JP2591461B2 - Organic thin film EL device and method of manufacturing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an organic thin film EL device used for a flat light source or a display.

[0002]

2. Description of the Related Art Attention has been focused on the use of EL elements as self-luminous planar display elements. Among them, an organic thin-film EL element using an organic substance as a light-emitting material achieves extremely high luminous efficiency and high-luminance light emission at an applied voltage of less than 10 V by laminating organic layers and separating functions. The basic element structure is anode / hole injection layer / light-emitting layer / cathode, or anode / light-emitting layer / electron injection layer / cathode, or anode /
It is a hole injection layer / light emitting layer / electron injection layer / cathode, and is usually formed sequentially from the anode.

[0003] Most of the conventional organic thin-film EL devices use a vapor-deposited film of an aromatic amine for the hole injection / transport layer. Among them, in particular, 1,1-bis (4-diparatolylaminophenyl) cyclohexane or N, N′-diphenyl-N, N′-bis (3-methylphenyl) -1,
1′-biphenyl-4,4′-diamine is preferably used because it has good hole injection properties and good film-forming properties.
The deposited films of these materials are uniform immediately after the deposition,
It is reported that flocculation will progress considerably in a few days ('9
3 Spring Applied Physics Society Preliminary Collection 30a-SZK-1, '93 Autumn Applied Physics Society Preliminary Collection 29a-ZC-8). Therefore, deterioration of the element occurs. Further, the glass transition point is 60 to
As low as 80 ° C., thermal degradation occurs when the device is driven.

Generally, aromatic amines have a glass transition point of 10
It is considered that the temperature is as low as 0 ° C. or less, which causes deterioration of the element. This means that the quality of the organic layer is an organic thin film EL.
This indicates that the element characteristics of the element are greatly affected, and it is important for an organic thin film EL element to produce a stable film having a uniform and good adhesion at an interface with another film in contact.

On the other hand, a method of using a porphyrin compound for the hole injection layer has been disclosed (Japanese Patent Application Laid-Open No. 57-51781). And aggregation resulted in deterioration of the device.

[0006] This is because, in general, a single organic molecule is unstable, and crystallization and aggregation progress with time, so that it is difficult to form a stable and uniform thin film state for a long time.

Therefore, in order to improve the thermal stability of the hole injection layer and suppress the deterioration of the device, the aforementioned 1,1-bis (4-diparatolylaminophenyl) cyclohexane or N, N'- A method in which a film in which an aromatic amine such as diphenyl-N, N'-bis (3-methylphenyl) -1,1'-biphenyl-4,4'-diamine is dispersed in a polymer binder is used as a hole transport layer. Conceivable. For forming the hole injection layer, a wet film forming method such as a spin coating method or a dip coating method can be used. Junji Kido et al. Have published the characteristics of an organic thin-film EL device having such a configuration (Japan Journal of Applied Physics, Vol. 31, '92, L960 (Japan Jou).
rnal of Applied Physics N
o. 31'92L960)). As described above, the light emitting characteristics of the device in which the aromatic amine resin dispersion film is introduced as the hole injection layer emits light with high luminance immediately after the device is fabricated, but the luminance is immediately increased when the light is continuously emitted with the applied voltage constant. It drops to less than 10% of the initial luminance. This was due to poor adhesion at the interface between the anode and the hole injection layer, causing the hole injection layer to immediately peel off from the anode and fail to make electrical contact. This also applies to a device in which a phthalocyanine resin dispersion film is introduced as a hole injection layer, and similarly, there is a problem in electrical contact between the anode and the hole injection layer.

In order to increase the stability of the device by reducing pinholes, a hole-injecting porphyrin compound layer and a hole-injecting aromatic 3
A structure of an organic thin film EL device using two layers of a secondary amine layer has been disclosed (Japanese Patent Application Laid-Open No. Sho 63-295695). However, even with this configuration, the stability of the element has not been sufficiently improved. That is, it is extremely difficult to simultaneously prevent separation of the interface between the anode and the hole injection transport layer and prevention of crystallization and aggregation of the hole injection layer.

