JPH0446314B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to an EL device that uses electrical light emission, that is, EL, and more specifically, the light emitting layer has a two-layer structure, and each layer has a two-layer structure, and each layer has a two-layer structure. The present invention relates to an EL device comprising a thin film of at least one electroluminescent organic compound having relatively different electronegativities. (Conventional technology) Conventional EL elements are made of Mn, Cu or ReF ₃
ZnS containing (Re; rare earth ion) etc. as an activator
It consists of a light-emitting layer with a light-emitting base material, and the light-emitting layer is broadly classified into powder type EL and thin film type EL depending on the basic structure of the light emitting layer. Among devices that have been put into practical use, thin-film ELs generally have higher brightness than powder-type ELs, but because thin-film ELs form a light-emitting layer by vapor-depositing a light-emitting base material onto a substrate, they are not suitable for large-area devices. It has the drawbacks that it is difficult to manufacture and the manufacturing cost is very high. For this reason, powder-type EL, in which a light-emitting base material, ie, ZnS, is dispersed in an organic binder, which is most easily mass-produced and costs only a few tenths of the cost of thin-film devices, has attracted attention. Generally, in EL light emission, the thinner the thickness of the light emitting layer, the better the light emission characteristics. However, in the case of powder type EL, the luminescent base material is a discontinuous powder, so if the luminescent layer is thinned, pinholes are likely to occur in the luminescent layer, making it difficult to reduce the layer thickness. However, the major drawback is that sufficient brightness characteristics cannot be obtained. Recently, an improved device in which an intermediate dielectric layer made of a vinylidene fluoride polymer is disposed within the light-emitting layer of the powder type EL has been disclosed in Japanese Patent Application Laid-open No. 172891/1982, but it has not yet been reported. We have not yet achieved sufficient performance in terms of luminance, power consumption, etc. On the other hand, recently, research and development has been actively conducted to control the chemical structure and higher-order structure of organic materials to create new materials for optical and electronics. Organic materials such as devices and ferroelectric liquid crystals that are comparable to or superior to metals and inorganic materials have been announced. As described above, there is a demand for the development of functional organic materials as new functional materials that surpass inorganic materials, and a cumulative film of monomolecular layers of anthracene derivatives and pyrene derivatives, which have hydrophilic and hydrophobic groups in their molecules, is being used as an electrode substrate. An EL element formed above is proposed in Japanese Patent Laid-Open No. 52-35587. However, the brightness of those EL elements,
In addition, in the case of organic EL devices, the density of carrier electrons or holes is extremely low, and excitation of functional molecules due to carrier recombination, etc. has not yet been achieved in terms of power consumption, etc. The probability becomes very small, and efficient light emission cannot be expected. (Disclosure of the Invention) Therefore, an object of the present invention is to solve the above-mentioned drawbacks of the prior art, to provide a light emitting device with sufficiently high brightness even when driven at a low voltage, and to be inexpensive and easy to manufacture.
The purpose is to provide an EL element. The above object of the present invention has been achieved by forming a light emitting layer of an EL element by combining specific materials and having a specific configuration. That is, the present invention provides an EL device comprising a two-layer structure of a light emitting layer, a transparent electrode layer and a back electrode layer sandwiching the light emitting layer, in which the first light emitting layer faces the transparent electrode layer and the second light emitting layer faces the transparent electrode layer. The second light-emitting layer is made of a monomolecular film made of at least one electroluminescent organic compound that is electron-accepting relative to the second light-emitting layer, or a cumulative film thereof, and the second light-emitting layer faces the back electrode layer. and the above-mentioned EL device, comprising a film deposited by a resistance heating vapor deposition method or a CVD method, comprising at least one electroluminescent organic compound that is electron-donating relative to the first light emitting layer. It is. To explain the present invention in detail, the electroluminescent organic compound used in the present invention and which mainly characterizes the present invention has a high luminous quantum efficiency and also has a π electron system that is susceptible to external perturbation. It is a compound that can be electrically excited, for example, basically:
fused polycyclic aromatic hydrocarbon, p-terphenyl,
2,5-diphenyloxazole, 1,4-bis(2-methylstyryl)-benzene, xanthine,
Coumarin, acridine, cyanine dye, benzophenone, phthalocyanine and its metal complex, porphyrin and its metal complex, 8-hydroxyquinoline and its metal complex, organic ruthenium complex,
Examples include organic rare earth complexes and derivatives of these compounds. Furthermore, compounds that can serve as electron acceptors or electron donors for the above compounds include heterocyclic compounds other than those mentioned above and derivatives thereof, aromatic amines and aromatic polyamines, compounds with a quinone structure, and tetracyanoquinodimethane. and tetracyanoethylene. In the present invention, the compound useful for forming the first light-emitting layer is obtained by chemically modifying the electroluminescent compound as described above by a known method as necessary.
