JP2926845B2 - Organic thin film EL device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an organic thin-film light emitting device used for a flat light source or a display.

[Conventional technology] EL (electroluminescence) elements made from organic materials have attracted attention as realizing inexpensive large-area film-shaped full-color display elements with their abundant number of materials and molecular-level synthesis technology. I have. For example, a DC-driven organic thin-film light-emitting element in which a thin film is formed by a LB method or a vacuum deposition method using a condensed polycyclic aromatic system such as anthracene or perylene as a raw material has been manufactured, and its light-emitting characteristics have been studied.

Furthermore, recently, a new type of organic thin-film light-emitting device having an organic thin film having two structures has been reported and has attracted strong interest (Applied Physic Letters, Vol. 51, 913).
Page, 1987). According to the report, as shown in FIG. 4, a metal chelate compound that emits strong fluorescence was used as the light emitting layer 44.
First, an amine-based material was used for the hole-injecting layer 43 made of a hole-conducting organic material, and these were used as transparent electrodes formed on a glass substrate 41.
It is disclosed that bright green light emission was obtained by inserting between the metal electrode 45 and the metal electrode 45, and a luminance of about 100 cd / m ² was obtained by applying a direct current of 6 to 7V.

This organic thin-film EL device has the potential to be manufactured by a simple vacuum vapor method and a low-temperature film-forming process at 100 ° C. or lower, and to provide a light-emitting device from red to blue at low cost.

[Problems to be Solved by the Invention] However, the light emission characteristics with respect to the applied voltage of the organic thin film EL device having the structure shown in FIG. 4 tend to shift to the higher voltage side with the voltage application time. Is observed even when the organic material is replaced, and is considered to be caused by the element structure itself. Such a phenomenon that the light emission characteristics change with the driving of the element causes the following problem. That is, an increase in the light emission threshold voltage makes it difficult to perform constant voltage driving, which is an easy driving method, and further causes a decrease in light emission efficiency. Increasing the drive voltage in order to compensate for a decrease in luminance has led to accelerated deterioration due to device light emission efficiency, dielectric breakdown, and heat generation.

An object of the present invention is to provide an organic thin-film EL device capable of overcoming the above-described problems and capable of emitting multicolor and high-brightness light at a constant voltage, having little device deterioration, and having a long service life. I do.

[Means for Solving the Problems] A means provided by the present invention for solving the above-mentioned problems is that a light emitting layer composed of an organic phosphor and a charge injection layer are formed between a pair of electrodes at least one of which is transparent. In the organic thin film EL device including the organic thin film layer to be formed, the mixed layer containing the electrode material and the charge injection material is inserted between the charge injection layer and the electrode, and / or the electrode is formed between the light emitting layer and the electrode. Organic thin film EL, characterized by inserting a mixed layer containing material and organic phosphor
Element.

[Operation] The phenomenon that the light emission characteristics of an organic thin film EL element with respect to an applied voltage shifts to a higher voltage side with a voltage application time is caused by an organic thin film
There is little dependence on the organic material used for the EL element, and there is a cause in the element structure itself.

That is, the shift of the emission characteristics on the high voltage side is caused by the fact that the energy barrier at the electrode interface increases with the voltage application time. As a result of various studies on this point, it has been found that a deep trap level is generated at the electrode interface by the strong electric field injection, thereby forming a homopolar charge layer, that is, a homocharge layer at the electrode interface. The newly formed homocharge layer increases the energy barrier at the interface. The homo-charge is accumulated with the driving of the device, and as a result, the driving voltage increases together with the voltage application time.

The relationship between the homocharge layer and the rise in the energy barrier is proportional to the amount of charge accumulated at the interface and the power of the charge layer width.
Insert a relatively low resistance layer between the electrode and the organic thin film,
By dispersing the homoelectrode layer concentrated at the electrode interface and increasing the width of the charge layer, an increase in the energy barrier width can be suppressed. In the present invention, the relaxation of the homocharge layer
As a method of neutralization, a mixed layer in which an organic charge transfer complex or an electrode material is added is inserted into an organic thin film layer in contact with an electrode.

In addition, by inserting the mixed layer at the interface with the electrode, the adhesion to the electrode is improved, and as a result, element deterioration due to electrode peeling can be significantly reduced.

[Example] Hereinafter, an example of the present invention will be described in detail.

Example 1 As shown in FIG. 1, after a transparent electrode 2 made of ITO or the like was formed on a glass substrate 1, N, N, N ', N'-tetraphenyl-4,4'-diaminodiphenyl ( Hereinafter, the hole injection layer 3 made of diamine is abbreviated to 600 °, and the light emitting layer 4 is formed to 500 m by using tris (8-hydroxyquinoline) aluminum (hereinafter abbreviated as aluminoquinoline) as an organic phosphor. did. Subsequently, a mixed layer 5 containing aluminum quinoline and an organic charge transfer complex composed of TTF (tetrathiofulvalene) and TCNQ (tetracyanoquinodimethane) was formed in a continuously changing distribution as shown in FIG.
Form 300mm. Finally, a metal electrode 6 of an alloy in which Mg and In are mixed at a ratio of 10: 1 is formed by an electron beam evaporation method at 1500 ° to complete an organic thin film light emitting device.

