JPH04264189A - Electroluminescent element - Google Patents

Abstract

PURPOSE:To obtain an element excellent in uniform luminescence and reliability by providing an electron-injection layer and a hole-injection layer in a charge- injection layer, the electron-injection layer injecting electrons from a negative pole into a luminescent layer, and using a specific compound as a hole-injection material. CONSTITUTION:An electroluminescent element which comprises a pair of electrodes, i.e., a positive pole 2 and a negative pole 5, and a luminescent layer 4 and a charge-injection layer held therebetween. The positive and negative poles 2 and 5 inject electrons and holes, respectively, and also applies an electric field to the luminescent layer 4 and the charge-injection layer to conduct the charges injected, the charge-injection layer comprising at least one of an electron-injection layer 7 and a hole-injection layer 3. The electron-injection layer 7 conducts electrons from the negative pole 5 to inject them into the luminescent layer 4. The hole-injection layer 3 is made of a compound given by the formula and conducts holes from the positive pole to inject them into the luminescent layer 4. The luminescent layer 4 reunites the electrons and holes injected. The compound of the formula can be synthesized, for example, by subjecting the corresponding carbonyl compounds to a coupling reaction.

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 electroluminescent device used as a light emitting source for various display devices, and more particularly to an electroluminescent device provided with a hole injection layer having excellent film formation properties.

0002

2. Description of the Related Art With the rapid increase in demand for flat displays to replace conventional cathode ray tubes, various display elements are being actively developed and put into practical use. Electroluminescent devices (hereinafter also referred to as EL devices) also meet these needs, and in particular, as all-solid-state light emitting devices, they are attracting attention due to their high resolution and high visibility, which are not found in other displays. What is currently in practical use is an EL element made of an inorganic material using a ZnS/Mn-based light emitting layer. However, this type of inorganic EL element requires a high driving voltage of about 200 V to emit light, so the driving method is complicated and the manufacturing cost is high. Furthermore, the efficiency of blue light emission is low, making it difficult to produce full color. E
In order to apply L elements as displays, characteristics such as low driving voltage, high brightness, long life, and full color are required. Full-color EL devices using organic materials can be expected due to the diversity of organic materials. In addition, it has been reported that high brightness was obtained even at low applied voltages in a multilayer structure using a quinolinol compound as a light-emitting material and a diamine compound as a charge injection material, and since then research has been active toward practical application. .

0003

[Problems to be Solved by the Invention] However, at present, an organic EL device that satisfies all required characteristics has not been obtained.
The search for charge injection materials is being actively pursued. As a light-emitting material, it is required to have excellent film-forming properties, high luminous efficiency, and stability, and as a charge-injection material, it is required to have excellent film-forming properties and to have high charge injection efficiency and stable charge injection into the light-emitting layer. Materials disclosed in Japanese Unexamined Patent Application Publication No. 57-51781 and Japanese Unexamined Patent Publication No. 59-194393 are known. The present invention has been made in view of the above-mentioned points, and its purpose is to provide an electroluminescent device with uniform luminescence and excellent reliability by developing a hole-injecting material with excellent film-forming properties.

0004

[Means for Solving the Problems] According to the present invention, the above-mentioned object has a pair of positive and negative electrodes, a light-emitting layer and a charge injection layer sandwiched between them, and the pair of positive and negative electrodes is configured to inject electrons, respectively. While injecting holes, an electric field is applied to the light emitting layer and the charge injection layer to conduct the injected charges, the charge injection layer is composed of at least one of an electron injection layer and a hole injection layer, and the electron injection layer is Electrons from the negative electrode are conducted and injected into the emissive layer, the hole injection layer is made of a compound represented by general chemical formula I, holes from the positive electrode are conducted and injected into the emissive layer, and the emissive layer is injected. This is achieved by recombining electrons and holes.

[0005]

[Case 2]

(In general chemical formula (I), R1, R
2, R7 and R8 each represent an optionally substituted alkyl group, aryl group, alkenyl group, aralkyl group or thenyl group, and R3, R4, R5 and R6 each represent a hydrogen atom, a halogen atom, an alkyl group, an alkoxy group, Represents a hydroxy group or an aryl group. )

Specific examples of the compounds represented by general chemical formula (I) are shown in chemical formulas 1 to 24.

