JPH04220995A - Organic el element - Google Patents

Abstract

PURPOSE:To enhance the light emission efficiency and prolong the lifetime by allowing hole injection layers to contain aromatic tertiary amine as presented by a specific expression, and laminating in the sequence with smaller ionization potential first started with the anode. CONSTITUTION:An anode 2a, hole injection layers 3a, 3b of 400Angstrom thick, light emission layer 4 of 600Angstrom thick, and a cathode 2b are laminated on a base board 1 made of glass, etc., - otherwise, cathode is laid over the anode, hole injection layers, and light emission layer while an electron implantation layer is interposed. The hole injection layers contain aromatic tertiary amine as given by Eq. I, wherein the ionization potential is smaller in the sequence of the layer 3a, and then 3b. In Eq. I, R1-R8 are H atom, halogen atom, alkyl radical having 1-8C atoms, or alkyl radical having 1-8C atoms, while (l), (m), (n), (o) are numerals between 0-3. The organic light emitting element according to this constitution is equipped with a high light emission efficiency and a long lifetime.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to organic electroluminescent devices used in light sources, displays, and the like.

0002

[Prior Art] Research on electroluminescent devices using organic materials has been conducted for a long time, focusing on anthracene.
These devices had high driving voltage and low luminance and efficiency, so they could not be put to practical use. However,
Recently, an organic electroluminescent device having a two-layer structure in which a light emitting layer and a hole injection layer are laminated has attracted attention because it exhibits high brightness and low driving voltage. (Applied Physics Letters,
51, p. 913, 1987). This organic electroluminescent device produces approximately 1000 cd/m when a DC voltage of 10 V or less is applied.
2 and a brightness efficiency of 1.5 lm/W, which is an order of magnitude higher in performance than conventional organic electroluminescent devices.

0003

[Problems to be Solved by the Invention] As mentioned above, an organic electroluminescent device consisting of two layers, a light emitting layer and a hole injection layer, exhibits high brightness. There were some problems. First, it has a short lifespan. The initial brightness of this organic electroluminescent device was set to 5
When set at 0 cd/m2 and driven with a constant current, the half-life time was as short as about 100 hours. Second,
The problem is that the luminous efficiency has not yet reached a practical level. For example, to obtain a luminance of approximately 100 cd/m2, a driving voltage of 6.5 V and a current density of 5 mA/cm2 were required. Increasing luminous efficiency required further reductions in driving voltage and current density. Regarding the lifespan, an improved version of the above-mentioned light emitting element has been proposed (Japanese Unexamined Patent Application Publication No. 1983-1999).
295695). According to this document, it is explained that stability is improved by forming a layer containing a hole-injecting porphyrin on the anode, and further forming a layer containing a hole-transporting aromatic tertiary amine thereon. However, it is still difficult to say that it is sufficient, and furthermore, the luminous efficiency has not been improved.

[0004] Therefore, the present invention has been made with the object of improving the problems in the above-mentioned conventional techniques. That is, an object of the present invention is to have high luminous efficiency and
An object of the present invention is to provide an organic electroluminescent device with a long life.

[0005]

[Means for Solving the Problems] As a result of intensive research, the present inventors used a certain aromatic tertiary amine in the hole injection layer, and the hole injection layer was sequentially applied to the ionization potential starting from the anode. The inventors have discovered that the above object can be achieved by laminating two or more layers in ascending order of size, and have completed the present invention.

That is, the organic electroluminescent device of the present invention has the following features:
In an organic electroluminescent device in which an anode, a hole injection layer, a light emitting layer, and a cathode are sequentially stacked on a substrate, or an anode, a hole injection layer, a light emitting layer, an electron injection layer, and a cathode are stacked on a substrate, The hole injection layer contains an aromatic tertiary amine represented by the following general formula (I), and is characterized in that two or more hole injection layers are laminated in order of decreasing ionization potential from the anode. .

