JP3243887B2 - Organic electroluminescent device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an organic electroluminescent device, and more particularly to a thin film device which emits light by applying an electric field to a light emitting layer made of an organic compound.

[0002]

2. Description of the Related Art Conventionally, as a thin film type electroluminescent (EL) element, Zn, which is a group II-VI compound semiconductor of an inorganic material, is used.
In general, S, CaS, SrS, and the like are doped with Mn or a rare earth element (Eu, Ce, Tb, Sm, or the like) which is a luminescence center. However, EL devices manufactured from the above inorganic materials include: 1) AC drive is required (50-1000 Hz), 2) High drive voltage (up to 200 V), 3) It is difficult to achieve full color (especially blue), 4) Cost of peripheral drive circuit is high. ing.

However, in recent years, in order to improve the above problems,
Development of EL devices using organic thin films has been started. In particular, the electrode type was optimized for the purpose of improving the efficiency of carrier injection from the electrode in order to increase the luminous efficiency, and the organic luminescent layer composed of an organic hole transport layer composed of an aromatic diamine and the aluminum complex of 8-hydroxyquinoline was used. Of an organic electroluminescent device provided with a layer (Appl. Phy
s. Lett. 51, p. 913, 1987), the luminous efficiency has been greatly improved as compared with a conventional electroluminescent element using a single crystal such as anthracene.

One of the features of the organic electroluminescent device is that blue light emission can be realized. Organic materials that exhibit blue electroluminescence include anthracene (Jpn. J. App.
l. Phys. 27, L269, 1988),
Tetraphenylbutadiene, pentaphenylcyclopentadiene (Appl. Phys. Lett., Vol. 56,
799, 1990), distyrylbenzene derivatives (Journal of the Chemical Society of Japan, p. 1162, 1992), and styrylamine-containing polycarbonates (Appl. Phys. L).
ett. , 61, 2503, 1992), oxadiazole derivatives (Jpn. J. Appl. Phy.
s. , 31, 1812, 1992; Journal of the Chemical Society of Japan, 1540, 1991), azomethine zinc complex (Jpn. J. Appl. Phys., 32, L51).
1 1993).

[0005]

When the above-mentioned blue light-emitting material is used as a light-emitting layer of an organic electroluminescent device, 1) Since the structure of organic molecules which emit blue light is often simple, the molecular weight becomes small and the molecular weight becomes small. Due to the unstable state, it is easy to crystallize and it is difficult to obtain a uniform film. 2) Low luminous efficiency and high luminance cannot be obtained. 3) Interaction with hole transport material to form exciplex. And 4) a short lifetime when driven as an element, and the like.

For the reasons described above, there are many problems in practical use of a blue light emitting organic electroluminescent device.

[0007]

Means for Solving the Problems In view of the above-mentioned circumstances, the present inventors have conducted intensive studies for the purpose of providing an organic electroluminescent device capable of maintaining stable blue light-emitting characteristics for a long period of time. The inventors have found that it is preferable to include an oxazole metal complex in the light-emitting layer, and have completed the present invention.

That is, the gist of the present invention is an organic electroluminescent device in which an anode, a hole transport layer, an organic light emitting layer, and a cathode are sequentially laminated, wherein the organic light emitting layer has the following general formula (I)

[0009]

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(Wherein R ¹ to R ⁸ each independently represent a hydrogen atom, a halogen atom, an alkyl group, an aralkyl group, an alkenyl group, an allyl group, a cyano group, an amino group,
Amide group, alkoxycarbonyl group, carboxyl group,
An alkoxy group, an aromatic hydrocarbon group which may have a substituent or an aromatic heterocyclic group which may have a substituent, and M is beryllium, zinc, cadmium, aluminum, gallium, indium, (Indicating scandium, yttrium, magnesium, calcium, strontium, cobalt, copper or nickel, and n is an integer from 1 to 3).

Hereinafter, the organic electroluminescent device of the present invention will be described with reference to the drawings. FIG. 1 is a cross-sectional view schematically showing an example of the structure of an organic electroluminescent device of the present invention, wherein 1 denotes a substrate, 2a and 2b denote conductive layers, 3 denotes a hole transport layer, and 4 denotes an organic light emitting layer. . The substrate 1 serves as a support for the organic electroluminescent device of the present invention, and may be a quartz or glass plate, a metal plate or metal foil, a plastic film or sheet, or the like. A transparent synthetic resin substrate such as acrylate, polycarbonate, and polysulfone is preferred.

