JPH06336586A - Organic electroluminescent device - Google Patents

Abstract

PURPOSE:To provide the subject device which can maintain stable blue-emitting characteristics over a long term. CONSTITUTION:This device is formed by successively laminating an anode, a hole transport layer, an org. luminescent layer, and a cathode to provide the organic luminescent layer which contains an oxazole metal complex of the formula (wherein R<1> to R<2> are each independently H, halogen, alkyl, aralkyl, alkenyl, allyl, cyano, amino, amide, alkoxycarbonyl, carboxyl, alkoxy, an optionally substd. arom. hydrocarbon group, or an optionally substd. arom. heterocyclic group; M is Be, Zn, Cd, Al, Ga, In, Sc, Y, Mg, Ca, Sr, Co, Cu, or Ni; and (n) is an integer of 1-3).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an organic electroluminescence device, and more particularly to a thin film type 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 II-VI group compound semiconductor of an inorganic material has been used.
It is general that S, CaS, SrS, etc. are doped with Mn or a rare earth element (Eu, Ce, Tb, Sm, etc.), which is the emission center, but the EL element made from the above inorganic material is 1). AC drive is required (50 to 1000 Hz), 2) high drive voltage (up to 200 V), 3) full colorization is difficult (especially blue is a problem), and 4) peripheral drive circuit costs are high. ing.

However, in recent years, in order to improve the above problems,
EL devices using organic thin films have been developed. 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 light emission efficiency, and the organic hole transport layer made of an aromatic diamine and the organic light emission made of an aluminum complex of 8-hydroxyquinoline. Of an organic electroluminescent device having a layer (Appl. Phy
s. Lett. , 51, p. 913, 1987), the luminous efficiency is significantly improved as compared with the conventional electroluminescent device using a single crystal such as anthracene.

One of the characteristics of the organic electroluminescent device is that blue light emission can be realized. As an organic material exhibiting blue electroluminescence, anthracene (Jpn. J. App
l. Phys. , 27, L269, 1988),
Tetraphenyl butadiene, pentaphenyl cyclopentadiene (Appl. Phys. Lett., Vol. 56,
799, p. 799), distyrylbenzene derivative (Chemical Society of Japan, p. 1162, 1992), styrylamine-containing polycarbonate (Appl. Phys. L).
ett. , 61, 2503, 1992), oxadiazole derivative (Jpn. J. Appl. Phy.
s. 31: 1812, 1992; Chemical Society of Japan, 1540, 1991), zinc azomethine complex (Jpn. J. Appl. Phys., 32, L51).
1 page, 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 emitting blue light is often simple, the molecular weight becomes small and the thin film is formed. 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 brightness cannot be obtained. 3) Interaction with the hole transport material to form an exciplex. The problems are that electroluminescence has a long wavelength, and 4) the device has a short life when driven.

Due to the above-mentioned reasons, many problems are present in the practical application of the blue light emitting organic electroluminescent device.

[0007]

In view of the above situation, the inventors of the present invention have made earnest studies as a result of providing an organic electroluminescent device capable of maintaining stable blue light emission characteristics for a long period of time. It has been found that it is suitable to contain an oxazole metal complex in the light emitting layer, and the present invention has been completed.

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, and the organic light emitting layer has the following general formula (I).

[0009]

[Chemical 2]

(In the formula, R ¹ to R ⁸ are each independently 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 represents beryllium, zinc, cadmium, aluminum, gallium, indium, An organic electroluminescent device is characterized by containing an oxazole metal complex represented by scandium, yttrium, magnesium, calcium, strontium, cobalt, copper or nickel, and n is an integer of 1 to 3).

The organic electroluminescent device of the present invention will be described below with reference to the drawings. FIG. 1 is a cross-sectional view schematically showing a structural example of an organic electroluminescence device of the present invention, where 1 is a substrate, 2a and 2b are conductive layers, 3 is a hole transport layer, and 4 is an organic light emitting layer. . The substrate 1 serves as a support for the organic electroluminescent element of the present invention, and a plate of quartz or glass, a metal plate or metal foil, a plastic film or sheet, or the like is used, but a glass plate, polyester, polymeta A transparent synthetic resin substrate such as acrylate, polycarbonate or polysulfone is preferable.