It is a compound that has at least one hydrophobic part and at least one hydrophilic part (both of which are in a relative sense) in its structure, for example, the following general formula ( ) and other compounds. [(X-R ₁ ) _n Z] _o -φ-R ₂ () In the above formula, X is a hydrogen atom, a halogen atom, an alkoxy group, an alkyl ether group, a nitro group; a carboxyl group, a sulfonic acid group, a phosphoric acid group. basis,
silicic acid group, 1st to 3rd amino group; metal salts thereof,
Primary to tertiary amine salts, acid salts; ester groups, sulfamide groups, amide groups, imino groups, quaternary amino groups and their salts, hydroxyl groups, etc.; R ₁ has 4 carbon atoms;
~30, preferably 10 to 25 alkyl groups, preferably linear alkyl groups; m is 1 or 2,
n is an integer from 1 to 4; Z is a direct bond or -
O-, -S-,

【formula】

【formula】

[Formula] is a linking group such as -CO-, -COO-, etc. (R ₃ is an arbitrary substituent such as a hydrogen atom, an alkyl group, or an aryl group); φ is the residue of an electroluminescent compound as exemplified later. R ₂ is a hydrogen atom or any other substituent like X; at least one of one or more X, φ and R ₂ is a hydrophilic moiety, and at least one is a hydrophobic part. Further, in the present invention, the organic compound useful for forming the second light emitting layer is selected from the same types of compounds as above, except for those chemically modified. Preferred examples of compounds of the general formula () useful for forming the first layer, basic skeletons of compounds useful for forming the second layer, and other compounds are as follows. (However, φ (basic skeleton) illustrated below is an alkyl group having 1 to 4 carbon atoms, an alkoxy group, an alkyl ether group, a halogen atom, a nitro group, a primary to tertiary amino group, a hydroxyl group, a carboxamide group, a sulfamide group. may have common substituents such as groups.) The above luminescent compounds can be used alone or as a mixture in each luminescent layer of the present invention. Note that these compounds are examples of preferred compounds, and as long as the same purpose is achieved,
Of course, other derivatives or other compounds may also be used. The present invention is characterized in that the luminescent compounds as described above are used separately in the first luminescent layer and the second luminescent layer of the EL device of the present invention, depending on their electronegativity. That is, since the above-mentioned luminescent compounds have different electronegativities, among them, the compound that is relatively electron-accepting is selected as the luminescent compound for forming the first luminescent layer. However, the light-emitting compound that is relatively electron-donating to these compounds may be selected as the second light-emitting layer forming compound.
Among such luminescent compounds, particularly preferable electron-donating compounds are those having electron-donating groups such as primary to tertiary amino groups, hydroxyl groups, alkoxy groups, alkyl ether groups, or nitrogen heterogeneous compounds. The main compounds are ring compounds, and the main electron-accepting compounds are compounds having electron-withdrawing groups such as carbonyl groups, sulfonyl groups, nitro groups, and quaternary amino groups. In the present invention, such luminescent compounds can be used alone or in combination in each luminescent layer. The other elements forming the EL element of the present invention, namely the transparent electrode layer and the back electrode layer, sandwich the light emitting layer, and any conventionally known elements can be used, but at least one layer is transparent. It must be. As the transparent electrode layer, any desired transparent electrode layer can be used, and preferred examples include transparent synthetic resins such as polymethyl methacrylate and polyester, and transparent films or sheets made of glass, etc. with oxidized material on the surface. It is coated entirely or in a pattern with a transparent conductive material such as indium, tin oxide, or indium-tin-oxide (ITO). When an opaque back electrode layer is used on one side, these opaque electrode layers may also be of conventionally known types, and are generally and preferably made of aluminum, silver, or gold with a thickness of about 0.1 to 0.3 μm. It is a vapor deposited film such as. In addition, the shape of the transparent electrode layer or back electrode layer may be plate-like, belt-like,
It may have any shape, such as a cylindrical shape, and can be selected depending on the purpose of use. Furthermore, the thickness of the transparent electrode layer is preferably about 0.01 to 0.2 μm; if the thickness is less than this range, the physical strength and electrical properties of the element itself will be insufficient, and if the thickness is more than the above range, There may be problems with transparency, lightness, compactness, etc. In the EL device of the present invention, a luminescent layer consisting of two layers is formed by separately using electroluminescent compounds having relatively different electronegativity as described above between the two electrode layers as described above. The first layer constituting the luminescent layer of the two-layer structure formed