When the light emission characteristics of this device were measured in dry nitrogen,
When a DC voltage of about 5 V was applied, green light emission of 300 cd / m ² was obtained. When an aging test was performed on the organic thin film light emitting device at a current density of 0.5 mA / cm ² , the luminance half life was 1,000 hours or more. The reliability of this device has been greatly improved, as it has been between 100 and 500 hours for conventional devices. In addition, the shift in the electrical characteristics was about 5 V, which was significantly lower than in the past.

In the present invention, tris (8-hydroxyquinoline) aluminum organic phosphor is used, but anthracene derivative, pyrene derivative, tetracene derivative, stilbene derivative, perylene derivative, quinone derivative, phenanthrene derivative, naphtan derivative, naphthalimide derivative, phthaloperinone derivative A similar effect was observed when an organic material that emits strong fluorescence in the visible region, such as a cyclopentadiene derivative, a cyanine derivative, or another organic material, was used as the material of the light emitting layer 4. Also, about 10 ^-5 to 10 ^-2 mol of rhodamine, cyanine, pyran, coumarin, fluorene, POP
The emission wavelength can be changed by further adding other highly fluorescent organic molecules such as OP and PBBO. Transparent electrode 2 is ITO
In addition, any conductive material having a work function of 4.5 eV or more such as ZnO: Al, SnO ₂ : Sb, In ₂ O ₃ , Au, CUI _x , and Pt may be used.

The metal electrode 6 may be made of a material other than MgIn as long as it has a work function lower than that of the transparent electrode 2.

Example 2 An organic thin film EL device having the same structure as in FIG. 1 and using a perylene derivative emitting strong fluorescence from 610 nm to 630 nm for the light emitting layer 4 and using a triphenylmethane derivative as the hole injection layer 3 was produced. The mixed layer 5 is made of 100% of perylene derivative to Mg100
% In the electrode direction. Finally, Mg and In
Are formed at a ratio of 10: 1 to form a metal electrode 6 of 1500 mm by electron beam evaporation to complete an organic thin film light emitting device.

The organic thin-film EL element thus produced was excellent in adhesion between the metal electrode and the organic light-emitting layer, and no peeling of the electrode was observed even after long-time operation.

Example 3 As shown in FIG. 3, a naphthalimide derivative that emits strong fluorescence from 530 nm to 550 nm was provided on the light emitting layer 34 and the electron injection layer 35.
Used was aluminum quinoline. The mixture 33 and 36, respectively naphthalimide derivatives and CuI _x, and alumiquinoline and TTF · TCNQ complex consisting of those changes are continuous. Finally, a back metal electrode 37 of an alloy in which Mg and In are mixed at a ratio of 10: 1 is formed by electron beam evaporation at a thickness of 1500 1 to complete an organic thin-film light emitting device.

As the material of the electron injection layer 35, anthracene, tetracene, or the like may be used. Furthermore, diamine, a hole injection layer between the fourth layer or ITO surface and the hole injection layer inserted into ITO surface which is a hole injection electrode, e.g., five layers was inserted a mixed layer obtained by adding a diamine CuI _x structure The same effect was obtained with the device of the above.

In this embodiment, the mixed layer in which the composition is continuously changed is used. However, the mixed layer may be a layer in which the composition is changed discontinuously or the mixed layer has a uniform composition. In addition, the material contained in the mixed layer is not limited to the components of the adjacent electrode or the organic layer, and may be any material that satisfies the required characteristics.

[Effects of the Invention] As described above, according to the present invention, it is possible to provide an excellent element having less characteristic drift with respect to the driving time of the emission characteristic than the conventional organic thin film EL element.
Furthermore, element deterioration due to electrode peeling is reduced, and a long-life organic thin film EL element can be provided.

[Brief description of the drawings]

1 is a sectional view of one embodiment of the present invention, FIG. 2 is a composition distribution diagram of a mixed layer in the embodiment of FIG. 1, FIG. 3 is a sectional view of another embodiment of the present invention, FIG. Is a conventional organic thin film
FIG. 3 is a sectional view of an EL element. 1,31,41… Glass substrate 2,32,42… Transparent electrode 3,43… Hole injection layer 4,34,44… Emitting layer 5,33,36… Mixed layer 6,37,45 …… Metal electrode 35 …… Electron injection layer

Claims (1)

(57) [Claims] 1. An organic thin-film EL device comprising an organic thin-film layer formed by a light-emitting layer composed of an organic phosphor and a charge injection layer between a pair of electrodes at least one of which is transparent. An organic thin film EL comprising a mixed layer containing an electrode material and an organic phosphor inserted between a light-emitting layer and an electrode, and / or a mixed layer containing an electrode material and a charge injection material is inserted between the light-emitting layer and the electrode. element.