[0009]

As described above, the organic thin-film EL device exhibits high luminance, but its life is shorter than other light-emitting devices, which hinders practical use.

In general, since the luminance is proportional to the current density, it is necessary to increase the applied voltage to increase the current in order to increase the luminance of the element, but the life of the element is further shortened. In order to expand the practicality of the organic thin film EL element, it is desirable to lower the driving voltage.

The present invention has been made in view of the above-mentioned conventional circumstances, and has a low driving voltage and a long life.
It is intended to provide an L element.

[0012]

Means for Solving the Problems The present inventors have proposed an organic thin film EL device having at least one hole injecting layer and a light emitting layer between a pair of transparent electrodes, wherein the hole injecting layer is in contact with the anode. And a film thickness of 100 nm made of a low-molecular organic P-type semiconductor containing a polymer binder in the voids
It has been found that a low driving voltage and a long life can be realized in an organic thin film EL device characterized by the following films.

An object of the present invention is to form a low-molecular organic P-type semiconductor on the anode to make the interface between the anode and the hole injection layer adhere to each other, and fill a gap between the low-molecular organic P-type semiconductor with a polymer binder. Thereby, crystallization and aggregation of the low-molecular organic P-type semiconductor are prevented. FIG. 1 shows an example of the organic thin film EL device of the present invention. The present invention is not limited to this configuration. FIG. 2 shows, as a conventional example, an example of an organic thin film EL device using a hole injection layer formed on an anode by dispersing a low-molecular organic P-type semiconductor in a polymer binder. In the device of FIG. 2, peeling occurs because the adhesion at the interface between the hole injection layer and the anode is weak, and the resistance at this interface increases to generate Joule heat, so that the peeling further proceeds. On the other hand, in the device shown in FIG. 1, the adhesion between the low-molecular organic P-type semiconductor and the anode is stronger than that in the device shown in FIG. Therefore, the device of FIG. 1 stabilizes the hole injection layer and improves the electrical contact.

Therefore, by adopting this structure, crystallization and aggregation of the low molecular weight organic P-type semiconductor can be prevented while keeping the interface at the interface with the anode, and the film is extremely stabilized. Therefore, the electric contact is good and stable, so that the hole injection efficiency is improved, the driving voltage is reduced, and the durability of the element is also increased.

In order to realize the above element configuration, the low-molecular organic P-type semiconductor film of the hole injection layer must be sufficiently thin. In general, when a low molecular weight organic film is formed on a substrate, voids are easily formed in the low molecular weight organic film as the film thickness decreases, and the voids decrease as the film thickness increases. This is because there are various influences from the substrate near the substrate, which hinders uniform film formation. Therefore, the smaller the thickness of the low molecular weight organic P-type semiconductor film of the hole injection layer, the easier it is to impregnate the polymer binder. When the film thickness is increased, it becomes very difficult to impregnate the entire low-molecular organic P-type semiconductor film with the polymer binder. If the polymer binder is not impregnated into the entire low molecular weight organic P-type semiconductor film, the portion near the non-impregnated anode remains open and the advantages of the present invention as described above are lost. Would. Therefore, the thickness of the low molecular weight organic P-type semiconductor film of the hole injection layer is 10
It is preferably 0 nm or less.

The voids in the film can be observed with an atomic force microscope or an electron microscope. A film having voids with a diameter of 1 nm or more is particularly preferable because it can be easily filled with a polymer binder and the film can be stabilized.

Hereinafter, a method for forming the hole injection layer will be described.

The low molecular weight organic P-type semiconductor of the hole injection layer refers to a material having a hole transporting property in the state of an organic thin film EL device. In general, an organic semiconductor is either P-type or N-type depending on the layer adjacent thereto. As one of the determination methods,
There is a method of measuring a resistance value in an oxygen atmosphere and a vacuum in the thin film state. Those having a lower resistance value in an oxygen atmosphere can be determined to be P-type.