A conductive layer 2a is provided on the substrate 1,
As the conductive layer 2a, usually, aluminum, gold,
Metals such as silver, nickel, palladium, and tellurium; metal oxides such as oxides of indium and / or tin; and copper iodide;
It is composed of carbon black or a conductive polymer such as poly (3-methylthiophene). Conductive layer 2
The formation of a is usually performed by a sputtering method, a vacuum deposition method, or the like in many cases. However, fine metal particles such as silver or copper iodide, carbon black, conductive metal oxide fine particles, conductive polymer fine powder, etc. In the case of (1), it can also be formed by dispersing in an appropriate binder resin solution and applying it on a substrate. Further, in the case of a conductive polymer, a thin film can be formed directly on a substrate by electrolytic polymerization or can be formed by coating on a substrate (Appl. Phys. L).
ett. 60, 2711, 1992).

The above-mentioned conductive layer 2a can be laminated with a different material. The thickness of the conductive layer 2a depends on the required transparency, but when transparency is required,
It is desirable that the visible light transmittance is 60% or more, preferably 80% or more. In this case, the thickness is usually 5 to 5.
It is 1000 nm, preferably about 10 to 500 nm. If opaque, the conductive layer 2a may be the same as the substrate 1.

In the example shown in FIG. 1, the conductive layer 2a serves as an anode (anode) for injecting holes. on the other hand,
The conductive layer 2b plays a role of injecting electrons into the organic light emitting layer 4 as a cathode (cathode). As the material used for the conductive layer 2b, the material for the conductive layer 2a can be used. However, for efficient electron injection, a metal having a low work function is preferable, and tin, magnesium, indium, aluminum, A suitable metal such as silver or an alloy thereof is used. The thickness and forming method of the conductive layer 2b are usually
This is the same as the conductive layer 2a. Although not shown in FIG. 1, a substrate similar to the substrate 1 can be further provided on the conductive layer 2b. However, the conductive layer 2a is used as the EL element.
At least one of the conductive layer 2b and the conductive layer 2b needs to have good transparency. For this reason, one of the conductive layer 2a and the conductive layer 2b preferably has a thickness of 10 to 500 nm, and good transparency is desired.

The hole transport layer 3 is provided on the conductive layer 2a. The hole transport material has a high hole injection efficiency from the conductive layer 2a and efficiently transports the injected holes. It must be a material that can be used. For that purpose, it is required that the ionization potential is small, the hole mobility is large, the stability is excellent, and impurities serving as traps are hardly generated at the time of manufacture or use.

Examples of such a hole transport compound include, for example, JP-A-59-194393 and US Pat.
175,960, U.S. Pat. No. 4,923,774 and U.S. Pat. No. 5,047,687.
N'-diphenyl-N, N '-(3-methylphenyl)
-1,1'-biphenyl-4,4'-diamine: 1,
1'-bis (4-di-p-tolylaminophenyl) cyclohexane: an aromatic amine compound such as 4,4'-bis (diphenylamino) quadrophenyl;
Hydrazone compounds described in JP-A-11591, silazane compounds and quinacridone compounds described in U.S. Pat. No. 4,950,950. These compounds may be used alone or, if necessary, in combination. In addition to the above compounds, polyvinyl carbazole and polysilane (Appl. Phys. Lett.,
59, p. 2760, 1991).

The hole transport layer 3 can be formed by laminating the above organic hole transport material on the conductive layer 2a by a coating method or a vacuum evaporation method. In the case of a coating method, one or two or more organic hole transporting compounds are added, and if necessary, additives such as a binder resin that does not trap holes and additives such as a coating property improving agent such as a leveling agent are added and dissolved. The solution is prepared, applied on the conductive layer 2a by a method such as spin coating, and dried to form the organic hole transporting layer 3. Examples of the binder resin include polycarbonate, polyarylate, and polyester. Since the binder resin reduces the hole mobility if the amount added is large,
A smaller amount is desirable, and usually 50% by weight or less is preferred.

In the case of the vacuum evaporation method, the organic hole transporting material is put into a crucible installed in a vacuum vessel, and the inside of the vacuum vessel is evacuated to 10 ^-6 Torr by a suitable vacuum pump.
The crucible is heated to evaporate the hole transporting material, and the organic hole transporting layer 3 is formed on the substrate placed facing the crucible. The thickness of the hole transport layer 3 is usually 10 to 300 nm,
Preferably it is 30 to 100 nm. In order to uniformly form such a thin film, a vacuum deposition method is often used.

