JP2000323275A - Method for improving characteristic of organic electroluminescent element by heating after gamma ray irradiation - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress exfoliation of an interface and crystallization of polymer generated during heat treatment by bridging action and to improve the interface and a film quality by irradiating an EL element with gamma rays before the heat treatment, slightly introducing a bridge between polymers, and treating them by heat. SOLUTION: A polymer thin film of thickness 100 nm formed by dispersing tris-(8-hydroxyquinolinats) aluminum to polyvinylcarbazole at the ratio 25:4 is prepared on an indium-tin oxide transparent electrode substrate by a pull method and an Mg/Ag electrode is laminated thereon so that an EL element is constructed. The device has gamma rays irradiated thereon, is treated by heat of 230 deg.C a little higher than the glass transition point of the polyvinylcarbazole for 10 minutes, and is impressed with a voltage so as to evaluate its light emitting characteristic. As the irradiation quantity is increased, the light emitting efficiency decrease by the heat treatment is reduced and if being heat treated after irradiated at, for example, 5 kGy, the interface is improved and the light emitting efficiency is improved by approximately 20%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION An organic electroluminescence (EL) device is composed of an organic thin film having a thickness of about 100 nm, and holes and electrons are injected from an anode and a cathode electrode into the thin film to emit light. Organic EL devices are promising as thin-film displays because they emit light by themselves.
However, heat generation during light emission causes separation or disorder of the interface between the hole transport layer / anode electrode and the interface between the hole transport layer / light emission and the electron transport layer, which causes a problem of deterioration of the element such as reduction in luminance.

[0002] By overcoming this deterioration by the method of the present invention, it is possible to produce a device with less deterioration and a long life which can be used even at a high temperature.

[0003]

2. Description of the Related Art Organic EL devices include a stacked type and a single layer type.
Further, the organic EL element has at least one layer using a polymer thin film having a hole transporting ability (combined of the polymers A and B) shown in FIG. There is an element in which an electron transport agent is dispersed.

That is, conventionally, a polymer compound having a chemical formula as shown in FIG. 1 has been used as a polymer compound for forming a hole transport layer of an organic EL device. As shown in FIG. 2A, the organic EL device having this hole transport layer is formed on an indium-tin oxide (ITO) electrode formed on a glass substrate, and the polymer compound A shown in FIG. , B, and a light-emitting and electron-transporting layer and a back electrode are sequentially formed on the hole-transporting layer. Alternatively, as shown in FIG. 2 (2), the organic EL element is obtained by dispersing a luminescent agent and an electron transporting agent in the polymer compound shown in FIG. 1 on an ITO electrode formed on a glass substrate. Is formed as a hole transport layer, and a back electrode is formed on the transport layer.

As for the hole transport layer, the size of an organic molecule having a hole transport ability is increased so as not to form a polycrystalline thin film (specifically, a triphenylamine derivative having a high glass transition point is synthesized: 58th Applied Physics) Proceedings of Academic Lecture Meeting of the Society, 1998.10.Akita University) was held. That is,
Since it is necessary to improve the heat resistance of the hole (hole) transport material in order to improve the reliability of the organic EL element, by using a high heat-resistant triphenylamine tetramer as the hole transport material. The drive life was greatly improved.

Furthermore, a thin film is formed on an indium-tin oxide transparent electrode substrate by vacuum evaporation, a light emitting and electron transporting layer is further formed, and a heat treatment is performed at a temperature slightly higher than the glass transition point to obtain an interface or a thin film. Improve film quality and improve EL
The longevity of the element was attempted (59th Annual Meeting of the Japan Society of Applied Physics, 1998.9. Hiroshima University).
That is, a long-life organic EL device was obtained by using a triphenylamine tetramer, which is a highly heat-resistant hole transporting material, as a hole transporting layer and performing a heat treatment in a vacuum.

[0007]

In an EL device comprising a polymer thin film as a component, heat treatment is carried out at a temperature slightly higher than the glass transition point, and if an attempt is made to improve the interface or film quality, the peeling of the interface, The structural stability of the polymer thin film was lost, and it was impossible to improve the characteristics of the EL element. That is, the separation or disorder of the interface between the hole transport layer / anode electrode and the interface between the hole transport layer / light emission and the electron transport layer caused deterioration of the element such as reduction in luminance.

