JP2000268959A - Method for changing emission color of organic EL element - Google Patents

Abstract

(57) [Summary] [Problem] To easily increase the degree of freedom of color selection of emission color. SOLUTION: A light emitting layer contains a plurality of fluorescent materials having different light emission levels depending on a difference in current density of a current flowing therein, and a current is caused to flow through the light emitting layer by changing at least one of its current value and voltage value. As a result, the current density is changed, and the degree of light emission of the fluorescent material is selectively changed to change the emission color. Since light of various colors can be emitted, the degree of freedom in selecting the emission color can be easily increased. As a result, the image on the display panel can be easily and vividly displayed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a method of emitting light from an organic EL device that can be used in a display panel, and more particularly to a method of changing the color of emitted light.

[0002]

2. Description of the Related Art Conventionally, a light emitting layer containing an organic fluorescent material has been provided between a transparent anode layer made of a transparent conductive material such as ITO (indium tin oxide) and a cathode layer made of Mg-Ag. There is an organic EL element sandwiched directly or indirectly. In such an organic EL device, light emission occurs in the following process.

First, holes are directly or indirectly injected from the anode layer into the light emitting layer, and electrons are directly or indirectly injected from the cathode layer into the light emitting layer. The holes and electrons transferred to the light emitting layer respectively recombine and release energy. The fluorescent material contained in the light emitting layer absorbs the emitted energy and is excited, but immediately returns from the excited state to the ground state. At this time, energy corresponding to the difference between the excited state energy level and the ground state energy level is emitted as light energy. The light energy thus emitted to the outside of the light emitting layer passes through the anode layer and is visually recognized as light emission.

In general, an anode layer is formed on a transparent substrate, and light is emitted through the transparent substrate. Further, it has been described that the light emitting layer is indirectly sandwiched between the anode layer and the cathode layer. However, in many organic EL devices, holes injected from the anode layer and electrons injected from the cathode layer are used. A hole transport layer that transports holes is provided between the anode layer and the light emitting layer, or an electron is transported between the cathode layer and the light emitting layer, such that carriers are easily transported to the light emitting layer. An electron transport layer is provided.

On the other hand, as disclosed in Japanese Patent Application Laid-Open No. 9-176630, there is an organic EL device in which the emission color is adjusted by doping a light emitting layer with a dye. In addition, there is an organic EL device having an electron transport layer having both electron transport and light emission separately from the light emitting layer.

[0006]

However, the conventional organic E
In the light emitting method of the L element, a current having a constant current value and a current having a constant voltage value are applied to emit light of one kind of color, and the degree of freedom in selecting the color of emitted light is not sufficient. . The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide a method of changing the emission color of an organic EL element, which can easily increase the degree of freedom in selecting the emission color.

[0007]

According to a first aspect of the present invention, there is provided a method for changing the emission color of an organic EL device, comprising the steps of: providing an organic EL device having a light emitting layer between an anode layer and a cathode layer; A method for changing the color of emitted light, wherein the light emitting layer contains a plurality of fluorescent materials having different degrees of light emission due to a difference in current density of a current flowing therethrough, and the light is emitted to the light emitting layer by at least a current value and a voltage value. It is characterized in that the current density is changed by flowing one of them while changing the other, and the emission color of the fluorescent material is selectively changed to change the emission color.

According to a second aspect of the present invention, there is provided a method for changing the emission color of an organic EL device, wherein the current density is at least 1 order. It is characterized by changing.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS (Effects and Effects) The light emitting layer of the organic EL device used in the present invention contains a plurality of fluorescent materials having different light emission levels due to the difference in current density of the flowing current. Therefore, by changing at least one of the current value and the voltage value of the current flowing through the light emitting layer to change the current density of the current flowing through the light emitting layer, the degree of light emission of the fluorescent material can be selectively changed. As a result, the color of light emitted from the light emitting layer can be changed.