The layer faces the transparent electrode layer and is a monomolecular film or a cumulative film thereof made of a compound that is electron-accepting relative to the second layer and arranged with highly ordered molecular orientation. The second layer faces the back electrode layer and is characterized in that it is a molecular deposited film made of a compound that is electron-donating relative to the first layer. In the present invention, a particularly preferred method for forming such a first layer monomolecular film or a cumulative film thereof is the Langmiur-Blodget method (LB method). In this LB method, when a molecule has a structure that has a hydrophilic group and a hydrophobic group within the molecule, and the balance between the two (balance of amphiphilicity) is maintained appropriately, the molecule will have a hydrophilic group and a hydrophobic group on the water surface. This is a method of forming a monomolecular film or a cumulative film thereof by utilizing the fact that the monomolecular layer is formed with the group facing downward. Specifically, in order to prevent the monomolecular film spread on the water layer from freely diffusing and spreading too much, a partition plate (or float) is provided to limit the spread area and control the aggregated state of the film material. is controlled, the surface pressure is gradually increased, and a surface pressure suitable for manufacturing a monomolecular film or a cumulative film thereof is set. By gently raising or lowering the clean substrate vertically while maintaining this surface pressure, the monolayer is transferred onto the substrate. Although a monomolecular film is manufactured in the above manner, a cumulative film of a monomolecular film is formed by repeating the above-described operations as a cumulative film having a desired degree of accumulation. In addition to the above-mentioned vertical dipping method, the monomolecular film can be transferred onto the substrate by methods such as a horizontal deposition method and a rotating cylinder method. The horizontal deposition method is a method in which the substrate is brought into horizontal contact with the water surface and transferred, and the rotating cylinder method is a method in which a cylindrical substrate is rotated on the water surface to transfer the monomolecular film onto the surface of the substrate. In the vertical immersion method described above, when a substrate with a hydrophilic surface is lifted out of water in a direction across the water surface, a monomolecular film with the hydrophilic groups of the molecules facing the substrate is formed on the substrate. When the board is moved up and down as described above, 1
The monolayers are layered one by one. Since the direction of the film-forming molecules is reversed between the pulling process and the dipping process, this method forms a Y-shaped film in which the hydrophilic groups of the molecules face each other, and the hydrophobic groups of the molecules face each other between the layers. be done. On the other hand, the horizontal adhesion method is a method in which a substrate is attached horizontally to the water surface and then transferred, and a monomolecular film with the hydrophobic groups of the molecules facing the substrate is formed on the substrate. In this method, even if monomolecular films are accumulated, there is no change in the direction of the film-forming molecules, and an X-shaped film is formed in which the hydrophobic groups face the substrate side in all layers. On the other hand, a cumulative film in which the hydrophilic groups in all layers face the substrate side is called a Z-type film. The rotating cylinder method is a method in which a monomolecular film is transferred to the basic surface by rotating the basic water surface of the cylinder method. The method of transferring the monomolecular film onto the substrate is not limited to these methods; in other words, when using a large-area substrate, a method of extruding the substrate from a substrate roll into a water layer may also be used. Furthermore, the directions of the hydrophilic groups and hydrophobic groups described above toward the substrate are in principle, and can be changed by surface treatment of the substrate. In the present invention, particularly preferred methods for forming the molecular deposition film constituting the second light emitting layer are resistance heating evaporation method and CVD method, for example,
With the vapor deposition method, a thin film of about 500 mm can be formed as the second light emitting layer. For example, when using the resistance heating evaporation method, the material is placed in a tungsten board placed in a vacuum chamber, placed at a distance of 30 cm or more from the substrate, resistively heated, and sublimable materials are set at the sublimation temperature, and fusible materials are set at the melting point. Vapor deposition is performed by setting the temperature above. Pre-vacuum degree is 2×
Reduce the temperature to below 10 ^-6 Torr, close the shutter before vapor deposition, heat the boat, let it air for about 2 minutes, then open the shutter and start vapor deposition. The speed during deposition is measured using a crystal oscillator film thickness monitor, but the preferred speed is 0.1.
It is carried out between Å/sec and 100 Å/sec. The degree of vacuum at that time is 10 ^-3 Torr or less to prevent oxidation etc.
