminescence generated in the organic layer. Glass such as 1737, or a resin film of polyester or the like is used.

Next, the present invention will be described in more detail based on specific embodiments. Based on the configuration in FIG. 1, a device was manufactured using 9-diethylamino-5-H-benzo [a] phenoxazin-5-one as a dopant.

(Example 1) N, N'-bis (4'-diphenylamino-4-) was formed on a glass substrate on which ITO was formed.
A hole transport layer of (biphenylyl) -N, N'-diphenylbenzidine having a thickness of 50 nm is formed. Subsequently, tris (8-hydroxyquinoline) aluminum was formed to a thickness of 1 nm as a blocking layer.

The light emitting layer was formed by using 1% by weight of tris (8-hydroxyquinoline) aluminum as a host material.
After doping with 9-diethylamino-5-H-benzo [a] phenoxazin-5-one to obtain a film thickness of 20 nm, evaporation of only the dopant was stopped and only the host material was evaporated to 30 nm to form an electron transport layer.

Finally, a negative electrode made of an AlLi alloy was formed. When a DC voltage was applied to this element and evaluated,
Only the maximum wavelength 630 nm inherent in the dopant was observed, and a red light-emitting device was obtained.

The chromaticity coordinates are (x, y) = (0.68,
0.32). The luminous efficiency was 3.3 cd / A, which was equivalent to that of an undoped green light-emitting device. Furthermore, when the initial luminance was set to 500 cd / m ² and continuous lighting was performed by DC constant current driving, the luminance half-life was 5000 hours, which is equivalent to that of the undoped element.

Example 2 In the formation of the hole transporting layer of Example 1, N, N'-bis (4'-diphenylamino-4-biphenylyl) -N, N'-diphenylbenzidine was replaced by N, N'-diphenylbenzidine. N'-diphenyl-N, N'-bis (3-methylphenyl) -1,1'-biphenyl-4
An organic light-emitting device was produced in the same manner as in Example 1 except that 4'-diamine was used.

When a DC voltage was applied to this device and evaluated, only the maximum wavelength 630 nm inherent to the dopant was observed, and a red light-emitting device was obtained. The luminous efficiency was 3.6 cd / A, which was equivalent to that of an undoped green light-emitting device.

The chromaticity coordinates are (x, y) = (0.64,
0.34). Further, the initial luminance is 500 cd / m.
When set to 2 and continuously lit by DC constant current drive,
The luminance half-life was 100 hours, which is equivalent to that of the undoped element.

Comparative Example 1 N, N'-bis (4'-diphenylamino-4-) was formed on a glass substrate on which ITO was formed.
A light emitting layer is formed following the formation of a hole transport layer having a thickness of 50 nm made of (biphenylyl) -N, N'-diphenylbenzidine.

The light emitting layer was formed by using tris (8-hydroxyquinoline) aluminum as a host material and adding 1% by weight of 9-
After doping with diethylamino-5-H-benzo [a] phenoxazin-5-one and obtaining a film thickness of 20 nm, deposition of only the dopant was stopped and only the host material was changed to 3 nm.
An electron transporting layer was formed by evaporating 0 nm.

Finally, a negative electrode made of an AlLi alloy was formed. When a DC voltage was applied to this element and evaluated,
Emission of 600 nm from the complex formed by TPD and phenoxazone 9 was observed, shifted from the intrinsic wavelength of the dopant, and the EL emission color was orange. The chromaticity coordinates were (x, y) = (0.51, 0.41).

[0051]

As described above, the present invention relates to a system in which a light emitting layer is doped with a fluorescent substance,
By providing a blocking layer between the fluorescent substance and the layer containing the compound that easily interacts,
It is possible to provide an organic light-emitting element capable of preventing a reaction between a fluorescent material, which is a guest material, and another material, and efficiently extracting an original light emission color of a dopant without causing a wavelength shift, a decrease in quantum efficiency, or the like.