Accordingly, in the present invention, light of various colors is emitted by appropriately selecting a plurality of fluorescent materials and appropriately changing at least one of the current value and the voltage value of the current flowing through the light emitting layer. Therefore, the degree of freedom in selecting the emission color can be easily increased. As described above, according to the present invention, an image on the display panel can be easily and vividly displayed. (Embodiment) The laminated structure of the organic EL element is not particularly limited. For example, as described above, a transparent anode layer made of a transparent conductive material such as ITO, a light emitting layer containing a fluorescent material, and a cathode layer made of Mg-Ag or the like are sequentially formed on a transparent substrate. Alternatively, on a transparent or opaque substrate, a cathode layer made of Mg-Ag, a light emitting layer containing the plurality of fluorescent materials, and a transparent anode layer made of a transparent conductive material such as ITO may be sequentially formed. The formed structure may be used.

The material for the anode layer is not particularly limited, but it is preferable to use a material having a high work function. An anode layer made of a material having a high work function easily emits holes, and can efficiently inject holes. The material of the cathode layer is not particularly limited, either, but it is preferable to use a material having a low work function. The cathode layer made of a material having a low work function easily emits electrons and can inject electrons efficiently.

As the transparent conductive material of the transparent anode layer, in addition to ITO, AZO (Al-added ZnO) or SnO ₂
And so on. The layer made of any of the materials mentioned here can be formed by an evaporation method such as a sputtering method. On the other hand, as a material of the cathode layer,
In addition to Mg-Ag, a conductive metal such as Al can be used. The layer made of any of the materials mentioned here can be formed by an evaporation method such as a sputtering method.

Here, the anode layer may be formed from an opaque conductive material such as Mg-Ag, or the cathode layer may be formed from a transparent conductive material such as ITO. Not good. The material of the light emitting layer is not particularly limited, except that a plurality of fluorescent materials having different light emission levels depending on the difference in current density of the flowing current are contained. The plurality of fluorescent materials may be used alone, or the plurality of fluorescent materials may be contained in another organic material. The ratio of the content of the first fluorescent organic material to the content of the second fluorescent organic material is not particularly limited. In the former form, the first fluorescent organic material may be doped with the second fluorescent organic material. The types of the plurality of fluorescent materials are not particularly limited by their types, and known materials can be used.

Particularly, in the present invention, the plurality of fluorescent materials include:
The energy level (HOM) of each predetermined highest occupied orbit
A first fluorescent organic material having an energy level of the lowest unoccupied orbital (LU level) and an energy level of the lowest unoccupied orbit, and capable of emitting first visible light of a predetermined wavelength with recombination of electrons and holes; Lower HOMO level and higher LUM than the first fluorescent organic material
It has an O level, and is accompanied by the recombination of electrons and holes with the first
Preferably, the second fluorescent organic material is capable of emitting second visible light having a longer wavelength than visible light. These first
By including the fluorescent organic material and the second fluorescent organic material in the light emitting layer, it is possible to selectively change the degree of light emission of the fluorescent material and change the emission color while maintaining an appropriate potential level. Therefore, both the emission characteristics and the emission color selectivity can be improved.

The first fluorescent organic material and the second fluorescent organic material have an appropriate HOMO level and an appropriate LUMO level, respectively, so that the work function can be easily matched with the anode layer and the cathode layer. It is preferable to use a fluorescent organic material having a high energy level. For example, when the anode layer is made of ITO having a work function of 4.9 eV and the cathode layer is made of Al having a work function of 3.7 eV, the HOMO level of the first fluorescent organic material is 5.8 eV.
Alq ₃ having a LUMO level of 3.0 eV at V (chemical formula 1)
And as the second fluorescent organic material, H
DCM2 (chemical formula 2) having an OMO level of 5.6 eV and a LUMO level of 3.5 eV can be given. Alq ₃
Can emit green light, and DCM2 can emit red light.

[0016]

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[0017]

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Further, the light emitting layer may have not only light emission but also an electron transport function of transporting electrons. The electron transport function may be performed by at least one of the first fluorescent organic material and the second fluorescent organic material, or may be performed by another organic material of the first fluorescent organic material and the second fluorescent organic material. . On the other hand, a hole injection layer or a hole transport layer is interposed between the electrode layer serving as the anode layer and the light emitting layer, and an electron injection layer or electron transport layer is provided between the electrode layer serving as the cathode layer and the light emitting layer. It is preferable to interpose such as. For these layers, it is preferable to use materials having appropriate HOMO levels and LUMO levels, respectively, so that the work functions can be easily matched with the anode layer and the cathode layer. Note that when the light-emitting layer has a function of transporting electrons, the electron transport layer may not be provided separately.

