JPS6144980A - El element - Google Patents

Abstract

PURPOSE:To provide a thin EL element which gives emission with sufficiently high brightness even by low-voltage driving, can be easily produced and is inexpensive, by using a luminescent layer having a three-layer laminate structure wherein electroluminescent org. compds. are used. CONSTITUTION:A luminescent layer 2 of 0.01-1mu in thickness is interposed between a transparent electrode 1 and a back electrode 3 to obtain an EL element. The luminescent layer 2 has a laminate structure composed of a first layer 4, a second layer 5 and a third layer 6 wherein (i) the first layer is formed by a monomolecular or built-up film of an electroluminescent org. compd. which is relatively electron-donating as compared with the second layer, (ii) the second layer is formed by a monomolecular or built-up film of an electroluminescent org. compd. which is relatively electron-accepting as compared with the second layer and relatively electron-donating as compared with the third layer and (iii) the third layer is formed by a molecular layered film of an electroluminescent org. compd. which is relatively electron-accepting as compared with the second layer.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention is directed to - electrical light emission, that is, EL using EL;
Regarding the device, more specifically, the light-emitting layer has a three-layer structure, and each layer is composed of a thin film of at least one electroluminescent organic compound having a different electronegativity relative to other adjacent layers. Regarding EL elements.

(Prior art) Conventional EL elements are made of Mfl, Cu or ReF.
Z containing 3 (Re; rare earth ion) etc. as an activator
It consists of a light-emitting layer using nS as a light-emitting base material, and is broadly classified structurally 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 an EL device, the thinner the light emitting layer is, the better the light emitting characteristics will be. However, in the case of the powder type EL, since the luminescent base material is a discontinuous powder, pinholes easily occur in the luminescent layer when the luminescent layer is thinned, and it is difficult to reduce the layer thickness. Therefore, it has a major drawback in 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. Sufficient performance in terms of luminance, power consumption, etc. has not yet been achieved. 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 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 published in Japanese Patent Application Laid-Open No. 52-3558.
This is proposed in Publication No. 7. 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 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 light emitting layer having a three-layer stacked structure and two electrode layers sandwiching the light emitting layer, at least one of which is transparent. at least one electron-donating material relative to the light-emitting layer of
The second emissive layer is relatively electron-accepting to the first emissive layer, and at the same time is relatively electron-accepting to the third layer. The third layer is composed of at least one electroluminescent organic compound that is electron-donating relative to the second layer, or a cumulative film thereof, and the third layer is electron-accepting relative to the second layer. The above-mentioned EL device is characterized in that it is made of a molecular deposited film made of various electroluminescent organic compounds.

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. For example, it 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.

In the present invention, the compounds useful for forming the first and second light-emitting layers are obtained by chemically modifying the electroluminescent compounds as described above by known methods as necessary, and at least It is a compound that has one hydrophobic part and at least one wood-loving part (both of which are in a relative sense), and is, for example, a compound represented by the following general formula (I). and other compounds.

[(,X-R,),Z],-φ-R2(I) In the above formula, Acid groups, silicic acid groups, primary to tertiary amino groups; metal salts thereof, primary to tertiary amine salts, acid salts; ester groups, sulfamide groups, amide groups, imino groups, quaternary amino groups and salts thereof; hydroxyl group, etc.; R1 is an alkyl group having 4 to 30 carbon atoms, preferably 10 to 25 carbon atoms, preferably a linear alkyl group; m is 1 or 2; D is an integer of 1 to 4; Z is a direct bond or group (R3 is any substituent such as a hydrogen atom, an alkyl group, or an aryl group)
and: φ is a residue of an electroluminescent compound as exemplified below; R2 is a hydrogen atom or any other substituent, like X; one or more of X, φ and R2
At least one of the bonds is a woody moiety, and at least one is a hydrophobic moiety.

Further, in the present invention, the organic compound useful for forming the third light-emitting layer is selected from the same types of compounds as above, except for those chemically modified.

Preferred examples of compounds of general formula (I) useful for forming the first and second layers, basic skeletons of compounds useful for forming the third layer, and other compounds include:
It is 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,
It may have general substituents such as a class amine group, a hydroxyl group, a carboxamide group, a sulfoamide group, etc. ) (Margin below) Z=NH, O, S Z=CO, NHz=co, N
H,0,8Z = NH,0,5 Z=NH,0,S Z=NH,0
1SZ=S% Se Z=S, Se
Z=S, 5eZ=NH,0,S Z=NH,Q
S Z=NH, O1SM-Mg t Z n , S
n i AZCZ M-”Hz i B e i
Mg, Ca, CdSn, AtC4YbCI M= Er, Tm Sm+ Eu, Tb, z
=o, NmM=A4 Ga, Ir, Ta, 'a=
3 M=Er, Sm, EuM=-Zn, Cd
, Mg, pb, a=2 Gd, Tb,
DyTm, Yb M=Er+ Sm, Eu, Gd M=Er, S
m, EuTb, Dy, Tm, Yb G
d, Tb, DyTm, Yb 2=0, S, SeO≦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.

In the present invention, the above-mentioned luminescent compounds are used as the first to third light-emitting compounds of the EL device of the present invention, depending on their electronegativity.
The light emitting layer is divided into two light emitting layers, and the light emitting layer has a three-layer laminated structure.

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 The compound means that the luminescent compounds having different electronegativity are selected as the compound for forming the third luminescent layer, and a compound having an intermediate electronegativity among the selected compounds is used for forming the second luminescent layer. It may be selected and used as a compound for use.

Among such light-emitting compounds, particularly preferable electron-donating compounds are those having electron-donating groups such as primary to tertiary amine 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 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), or the like, on the entire surface or in a pattern. If an opaque cut-off electrode is used on one side, these opaque electrodes may also be of conventionally known types, and are generally and preferably made of aluminum with a thickness of about 0.1 to 0.3 pm; It is a vapor deposited film of silver, 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 approximately 0.02 to 0.2 pm; if the thickness is less than this range, the physical strength and electrical properties of the element itself will be insufficient; 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 light-emitting layer consisting of three layers using separately the electroluminescent compounds having relatively different electronegativities as described above, and among the light-emitting layers of the formed three-layer structure, The present invention is characterized in that the molecules constituting the first and second layers are each a monomolecular film or a cumulative film thereof in which molecules are arranged with highly ordered molecular orientation, and the third layer is a molecular deposition film.

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 the balance between the two (amphiphilic balance) is moderately maintained in a molecule with a structure that has a lignophilic group and a hydrophobic group in the molecule, 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 a monomolecular layer is formed with the hydrophilic 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 transverse to the water surface, a monomolecular film is formed on the substrate with the tree-philic groups of the molecules facing toward the 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 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 each layer. 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 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 present invention, particularly preferred methods for forming the molecular deposition film constituting the third light-emitting layer are resistance heating evaporation method and CVD method. It is possible to form a thin film of about

For example, when using the resistance heating vapor deposition method, the material is placed in a tungsten board placed in a vacuum chamber, and the material is placed 30 cm from the substrate.
Otherwise, resistance heating is performed, and sublimable materials are set at the sublimation temperature, and meltable materials are vapor deposited at a temperature higher than the melting point. The pre-vacuum level is set to 2 x 10-' Torr or less, the port is closed with a shutter before vapor deposition, the port is heated and the air is blown for 2 minutes, and then the shutter is opened to perform vapor deposition.

The speed during vapor deposition is measured using a crystal oscillator film thickness monitor, but a suitable speed is 0°1 A/sec.
In order to prevent oxidation, the degree of vacuum is maintained at less than IO Torr, preferably about 1 O Torr.

In the EL device of the present invention, the above-mentioned light-emitting layer forming material is preferably formed by the above-mentioned LB method and molecular deposition film, and between the above-mentioned two electrode layers, three compounds having different electronegativities are formed. 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 light emitting layer has a three-layer structure, the first and second light emitting layers are formed as a monomolecular film or a cumulative film thereof using compounds with different electronegativities as described above, and the third layer is formed by molecular deposition. It has been found that the performance of conventional EL elements can be significantly improved by forming the film as a film.

One important aspect of the present invention is an aspect in which the first and second light-emitting layers are monomolecular films made of the above-mentioned light-emitting material. In the EL device of this embodiment, first, a material that is relatively electron-donating to the second layer to be formed as an intermediate layer is dissolved in a suitable organic solvent such as chloroform, dichloromethane, dichloroethane, etc. -'-tσN, the solution is developed on an aqueous phase of an appropriate pH (for example, pn about 1 to 8) that may contain various metal ions, and the solvent is removed by evaporation. to form a monomolecular film, and then transferred onto one electrode substrate using the LB method as described above.
layer, and then a monomolecular film made of a material that is electron-accepting and donating relative to the first layer thus formed and at the same time electron-donating relative to the third layer. , a second layer is formed in the same manner as above, and a third layer is formed on the surface of the second layer by a molecular deposition method as described above from a compound that is electron-accepting relative to the second layer. Finally, a back electrode layer is obtained by depositing an electrode material such as aluminum, silver, gold, etc., preferably by vapor deposition. The thickness of the luminescent layer consisting of three monomolecular layers of the EL device thus obtained varies depending on the type of material used, but is generally about 0.01-1.
A thickness of gm is preferred.

Another important aspect is an aspect in which at least one layer, preferably both layers, of the first and second layers constituting the light-emitting layer of the EL device of the present invention are a cumulative film of the above-mentioned monomolecular film. be. This embodiment can be obtained by accumulating the above-mentioned monomolecular film by various methods to the required number of times and forming a molecular deposition film using the LB method and the molecular deposition method.

Although the thickness of the light-emitting layer of the EL device thus obtained and the cumulative number of monolayers in the first layer and the second layer can be changed arbitrarily, in the present invention, Preferably, one layer has a cumulative number of about 4 to 150, the second layer has a cumulative number of about 4 to 150, and the total thickness of the three layers is about 0.03 to Igm.

Note that the adhesion between the electrode layer used as a substrate and the light emitting layer is as follows.
The LB method and molecular deposition method are sufficiently strong and the light emitting layer will not peel or fall off, but in order to strengthen the adhesion, the substrate surface 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, and the first and second layers have a high degree of molecular order and function necessary for the operation of an EL device, and have excellent light-emitting performance.

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 that is a single layer, as schematically shown in FIG. Since the third light-emitting layer and the third light-emitting layer are stacked with uniform interfaces, various interactions between the three layers with different electronegativities are extremely easy, which is impossible to achieve with conventional technology. It exhibits excellent luminous performance. That is, the first to third
By variously changing the difference in electronegativity between the light emitting layer and the light emitting layer, the light emission intensity can be improved, the light emission color can be arbitrarily changed, and the service life 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 in at least one layer, a luminescent layer with excellent luminescence properties, film-forming properties, and film strength can be obtained.

The EL device of the present invention described above can achieve excellent EL emission 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 that.

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 nm on the surface of a 5011!1 square glass plate by sputtering.

After thoroughly cleaning this film-forming substrate, Joyce-Loebe
pH 6 of Langmuir-Trough4 made by 1 company
, immersed in an aqueous phase adjusted to 5. Next, ((mountain) 6(ON~AB) The above compounds A and B were dissolved in chloroform at a molar ratio of 1:l (10'', 3+sol/l), and then developed on the above aqueous phase. After removing the solvent chloroform by evaporation, the surface pressure was increased (30 dyne/cm) to precipitate the above mixed molecules in the form of a film.Then, while keeping the surface pressure constant, the film-forming substrate was placed on the water surface. The mixed monomolecular film was transferred onto the substrate by gently moving it up and down in the transverse direction (vertical speed 20 m/win), and a mixed monomolecular cumulative film of 7N was created and used as the first layer. 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 mixed monolayer remaining on the aqueous phase was completely removed, and a new CD was prepared using the same concentration and method as above, using the above compounds C and D in a molar ratio of 1:1. The mixed monomolecular film alone and the second layer were formed by accumulating eight layers.

Next, anthraquinone (E) (mp, 286° C.) was vapor-deposited to a thickness of 500 μm on the transparent electrode substrate provided with the above-mentioned mixed monomolecular film and its cumulative film using a resistance heating vapor deposition device. It has three layers. This vapor deposition is performed once in the vapor deposition tank.
Reduce the pressure to a vacuum level of 0-''' Torr, gradually increase the temperature of the resistance heating board (MO), keep it constant at 286°C, and adjust the pumping speed to increase the vacuum level to 9x.
10-'Tart maintained, deposition rate 5λ/see
A deposited film was formed by adjusting the current flowing through the board containing anthraquinone so that The degree of vacuum during vapor deposition is
It was 9×10-' Torr. Further, the temperature of the substrate holder was kept constant by circulating 20° C. water.

Finally, the substrate with the thin film formed as described above was placed in a deposition tank, and the iron tank was once depressurized to a vacuum level of 10-' Torr.The vacuum level was then adjusted to 10-' Torr, and the deposition rate was set at 20λ. /sec, Al was evaporated onto the thin film to a thickness of 1500 to form a back electrode. As illustrated in FIG. 3, the produced EL device is sealed with a sealing glass, and then purified, degassed, and dehydrated silicone oil is injected into the seal according to a conventional method to produce an EL light-emitting cell of the present invention. Formed. IOV, 4 for these EL light emitting cells
When an AC voltage of 00Hz was applied, the first and second
When the layer is a monolayer, the current density is 0.10 m'
EL emission with a brightness of 1.7 ft-L (7) was observed at A/crIf, and when the first and second layers were cumulative films, the brightness was 19.1 ft-L at a current density of 0 and 08 mA/crn''.
EL emission was observed.

The above-mentioned EL element of the present invention had a lower driving voltage and better luminance characteristics than the conventional EL element using ZnS as a light emitting matrix.

Comparative Example 1 In Example 1, except that only Compound A was used as the luminescent compound and a single layer was used, the rest was Example 1.
An EL element for comparison was obtained in the same manner as in Example 1, and evaluated in the same manner as in Example 1. The current density was 0 and 13 mA/ct.
n'c' luminance was 1 ft-L or less. Example 2 The following compounds F, G, and H were used in place of compounds A, C, and E in Example 1, and FQ
EL elements of the present invention (however, the cumulative number of each was 7 and 8) were obtained in the same manner as in Example 1, and evaluated under the same conditions as in Example 1. As a result, the current density was 0°10.
mA/crn', the brightness (Ft-1,) is 19.1-t
'there were.

[Brief explanation of the drawing]

FIG. 1 schematically shows an EL device using the LB method of the prior art, and FIG. 2 shows an EL device according to the present invention.
The device is diagrammatically shown, and FIG.
It is a diagram schematically showing a cross section of an L element. 1; Transparent electrode 2: Luminescent layer 3; Back electrode 4; Luminescent compound 5; Luminescent compound 6; Luminescent compound 7; Seal glass 8
; Silicone insulating oil 9 ; Glass plate patent applicant Canon Co., Ltd. agent Patent attorney 1) Katsuhiro-) C Figure 1 Figure 2 Figure 3

Claims (1)

[Claims] In an EL device consisting of a light-emitting layer with a three-layer stacked structure and two electrode layers sandwiching the light-emitting layer, at least one of which is transparent, the first light-emitting layer is opposite to the second light-emitting layer. Consisting of a monomolecular film or a cumulative film thereof consisting of at least one relatively electron-donating electroluminescent organic compound,
a monomolecule comprising at least one electroluminescent organic compound in which the second emissive layer is electron-accepting relative to the first emissive layer and simultaneously electron-donating relative to the third layer; The above EL, characterized in that the third layer is a molecular deposited film made of at least one electroluminescent organic compound that is electron-accepting relative to the second layer. element.