The low-molecular organic P-type semiconductor of the hole injection layer is appropriately selected from known materials. For example, condensed polycyclic dyes such as spiro compounds, azo compounds, quinone compounds, indigo compounds, diphenylmethane compounds, polymethine compounds, acridine compounds, and porphine compounds described in "Dye Handbook: Kodansha '86" can be applied. . In addition, "Organic Semiconductors: Ferrac Chemie, Inc. '74 (ORGANIC SEMICONDUCTORS:
Low-molecular-weight organic P-type semiconductors described in "VERLAG CHEMIE '74)" can also be used. Among these low-molecular-weight organic P-type semiconductors, materials that are separated from the anode when impregnated with a polymer binder are used. In addition, it is preferable that a film is easily formed in a thin film state.

Known low-molecular organic P-type semiconductor films can be formed by, for example, a vapor deposition method. This is, specifically, resistance heating, electron beam, sputtering, ion plating, MBE or the like.

Also, it can be formed by a wet film forming method.
For example, a film can be formed by using an apparatus such as a spin coater, an applicator, a spray coater, a bar coater, a dip coater, a doctor blade, a roller coater, a curtain coater, and a bead coater.

In this case, a known solvent is appropriately used as a solvent used for preparing the coating liquid. For example, alcohols, aromatic hydrocarbons, ketones, esters, aliphatic halogenated hydrocarbons, ethers, amides, sulfoxides and the like are applied.

The polymer binder filling the voids in the low molecular weight organic P-type semiconductor film is appropriately selected from known materials. For example, vinyl resin, acrylic resin, epoxy resin, silicon resin, styryl resin, polyimide, polysilylene, polyvinyl carbazole, polycarbonate, cellulose resin, polyolefin resin, etc.
Natural resins such as glue and gelatin can be used.

As a method for impregnating the low molecular weight organic P-type semiconductor film with the polymer binder, a known method can be applied. For example, the above-mentioned wet film forming method can be used. In this case, as the wet film forming method and the solvent, known methods and materials as described above are appropriately selected. In this case,
It is necessary to prevent the low-molecular organic P-type semiconductor film from being dissolved in the solvent. That is, it is necessary to select each material so that the solvent dissolves the polymer binder and does not dissolve the low-molecular organic P-type semiconductor film.

As a method for impregnating a low molecular weight organic P-type semiconductor film with a polymer binder, first, a layer of a high molecular weight binder is formed on the low molecular weight organic P-type semiconductor film, and then the substrate is heated. Thereby, the polymer binder can be impregnated. In order to form the polymer binder layer on the low molecular weight organic P-type semiconductor film, a vapor deposition method or the like can be used. May be placed on top. In this case, the material used for the low molecular weight organic P-type semiconductor film is selected from materials whose melting point is equal to or higher than the melting point of the polymer binder.

When the polymer binder is impregnated, one or more organic P-type semiconductors may be mixed. If an organic P-type semiconductor is appropriately selected, the hole injection efficiency is further improved. This is because the holes injected into the hole injection layer further flow through the organic P-type semiconductor in the polymer binder, so that the barrier is lowered. At this time, as the organic P-type semiconductor mixed with the polymer binder, it is desirable to select an organic P-type semiconductor whose oxidation potential is between the low-molecular-weight organic P-type semiconductor used for the hole injection layer and the light emitting material. The organic P-type semiconductor is appropriately selected from known materials. For example, in addition to the above-mentioned low molecular weight organic P-type semiconductor, a high molecular weight organic P-type semiconductor such as polyvinyl carbazole and polysilylene can be applied. In the present invention, the role of the polymer binder is to suppress crystallization and aggregation of the hole injection layer, and if the proportion of the organic P-type semiconductor mixed with the polymer binder is too large, mechanical properties expected of the polymer binder are expected. Physical properties are degraded, and their functions are lost.