This is preferably carried out by maintaining the temperature at about 10 ^-5 Torr. In the EL element of the present invention, a relatively electron-accepting compound is selected from the above-mentioned compounds as the first layer facing the transparent electrode layer among the two electrode layers as described above, and the EL element is formed by, for example, the LB method. , a monomolecular film or a cumulative film thereof is formed, and a relatively electron-donating compound is selected from the above compounds as the second layer facing the back electrode layer, and a molecular deposited film is formed by the method described above. This can be obtained by forming a light-emitting layer with a two-layer structure. As mentioned in the conventional technology section, EL is achieved by the LB method.
Although it is known to form an EL element, the known method cannot obtain an EL element with sufficient performance, and as a result of various studies, the inventor has determined that the light-emitting layer has a two-layer structure, and that the first layer has a two-layer structure. The light-emitting layer is formed as a monomolecular film or a cumulative film thereof using a compound that is relatively electron-accepting as described above, and the second layer is relatively electron-donating with respect to the first layer. By forming a molecular deposition film from a compound, it is possible to overcome the conventional technology.
It was discovered that the performance of EL elements was significantly improved. One embodiment of the present invention is an embodiment in which the first light-emitting layer is a monomolecular film made of the luminescent material. The EL device of this embodiment is prepared by first applying a material that is relatively electron-accepting to the second layer in a suitable organic solvent, such as chloroform, dichloromethane, or
It is dissolved in dichloroethane etc. to a concentration of about 10 ^-4 to ^{10 -2} M, and the solution is poured over an aqueous phase at an appropriate pH (for example, pH about 1 to 8) which may contain various metal ions. The solvent was removed by evaporation to form a monomolecular film, which was then transferred onto one of the transparent electrode substrates as the first layer using the LB method as described above. A second layer is formed as a deposited film using a material that is electron-donating relative to the formed first layer by the molecular deposition method described above, and then, for example, aluminum is deposited on the surface of this second layer. The back electrode layer is obtained by depositing an electrode material such as , silver, or gold, preferably by vapor deposition or the like. The thickness of the two-layer light emitting layer of the EL device thus obtained varies depending on the type of material used, but is generally preferably about 0.01 to 1 μm thick. Another important aspect is that the first layer constituting the light-emitting layer of the EL device of the present invention is a cumulative film of the above-mentioned monomolecular films. This embodiment can be obtained by using the LB method described above to accumulate monolayers as described above to the required number of layers by various methods, and then forming a second layer as described above. The thickness of the light-emitting layer of the EL device of the present invention obtained in this way can be changed arbitrarily, but
In the present invention, the cumulative number of monomolecular films in the first layer is approximately 4 to 200, and the thickness of the second layer is approximately 0.01 to 0.5 μm.
The thickness of the entire light emitting layer is preferably about 0.02 to 1 μm. Note that the adhesion between one or both electrode layers used as a substrate and the light-emitting layer is sufficiently strong in the LB method and molecular deposition method, and the light-emitting layer does not peel or fall off. However, in order to strengthen the adhesion, the surface of the substrate may be treated in advance, or a suitable adhesive layer may be provided between the substrate and the light emitting layer. Furthermore,
In the LB method, adhesion can also be improved by adjusting various conditions such as the type of material used to form the luminescent layer, the pH of the water layer used, ion species, water temperature, transfer rate of the monomolecular film, and surface pressure of the monomolecular film. power can be strengthened. The performance of the EL element formed as described above may deteriorate due to the influence of moisture and oxygen in the air, so conventionally known means can be used to make it moisture resistant.
It is desirable to have an oxygen-resistant sealed structure. In the EL device of the present invention as described above, the structure of the light emitting layer is an ultra-thin film, and the first layer has a high degree of molecular order and function necessary for the operation of the EL device, and The second layer and the first layer exhibit excellent light emitting performance through various electrical interactions. Furthermore, as schematically shown in FIG. 1, the light-emitting layer of the EL device of the present invention is different from the light-emitting layer of the prior art consisting of a single layer, as schematically shown in FIG. Since the light-emitting layer and the second light-emitting layer have a uniform interface, it is extremely easy to use these two layers with different electronegativities in various ways, which is superior to conventional techniques. It exhibits excellent light emitting performance. That is, the first light emitting layer and the second light emitting layer
By variously changing the difference in electronegativity between the light emitting layer and the light emitting layer, the emission intensity can be improved or the emission color can be arbitrarily changed, and the service life can also be significantly extended. Furthermore, although the conventional technology has excellent luminescence,
Materials with insufficient film-forming properties and film strength could not be practically used, but in the present invention, even materials with poor film-forming properties and film strength but excellent luminescence properties can be used.