A conductive layer 2a is provided on the substrate 1,
The conductive layer 2a is usually aluminum, gold,
Metals such as silver, nickel, palladium and tellurium, metal oxides such as indium and / or tin oxides 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, but metal fine particles such as silver, copper iodide, carbon black, conductive metal oxide fine particles, conductive polymer fine powder, etc. In this case, it can also be formed by dispersing it 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 directly formed on the substrate by electrolytic polymerization or can be formed by coating on the substrate (Appl. Phys. L).
ett. , 60, 2711, 1992).

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

In the example of FIG. 1, the conductive layer 2a plays a role of hole injection as an anode. on the other hand,
The conductive layer 2b plays a role of injecting electrons into the organic light emitting layer 4 as a cathode. The material used for the conductive layer 2b can be the material for the conductive layer 2a, but a metal having a low work function is preferable for efficient electron injection, 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
It is similar to the conductive layer 2a. Although not shown in FIG. 1, a substrate similar to the substrate 1 may be further provided on the conductive layer 2b. However, as the EL element, the conductive layer 2a is used.
At least one of the conductive layer 2b and the conductive layer 2b needs to have good transparency. From this, it is preferable that either one of the conductive layer 2a and the conductive layer 2b has a film thickness of 10 to 500 nm, and it is desired that the film has good transparency.

The hole transport layer 3 is provided on the conductive layer 2a. As the hole transport material, the hole injection efficiency from the conductive layer 2a is high, and the injected holes are efficiently transported. It must be a material that can be. For that purpose, it is required that the ionization potential is small, the hole mobility is large, the stability is excellent, and the impurities serving as traps are not easily generated during manufacturing or use.

Examples of such hole transport compounds include, for example, JP-A-59-194393, US Pat.
175,960, U.S. Pat. No. 4,923,774 and U.S. Pat. No. 5,047,687, N,
N'-diphenyl-N, N '-(3-methylphenyl)
-1,1'-biphenyl-4,4'-diamine: 1,
Aromatic amine compounds such as 1'-bis (4-di-p-tolylaminophenyl) cyclohexane: 4,4'-bis (diphenylamino) quadrophenyl, JP-A 2-3
The hydrazone compound shown in 11591 gazette, the silazane compound shown by US Patent 4,950,950, a quinacridone compound, etc. are mentioned. These compounds may be used alone, or may be mixed and used if necessary. In addition to the above compounds, polyvinylcarbazole and polysilane (Appl. Phys. Lett.,
59, page 2760, 1991) and the like.

The hole transport layer 3 can be formed by stacking the above organic hole transport material on the conductive layer 2a by a coating method or a vacuum deposition method. In the case of the coating method, one or more organic hole transport compounds are added, and if necessary, a binder resin that does not become a hole trap and an additive such as a coating improver such as a leveling agent are added and dissolved. A solution is prepared, coated on the conductive layer 2a by a method such as spin coating, and dried to form the organic hole transport layer 3. Examples of the binder resin include polycarbonate, polyarylate, polyester and the like. If the binder resin is added in a large amount, it lowers the hole mobility.
A smaller amount is desirable, and usually 50% by weight or less is preferable.

In the case of the vacuum deposition method, the organic hole transporting material is placed in a crucible installed in a vacuum container, the interior of the vacuum container is evacuated to 10 ^-6 Torr by an appropriate vacuum pump,
The crucible is heated to evaporate the hole transport material, and the organic hole transport 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,
It is preferably 30 to 100 nm. In order to uniformly form such a thin film, the vacuum evaporation method is often used.

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

The thickness of the inorganic hole transport layer 3 is usually 10 to 300 nm, preferably 30 as in the organic hole transport layer 3.
-100 nm. The 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 of a compound that can be transported in the direction of.