The hole injection layer is made of copper phthalocyanine (CuP
c) or, VO _X, MO _X, can be formed from such RuO _X. The hole transport layer is preferably formed from a material having a low ionization potential, and is preferably a carrier block layer so that the light emitting layer can efficiently emit light. Examples of such a material include a tertiary amine derivative such as a triphenyldiamine derivative, MTDATA, hydrazone, polyvinyl carbazole (PVCz; chemical formula 3), and the like.

[0020]

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For example, when the anode layer is made of ITO, it is preferable to use a material having a HOMO level of 4.9 eV or more from the work function for the hole injection layer and the hole transport layer. As a material for such a hole injection layer, for example, copper phthalocyanine (4.8 eV) can be given. Also,
As a material for the hole transport layer, for example, PVCz (5.9e)
V).

On the other hand, the electron injection layer can be formed from LiF or the like. The electron transport layer is preferably formed from a material having a high electron transport ability. Further, a material which itself becomes a light emitting layer may be used. As such materials,
An aluminum quinolinol complex can be mentioned.
Each layer constituting the above-described organic EL element can be formed by a known evaporation method such as a vacuum evaporation method, a Langmuir-Blodgett evaporation method, and an organic molecular beam epitaxy method. Also, the thickness of each layer is not particularly limited, and is appropriately selected so as to obtain desired light emitting characteristics.

In the present invention, a current is caused to flow through the light emitting layer while changing at least one of the current value and the voltage value, so that the degree of light emission of the fluorescent material is selectively changed to change the light emission color. For example, when the light emitting layer is Al, which is the first fluorescent organic material,
If the q ₃ second is a fluorescent organic material DCM2, which are doped, by passing in a current to the light emitting layer is changed at least one of the current and voltage values, by changing the light emission color as follows Can be.

When both the current value and the voltage value are small and the current density is low, the first fluorescent organic material selectively emits light to become green. When at least one of the current value and the voltage value is large and the current density is large, both the first fluorescent organic material and the second fluorescent organic material emit light, and green and red combine to become yellow. In the present invention, it is preferable that at least one of the current value and the voltage value is changed by at least one order. By changing the current value and the voltage value in this manner, the current density of the current flowing through the light emitting layer can be sufficiently changed. As a result, the degree of light emission of the fluorescent material can be sufficiently changed, and the light emission color can be sufficiently changed.

[0025]

The present invention will be described below in detail with reference to examples. (Example 1) As shown in FIG. 1, an organic EL element used in this example is formed on a transparent substrate 10 (plate thickness 1.1 mm) made of transparent glass and an anode layer 12 made of ITO.
(Layer thickness 130 nm), a hole transport layer 14 (layer thickness 20 nm) made of PVCz (PVK), and DCM2 on Alq ₃ .
Light-emitting layer 16 (layer thickness: 60 nm) doped with and an electron injection layer 18 of lithium fluoride (layer thickness: 0.5 nm)
And a cathode layer 20 of aluminum (layer thickness 150 n
m). FIG. 2A shows an energy level diagram of the organic EL device.

The organic EL element is sealed by a box-shaped enclosing member (not shown) having an opening on one side. This encapsulating member is made of stainless steel (SUS), and has a flange-shaped protrusion on the periphery of the opening. Glued. A space (not shown) is formed between the organic EL element and the enclosing member, and the space is filled with nitrogen gas. This organic EL device was manufactured in the following procedure. [Formation of Organic EL Element] First, a transparent substrate 10 is prepared,
The anode layer 1 is formed on the transparent substrate 10 by sputtering.
2 was formed. The hole transport layer 14 was formed on the surface of the anode layer 12 by spin coating.

Subsequently, the light emitting layer 1 is formed on the hole transport layer 14.
6. The electron injection layer 18 and the cathode layer were sequentially formed by a vacuum evaporation method. [Sealing of Organic EL Element] An adhesive that cures when irradiated with ultraviolet light is applied to at least one of the adhesive surfaces of the transparent substrate and the enclosing member so that the adhesive is interposed between the adhesive surfaces without any gap. Then, the adhesive surfaces of the transparent substrate and the encapsulating member were overlapped in an atmosphere of nitrogen gas.