[0008]

[Chemical formula 3]

[0009]

[C4]

0010

[C5]

[0011]

[Operation] In order to achieve the above object, the present inventors conducted a number of experiments while intensively studying various organic materials, and as a result, the present inventors found that the compound represented by the above general chemical formula (I) can be used as a hole-injecting substance. It has been found that good film formation is possible by this method.

0012

EXAMPLES The compound of general chemical formula (I) used in the present invention can be synthesized by a known method. For example, each corresponding carbonyl compound is TiCl4
(Chemical Letters Chem.Le
tt. p1041 (1973)).

FIG. 1 is a sectional view showing an electroluminescent device according to an embodiment of the present invention. Light emission proceeds in the direction of the arrow in the figure. A semi-transparent film made of gold, nickel, etc. or indium tin oxide (ITO) is placed on an insulating transparent substrate 1 made of glass or the like.
), a positive electrode (hole injection electrode) 2 made of a transparent conductive film such as tin oxide (SnO2) is formed by resistance heating evaporation, electron beam evaporation, or sputtering. This positive electrode 2 preferably has a thickness of 100 to 3000 Å in order to have transparency. Next, an organic thin film including a hole injection layer 3 and a light emitting layer 4 is formed in this order. Both layers can be formed by spin coating, casting, LB method resistance heating evaporation, electron beam evaporation, etc., but resistance heating evaporation is preferred from the viewpoint of uniformity of the film. Further, the film thicknesses of both layers are each 200 to 2000 Å, preferably 300 to 800 Å. Finally, the negative electrode 5 is formed by vapor deposition. Note that as the material for the negative electrode, Mg, Mg/Ag, In, Ca, Al, etc. having a small work function are used.

After forming ITO with a thickness of about 1000 Å on a 50 mm square glass substrate, the substrate was placed in a resistance heating vapor deposition apparatus, and a hole injection layer and a light emitting layer were sequentially formed. During film formation, the pressure inside the vacuum chamber was reduced to 6 x 10-6 Torr. For the hole injection layer, a compound represented by the chemical formula 1 was used, and a film thickness of 600 Å was formed by heating at a boat temperature of 150° C. to 180° C. at a film formation rate of 2 Å/sec. Next, without breaking the vacuum of the vacuum layer, tris(8-hydroxyquinoline) aluminum was added as a light emitting layer at a boat temperature of 100 to 300.
600 Å at a deposition rate of 2 Å/sec.
Formed. After this, the sample was removed from the vacuum layer and the diameter
A stainless steel mask having a dot pattern of mm was attached and newly set in a resistance heating vapor deposition apparatus to form Mg/Ag (ratio of 10:1) as a negative electrode.

The hole injection layer made of the compound represented by the chemical formula 1 becomes a uniform vapor deposited film, and the obtained E
When a DC voltage of 10 V was applied to the L element, uniform green (emission center wavelength: 530 nm) light emission was obtained.
Good stability was obtained even during continuous light emission over a period of time.

Similar results were obtained for an EL device obtained using the compound represented by the chemical formula 3 in the same manner as above.

FIG. 2 is a sectional view showing an electroluminescent device according to another embodiment of the invention. A positive electrode 2 made of a semi-transparent film such as gold or nickel or a transparent conductive film such as indium tin oxide (ITO) or tin oxide (SnO2) is formed on a transparent substrate 1 such as glass, and a hole injection layer 3 is formed. ，
Three layers including a light emitting layer 4 and an electron injection layer 7 are formed. The hole injection layer 3, the light emitting layer 4 and the electron injection layer each have a thickness of 200 to 2000 Å, preferably 300 to 800 Å. Finally, the negative electrode 5 is deposited using Mg, Mg/Ag, In, Ca, Al, or the like.