Further, as a constitution of the organic light emitting device, by using a blocking layer in a combination of an amine-based hole transporting layer and an electron transporting light emitting layer made of Alq, good characteristics of high efficiency and long life are obtained. Is obtained.

Further, by providing a blocking layer to prevent a reaction with other materials, it becomes possible to use various fluorescent substances, and a wider range of materials can be selected for a fluorescent substance as a dopant, thereby enabling colorization. Become.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a structure of an organic light-emitting device in which an electron transport layer is doped according to a first embodiment of the present invention and in which a light-emitting layer and an electron transport layer are functionally separated.

FIG. 2 is a cross-sectional view of the structure of an organic light-emitting device according to a second embodiment of the present invention in which an electron transport layer is doped and serves as both a light-emitting layer and an electron transport layer.

FIG. 3 is a cross-sectional view of a structure of an organic light-emitting device in which a hole-transporting layer is doped and a light-emitting layer and a hole-transporting layer are separated according to a third embodiment of the present invention.

FIG. 4 is a cross-sectional view of the structure of an organic light emitting device in which a hole transport layer is doped and serves as both a light emitting layer and a hole transport layer according to a fourth embodiment of the present invention.

[Explanation of symbols]

 11, 21, 23 Hole transport layer 12, 14, 24 Electron transport layer 13, 20, 22, 40 Light emitting layer

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Hitoshi Hisada 1006 Kazuma Kadoma, Kadoma City, Osaka Inside Matsushita Electric Industrial Co., Ltd.

Claims (10)

[Claims] An organic light emitting device having at least an organic charge transport layer and an organic light emitting layer between a positive electrode and a negative electrode, wherein the organic light emitting layer is provided at an interface between the organic light emitting layer and the organic charge transport layer. An organic light-emitting device comprising a blocking layer made of a compound that does not interact with each other. 2. The method according to claim 1, wherein the blocking layer has a thickness of 0.1 to 20 nm.
An organic light-emitting device characterized by having a thickness of: 3. An organic light emitting device having at least a hole transport layer and an organic light emitting layer between a positive electrode and a negative electrode, wherein the organic light emitting layer is doped with a fluorescent substance. Between the layers, the electron transport layer
An organic light emitting device having a thickness of 20 nm. 4. The organic light emitting device according to claim 3, wherein said fluorescent substance has a phenoxazone skeleton. 5. The method according to claim 1, wherein the fluorescent substance is 9-diethylamino-5.
The organic light-emitting device according to claim 3, wherein the organic light-emitting device is -H-benzo [a] phenoxazin-5-one. 6. The organic light emitting device according to claim 3, wherein said hole transport layer is composed of at least a triphenylamine derivative. 7. The method according to claim 1, wherein the hole transport layer has at least the following general formula: (Wherein R1, R2, R3, R4, and R5 may be the same or different; R1, R2, and R3 represent a hydrogen atom, a lower alkyl group, or a lower alkoxy group; R4 and R5 represent a hydrogen atom or a lower alkyl group; , A lower alkoxy group or a chlorine atom). 8. The method according to claim 1, wherein said hole transport layer comprises at least N, N '.
-Bis (4'-diphenylamino-4-biphenylyl)
4. The organic light-emitting device according to claim 3, comprising -N, N'-diphenylbenzidine. 9. The method according to claim 1, wherein the hole transport layer has at least the following general formula: (Wherein R 1 and R 2 may be the same or different and represent a hydrogen atom, a lower alkyl group, a lower alkoxy group, a substituted or unsubstituted aryl group, and R 3 represents a hydrogen atom, a lower alkyl group, a lower alkoxy group, or 4. A compound represented by the formula:
The organic light-emitting device as described in the above. 10. The method according to claim 1, wherein the hole transport layer comprises at least N,
4. The organic light emitting device according to claim 3, comprising N'-diphenyl-N, N'-bis (3-methylphenyl) -1,1'-biphenyl-4,4'-diamine.