The compound used for the organic light emitting layer 4 needs to be a compound having a high electron injection efficiency from the conductive layer 2b and capable of efficiently transporting the injected electrons. For that purpose, it is required that the compound has a high electron affinity, a high electron mobility, excellent stability, and an impurity that becomes a trap and is less likely to be generated at the time of production or use. It is also required to play a role of emitting blue light when the holes and the electrons are recombined. Further, it is important to provide a uniform thin film shape from the viewpoint 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, by containing the oxazole metal complex represented by the above general formula (I) in the organic light emitting layer 4 of the organic electroluminescent device, stable blue light emitting characteristics can be obtained. In the general formula (I), R ¹ to R ⁸ are each independently preferably a halogen atom such as a hydrogen atom, a chlorine atom, a bromine atom or an iodine atom, or a carbon number of 1 to 1 such as a methyl group or an ethyl group. 6 alkyl group; aralkyl group such as benzyl group and phenethyl group; cyano group, amino group, dimethylamino group; carbon number 1 to 6 such as methoxycarbonyl group, ethoxycarbonyl group
Alkoxycarbonyl 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, anthryl group; pyridyl group, quinolyl group, thienyl And aromatic heterocyclic groups such as groups, carbazole groups, indolyl groups and furyl groups. Substituents for substituting 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, etc. And aryloxy groups such as benzyloxy group; aryl groups such as phenyl group and naphthyl group; and 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 or indium. n varies depending on the valence of the metal atom, and in the case of a divalent metal, 2,
It is 3 in the case of trivalent metal. 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]

[Chemical 3]

The oxazole compound represented by the above general formula (II) is described in, for example, J. Chem. Soc. Perk
in I, page 1291, 1976. Specific preferred examples of the oxazole compound represented by the general formula (II) are shown in Tables 1 to 4 below, but are not limited thereto.

[0026]

[Table 1]

[0027]

[Table 2]

[0028]

[Table 3]

[0029]

[Table 4]

Examples of the metal salt forming a complex with the oxazole compound represented by the general formula (II) include halides such as chlorides and bromides, sulfates, nitrates and the like.
The complex formation reaction is performed, for example, by "fluorescence / ultraviolet absorption analysis",
Kyoritsu Shuppan, page 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.
It is 0 nm, preferably 30 to 100 nm. The organic light emitting layer 4 can also be formed by the same method as the organic hole transport layer 3, but a vacuum deposition method is usually used. For the purpose of improving the luminous efficiency of the device and changing the luminescent color, for example, by doping an aluminum complex of 8-hydroxyquinoline as a host material with a fluorescent dye for laser such as coumarin (J. Appl. Phys., Vol. 65). , 361
Page 0, 1989). Also in the present invention, by using the above oxazole metal complex as a host material and doping with a blue fluorescent dye in a concentration of 10 ⁻³ to 10 mol%, the emission characteristics of the device can be further improved.

As a method for further improving the luminous efficiency of the organic electroluminescent device, it is conceivable to further stack the electron transport layer 5 on the organic light emitting 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 further having a high electron transporting ability. As such an electron transport material,

[0033]

[Chemical 4]

[0034]

[Chemical 5]

Oxadiazole derivatives such as (Appl.
Phys. Lett. , 55, 1489, 1989
Year; Jpn. J. Appl. Phys. , Volume 31, 18
P. 12, 1992) or a system in which they are dispersed in a resin such as polymethylmethacrylate (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
˜200 nm, preferably 10 to 100 nm.

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

[0037]

EXAMPLES Next, the present invention will be explained more specifically with reference to synthesis examples and examples, but the present invention is not limited to the description of the following examples unless it exceeds the gist. Synthetic Example Zinc sulfate (heptahydrate) 3.6 g dissolved in 40 ml of water, and 2- (o-hydroxyphenyl) shown below
-Benzoxazole (Compound (1) in Table-1)

[0038]

[Chemical 6]

A solution prepared by dissolving 4.1 g in 300 ml of ethyl alcohol was mixed and stirred at 40 ° C. for 140 minutes to react. The pH of this mixed solution was 5. After the reaction was completed, 14 ml of ammonia water was added to adjust the pH to 9, and a pale yellow precipitate was produced. The precipitate was air-dried and then subjected to sublimation purification three times to obtain 1.2 g of a pale yellow powder. The elemental analysis results 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 Analytical 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]