As a material for the hole transport layer 3, an inorganic material can be used instead of an organic compound. The conditions required for the inorganic material are the same as those for the organic hole transport compound. Examples of the inorganic material used for the hole transport layer 3 include p
Hydrogenated amorphous silicon, p-type hydrogenated amorphous silicon carbide, p-type hydrogenated microcrystalline silicon carbide, p-type zinc sulfide, p-type zinc selenide, and the like. These inorganic hole transport layers 3 are formed by a CVD method, a plasma CVD method, a vacuum evaporation method, a sputtering method, or the like.

The film thickness of the inorganic hole transporting layer 3 is, like the organic hole transporting layer 3, usually 10 to 300 nm, preferably 30 to 300 nm.
１００100 nm. An organic light emitting layer 4 is provided on the hole transport layer 3, and the organic light emitting layer 4 efficiently transfers electrons from the cathode between the electrodes to which an electric field is applied.
Formed from compounds that can be transported in the direction of

The compound used in the organic light emitting layer 4 needs to be a compound having high electron injection efficiency from the conductive layer 2b and capable of transporting the injected electrons efficiently. For this purpose, it is required that the compound has a high electron affinity, a high electron mobility, a high stability, and hardly generates impurities serving as traps during production or use. It is also required to emit blue light upon recombination of holes and electrons. Furthermore, providing a uniform thin film shape is also important in terms of device stability. The present inventors have found that an oxazole metal complex is suitable as a material satisfying such conditions.

In the present invention, stable blue light emission characteristics can be obtained by incorporating the oxazole metal complex represented by the general formula (I) into the organic light emitting layer 4 of the organic electroluminescent device. In the general formula (I), R ¹ to R ⁸ are preferably each independently a halogen atom such as a hydrogen atom, a chlorine atom, a bromine atom, or an iodine atom; An aralkyl group such as a benzyl group and a phenethyl group; a cyano group, an amino group, a dimethylamino group; and a carbon number of 1 to 6 such as a methoxycarbonyl group and an ethoxycarbonyl group.
Carboxyl group; alkoxy group having 1 to 6 carbon atoms such as methoxy group and ethoxy group; aromatic hydrocarbon group such as phenyl group, naphthyl group, acenaphthyl group and anthryl group; pyridyl group, quinolyl group and thienyl And aromatic heterocyclic groups such as a carbazole group, an indolyl group and a furyl group. Substituents for these aromatic hydrocarbon groups or aromatic heterocyclic groups include alkyl groups having 1 to 6 carbon atoms such as methyl group and ethyl group; lower alkoxy groups such as methoxy group; phenoxy group, trioxy group and the like. Aryloxy groups such as benzyloxy group; aryl groups such as phenyl group and naphthyl group; substituted amino groups such as dimethylamino group. Particularly preferably, a halogen atom such as a hydrogen atom and a chlorine atom, an alkyl group having 1 to 6 carbon atoms, and an alkoxy group having 1 to 6 carbon atoms are selected.

In the general formula (I), M is
Preferably, it is selected from beryllium, zinc, cadmium, aluminum, gallium and indium. n depends on the valence of the metal atom, and in the case of a divalent metal, 2,
In the case of a trivalent metal, it is 3. The oxazole metal complex represented by the general formula (I) is synthesized by a complex formation reaction between a corresponding metal salt and an oxazole compound represented by the following general formula (II).

[0024]

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The oxazole compound represented by the general formula (II) is described, for example, in J. Am. Chem. Soc. Perk
in I, p. 1291, 1976. Preferred specific examples of the oxazole compound represented by the general formula (II) are shown in the following Tables 1 to 4, but are not limited thereto.

[0026]

[Table 1]

[0027]

[Table 2]

[0028]

[Table 3]

[0029]

[Table 4]

Examples of the metal salt which forms a complex with the oxazole compound represented by the general formula (II) include halides such as chlorides and bromides, sulfates, nitrates and the like.
Complex formation reactions are described, for example, in "Fluorescence and UV absorption analysis",
It is carried out by the method shown in Kyoritsu Shuppan, p. 43, 1965, and is usually obtained as a precipitate from a water-alcohol solution of an oxazole compound and a metal salt.

The thickness of the organic light emitting layer 4 is usually 10 to 20.
0 nm, preferably 30-100 nm. The organic light emitting layer 4 can be formed by the same method as that of the organic hole transport layer 3, but usually, a vacuum evaporation method is used. For the purpose of improving the luminous efficiency of the device and changing the luminescent color, for example, doping a fluorescent dye for laser such as coumarin using 8-hydroxyquinoline aluminum complex as a host material (J. Appl. Phys., Vol. 65) , 361
0, 1989). Also in the present invention, the light emitting characteristics of the device can be further improved by doping a blue fluorescent dye with 10 ^-3 to 10 mol% using the oxazole metal complex as a host material.