[0008]

Means for Solving the Problems The present inventor has already proposed EL
A method for irradiating the device with gamma rays to prevent polycrystallization of the polymer thin film when the EL device is used has been filed (Japanese Patent Application No. 11-28151). Gamma rays to introduce a slight cross-link between the polymers, and then perform a heat treatment to suppress the separation of the interface and the crystallization of the polymer during the heat treatment due to the cross-linking action. This is a method for improving the characteristics of the EL element by improving the quality of the EL element. An electron beam can be used for irradiating the EL element.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION Irradiation of gamma rays causes cross-linking in a polymer thin film which is a component of an EL element, and is caused by foaming and crystallization of the polymer thin film during heat treatment after the cross-linking. Separation and turbulence at the interface are suppressed. As a result, the interface and the film quality are improved by heating the EL element at a temperature just above the glass transition point after the irradiation of the gamma ray, thereby improving the characteristics of the EL element.

[0010]

EXAMPLES The present invention will be described below with reference to examples.

[0011]

Example 1 Tris- (8-
(Hydroxyquinolinato) aluminum is dispersed in a ratio of 25: 4, and a polymer thin film having a thickness of 100 nm is formed on an indium-tin oxide transparent electrode substrate by a pull-up method.
An EL element was constructed by laminating Mg / Ag electrodes thereon. The device was irradiated with gamma rays, heat-treated at 230 ° C. slightly higher than the glass transition point of polyvinyl carbazole for 10 minutes, and then a voltage was applied to evaluate the light emission characteristics.

When gamma rays are not irradiated, the heat treatment reduces the luminous efficiency of 0.9 cd / A to 20%,
It was 0.18 cd / A. As the irradiation dose was increased, the decrease in the luminous efficiency due to the heat treatment was reduced to 5 kGy.
By the irradiation, the interface was improved by the heat treatment, and the luminous efficiency increased by about 20% to reach 1.1 cd / A (FIG. 3).

[0013]

EXAMPLE 2 A thin film of N-phenyl-N'-polyacryloyloxymethylene-NN'-bis (3-methylphenyl) -1,1'-biphenyl-4,4-diamine was indium-coated to a thickness of 60 nm. Prepared on a tin oxide transparent electrode substrate, tris- (8-hydroxyquinolinato) aluminum was deposited thereon by vacuum evaporation to a thickness of 60 nm,
Further, an EL element was constructed by laminating Mg / Ag electrodes.
After irradiating 5 Kgy with gamma rays, heat treatment is performed at 140 ° C. for 10 minutes, so that the luminous efficiency is 0.6 cd / A to 2.
Increased to 3 cd / A.

[0014]

According to the present invention, crosslinking is introduced into the polymer thin film constituting the EL element by irradiation with gamma rays, and heat treatment is performed at a temperature just above the glass transition point, whereby crystallization or foaming of the polymer thin film due to heating is performed. In this state, the adhesion of the interface between the electrode and the other organic thin film layer in contact with the polymer thin film is improved, and as a result, the characteristics of the EL element are improved and the life of the EL element can be extended. According to this method, an EL element capable of continuous light emission in a higher temperature environment can be developed, so that the use of the EL element is expanded.

[Brief description of the drawings]

FIG. 1 is a diagram showing an example of a polymer having a hole transporting ability.

FIG. 2 is a diagram schematically showing an EL element. In FIG. 2A, a hole transporting layer and a light emitting and electron transporting layer are provided on the ITO electrode. In FIG. 2B, a hole transporting layer and a light emitting and electron transporting layer are provided on the ITO electrode.

FIG. 3 is a diagram showing a gamma-ray irradiation effect on heat treatment of an EL element.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI theme coat ゛ (reference) H05B 33/14 H05B 33/14 A // C08L 39:04 F term (reference) 3K007 AB00 AB15 CA01 CB01 DA00 DB03 FA00 FA01 FA03 4F073 AA05 BA18 BA34 BB01 CA41 GA01 HA04 HA05

Claims (1)

[Claims] 1. A method for improving the characteristics of an organic electroluminescence device by irradiating the device with gamma rays and then heating the device.