[Chemical 7]

Example 1 A glass substrate was ultrasonically washed with acetone, washed with pure water, ultrasonically washed with isopropyl alcohol, and dried with dry nitrogen. U
After performing V / ozone cleaning, it was installed in a vacuum vapor deposition 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) produced in the synthesis example was placed in a ceramic crucible and heated by a tantalum wire heater around the crucible to carry out vapor deposition. The temperature of the crucible at this time was controlled in the range of 180 to 200 ° C. The degree of vacuum during vapor deposition is 2 × 10 ^-6 Torr, and the vapor deposition time is 3 minutes 4
A uniform and transparent film having a film thickness of 78 nm was obtained in 0 second. This vapor-deposited film was uniform even after storage in vacuum for 183 days, and no crystallization was observed. Comparative Example 1 A vapor deposition 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 film was obtained from the beginning. Comparative Example 2 Example 1 except that pentaphenylcyclopentadiene was used instead of the oxazole zinc complex (E1).
A vapor-deposited film was prepared in the same manner as in. A cloudy, non-transparent 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.

An indium tin oxide (ITO) transparent conductive film having a thickness of 120 nm deposited on a glass substrate is ultrasonically cleaned with acetone, washed with pure water, ultrasonically cleaned with isopropyl alcohol, dried with dry nitrogen, and UV / ozone. After the cleaning, it was installed in a vacuum vapor deposition apparatus and evacuated using an oil diffusion pump until the degree of vacuum in the apparatus became 2 × 10 ⁻⁶ Torr or less.

As the 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]

[Chemical 8]

[0048] The sample was placed in a ceramic crucible and heated by a tantalum wire heater around the crucible for vapor deposition. The temperature of the crucible at this time was controlled in the range of 160 to 170 ° C. The degree of vacuum during vapor deposition is 1.5 to 1.6 × 10 ^-6 Torr
Then, an organic hole transport layer 3 having a film thickness of 60 nm was obtained with a vapor deposition time of 3 minutes and 10 seconds. Next, the oxazole zinc complex (E1) shown in the synthesis example was vapor-deposited on the organic hole transport layer 3 in the same manner to form the organic light emitting layer 4. The temperature of the crucible at this time was controlled in the range of 180 to 200 ° C. The degree of vacuum during 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 mixture of magnesium and silver
Gold electrode is vapor-deposited with a film thickness of 150 nm by the two-source simultaneous vapor deposition method.
did. Vapor deposition uses a molybdenum boat and the degree of vacuum is 4 ×
10 ^-6Torr, vapor deposition time is 4 minutes and 20 seconds, and it is a glossy film.
was gotten. The atomic ratio of magnesium to silver is 10: 1.5.
Met. Of the organic electroluminescent device produced in this manner
Plus ITO electrode (anode), magnesium / silver alloy electrode
If a negative DC voltage is applied to the pole (cathode),
Element emits a uniform blue light, and the peak wavelength of the light emission is
It was 480 nm.

Table 5 shows the results of the emission characteristics immediately after the above device was manufactured and after the device was stored in a vacuum for a long period of time. No significant increase in driving voltage was observed, no decrease in luminous efficiency was observed, and stable storage stability of the device was obtained.

[0050]

[Table 6]

[0051]

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

Therefore, the organic electroluminescent device of the present invention is a light source (for example, a light source for a copying machine, a liquid crystal display) which makes use of the characteristics of a flat panel display (for example, for OA computers or wall-mounted televisions) and a surface light emitter. Light sources for backlights of instruments and instruments), display boards, and sign lights are considered, and their technical value is extremely large.

[Brief description of drawings]

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

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

[Explanation of symbols]

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

Claims (1)

[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, and the organic light emitting layer has the following general formula (I): (In the formula, R ¹ to R ⁸ are each independently 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, where n is an integer from 1 to 3). An organic electroluminescent device characterized by the above.