Next, an ultraviolet lamp was prepared, and the ultraviolet light emitted from the ultraviolet lamp was irradiated on the adhesive while shielding the organic EL element with a mask. As a result, the adhesive was cured, and the organic EL element was sealed. (Comparative Example 1) except that the light-emitting layer was formed only from Alq ₃ was fabricated an organic EL device in the same manner as in Example 1. The energy level diagram of this organic EL device is shown in FIG.
(Comparative Example 2) An organic EL device was manufactured in the same manner as in Example 1 except that the light emitting layer was formed only from DCM2. [Emission Color Variable Test of Organic EL Element] With respect to the organic EL elements of Example 1 and Comparative Examples 1 and 2, the current value of the current flowing through the light emitting layer of each organic EL element was appropriately set, and the current was measured. At a current density of 8.3 mA / cm ² . Next, a current value flowing through the light emitting layer is appropriately set, and the current density is set to 83.3 mA / cm ^2.
The organic EL device was caused to emit light. When light was emitted at these two current densities, the emission luminance was measured with a luminance meter,
The external quantum efficiency was calculated. Table 1 shows the measurement results.

[0029]

[Table 1] The light emitted from the organic EL devices of Example 1 and Comparative Examples 1 and 2 was 400 to 800 n.
The spectral intensity in the wavelength range of m was measured using a spectroscope. Here, FIG. 3 shows only the measurement results of the organic EL element of Example 1. Table 1 shows the wavelength (EL peak wavelength) at which the spectral intensity of each organic EL element has the maximum (peak).

Further, the chromaticity of the light emitted from the organic EL device was calculated. The measurement result is superimposed on the XYZ color system chromaticity diagram CIE (1931) of FIG. In the organic EL device of Comparative Example 1 (not including DCM2), PV
Cz becomes a carrier block layer, and only green light emission derived from Alq ₃ is visually recognized. On the other hand, in the organic EL device of Example 1, when the current density was set to 8.3 mA / cm ² , emission of yellow light was visually recognized. This yellow light
From the emission spectra of the light emitted from each of the organic EL devices of Comparative Example 1 and Comparative Example 2, Alq ₃ (535 nm) and D
It can be seen that the light was emitted from CM2 (610 nm).

When the current density was 83.3 mA / cm ² , green light emission was visually recognized. This green light is A
This indicates that the light emitted by the luminance of lq ₃ is relatively increased from that of DCM2. Further, it was also found that the change in the emission color between yellow and green occurred reversibly. It is considered that such a change in the color light occurs as follows.

At a low current value, many carriers are injected into the electron level of DCM 2 located at a low energy position, but as the current value increases, carriers are transferred to Alq ₃ at a high energy level. Is injected, and light emission from Alq ₃ becomes stronger. Therefore, the emission color changes toward the short wavelength side as the current value increases.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view schematically showing an organic EL device used in Example 1.

FIG. 2 is a longitudinal sectional view schematically showing an energy level diagram of each organic EL element used in Example 1 and Comparative Example 1.

FIG. 3 is a graph showing a relationship between a wavelength of light emitted from an organic EL element and a spectrum intensity in Example 1.

FIG. 4 is a graph showing a change in color coordinates of light emitted from the organic EL element in the XYZ color system chromaticity diagram CIE (1931) in Example 1.

[Explanation of symbols]

10: transparent substrate 12: anode layer 14: hole transport layer 1
6: electron injection layer 18: cathode layer

Claims (2)

[Claims] 1. A method for varying the emission color of an organic EL device comprising a light-emitting layer between an anode layer and a cathode layer, wherein the light-emitting layers have different light-emission levels depending on the current density of the current flowing through the light-emitting layer. Including multiple fluorescent materials,
The current density is changed by flowing a current through the light-emitting layer while changing at least one of the current value and the voltage value, and the luminescent color is changed by selectively changing the degree of light emission of the fluorescent material. A method for changing the emission color of an organic EL element. 2. The method according to claim 1, wherein the current density is changed by at least one order.