JPH0446313B2 - - 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 in which at least one electroluminescent organic compound having a relatively different electronegativity is arranged in a highly ordered molecular orientation. (Conventional technology) Conventional EL elements are made of Mn, Cu or ReF ₃
ZnS containing (Re; rare earth ion) etc. as a deactivating agent
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. , ferroelectric liquid crystals, and other organic materials 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 small, and the probability of excitation of functional molecules due to carrier recombination is low. 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 has a two-layered light-emitting layer, and at least one layer sandwiching the light-emitting layer, which is transparent.
In an EL device consisting of a layer of electrode layers, the first
The light-emitting layer consists of a monomolecular film or a cumulative film thereof made of at least one electroluminescent organic compound that is electron-accepting relative to the second light-emitting layer,
The above-mentioned EL, wherein the second light-emitting layer is made of a monomolecular film or a cumulative film thereof made of at least one electroluminescent organic compound that is electron-donating relative to the first light-emitting layer. It is element. 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,
Mention may be made of 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 having a quinone structure, such as tetracyanoquinodimethane and tetracyanoethylene, can be mentioned. Particularly useful compounds in the present invention are those obtained by chemically modifying the electroluminescent compounds as described above by a known method as required, and having at least one compound in the structure.
A compound having at least one hydrophobic moiety and at least one hydrophilic moiety (both of which are in a relative sense), such as a compound represented by the following general formula () and Includes 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, or a phosphoric acid group. base,
silicic acid group, 1st to 3rd amino group; metal salts thereof,
Primary to tertiary amine groups, 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. Preferred examples of the compound φ of the general formula () and other compounds are as follows. 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, when one or more of the above-mentioned compounds are employed as the luminescent compound for forming the first luminescent layer, the luminescent properties of these employed Compound refers to the luminescent compound having a different electronegativity as a second compound.
may be selected as the compound for forming the light emitting layer. 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-donating compounds. Heterocyclic compounds are the main ones, and electron-accepting compounds are mainly 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, that is, the electrode layers that sandwich the light emitting layer, can use any conventionally known elements, but at least one layer must be transparent. There is. As the transparent electrode, any desired transparent electrode layer can be used, and preferred examples include transparent synthetic resins such as polymethyl methacrylate and polyester, transparent films or sheets such as glass, and indium oxide on the surface. , tin oxide,
It is coated entirely or in a pattern with a transparent conductive material such as indium tin oxide (ITO). If opaque electrodes are used on one side, these opaque electrodes may be of conventionally known type, and are generally and preferably have a thickness of about 0.1 mm.
It is a vapor deposited film of aluminum, silver, gold, etc. with a thickness of ~0.3 μm. In addition, the shape of the transparent electrode or opaque electrode is
It may have any shape such as a plate, a belt, or a cylinder, and can be selected depending on the purpose of use. In addition, the thickness of the transparent electrode 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, it will become transparent. Problems may arise in terms of performance, lightness, compactness, etc. In the EL device of the present invention, electroluminescent compounds having relatively different electronegativities as described above are used separately between the two electrode layers as described above to form a luminescent layer consisting of two layers. It is characterized by being a monomolecular film or a cumulative film thereof, in which the molecules constituting the two-layer structure are arranged with highly ordered molecular orientation. In the present invention, a particularly preferred method for forming such a monomolecular film or a cumulative film thereof is the Langmuir project method (LB method).
It is. In this LB method, when a molecule with a structure that has a hydrophilic group in the molecule maintains an appropriate balance between the two (amphiphilic balance), the molecule is placed above the water surface with the hydrophilic group below. This is a method of forming a monomolecular film or a cumulative film thereof by utilizing the fact that the monomolecular layer becomes a monomolecular layer. Specifically, 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 allow the film material to aggregate. The conditions are controlled, the surface pressure is gradually increased, and a surface pressure suitable for producing a monomolecular film or a cumulative film 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 as a cumulative film having a desired degree of accumulation by repeating the above-described operations. 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 film formation is reversed between the pulling process and the dipping process, this method creates 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. is formed. 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 contrary, 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 the monomolecular film is transferred to the surface of the substrate by rotating the cylinder above the water surface of the substrate. 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 EL element of the present invention, a two-layer structure is formed using the above-mentioned material for forming a light-emitting layer, preferably by the above-mentioned LB method, between the two electrode layers, each comprising a compound having a different electronegativity. It can be obtained by doing. 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.As a result of various studies, the inventor of the present invention created a two-layer structure for the light-emitting layer, and each light-emitting layer It has been found that the performance of conventional EL devices can be significantly improved by forming a monomolecular film or a cumulative film thereof using compounds having different electronegativities as described above. One important embodiment of the present invention is an embodiment in which each luminescent layer is a monomolecular film made of the luminescent material. The EL device of this embodiment is prepared by first preparing a material that is electron-accepting relative to the other layers in a suitable organic solvent, such as chloroform, dichloromethane, dichloroethane, etc., at about 10 ^-4 to 10 ^-2 The solution is developed on an aqueous phase with an appropriate pH (e.g., pH about 1 to 8) that may contain various metal ions, and the solvent is removed by evaporation to form a monomolecular film. was formed and transferred onto one electrode substrate as the first layer by the LB method as described above, dried sufficiently, and then made to have an electron-donating property relative to the first layer formed in this way. A material, such as aluminum, is similarly transferred as a monomolecular film onto the surface of the first light-emitting layer, and then, for example, aluminum,
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 luminescent layer consisting of two monomolecular films of the EL device obtained in this way varies depending on the type of material used, but in general, a thickness of about 0.01 to 1 μm is suitable. be. Another important aspect is that at least one, preferably both, of the two layers constituting the light-emitting layer of the EL device of the present invention is a cumulative film of the above-mentioned monomolecular film. This embodiment can be obtained by using the above-mentioned LB method and by accumulating the above-mentioned monomolecular film by various methods up to the required number of layers. The thickness of the light-emitting layer of the EL device obtained in this way, that is, the cumulative number of monolayers, can be changed arbitrarily. suitable. Note that the adhesion between one or both of the electrode layers used as a substrate and the light-emitting layer is sufficiently strong in the LB method, and the light-emitting layer will not peel or fall off. In order to strengthen the adhesive force, 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, the adhesive force can be strengthened by adjusting various conditions such as the material for forming the luminescent layer, the pH of the water layer used, ion species, water temperature, monomolecular film transfer rate, or monomolecular film surface pressure. be able to. 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. The EL device of the present invention as described above has a light-emitting layer structure that is an ultra-thin film, has a high degree of molecular order and functionality necessary for the operation of the EL device, and has excellent light-emitting performance. It is. In addition, in terms of manufacturing, the thickness of the light emitting layer is uniform over a large area,
The EL element can be made without defects, and can be used at room temperature.
Since it can be produced under normal pressure or conditions close to it, it has the advantage that it is possible to use light-emitting functional materials that are relatively non-heat resistant. 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, various interactions between these two layers with different electronegativities are extremely easy, which is superior to the level that cannot be achieved with conventional technology. It exhibits excellent light emitting performance. In other words, by variously changing the difference in electronegativity between the first light-emitting layer and the second light-emitting layer, the light emission intensity can be improved or the light emission color can be changed arbitrarily, and its durability can be improved. Lifespan 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-forming properties for either layer, it is possible to obtain a light-emitting layer that is excellent in all of luminescent properties, film-forming properties, and film strength. The EL device of the present invention described above has excellent properties by applying electrical energy such as alternating current, pulse, 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 M Cdcl ₂ and adjusted to pH 6.5. Next, the above compound A was dissolved in chloroform (10 ^-3 mol/) and then developed on the above water phase. After the solvent chloroform was removed by evaporation, the surface pressure was increased (30 dyne/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), and the dye monomolecular film was transferred onto the substrate. A cumulative film was created. In this cumulative process, each time the substrate is pulled out of the water tank,
The water adhering to the substrate was removed by evaporation by leaving it for 30 minutes or more. Next, the monomolecular film remaining on the surface of the aqueous phase was completely removed, and the pyrene derivative newly dissolved in chloroform was added. was developed on the aqueous phase. By the same method as above,
Only new functional monolayers were formed on the surfaces of the monolayers and monolayers that had already been produced, and cumulative films were formed by stacking 5, 10, and 15 layers of monolayers. The substrate with the thin film formed as described above was placed in a vapor deposition tank, and the tank was once depressurized to a degree of vacuum of 10 ^-6 Torr, and then the degree of vacuum was adjusted to 10 ^-5 Torr, and the deposition rate was 20 Å/sec. , A1 was evaporated onto the thin film at a film thickness of 1500 Å to form a back electrode. As illustrated in Fig. 3, the produced EL element is sealed with a sealing glass, and then purified and degassed according to conventional methods.
Dehydrated silicone oil was injected into the seal to form four EL light emitting cells of the present invention. When an AC voltage of 10 V and 400 Hz was applied to these EL cells, EL light with a color unique to the dye was obtained. The evaluation results are shown in Table 1. 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. Example 2 By the same process as Example 1,

