JPS6137887A - El element - Google Patents

Abstract

PURPOSE:An inexpensive EL element with a good light emission efficiency by low-voltage driving, made by holding a light-emitting layer of an electron-acceptive, electroluminescent org. compd. and a light-emitting layer of an electron- donating electroluminescent org. compd. between two electrode layers. CONSTITUTION:A light-emitting layer 2, 0.01-1mum in thickness, comprising a first light-emitting layer 4 consisting of a monomolecular film or monomolecular cumulative film of an electroluminescent org. compd. such as a compd. of formula I which is more electron-acceptive than a second light-emitting layer, and the second light-emitting layer 5 consisting of a monomolecular film or monomolecular cumulative film of an electroluminescent org. compd. which is more electron-donating than the first light-emitting layer, are held between electrode layers 1 and 2 of which at least one is transparent.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to an EL element using electrical light emission, that is, EL, and more specifically, the present invention relates to an EL element using electrical light emission, that is, EL, and more specifically, the light emitting layer has a two-layer structure,
each layer comprises at least one electroluminescent organic compound having a different electronegativity relative to other adjacent layers;
E consists of a thin film arranged with highly ordered molecular orientation.
Regarding L elements.

(Prior art) Conventional EL elements are made of Mn, Cu or Re F3.
Zn containing (Re; rare earth ion) etc. as an activator
It consists of a light-emitting layer using S as a light-emitting base material, and is structurally 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, that is, 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 the powder type EL, since the luminescent base material is a discontinuous powder, pinholes are likely to occur in the luminescent layer when the luminescent layer is thinned.
It has a major drawback in that it is difficult to reduce the layer thickness, and therefore 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 JP-A-58-172891.
It has 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 piezoelectric elements, pyroelectric elements, nonradiometer optical elements, and ferroelectric liquid crystals, have been announced that are comparable to or superior to metals and inorganic materials. 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 a parent tree group and a hydrophobic group in the molecule, is being used as an electrode. The EL element formed on the substrate was disclosed in Japanese Patent Application Laid-Open No. 52-35587.
The proposal has been published in the official bulletin. However, these EL devices have not yet achieved sufficient performance as a real EL device in terms of brightness, power consumption, etc. Furthermore, in the case of organic EL devices, the density of carrier electrons or holes is extremely low. The probability of excitation of functional molecules due to carrier recombination or the like becomes extremely 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, and to provide an EL element that can emit light with sufficiently high brightness even when driven at a low voltage, is inexpensive, and is easy to manufacture. It is to provide.

The object of the present invention has been achieved by forming the light-emitting layer of an EL element by combining specific materials and forming it into a specific configuration.

That is, the present invention provides an EL device comprising a two-layered light-emitting layer and two electrode layers sandwiching the light-emitting layer, at least one of which is transparent. consisting of a monomolecular film or a cumulative film thereof comprising at least one electroluminescent organic compound that is electron-accepting relative to the layer, 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 made of a monomolecular film or a cumulative film thereof made of at least one electroluminescent organic compound having an electron-donating property.

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 luminescence quantum efficiency and is easily susceptible to external perturbation.
It is a compound that has an electronic system and can be electrically excited, and basically includes fused polycyclic aromatic hydrocarbons, p-terphenyl, 2.5-diphenyloxazole, 1,4-
Bis(2-methylstyryl)-benzene, xanthine,
Examples include coumarin, acridine, cyanine dyes, benzophenone, phthalocyanine and its metal complexes, porphyrin and its metal complexes, 8-hydroxyquinoline and its metal complexes, organic ruthenium complexes, 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.

Particularly useful compounds in the present invention are those obtained by chemically modifying the electroluminescent compound as described above by a known method if necessary, and having at least one hydrophobic moiety and at least one hydrophilic moiety in its structure. It is a compound having a sexual moiety (all of these are in a relative sense), and includes, for example, a compound represented by the following general formula (I) and other compounds.