[0007]

[Formula 2] (wherein, R1 to R6 are hydrogen atoms, halogen atoms,
Alkyl group having 1 to 8 carbon atoms or 1 to 8 carbon atoms
represents an alkoxy group, and l, m, n and o each mean 0 to 3. ) In the present invention, the ionization potential Ip of the light emitting layer is larger than the work function of the anode,
Moreover, it has been found that this is particularly effective when Ip of the hole injection layer is located between Ip of the light emitting layer and the work function of the anode.

The present invention will be explained in detail below. In the present invention, some specific examples of the aromatic tertiary amine contained in the hole injection layer are shown below, but the invention is not limited thereto.

[0009]

[Chemical formula 3]

0010

[C4]

[0011]

[C5]

0012

[C6]

[0013]

[C7]

[0014]

[Chemical formula 8]

[0015]

[Chemical formula 9]

[0016]

[Chemical formula 10]

The hole injection layer is usually formed by vacuum evaporation, spin coating, casting, or the like. Table 1 shows some of the results of measurements of Ip of thin films formed by these methods using an atmospheric photoelectronic surface analyzer AC-1 (manufactured by Riken Keiki Co., Ltd.).

[0018]

[Table 1]

The organic electroluminescent device of the present invention includes a plurality of hole injection layers having different ionization potentials Ip, and the thickness of each hole injection layer is usually 10 to 300 nm.
0 angstrom, preferably 50 to 1000 angstrom.

Any compound can be used for the light-emitting layer as long as it exhibits strong fluorescence in a solid state. Examples include anthracene, naphthalene, pyrene, perylene, butadienes, stilbenes, metal chelating oxinoids, and derivatives thereof.

In the present invention, an electron injection layer may be further laminated on the light emitting layer. This is effective when the compound used for the light emitting layer has poor electron injection/transport properties. As the compound used for the electron injection layer, any compound can be used as long as it has electron transport properties. Examples include fluorenone, anthraquinone, perylene, anthrone and derivatives thereof.

[0022] As the electrode, metals such as gold, silver, copper, aluminum, indium, alloys thereof, ITO,
Metal oxides such as SnO2 are used. It is preferable that at least one of the anode and the cathode of these electrodes is transparent or semitransparent.

The organic electroluminescent device of the present invention is formed by forming each of the layers described above on a substrate made of glass, ceramics, plastic, or the like.

[0024]

[Examples] The present invention will be specifically explained below with reference to Examples, but the present invention is not limited thereto.

FIG. 1 is a schematic cross-sectional view of an example of the organic electroluminescent device of the present invention. In the figure, 1 is a substrate, 2a is a transparent electrode, 2b is a back electrode, 3a is a first hole injection layer, 3
b is a second hole injection layer, and 4 is a light emitting layer.

Example 1 An electroluminescent device having the structure shown in FIG. 1 was manufactured according to the following procedure. (1) HTM-15 was deposited at 1×10 −5 on a glass substrate having an ITO transparent conductive film with a sheet resistance value of 10 Ω/sq.
It was deposited to a thickness of 400 angstroms under a vacuum of Torr. The deposition rate was 2-5 angstroms/sec. (First Hole Injection Layer 3a) (2) HTM-1 was deposited on the formed deposited film under the same conditions to a thickness of 400 angstroms. (Second hole injection layer 3b) (3) Furthermore, tris(8-quinolinol)aluminum was vapor-deposited thereon under the same conditions to a thickness of 600 angstroms. (Light-emitting layer) (4) Finally, magnesium and silver are co-evaporated onto the above-mentioned vapor-deposited film at an atomic ratio of 10:1 to a thickness of 1500 angstroms, thereby forming an organic electroluminescent device. The production has been completed.

In the above case, the work function of ITO and Ip of the tris(8-quinolinol) aluminum film are expressed as follows:
When measured using the device AC-1, each
．． They were 49eV and 5.83eV.

When 4.5 V DC was applied to this organic electroluminescent device using the ITO electrode as the anode and the Mg:Ag electrode as the cathode, a current of 4 mA/cm2 flowed and green light emission with a brightness of 100 cd/m2 was observed. It was done. Further, when this organic electroluminescent device was driven at a constant current of 4 mA/cm2 for 500 hours, the initial brightness of 100 cd/m2 decreased to only 80 cd/m2 at the end. Furthermore, during this time the voltage only increased slightly from 4.5V to 5.5V.