[expression] and An EL device of the present invention was fabricated by creating a pure hetero-cumulative film in which each of these was used as a monomolecular film, and using it as a light-emitting layer.
The evaluation results are shown in Table 1. Comparative Example 1 In Example 1, except that only Compound B was used as the luminescent compound and was used alone,
Otherwise, an EL element for comparison was obtained in the same manner as in Example 1.
In addition, evaluation was made in the same manner as in Example 1. The evaluation result is the first
Shown in the table. Comparative Example 2 A comparative EL device was obtained in the same manner as in Example 2, except that only Compound D was used as the light-emitting compound and a single layer was used. It was evaluated in the same way. The evaluation results are shown in Table 1.

【table】

[Table] Example 3 Instead of compounds A and B in Example 1,
Using the following compounds E and F, Otherwise, the EL devices of the present invention (however, the cumulative number of each was 10) were obtained in the same manner as in Example 1. When evaluated under the same conditions as in Example 1, the current density was 0.09 mA/cm ²
The brightness (Ft-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 an EL element, and FIG. 3 diagrammatically shows a cross section of the EL element 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 Consisting of a two-layered light emitting layer and two electrode layers sandwiching the light emitting layer, at least one of which is transparent.
In the EL element, the first light-emitting layer is
The second light emitting layer is composed of a monomolecular film made of at least one electroluminescent organic compound that is electron-accepting relative to the light emitting layer, or a cumulative film thereof, and the second light emitting layer is relatively to the first light emitting layer. The above-mentioned EL device is characterized in that it is composed of a monomolecular film made of at least one electroluminescent organic compound that is relatively electron-donating, or a cumulative film thereof.