[(X −R,), Z ]]? +-φ-R2I) or in the above formula, a hydrogen atom, /\rogen atom, alkoxy group, alkyl ether group, nitro group; carboxyl group, sulfonic acid group, phosphoric acid group, silicate group, 1st to 3rd amino acid group groups; metal salts thereof, primary to tertiary amine salts, acid salts; ester groups, sulfamide groups, amide groups, imino groups, quaternary amine groups and their salts, hydroxyl groups, etc.; R5 has 4 to 30 carbon atoms; , preferably 10 to 25 alkyl groups, preferably linear alkyl groups, and -m is 1 or -SO
Linking groups (R
3 is an arbitrary substituent such as a hydrogen atom, an alkyl group, or an aryl group; φ is a residue of an electroluminescent compound as exemplified later; R2 is a hydrogen atom or any other arbitrary substituent, like X at least one of one or more of X, φ and R2 is a hydrophilic moiety, and at least one is a hydrophobic moiety.

Preferred examples of the compound φ of the general formula (I) and other compounds are as follows.

(Margin below) Z=NH10, S Z=CO, NHZ=CO, N
u-I,0,Sz = NH,O,5 Z=NT(,0,S Z=N
H, 0, SZ= 8% Se Z= S,
Se Z= 8% 5eZ=NH,O
,S Z=NH1QS Z=NH,0,SM-M
g , Z n , S n i AZCZ M-Hz
i Be , Mg , Ca , CdSn, AtC M=A4 Gav Iri Teas a=3
M=Er, Sm, Eu1ken=Zn, Cdi Mgi
pb, a=2 Gds Tb, DyTm%y
b M=Er, Sm, Eut Gd M= Er
%Sm, EuTb, Dy, Tm, Yb
Gd, Tb, DyTm, yb Z=0. S, Se O≦p≦2 The above luminescent compounds can be used alone or as a mixture in each luminescent layer in the present invention. In addition,
These compounds are examples of preferred compounds, and it goes without saying that other derivatives or other compounds may be used as long as the same purpose is achieved.

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 As for the compound, the above-mentioned luminescent compounds having different electronegativities may be selected as the compound for forming the second luminescent 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 amide 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 two electrode layers sandwiching the light emitting layer, can be any conventionally known element, but at least one of the layers must be transparent. There is a need. 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, and transparent films or sheets made of glass, etc. with indium oxide on the surface. , tin oxide, indium-tin-oxide (ITo), etc., on the entire surface or in a pattern. When opaque electrodes are used on one side, these opaque electrodes may also be of conventionally known types, and are generally and preferably made of aluminum, silver, or aluminum having a thickness of approximately 0.1 to 0.3 gm.
It is a vapor-deposited film of gold, etc. Further, the shape of the transparent electrode or the opaque electrode may be any shape such as a plate, a belt, or a cylinder, and can be selected depending on the purpose of use. The thickness of the transparent electrode is preferably about 0.01 to 0.2 gm. 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, Otherwise, problems may arise in terms of transparency, light weight, compactness, etc.

In the EL element of the present invention, between the two electrode layers as described above,
It is obtained by forming a luminescent layer consisting of two layers using separately the electroluminescent compounds having relatively different electronegativities as described above, and the luminescent layer of the formed two-layer structure is composed of It is characterized by a monomolecular film or a cumulative film thereof, in which molecules 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-Prodgett method (LB method). In this LB method, when a molecule has a structure that has a hydrophilic group and a hydrophobic group in the molecule, and the balance between the two (balance of amphiphilicity) is maintained appropriately, the molecule is placed 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 functional groups facing downward. 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 gather. The conditions are 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 above 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 is formed on the substrate with molecules facing the hydrophilic substrate. When the substrate is moved up and down as described above, one monolayer layer is overlapped with each step. Since the direction of the film-forming molecules is reversed during the pulling process and the dipping process, in this method, between each layer, the woody groups and hydrophilic groups of the molecules, and the hydrophobic groups and hydrophobic groups of the molecules face each other. A Y-shaped film 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 the monolayer is 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 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 above-mentioned hydrophilic groups and hydrophobic groups toward the substrate are in principle, and can be changed by surface treatment of the substrate, etc.