An object of the present invention is to stabilize a hole injection layer in an organic thin film EL device, that is, to improve electrical contact and consequently to improve charge injection efficiency. Any combination can be applied. For example, after forming the hole injection layer of the present invention,
Further, the hole transport layer may be formed in one or more layers, the light emitting layer may be doped with an appropriate fluorescent dye, and the electron injecting layer may be formed in one or more layers on the light emitting layer. It may be, but not limited to this.

With this configuration, the stability of the hole injection layer is improved, so that the efficiency of hole injection into the light emitting layer is greatly improved, and the driving voltage is reduced.

[0029]

Embodiments of the present invention will be described below in detail.

(Example 1) An ITO (indium tin oxide) was formed on a glass substrate by sputtering to form an anode. The resistance value was 15Ω / □. A sublimation-purified metal-free phthalocyanine was formed thereon as a hole injecting layer by resistance heating vacuum evaporation to a thickness of 30 nm. At this time, the substrate was not heated. Thereafter, a coating solution was prepared by dissolving polycarbonate z200 (manufactured by Mitsubishi Gas Chemical Company) in dichloromethane at 2% by weight, and this was impregnated into the metal-free phthalocyanine layer by dip coating.
Dry at 80 ° C. for 30 minutes. The thickness of the hole injection layer + polymer binder was 32 nm. Further, tris- (8-hydroxyquinolinol) aluminum was formed thereon as a light emitting layer by vacuum evaporation at 75 nm, and finally MgAg was formed as a cathode by vacuum evaporation (evaporation rate ratio: 10:
1) was formed into a 250 nm film.

When the light emission characteristics of this device were measured in air, it was 1100 cd / m ² at a DC voltage of 10 V applied.
A very bright green luminescence was obtained. In addition, 6 mA /
When emitted continuously at a constant current density of 5 cm ² , 5
Even after 00 hours, the state of surface light emission was maintained, at which point the applied voltage increased from 7.8 V to 8.7 V, and the emission luminance dropped from 200 cd / m ² to 120 cd / m ² . This indicates that both the charge injection efficiency and the durability of the organic thin film EL device of the present invention are improved as compared with the comparative example.

(Example 2) An element was prepared in the same manner as in Example 1, except that the temperature of the substrate was 100 when the hole injection layer was vacuum deposited.
° C. When the light emission characteristics of this device were measured in air, a very bright green light emission of 1000 cd / m ² was obtained at a DC voltage of 10 V applied. In addition, 6mA / c
When continuous light emission at constant current density of m ^2, 50
After 0 hour, the state of surface light emission was maintained, at which point the applied voltage increased from 7.7 V to 8.7 V, and the emission luminance dropped from 190 cd / m ² to 110 cd / m ² . This was almost the same as the characteristics of the device shown in Example 1, and was not affected by the deposition conditions of the hole injection layer.

Example 3 A device was prepared in the same manner as in Example 1, except that chlordiane blue purified by sublimation was formed as a hole injection layer by vacuum evaporation to a thickness of 30 nm.

When the light emission characteristics of the device were measured in air, a very bright green light emission of 840 cd / m ² was obtained at a DC voltage of 10 V applied. In addition, 6mA / c
When continuous light emission at constant current density of m ^2, 50
After 0 hour, the state of surface light emission was maintained. At that time, the applied voltage was increased from 8.5 V to 10.1 V, and the emission luminance was decreased from 180 cd / m ² to 110 cd / m ² .

Example 4 An organic thin film E was produced in the same manner as in Example 1.
An L element was prepared, except that a film of AlLi (deposition rate ratio 3: 1) was formed to a thickness of 25 nm by vacuum deposition as a cathode.