As a method for further improving the luminous efficiency of the organic electroluminescent device, it is conceivable to further laminate an electron transport layer 5 on the organic luminescent layer 4 (see FIG. 2). The compound used for the electron transport layer 5 is required to be capable of easily injecting electrons from the cathode and having a higher electron transport ability. As such an electron transport material,

[0033]

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[0034]

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Oxadiazole derivatives such as (Appl.
Phys. Lett. 55, 1489, 1989.
Year; Jpn. J. Appl. Phys. , 31 volumes, 18
12, 1992) or a system in which they are dispersed in a resin such as polymethyl methacrylate (Appl. Phys. Le).
tt. 61, 2793, 1992) or n-type hydrogenated amorphous silicon carbide, n-type zinc sulfide, n-type zinc selenide and the like. The thickness of the electron transport layer 5 is usually 5
２００200 nm, preferably 10-100 nm.

It is to be noted that the structure opposite to that of FIG. 1, that is, the conductive layer 2b, the organic light emitting layer 4, the hole transport layer 3, and the conductive layer 2a can be laminated on the substrate in this order. It is also possible to provide the organic electroluminescent device of the present invention between two substrates, at least one of which is highly transparent. Similarly, it is also possible to laminate in a structure opposite to that of FIG.

[0037]

Next, the present invention will be described in more detail with reference to Synthesis Examples and Examples, but the present invention is not limited to the following Examples unless it exceeds the gist thereof. Synthesis Example 3.6 g of zinc sulfate (heptahydrate) dissolved in 40 ml of water, and 2- (o-hydroxyphenyl) shown below
-Benzoxazole (compound (1) in Table 1)

[0038]

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A solution prepared by dissolving 4.1 g in 300 ml of ethyl alcohol was mixed, and reacted by stirring at 40 ° C. for 140 minutes. The pH of this mixed solution was 5. After completion of the reaction, 14 ml of aqueous ammonia was added to adjust the pH to 9, and a pale yellow precipitate was formed. After the precipitate was air-dried, sublimation purification was repeated three times to obtain 1.2 g of a pale yellow powder. The results of elemental analysis of this final purified product are shown below.

[0040]

TABLE 5 Molecular _{formula: Zn (C 13 H 8 NO} 2) 2 Calculated [%] C: 64.28 H: 3.32 N: 5.77 O: 1
3.17 Zn: 13.46 Analysis value [%] C: 64.49 H: 3.20 N: 5.80 O: 1
3.20 Zn: 13.3 The structure of this oxazole zinc complex (E1) is shown below.

[0041]

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Example 1 A glass substrate was ultrasonically washed with acetone, washed with pure water, ultrasonically washed with isopropyl alcohol, dried with dry nitrogen, and dried.
After performing the V / ozone cleaning, the apparatus was set in a vacuum evaporation apparatus, and evacuated using an oil diffusion pump until the degree of vacuum in the apparatus became 2 × 10 ⁻⁶ Torr or less.

The oxazole zinc complex (E1) prepared in the synthesis example was placed in a ceramic crucible and heated by a tantalum wire heater around the crucible to perform vapor deposition. At this time, the temperature of the crucible was controlled in the range of 180 to 200 ° C. The degree of vacuum during the deposition was 2 × 10 ⁻⁶ Torr, and the deposition time was 3 minutes 4
In 0 seconds, a uniform and transparent film having a thickness of 78 nm was obtained. This deposited film was uniform and no crystallization was observed even after storage for 183 days in a vacuum. Comparative Example 1 An evaporated film was produced in the same manner as in Example 1 except that tetraphenylbutadiene was used instead of the oxazole zinc complex (E1). A cloudy, non-transparent thin film was obtained from the beginning. Comparative Example 2 Example 1 except that pentaphenylcyclopentadiene was used instead of the oxaazole zinc complex (E1).
A vapor deposition film was produced in the same manner as described above. A cloudy, non-transparent thin film was obtained from the beginning. Example 2 An organic electroluminescent device having the structure shown in FIG. 1 was produced by the following method.

A transparent conductive film of indium tin oxide (ITO) having a thickness of 120 nm deposited on a glass substrate is subjected to ultrasonic cleaning with acetone, water cleaning with pure water, ultrasonic cleaning with isopropyl alcohol, drying with dry nitrogen, UV / ozone. After the cleaning, the apparatus was set in a vacuum evaporation apparatus, and evacuated using an oil diffusion pump until the degree of vacuum in the apparatus became 2 × 10 ⁻⁶ Torr or less.