In the EL device of the present invention, the light-emitting layer forming material as described above is preferably used by the LB method as described above, and two compounds having different electronegativities are formed between the two electrode layers as described above.
This can be obtained by forming it as a layered structure.

As mentioned in the prior art section, it is known to form an EL element by the LB method, but this known method does not allow for obtaining an EL element with sufficient performance, and the present inventor has conducted various research studies. As a result, the performance of conventional EL devices has been significantly improved by forming the light-emitting layer into a two-layer structure and forming each light-emitting layer as a monomolecular film or a cumulative film thereof using compounds with different electronegativities as described above. This is what we found out.

One important embodiment of the present invention is an embodiment in which each luminescent layer is a monomolecular film made of the luminescent material. In the EL device of this embodiment, first, a material that is electron-accepting relative to other layers is dissolved in an appropriate organic solvent such as chloroform or dichloromethane, and the solution is mixed with various metal ions. (e.g. pHv
Jl~8) was developed on the aqueous phase, the solvent was removed by evaporation to form a monomolecular film, and the film was transferred onto one electrode substrate as the first layer using the LB method as described above. dried and then the material which is relatively electron-donating to the first layer thus formed is transferred in the same manner as a single molecule onto the surface of the first emissive layer, and then The back electrode layer is obtained by depositing an electrode material such as aluminum, silver, gold, etc., preferably by vapor deposition, on the surface of the second layer.

The thickness of the luminescent layer consisting of two monomolecular films of the EL device thus obtained varies depending on the type of material used, but generally a thickness of about 0.01 to IJLm 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 accumulating monolayers as described above in various ways up to the required number of layers by using the LB method described above.

The thickness of the light-emitting layer of the EL device obtained in this way, that is, the cumulative number of single molecules, can be changed arbitrarily, but in the present invention, the cumulative number of single molecules is about 4 to 400 in total. number is preferred.

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 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 light-emitting layer, the pH of the water layer used, the ion species, the water temperature, the transfer rate of the monomolecular film, or the surface pressure of the monomolecular film. can.

The performance of the EL element formed as described above may deteriorate due to the influence of moisture and oxygen in the air if left as is, so it is desirable to create a moisture- and oxygen-proof hermetically sealed structure using conventionally known means. .

In the EL device of the present invention as described above, the structure of the light emitting layer is as follows:
It is an ultra-thin film, has a high degree of molecular order and functionality necessary for the operation of an EL device, and has excellent light-emitting performance. In addition, in terms of manufacturing, the thickness of one light-emitting layer is uniform over a large area, making it possible to create an EL device without defects.Also, it can be manufactured at room temperature, normal pressure, or similar conditions, so it is relatively heat-resistant. There is an advantage that even a neutral luminescent functional material can be used.

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 consisting of a single layer in the prior art, as schematically shown in FIG. light emitting layer and second
Since the light-emitting layer has a uniform interface, various interactions between these two layers with different electronegativity are extremely easy, and it exhibits excellent light-emitting performance that cannot be achieved with conventional technology. It is something to do. That is, 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 the service life can also be changed. It can be significantly extended.

Furthermore, in the conventional technology, it was virtually impossible to use materials that had excellent luminescence but insufficient film formability or film strength; however, in the present invention, materials with poor film formability or film strength could not be used. but,
Even if a material has excellent luminescence properties, by using a material with excellent film-forming properties for one of the layers, a light-emitting layer with excellent luminescence properties, film-forming properties, and film strength can be obtained.

The EL device of the present invention described above can exhibit excellent EL 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. It shows luminescence.

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-L
Langmuir-Trough4 manufactured by oebe1
4X10Mc7)Cdcu, p)16,5
The sample was immersed in a water phase containing water.

(A) Next, the above compound (A) was dissolved in chloroform (
10 mol/Jj) and then developed on the above water phase. After removing the solvent chloroform by evaporation, the surface pressure was increased (30 d
yne/c+s), the above dye molecules were precipitated into a film. Thereafter, while keeping the surface pressure constant, the film-forming substrate is gently moved up and down in the direction across the water surface (vertical speed 2 (:
m/ff1in), the dye monomolecular film was transferred onto a substrate, and a monomolecular cumulative film of 5.10 and 15 layers was created. In this accumulation process, each time the substrate was pulled out of the water tank, it was left to stand for 30 minutes or more to evaporate and remove the water adhering to the substrate.