When the light emission characteristics of this device were measured in air, it was found that the applied voltage was 1500 cd / m ² at a DC voltage of 10 V.
A very bright green luminescence was obtained. In addition, 6 mA /
When emitted continuously at a constant current density of 5 cm ² , 5
Even after 00 hours, the state of surface light emission continued, at which point the applied voltage increased from 6.9 V to 7.6 V, and the emission luminance dropped from 220 cd / m ² to 150 cd / m ² .

(Example 5) As in Example 1, the organic thin film E
An L element was prepared, except that SnO ₂ was formed as an anode by sputtering so that the resistance was 15 Ω / □.

When the light emission characteristics of this device were measured in the air, it was determined that the applied voltage was 1500 cd / m ² at a DC voltage of 10 V.
A very bright green luminescence was obtained. In addition, 6 mA /
When emitted continuously at a constant current density of 5 cm ² , 5
After 00 hours, the state of surface light emission was maintained, at which point the applied voltage increased from 7.3 V to 9.0 V, and the emission luminance dropped from 220 cd / m ² to 150 cd / m ² .

Example 6 An ITO was formed on a glass substrate by sputtering to form an anode. The resistance value was 15Ω / □. As a hole injection layer, chlordiane blue purified by sublimation was formed thereon by vacuum evaporation at 30 nm. At this time, the substrate was not heated. Viscol 660-P (manufactured by Sanyo Chemical Industry Co., Ltd.) was dispersed thereon in isopropyl alcohol to form a 20 nm film by spin coating, and then the substrate was heated to 160 ° C. to be dissolved and impregnated in the chlordiane blue layer. . This heating was performed for one hour. Further, a tris-
(8-hydroxyquinolinol) aluminum 75n
m by vacuum evaporation, and finally MgAg
(Evaporation rate ratio: 10: 1) to form a 250 nm film.

When the light emission characteristics of this device were measured in air, a very bright green light emission of 760 cd / m ² was obtained at a DC voltage of 10 V applied. In addition, 6mA / c
When continuous light emission at constant current density of m ^2, 50
After 0 hour, the state of surface light emission was maintained. At that time, the applied voltage increased from 8.1 V to 9.9 V, and the emission luminance dropped from 190 cd / m ² to 140 cd / m ² . This indicates that both the charge injection efficiency and the durability of the organic thin film EL device of the present invention are improved as compared with the comparative example.

Example 7 An ITO was formed on a glass substrate by sputtering to form an anode. The resistance value was 15Ω / □. A sublimation-purified metal-free phthalocyanine was formed thereon as a hole injection layer by vacuum evaporation at 30 nm. At this time, the substrate was not heated. Then, 1,1-bis (4-diparatolylaminophenyl)
A coating solution was prepared by dissolving cyclohexane and poly (N-vinylcarbazole) at a weight ratio of 5: 1 in dichloromethane at 2% by weight, and this was impregnated into the metal-free phthalocyanine layer by dip coating. Dried for minutes. The thickness of the hole injection layer + polymer binder is 33 nm.
Met. Further, a tris- (8-
(Hydroxyquinolinol) aluminum was formed into a film by vacuum deposition of 75 nm, and finally, 250 nm of MgAg (deposition rate ratio: 10: 1) was formed as a cathode.

When the light emission characteristics of this element were measured in air, it was found that the applied voltage was 2000 cd / m ² at a DC voltage of 10 V.
A very bright green luminescence was obtained. In addition, 6 mA /
When emitted continuously at a constant current density of 5 cm ² , 5
Even after 00 hours, the state of surface light emission was maintained, at which point the applied voltage increased from 6.5 V to 7.2 V, and the emission luminance dropped from 220 cd / m ² to 160 cd / m ² . This indicates that both the charge injection efficiency and the durability of the organic thin film EL device of the present invention are improved as compared with the comparative example.