By using a material with excellent film formability for either layer, it is possible to obtain a light emitting layer that is excellent in all of luminescent properties, film formability, and film strength. The EL device of the present invention described above has excellent properties by applying electrical energy such as alternating current or panel or direct current between the electrode layers so that electrical energy such as a suitable electric field acts on the light emitting layer.
This shows EL light emission. Next, the present invention will be explained in more detail with reference to Examples. Note that the words in the middle of the text are based on weight. Example 1 A transparent electrode was formed by depositing an ITO layer with a thickness of 1500 Å on the surface of a 50 mm square glass plate by sputtering. After thoroughly cleaning this film-forming substrate, Joyce
−Langmuir−Trough4 made by Loebel 4×
It was immersed in an aqueous phase containing 10 ^-4 mol of KI and adjusted to pH 6.5. next, The above compound (A) was dissolved in chloroform (10 mol ^-3 /) and then developed on the above water phase.
After the solvent chloroform was removed by evaporation, the surface pressure was increased (30 dync/cm) to precipitate the dye molecules in the form of a film. Then, while keeping the surface pressure constant, the film-forming substrate was gently moved up and down in the direction across the water surface (vertical speed 2 cm/min) to transfer the dye monomolecular film onto the substrate. A molecular cumulative film was created. In this cumulative process, each time the substrate was pulled out of the water tank, it was allowed to stand for 30 minutes or more to evaporate and remove moisture attached to the substrate. Next, anthracene (B) was vapor-deposited to a thickness of 500 Å on the transparent electrode substrate provided with the above monomolecular cumulative film using a resistance heating vapor deposition apparatus. This vapor deposition is
After reducing the pressure in the deposition tank to a vacuum level of 10 ^-6 Torr,
A vapor deposited film was formed by gradually raising the temperature of the resistance heating board (M ₀ ) and adjusting the current flowing through the board containing anthracene so that the vapor deposition rate was 5 Å/sec. The degree of vacuum during vapor deposition was 9×
It was 10 ^-6 Torr. Additionally, the temperature of the substrate holder was kept constant by circulating 20°C water. Finally, the thin film formed as described above is placed in a deposition tank, and the tank is once depressurized to a vacuum level of 10 ^-6 Torr, and then the vacuum level is adjusted to 10 ^-5 Torr to speed up the deposition process.
A1 was deposited on the thin film at 20 Å/sec to a thickness of 1500 Å to form a back electrode. Figure 3 shows the created EL device.
As illustrated in , after sealing with a sealing glass, purified, degassed, and dehydrated silicone oil was injected into the seal according to a conventional method to form an EL light emitting cell of the present invention. When 10V, 400Hz AC voltage was applied to these EL light emitting cells,
It exhibited EL emission with a brightness of 15 ft/L at a current density of 0.09 mA/cm ² . The EL device of the present invention described above has a lower driving voltage than the conventional EL device using ZnS as a light emitting matrix.
The EL element had good luminance characteristics. Comparative Example 1 A comparative example was prepared in the same manner as in Example 1 except that the second layer was not formed.
When an EL element was obtained and evaluated in the same manner as in Example 1, the luminance was 1 ft-L or less at a current density of 0.2 mA/cm ² . Example 2 Instead of compounds A and B in Example 1,
Using the following compounds C and D, The EL device of the present invention (however, the cumulative number was 6) was obtained in the same manner as in Example 1, and evaluated under the same conditions as in Example ¹ . L) was 18.

[Brief explanation of the drawing]

FIG. 1 schematically shows an EL device using the conventional LB method, and FIG. 2 shows an EL device according to the present invention.
This diagrammatically shows the device, and FIG. 3 diagrammatically shows a cross section of the EL device of the present invention. 1; transparent electrode, 2; light emitting layer, 3; back electrode,
4; Luminescent compound, 5; Luminescent compound, 6; Seal glass, 7; Silicone insulating oil, 8; Glass plate.

Claims (1)

[Claims] 1. In an EL device consisting of a two-layer light emitting layer, a transparent electrode layer and a back electrode layer sandwiching the light emitting layer, the first light emitting layer faces the transparent electrode layer,
and a monomolecular film made of at least one electroluminescent organic compound that is electron-accepting relative to the second light-emitting layer, or a cumulative film thereof, and the second light-emitting layer is formed of the above-mentioned back electrode layer. at least one electron-donating layer facing the first light-emitting layer and relatively electron-donating to the first light-emitting layer.
The above-mentioned EL device is characterized in that it is made of a film deposited by a resistance heating vapor deposition method or a CVD method, which is made of a certain electroluminescent organic compound.