After forming ITO with a thickness of about 1000 Å on a 50 mm square glass substrate, the substrate was placed in a resistance heating vapor deposition apparatus, and a hole injection layer, a light emitting layer, and an electron transport layer were sequentially formed. During film formation, the pressure inside the vacuum chamber was reduced to 6 x 10-6 Torr. For the hole injection layer, a compound represented by the chemical formula 2 was used, and a film thickness of 600 Å was formed by heating at a boat temperature in the range of 150° C. to 180° C. at a film formation rate of 2 Å/sec. next,
Without breaking the vacuum of the vacuum layer, Tris (
8-Hydroxyquinoline) aluminum at boat temperature 1
The film was heated in the range of 00 to 300°C, and a film thickness of 600 Å was formed at a film formation rate of 2 Å/sec. Furthermore, without breaking the vacuum of the vacuum layer, a perylenetetracarboxylic acid derivative represented by the following chemical formula 25 was subsequently formed to a thickness of 700 Å as an electron injection layer. After this, the sample was taken out from the vacuum layer, a stainless steel mask consisting of a dot pattern with a diameter of 5 mm was attached, and it was newly set in a resistance heating evaporation apparatus, and Mg was used as a negative electrode.
/Ag (10:1 ratio).

[0019]

[C6]

The hole injection layer made of the compound represented by the chemical formula 2 becomes a uniform vapor deposited film, and when a DC voltage of 10 V is applied to the obtained organic EL device with a diameter of 5 mm, it turns green (
Emission center wavelength: 530 nm) uniform light emission was obtained, and 1
Good stability was obtained even during continuous light emission for over 00 hours.

[0021]

According to the present invention, a pair of positive and negative electrodes, a light-emitting layer and a charge injection layer are sandwiched between them, and the pair of positive and negative electrodes injects electrons and holes, respectively. An electric field is applied to the charge injection layer to conduct the injected charges, the charge injection layer is composed of at least one of an electron injection layer and a hole injection layer, and the electron injection layer conducts electrons from the negative electrode. injected into the light emitting layer, and the hole injection layer has the general chemical formula I
The compound represented by the general chemical formula (I) conducts holes from the positive electrode and injects them into the light-emitting layer, and the light-emitting layer recombines the injected electrons and holes. Due to the good film-forming properties, a hole-injection layer without pinholes can be obtained, and an electroluminescent device with uniform luminescence and excellent reliability can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing an electroluminescent device according to an embodiment of the present invention.

FIG. 2 is a sectional view showing an electroluminescent device according to a different embodiment of the present invention.

[Explanation of symbols]

1 Insulating transparent substrate 2 Positive electrode 3 Hole injection layer 4. Luminescent layer 5 Negative electrode 6. Drive DC power supply 7 Electron injection layer

Claims (6)

[Claims] Claim 1: A device comprising a pair of positive and negative electrodes and a light-emitting layer and a charge injection layer sandwiched between them, the pair of positive and negative electrodes injecting electrons and holes, respectively, into the light-emitting layer and the charge injection layer. Applying an electric field to conduct the injected charges, the charge injection layer comprising at least one of an electron injection layer and a hole injection layer, and the electron injection layer conducts electrons from the negative electrode and injects them into the light emitting layer. , the hole injection layer is made of a compound represented by the general chemical formula (I), conducts holes from the positive electrode and injects them into the light emitting layer, and the light emitting layer recombines the injected electrons and holes. An electroluminescent element characterized by: [Formula 1] (In general chemical formula (I), R1 , R2 , R7
and R8 are each an optionally substituted alkyl group,
It represents an aryl group, an alkenyl group, an aralkyl group or a thenyl group, and R3, R4, R5 and R6 each represent a hydrogen atom, a halogen atom, an alkyl group, an alkoxy group, a hydroxy group or an aryl group. ) 2. The device according to claim 1, wherein the compound represented by the general chemical formula I is R1, R2, R7, R8.
are each a phenyl group, R3 , R4 , R5 , R
An electroluminescent device characterized in that each of 6 is a hydrogen atom. 3. The device according to claim 1, wherein the compound represented by general chemical formula I comprises R1, R2, R7, R8
are each an ethyl group, R3 , R4 , R5 , R
An electroluminescent device characterized in that each of 6 is a hydrogen atom. 4. The device according to claim 1, wherein the compound represented by the general chemical formula I comprises R1, R2, R7, R8
are each a methyl group, R3 , R4 , R5 , R
An electroluminescent device characterized in that each of 6 is a hydrogen atom. 5. The electroluminescent device according to claim 1, wherein the light emitting layer is tris(8-hydroxyquinoline)aluminum. 6. The electroluminescent device according to claim 1, wherein the electron injection layer is a perylenetetracarboxylic acid derivative.