Example 8 An ITO was formed on a glass substrate by sputtering to form an anode. The resistance value was 15Ω / □. A sublimation-purified titanyl phthalocyanine was formed thereon as a hole injection layer by vacuum evaporation at 30 nm. At this time, the substrate was not heated. Then, the metal-free phthalocyanine and poly (N-vinylcarbazole) were added to tetrahydrofuran at a weight ratio of 1: 1 to 2
A coating solution was prepared by dissolving it in a weight% and impregnating the titanyl phthalocyanine layer by dip coating.
Dry at 0 ° C. for 30 minutes. The thickness of the hole injection layer + polymer binder was 33 nm. Further, tris- (8-hydroxyquinolinol) aluminum was formed thereon as a light emitting layer by vacuum evaporation at 75 nm, and finally MgAg (evaporation rate ratio 10: 1) was formed as a cathode at 250 nm.

When the light emission characteristics of this device were measured in air, it was found that the applied voltage was 1800 cd / m ² at a DC voltage of 10 V.
A very bright green luminescence was obtained. In addition, 6 mA /
When emitted continuously at a constant current density of 5 cm ² , 5
After 00 hours, the state of surface light emission was maintained, at which point the applied voltage increased from 7.5 V to 8.3 V, and the emission luminance dropped from 170 cd / m ² to 120 cd / m ² . This indicates that both the charge injection efficiency and the durability of the organic thin film EL device of the present invention are improved as compared with the comparative example.

Comparative Example 1 An organic thin film E was prepared in the same manner as in Example 1.
An L element was prepared, but the polymer binder was not impregnated into the hole injection layer.

When the light emission characteristics of this device were measured in air, a green light emission of 95 cd / m ² was obtained at a DC voltage of 10 V applied. When light was continuously emitted at a constant current density of 6 mA / cm ² , the applied voltage increased from 10.8 V to 17.2 V after 36 hours, and the emission luminance increased from 180 cd / m ² to 23 cd / m ² . The element was broken down when it fell.

Comparative Example 2 An organic thin film EL device was prepared in the same manner as in Example 1. However, the polymer binder was not impregnated into the hole injection layer, and 1,1-bis ( 4-
Diparatolylaminophenyl) cyclohexane was deposited to a thickness of 5 nm by vacuum evaporation.

When the light emission characteristics of this device were measured in air, green light emission of 270 cd / m ² was obtained at a DC voltage of 10 V applied. When the light was continuously emitted at a constant current density of 6 mA / cm ² , after 156 hours,
When the applied voltage increased from 9.3 V to 16.8 V and the light emission luminance decreased from 190 cd / m ² to 45 cd / m ² , the device was broken down.

[0049]

As described above, according to the present invention, an organic thin film EL device having improved charge injection efficiency and durability due to improved stability of the hole injection layer can be obtained.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing one example of an organic thin film EL device of the present invention.

FIG. 2 is a cross-sectional view showing one example of a conventional organic thin film EL device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Substrate 2 Anode 3 Hole injection layer 4 Light emitting layer 5 Cathode 6 Polymer binder 7 Low molecular organic P-type semiconductor

Claims (4)

(57) [Claims] An organic thin film having at least a hole injection layer and a light emitting layer between a pair of transparent electrodes.
In the L element, the hole injection layer is in contact with the anode, and
An organic thin-film EL device comprising a low-molecular organic P-type semiconductor containing a polymer binder in a void and having a thickness of 100 nm or less. 2. The organic thin film E according to claim 1, wherein the polymer binder contains a low-molecular organic P-type semiconductor.
L element. 3. A low-molecular-weight organic P-type semiconductor having a thickness of 100 nm.
2. The method according to claim 1, wherein the hole injection layer is formed by impregnating a polymer binder after forming the film. 4. A low-molecular organic P-type semiconductor having a thickness of 100 nm
3. The method for producing an organic thin film EL device according to claim 2, wherein a hole injection layer is formed by impregnating a polymer binder containing an organic P-type semiconductor after forming the film.