Comparative Example 1 An electroluminescent device was produced in the same manner as in Example 1 except that (2) was omitted. However, the thickness of the hole injection layer is 8
00 angstrom. To observe green light emission with a brightness of 100 cd/m2 from this organic electroluminescent device,
A voltage of 8V was required. Further, the current flowing at this time was 7 mA/cm2. Initial brightness 100cd
/m2, and the half-life time when driven at constant current was 30 hours.

Comparative Example 2 An electroluminescent device was produced in the same manner as in Example 1 except that (1) was omitted. However, the thickness of the hole injection layer is 8
00 angstroms. The half-life time of this organic electroluminescent device was 100 hours when the luminance was set to 100 cd/m 2 and the device was driven at a constant current.

Comparative Example 3 An organic electroluminescent device was produced in the same manner as in Example 1 except that the order of (1) and (2) was reversed. This organic electroluminescent device required a voltage of 7 V to observe green light emission with a brightness of 100 cd/m2. Further, the current flowing at this time was 8 mA/cm2. The half-life time when driving at constant current with the initial brightness set at 100 cd/m2 was 40 hours.

From the above comparison of Example 1 and Comparative Examples 1 to 3, it is understood that the organic electroluminescent device of the present invention has excellent stability and high efficiency.

Examples 2 to 6 Organic electroluminescent devices were produced in the same manner as in Example 1 (2) except that the compounds shown in Table 2 were used. For these organic electroluminescent elements, the luminance is 100 cd/m
The voltage required to observe the green light emission of 2 and the current flowing at this time were measured. The results are shown in Table 2.

[0034]

[Table 2]

From Table 2, it is understood that all organic electroluminescent devices have high efficiency. Furthermore, when these organic electroluminescent devices were tested under the same conditions as in Example 1, they showed equivalent stability.

Example 7 An HTM-4 layer was formed as a third hole injection layer between (2) and (3) of Example 1 in the same manner. However, the thickness of each hole injection layer was 250 angstroms. This organic electroluminescent device required a voltage of 4V to observe green light emission with a brightness of 100 cd/m2. Also,
The current flowing at this time was 3.5 mA/cm2. When this organic electroluminescent device was driven at a constant current of 3.5 mA/cm2 for 500 hours, the initial 100 cd/cm2
In the end, the brightness of m2 decreased only to 87 cd/m2.

It is understood that the stability and efficiency of this organic electroluminescent device are further improved compared to the case where the hole injection layer is made of two layers (Example 1).

Although tris(8-quinolinol)aluminum was used in the light-emitting layer in the above example, the same effect as above was also observed when other fluorescent dyes were used.

[0039]

Effects of the Invention The organic electroluminescent device of the present invention has excellent stability and high luminous efficiency compared to conventional organic electroluminescent devices. Therefore, the organic electroluminescent device of the present invention can be used for displays. It is possible to apply this method to other applications, and its industrial value is high.

[Brief explanation of the drawing]

FIG. 1 is a schematic cross-sectional view of an example of an organic electroluminescent device used in Examples of the present invention.

[Explanation of symbols]

1. ．． ．． ．． Substrate, 2a. ．． ．． ．． transparent electrode, 2b. ．． ．． ．．
Back electrode, 3a. ．． ．． ．． hole injection layer, 3b. ．． ．． ．． hole injection layer, 4. ．． ．． ．． Luminous layer.

Claims (1)

[Claims] 1. An anode, a hole injection layer, a light emitting layer, and a cathode are sequentially stacked on a substrate, or an anode,
In an organic electroluminescent device formed by laminating a hole injection layer, a light emitting layer, an electron injection layer and a cathode, the hole injection layer contains an aromatic tertiary amine represented by the following general formula (I),
An organic electroluminescent device characterized in that two or more hole injection layers are laminated in order of decreasing ionization potential starting from the anode. [Formula 1] (wherein, R1 to R6 are hydrogen atoms, halogen atoms,
Alkyl group having 1 to 8 carbon atoms or 1 to 8 carbon atoms
represents an alkoxy group, and l, m, n and o each mean 0 to 3. )