Next, the monomolecular film remaining on the surface of the aqueous phase was completely removed, and a new pyrene derivative dissolved in chloroform was spread on the aqueous phase. By the same method as above, only a new functional monolayer was formed on the surface of the monolayer and the monolayer accumulated, which had already been produced, to form a cumulative film in which 5.10 and 15 layers of monomolecular films were accumulated.

The substrate having the thin film formed as described above was placed in a vapor deposition tank, and the nuclide was once depressurized to a vacuum level of 10 Torr, and then the vacuum level was adjusted to 10 Torr and the vapor deposition rate was set at 20^/sec.
At c, AI was deposited on the thin film to a thickness of 1500^ to form a back electrode. As illustrated in FIG. 3, the produced EL elements are sealed with a sealing glass, and then purified, degassed, and dehydrated silicone oil is injected into the seal according to the conventional method to form the four EL elements of the present invention. A light emitting cell was formed. When an alternating current 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・E of the above invention
The L element had a lower driving voltage and better luminance characteristics than the conventional EL element using ZnS as a light emitting matrix.

Example? By the same process as in Example 1, a pure hetero-cumulative film having each of these as a monomolecular film was prepared, and an EL device of the present invention was prepared using the following as a light-emitting layer. The evaluation results are shown in Fang IL.

Comparative Example 1 In Example 1, except that only Compound B was used as the luminescent compound and a single layer was used, the rest was Example 1.
A comparative EL device was obtained in the same manner as in Example 1, and evaluated in the same manner as in Example 1. The evaluation results are shown in Table 1.

Comparative Example 2 In Example 2, except that Compound Rino was used as the luminescent compound and a single layer was used, the other changes were as in Example 2.
A comparative EL device was obtained in the same manner as in Example 2, and evaluated in the same manner as in Example 2. The evaluation results are shown in Table 1.

(Left below) 1 Shiranno Cumulative level 111 ■ Eel ■ 1 Toyoho 1 1st layer 2nd layer Tomoe (Bou ■ 2 1 1 5V, 4
QOHz 1.5 0.85 5 10V,
400Hz 12 0.1510 10 10
V, 400Hz 1B 0.115 15
10V, 400Hz 18 0.09℃0 1 1 5V, 400Hz L, S O
, 5551QV, 400) 1z 1B 0.1
410 10 10V, 40GHz 1B
0.115 15 10V, 400Hz 18
0.08J■已2 5V, 400Hz 1 or less 10
10V, 400Hz tt20
1QV, 40QHz tt30
10V, 400Hz tt2
5V, 400H21 or less 10 10V, 400Hz
tt20 10V, 400Hz
tt30】OV, 400Hz
tt Example 3 The following compounds E and F were used in place of compounds A and B in Example 1, and the other conditions were the same as in Example 1, to prepare an EL device of the present invention (however,
The cumulative number of each was 10) and evaluated under the same conditions as in Example 1, and the current density was 0.09 mA/cI112te, and the brightness (Ft-L) was 18.

[Brief explanation of the drawing]

FIG. 1 schematically shows an EL device using the LB method of the prior art, FIG. 2 schematically shows an EL device according to the present invention, and FIG. 3 schematically shows an EL device according to the present invention. It is a diagram schematically showing a cross section of an EL element.

Claims (1)

[Claims] A two-layer structure light-emitting layer and at least one light-emitting layer sandwiching the light-emitting layer.
In an EL device consisting of two transparent electrode layers, the first light-emitting layer is made of at least one electroluminescent organic compound that is electron-accepting relative to the second light-emitting layer. A monomolecular film consisting of a monomolecular film or a cumulative film thereof, in which the second light-emitting layer is made of at least one electroluminescent organic compound that is electron-donating relative to the first light-emitting layer. or a cumulative film thereof.