As an organic hole transport layer material, N, N'-diphenyl-N, N '-(3-methylphenyl) -1,1'-biphenyl-4,4'-diamine represented by the following structural formula: (H1)

[0046]

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Was put in a ceramic crucible, and heated by a tantalum wire heater around the crucible to perform vapor deposition. At this time, the temperature of the crucible was controlled in the range of 160 to 170 ° C. The degree of vacuum at the time of vapor deposition is 1.5 to 1.6 × 10 ⁻⁶ Torr.
Thus, an organic hole transport layer 3 having a thickness of 60 nm was obtained in a deposition time of 3 minutes and 10 seconds. Next, on the organic hole transport layer 3, the oxazole zinc complex (E1) shown in the synthesis example was vapor-deposited in the same manner to form the organic light emitting layer 4. At this time, the temperature of the crucible was controlled in the range of 180 to 200 ° C. The degree of vacuum at the time of vapor deposition was 1.3 to 1.2 × 10 ⁻⁶ Torr, the vapor deposition time was 3 minutes and 10 seconds, and the film thickness was 75 nm.

Finally, as a cathode, a combination of magnesium and silver
A gold electrode is deposited in a thickness of 150 nm by a binary simultaneous vapor deposition method.
did. Vapor deposition was performed using a molybdenum boat and the degree of vacuum was 4 ×
10 ^-6Torr, deposition time: 4 minutes 20 seconds, glossy film
was gotten. The atomic ratio of magnesium to silver is 10: 1.5
Met. The organic electroluminescent device thus manufactured
Plus magnesium and silver alloy electrode for ITO electrode (anode)
When a negative DC voltage is applied to the pole (cathode),
Device emits uniform blue light, and the peak wavelength of the light is
It was 480 nm.

Table 5 shows the results of the luminescence characteristics immediately after the production of the above-mentioned device and after storage in a vacuum for a long period of time. No remarkable rise in the driving voltage was observed, and no decrease in luminous efficiency was observed, and stable storage stability of the device was obtained.

[0050]

[Table 6]

[0051]

According to the organic electroluminescent device of the present invention, an anode, a hole transport layer, an organic light emitting layer, and a cathode are sequentially provided on a substrate,
Moreover, since the organic light emitting layer contains a specific compound, stable blue light emission characteristics can be obtained over a long period of time when a voltage is applied using both conductive layers as electrodes.

Accordingly, the organic electroluminescent device of the present invention can be used as a light source (for example, a light source of a copier, a liquid crystal display, or the like) in the field of a flat panel display (for example, for an OA computer or a wall-mounted television) or as a surface illuminant. And backlights for instruments and the like, display boards, and marker lights, and their technical value is extremely large.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view showing an example of the organic electroluminescent device of the present invention.

FIG. 2 is a schematic sectional view showing another example of the organic electroluminescent device of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Substrate 2a, 2b Conductive layer 3 Hole transport layer 4 Organic light emitting layer 5 Electron transport layer

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-6-322362 (JP, A) JP-A-6-2311881 (JP, A) JP-A-6-215874 (JP, A) JP-A-6-215874 33050 (JP, A) JP-A-5-331460 (JP, A) JP-A-5-17764 (JP, A) JP-A-5-214335 (JP, A) JP-A-4-298596 (JP, A) JP-A-60-211340 (JP, A) JP-A-59-40150 (JP, A) JP-A-6-172751 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C09K 11/06 H05B 33/14 REGISTRY (STN) CA (STN)

Claims (3)

(57) [Claims] 1. An organic electroluminescent device in which an anode, a hole transport layer, an organic light emitting layer, and a cathode are sequentially laminated, wherein the organic light emitting layer is represented by the following general formula (I): (Wherein, R ¹ to R ⁸ each independently represent a hydrogen atom, a halogen atom, an alkyl group, an aralkyl group, an alkenyl group, an allyl group, a cyano group, an amino group, an amide group, an alkoxycarbonyl group, a carboxyl group, An alkoxy group, an aromatic hydrocarbon group which may have a substituent or an aromatic heterocyclic group which may have a substituent, and M is beryllium, zinc, cadmium, aluminum, gallium, indium, scandium , Yttrium, magnesium, calcium, strontium, cobalt, copper or nickel, and n is an integer of 1 to 3). 2. The organic light emitting layer further contains a fluorescent dye.
The organic electroluminescent device according to claim 1, wherein 3. An electron transport layer between an organic light emitting layer and a cathode.
The organic electroluminescent device according to claim 1, wherein the device has: