JPH07263145A - Driving method of EL element - Google Patents

Abstract

(57) [Abstract] [PROBLEMS] To provide a method for driving an EL element, which eliminates the drawbacks of the prior art and provides light emission with sufficiently high brightness even at low voltage driving, is inexpensive, and is easy to manufacture. In a method for driving an EL device comprising a light emitting layer having a two-layer structure and a transparent electrode layer and a back electrode layer sandwiching the light emitting layer, a first light emitting layer faces the transparent electrode layer,
And, the second light emitting layer is formed of a deposited film by a resistance heating vapor deposition method or a CVD method, which is made of at least one electroluminescent organic compound relatively electron-accepting to the second light emitting layer, The transparent electrode layer and the transparent electrode layer are formed of a monomolecular film or an accumulated film of the at least one electroluminescent organic compound facing the electrode layer and relatively electron donative to the first light emitting layer. The method for driving an EL element, wherein a voltage of 10 V or less is applied to the back electrode layer to cause the light emitting layer to emit light.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for driving an EL device using electroluminescence, that is, EL, and more specifically, a light emitting layer has a two-layer structure, and each layer is adjacent to another layer. At least 1 with different electronegativity to
The present invention relates to a method for driving an EL device including a thin film of a certain kind of electroluminescent organic compound.

[0002]

2. Description of the Related Art A conventional EL device comprises a light emitting layer containing ZnS containing Mn, Cu or ReF ₃ (Re; rare earth ions) as an activator as a light emitting base material. Due to the difference in structure, they are roughly classified into powder type EL and thin film type EL structurally. Among the practically used devices, the thin film EL generally has higher brightness than the powder type EL, but the thin film EL is a large area device because the light emitting base material is deposited on the substrate to form the light emitting layer. However, there are drawbacks such as the difficulty in manufacturing and the extremely high manufacturing cost.

Therefore, a powder type EL in which a light emitting base material, that is, ZnS, is dispersed in an organic binder, which has the highest mass productivity and requires only a few tenths of a thin film type element, is gaining attention. became. Generally, in EL light emission, the thinner the thickness of the light emitting layer, the better the light emitting characteristics. However, in the case of the powder type EL, since the light emitting base material is a discontinuous powder, if the light emitting layer is made thin, pinholes are easily generated in the light emitting layer, and it is difficult to make the layer thickness thin. It has a major drawback that sufficient luminance characteristics cannot be obtained.

Recently, an improved element in which an intermediate dielectric layer made of a vinylidene fluoride polymer is arranged in the light emitting layer of the powder type EL is disclosed in Japanese Patent Laid-Open No. 172891/1983. However, it has not yet achieved sufficient performance in terms of light emission brightness, power consumption, and the like. On the other hand, recently, research and development of new optical and electronic materials by controlling chemical structures and higher-order structures of organic materials have been actively conducted, and EC elements, piezoelectric elements, pyroelectric elements, nonlinear optical elements, Is it comparable to metals and inorganic materials such as ferroelectric liquid crystals?
Or, organic materials surpassing them have been announced. In this way, amid the demand for the development of functional organic materials as new functional materials surpassing inorganic materials, the cumulative film of monomolecular layers of anthracene derivatives and pyrene derivatives having a hydrophilic group and a hydrophobic group in the molecule is used as an electrode substrate. The EL element formed above is disclosed in Japanese Patent Laid-Open No.
It is proposed in JP-A-2-35587.

[0005]

However, these EL elements have not yet achieved sufficient performance as an actual EL element, such as their brightness and power consumption.
In the case of the L element, the density of carrier electrons or holes is extremely low, the probability of excitation of functional molecules due to carrier recombination, etc. is extremely low, and efficient light emission cannot be expected.

Therefore, an object of the present invention is to solve the above-mentioned drawbacks of the prior art, to obtain a light emission with sufficiently high brightness even at low voltage driving, and at a low cost, and to easily manufacture an EL element driving method. Is to provide.

[0007]

The above-mentioned object of the present invention is E
This has been achieved by forming the light emitting layer of the L element by combining specific materials and forming a specific configuration. That is,
The present invention provides a method for driving an EL element comprising a light emitting layer having a two-layer structure and a transparent electrode layer and a back electrode layer sandwiching the light emitting layer, wherein the first light emitting layer faces the transparent electrode layer.
And, the second light emitting layer is formed of a deposited film by a resistance heating vapor deposition method or a CVD method, which is made of at least one electroluminescent organic compound relatively electron-accepting to the second light emitting layer, The transparent electrode layer and the transparent electrode layer are formed of a monomolecular film or an accumulated film of the at least one electroluminescent organic compound facing the electrode layer and relatively electron donative to the first light emitting layer. The EL element driving method is characterized in that a voltage of 10 V or less is applied to the back electrode layer to cause the light emitting layer to emit light.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in more detail with reference to the preferred embodiments. The electroluminescent organic compound used in the present invention and mainly characterizing the present invention is a compound which has a high emission quantum efficiency and further has a π-electron system which is easily subjected to external perturbation and which can be electrically excited. And
For example, basically, condensed polycyclic aromatic hydrocarbons, 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, organic rare earth Examples thereof include complexes and derivatives of these compounds. Further, as the compound capable of becoming an electron acceptor or an electron donor with respect to the above compound, a heterocyclic compound other than the above and derivatives thereof, an aromatic amine and an aromatic polyamine, a compound having a quinone structure, tetracyanoquinodimethane And tetracyanoethylene. Further, in the present invention, the organic compound useful for forming the first light emitting layer is selected from the same kinds of compounds as described below, except that it is chemically modified.

In the present invention, the compound useful for forming the second light emitting layer is obtained by chemically modifying the electroluminescent compound as described above, if necessary, by a known method, and at least the structure thereof. A compound in which one hydrophobic moiety and at least one hydrophilic moiety (both of which are in a relative meaning) are combined together, and for example,
The compounds represented by the following general formula (I) and other compounds are included. _{[(X-R 1) m} Z] n -φ-R 2 (I)

X in the above formula 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, a silicic acid group, a primary to tertiary amino group; Metal salt, primary to tertiary amine salt, acid salt; ester group, sulfamido group, amide group, imino group, quaternary amino group and salts thereof, hydroxyl group and the like; R ₁ has 4 to 30 carbon atoms, preferably Is 10-2
5 alkyl groups, preferably linear alkyl groups; m is 1 or 2, n is an integer of 1 to 4; Z is a direct bond or -O-, -S-, -N (R ₃ ) −, −C
_{H 2 N (R 3) -} , - SO 2 N (R 3) -, - CO -, -
COO- or the like (R ₃ is a hydrogen atom, an alkyl group, an aryl or other optional substituent); φ is a residue of an electroluminescent compound as exemplified later; R ₂ is X Hydrogen atom or any other substituent; at least one of one or more of X, φ and R ₂ is a hydrophilic moiety and at least one is a hydrophobic moiety.

Preferred examples of φ of the compound of the general formula (I) useful for forming the first layer, the basic skeleton of the compound useful for forming the second layer, and other compounds are shown below. (However, φ illustrated below
(Basic skeleton) 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 a general substituent such as a primary to tertiary amino group, a hydroxyl group, a carboxamide group and a sulfamide group. ).

[0012]

[0013]

[0014]

[0015]

The luminescent compounds as described above can be used alone or as a mixture in each luminescent layer in the present invention. It should be noted that these compounds are examples of preferable compounds, and other derivatives or other compounds may be used as long as the same purpose is achieved. In the present invention,
The luminescent compound as described above is characterized by being used separately for 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 light-emitting compounds have different electronegativities, the compounds having relatively electron-accepting property among them are used for forming the first light-emitting layer. The light emitting compound that is adopted as a compound and is relatively electron donative to these may be selected as the second light emitting layer forming compound. Among such light-emitting compounds, a compound particularly preferable as an electron-donating compound is a compound having an electron-donating group such as a primary to tertiary amino group, a hydroxyl group, an alkoxy group, an alkyl ether group, or nitrogen. Heterocyclic compounds are the main ones, and as electron-accepting compounds, those having an electron-withdrawing group such as a carbonyl group, a sulfonyl group, a nitro group and a quaternary amino group are the main ones. In the present invention, such a luminescent compound may be used alone or as a mixture of a plurality of luminescent layers.

The other elements forming the EL device of the present invention, that is, the transparent electrode layer and the back electrode layer, sandwich the light emitting layer, and any conventionally known one can be used.
At least one of the layers should be transparent. As the transparent electrode layer, any transparent electrode layer for the same purpose as in the past can be used, and preferred examples thereof include transparent synthetic resins such as polymethylmethacrylate and polyester, and transparent films such as glass and the like or oxidation on the surface of the sheet. Indium, tin oxide, indium-tin-oxide (I
A transparent conductive material such as TO) is coated on the entire surface or in a pattern. When an opaque back electrode layer is used on one side, these opaque electrode layers may also be those known in the art, with a typical and preferred thickness being about 0.
It is a vapor deposition film of 1 to 0.3 μm of aluminum, silver, gold or the like.

The transparent electrode layer or the back electrode layer may have any shape such as a plate shape, a belt shape and a cylindrical shape.
It can be selected according to the purpose of use. In addition, the thickness of the transparent electrode layer is preferably about 0.01 to 0.2 μm, and if the thickness is less than this range, the physical strength and electrical properties of the device itself will be insufficient, and the thickness above the above range will be insufficient. The thickness may cause problems in transparency, lightness, compactness, and the like.

In the EL device of the present invention, the electroluminescent compounds having relatively different electronegativities as described above are separately used between the two electrode layers as described above, and the electroluminescent compound is composed of two layers. And a first layer constituting the formed two-layered light-emitting layer facing the transparent electrode layer and having a relatively electron-accepting property with respect to the second layer. Or a monolayer film in which the second layer faces the back electrode layer and is arranged with a highly ordered molecular orientation composed of a compound that is relatively electron donative to the first layer. It is characterized by being a cumulative film.

In the present invention, a particularly preferable method for forming the molecular deposited film forming the first light emitting layer is a resistance heating vapor deposition method or a CVD method. For example, in the vapor deposition method, the first light emitting layer is used. As a result, a thin film having a thickness of about 500Å can be formed. For example, in the case of the resistance heating vapor deposition method, the material is put in a tungsten board placed in a vacuum chamber, placed 30 cm or more from the substrate, resistance heated, sublimable ones are set to sublimation temperature, and fusible ones have a melting point. Deposition is performed at the above temperature. The total degree of vacuum is set to 2 × 10 ⁻⁶ Torr or less, the shutter is closed before vapor deposition, the board is heated and emptied for about 2 minutes, and then the shutter is opened for vapor deposition.

The speed during the vapor deposition is carried out while measuring the film thickness of the crystal oscillator with a monitor, and the preferred speed is 0.1.
Perform between Å / sec and 100Å / sec. At that time, the degree of vacuum is kept at 10 ⁻³ Torr or less, preferably about 10 ⁻⁵ Torr in order to prevent oxidation and the like.

In the present invention, the Langmuir-Blodgett method (LB) is particularly preferable as a method for forming such a second layer monomolecular film or a cumulative film thereof.
Law). In the LB method, in a molecule having a structure having a hydrophilic group and a hydrophobic group in the molecule, when the balance between them (the balance of amphipathicity) is appropriately maintained, the molecule is hydrophilic on the water surface. This is a method of forming a monomolecular film or a cumulative film thereof by utilizing the fact that the base is turned downward to form a monomolecular layer.

Specifically, a partition plate (or a float) is provided to limit the spread area so that the monomolecular film spread on the water phase does not freely diffuse and spread over the water phase. By controlling the aggregated state of the substances and gradually increasing the surface pressure, the surface pressure suitable for the production of the monomolecular film or its cumulative film is set. The monolayer is transferred onto the substrate by gently raising or lowering the clean substrate vertically while maintaining this surface pressure. Although the monomolecular film is manufactured as described above, the cumulative film of the monomolecular film is formed as a cumulative film having a desired degree of accumulation by repeating the above operation.

In order to transfer the monomolecular film onto the substrate, it is possible to use a method such as a horizontal attachment method or a rotating cylinder method in addition to the above-mentioned vertical dipping method. The horizontal attachment method is a method of transferring a substrate by bringing the substrate into horizontal contact with the water surface, and the rotating cylinder method is a method of rotating a cylindrical substrate on the water surface to transfer a monomolecular film to the surface of the substrate. In the vertical dipping method described above, when a substrate having a hydrophilic surface is pulled out of water in a direction crossing the water surface, a monomolecular film in which hydrophilic groups of molecules are directed to the substrate is formed on the substrate.
When the substrate is moved up and down as described above, the monomolecular films are superposed one by one in each process.

Since the directions of the film-forming molecules are opposite in the pulling process and the dipping process, according to this method, the hydrophilic groups and hydrophilic groups of the molecules and the hydrophobic groups and hydrophobic groups of the molecules face each other between layers. A mold film is formed. On the other hand, the horizontal attachment method is a method of horizontally adhering the substrate to the water surface and transferring it, and a monomolecular film in which the hydrophobic groups of the molecules face the substrate is formed on the substrate. In this method, even if the monomolecular film is accumulated, there is no change in the direction of the film-forming molecules, and in all the layers, an X-type film in which the hydrophobic group faces the substrate is formed.

On the contrary, a cumulative film in which hydrophilic groups in all layers face the substrate is called a Z-type film. The rotating cylinder method
In this method, a cylindrical substrate is rotated on the water surface to transfer the monomolecular film to the substrate surface. The method of transferring the monomolecular film onto the substrate is not limited to these. That is, when a large-area substrate is used, a method of extruding the substrate from the substrate roll into the aqueous phase may be used. Further, the orientation of the hydrophilic group and the hydrophobic group described above to the substrate is in principle, and can be changed by surface treatment of the substrate. In the EL device of the present invention, of the two electrode layers as described above, a compound having a relatively electron-accepting property is selected from the above compounds as the first layer facing the transparent electrode layer, and the method as described above is used. It is obtained by forming a molecular deposition film and forming a monomolecular film or a cumulative film thereof by the LB method as the second layer facing the back electrode layer, and by forming the light emitting layer into a two-layer structure.

As described in the section of the prior art, it is known to form an EL element by the LB method, but the known method cannot obtain an EL element having sufficient performance, and the present inventors have As a result of various studies, the light emitting layer has a two-layer structure, the first light emitting layer is formed as a molecular deposition film using the compound having a relatively electron-accepting property as described above, and the second layer is formed as follows. By forming a monomolecular film or a cumulative film thereof from a compound that is relatively electron donative to the first layer,
It has been found that the performance of the EL element of the prior art is significantly improved.

One aspect of the present invention is an aspect in which the second light emitting layer is a monomolecular film made of the light emitting material. In the EL device of this aspect, first, a material that is relatively electron-accepting to the first layer is formed as a deposited film by the molecular deposition method as described above, and then the A material having an electron donating property relative to the first layer formed in the above is dissolved in a suitable organic solvent such as chloroform, dichloromethane, dichloroethane or the like at a concentration of about 10 ⁻⁴ to 10 ⁻² M, The solution is spread on an aqueous phase having a suitable pH (for example, pH about 1 to 8) which may contain various metal ions, and the solvent is removed by evaporation to form a monomolecular film. LB method, it is transferred onto one transparent electrode substrate to form a second layer and dried sufficiently, and then an electrode material such as aluminum, silver or gold is preferably applied to the surface of the second layer. By forming the back electrode layer by vapor deposition such as vapor deposition, Obtained.

The thickness of the light emitting layer composed of two layers of the EL device thus obtained varies depending on the type of material used, but generally a thickness of about 0.01 to 1 μm is suitable. . Another important aspect is an aspect in which the second layer constituting the light emitting layer of the EL device of the present invention is a cumulative film of the above monomolecular film. This mode can be obtained by using the above-mentioned LB method to form a monomolecular film on the first layer as described above by accumulating up to the required number of layers by various methods. Although the thickness of the light emitting layer of the EL device of the present invention thus obtained can be arbitrarily changed, in the present invention, the cumulative number of the monolayer of the second layer is about 4 to 200, It is preferable that the thickness of the first layer is about 0.01 to 0.5 μm, and the thickness of the entire light emitting layer is about 0.02 to 1 μm.

The adhesion between one of the electrode layers used as the substrate or both of the electrode layers and the light emitting layer is sufficiently strong in the LB method and the molecular deposition method, and the light emitting layer is peeled or peeled off. However, the surface of the substrate may be pretreated or an appropriate adhesive layer may be provided between the substrate and the light emitting layer for the purpose of enhancing the adhesive force. Further, in the LB method, by adjusting various conditions such as the kind of the material for forming the light emitting layer, the pH of the aqueous phase to be used, the ionic species, the water temperature, the transition speed of the monomolecular film or the surface pressure of the monomolecular film. Can also strengthen the adhesion. Since the performance of the EL element formed as described above may deteriorate under the influence of moisture and oxygen in the air as it is, it is desirable to use a conventionally known means to form a moisture resistant and oxygen resistant sealed structure. .

In the EL device of the present invention as described above, the structure of the light emitting layer is an ultrathin film, and the second layer has a high degree of molecular order and function necessary for the operation of the EL device. In addition, the first layer and the second layer exhibit various electrical interactions, thereby exhibiting excellent light emitting performance.

Further, the light emitting layer of the EL device of the present invention is different from the light emitting layer composed of a single layer of the prior art as shown schematically in FIG. 1, and as shown schematically in FIG. Since the light emitting layer and the second light emitting layer have a uniform interface, various interactions are extremely easy between the two layers having different electronegativities, which is excellent to the extent that cannot be achieved by the conventional technique. It exhibits light emitting performance. That is, the emission intensity can be improved or the emission color can be arbitrarily changed by changing the electronegativity difference between the first light emitting layer and the second light emitting layer, etc. It can be significantly extended.

Further, in the prior art, a material having excellent light-emitting property but insufficient film-forming property or film strength could not be practically used. Even if the material is poor in strength, it is possible to obtain a light-emitting layer having excellent light-emitting property, film-forming property, and film strength by using a material having excellent film-forming property in one of the layers even if the material has excellent light-emitting property. Obtainable. In the EL device of the present invention described above, electric energy such as a suitable electric field acts on the light emitting layer,
By applying electric energy such as AC or pulse or DC current between the electrode layers, excellent EL light emission is exhibited.

[0035]

EXAMPLES Next, the present invention will be described in more detail with reference to production examples, examples and comparative examples. It should be noted that the word "central part" is based on weight. Production Example 1 A transparent electrode layer was formed by depositing an ITO layer having a thickness of 1500 Å on the surface of a glass plate of 50 mm square by a sputtering method.

Next, using a resistance heating vapor deposition device, imidazole (A) was vapor-deposited to a film thickness of 500 Å on the transparent electrode substrate. This vapor deposition was performed once at 10 ⁻⁶ T in the vapor deposition tank.
After decompressing to a vacuum degree of orr, the resistance heating board (M
o) The temperature is gradually increased and the deposition rate is 5Å / sec.
Thus, the vapor deposition film was formed by adjusting the current flowing through the board containing imidazole. Vacuum degree during deposition is 9
It was × 10 ⁻⁶ Torr. The temperature of the substrate holder was kept constant by circulating water at 20 ° C.

Next, Chloroform solution containing at a ratio of 1: 1 (10 ^-3 mol /
Liters) of the Langmuir-Trough 4 from Joyce-Loebel.
Of 4 × 10 ⁻⁴ mol of CdCl ₂ at pH 6.5.
It was developed on the above water phase adjusted to. After removing the solvent chloroform by evaporation, increase the surface pressure (30 dyne / c
m), the above mixed molecules were deposited in the form of a film. After that, while keeping the surface pressure constant, the film-forming substrate on which imidazole has already been deposited is gently moved up and down in the direction crossing the water surface (vertical speed 2 cm / min), and the mixed monomolecular film is transferred onto the substrate. A monomolecular accumulating film accumulating in 6 layers was prepared. In this accumulating process, each time the substrate is pulled up from the water tank,
It was left for 30 minutes or more to evaporate and remove the water adhering to the substrate.

Finally, the thin film formed as described above is put into a vapor deposition tank, the pressure of the tank is once reduced to 10 ^-6 Torr, the vacuum degree is adjusted to 10 ^-5 Torr, and the vapor deposition rate is 20 Å. / Sec at a film thickness of 1500Å, Al was vapor-deposited on the thin film to form a back electrode layer. As shown in FIG. 3, the prepared EL device was sealed with sealing glass, and then purified, degassed and dehydrated silicon oil was injected into the seal according to a conventional method to obtain the EL light emitting cell of the present invention. Formed.

Example 1 When an AC voltage of 10 V and 400 Hz was applied to the EL light emitting cell of Production Example 1, EL light emission with a current density of 0.09 mA / cm ² and a brightness of 15 ft-L was exhibited. The EL element of the present invention described above was an EL element having a low driving voltage and good emission luminance characteristics, as compared with the EL element using ZnS of the conventional example as a light emitting base material.

Comparative Example 1 An EL device for comparison was obtained in the same manner as in Production Example 1 except that the first layer was not formed in Production Example 1.
When evaluated in the same manner as in Example 1, the current density was 0.1.
The luminance was 1 ft-L or less at mA / cm ² .

Production Example 2 E was used in place of the compound A in Production Example 1, F was used in place of B and C, Otherwise, in the same manner as in Production Example 1, the EL element of the present invention (however,
The cumulative number was 6). Example 2 The EL device of Production Example 2 was evaluated under the same conditions as in Example 1. As a result, the current density was 0.11 mA / cm ² and the brightness (Ft-L) was 34.

[0042]

[Brief description of drawings]

FIG. 1 is a diagram schematically showing an EL element according to a conventional LB method.

FIG. 2 is a diagram schematically showing an EL element used in the present invention.

FIG. 3 is a diagram schematically showing a cross section of an EL element used in the present invention.

[Explanation of sign]

 1: Transparent Electrode Layer 2: Light Emitting Layer 3: Back Electrode Layer 4: Light Emitting Compound 5: Light Emitting Compound 6: Seal Glass 7: Silicon Insulating Oil 8: Glass Plate

Claims (1)

[Claims] 1. A method of driving an EL device comprising a light emitting layer having a two-layer structure, and a transparent electrode layer and a back electrode layer sandwiching the light emitting layer, wherein the first light emitting layer faces the transparent electrode layer. And a deposited film formed by a resistance heating vapor deposition method or a CVD method, which is composed of at least one electroluminescent organic compound relatively electron-accepting to the second light emitting layer.
Of the electroluminescent layer, which is a monomolecular film facing the back electrode layer and is made of at least one electroluminescent organic compound relatively electron donative to the first electroluminescent layer, or a cumulative film thereof. The method for driving an EL element, wherein a voltage of 10 V or less is applied to the transparent electrode layer and the back electrode layer to cause the light emitting layer to emit light.