JPH04328295A - Organic electroluminescence(el) element - Google Patents

Abstract

PURPOSE:To realize light emission stabilized for a long time and with high luminance by a method wherein an electron transport layer is provided with a film thickness which produces a secondary maximum value of film thickness luminance attenuation characteristic and has a film thickness within the range in which the amplitude of the layer generates such luminance as exceeds the convergent luminance value of the layer. CONSTITUTION:An organic EL element is made up by laminating and filming an electron transport layer 5, an EL layer 3 and a hole transport layer 4 as a thin film between a pair of metal cathode 1 and a transparent anode 2. The most suitable organic EL element is that whose film thickness of the EL layer is fixed as 200Angstrom , whose thickness of the electron transport layer is set as 200Angstrom + or -300Angstrom corresponding to a secondary maximum value. This range of film thickness is the range where the secondary maximum value amplitude of the second high luminance of the film thickness luminance attenuation curve of the luminance/ current characteristic for a film thickness corresponding to the electron transport layer 5 and the material of the EL layer 3 exceeds a convergent value, and especially, it is possible to realize the organic EL element of high reliability and luminance by setting the film thickness of the electron transport layer 5 as a film thickness corresponding to the neighborhood of the secondary maximum value which indicates the second high luminance in a film thickness luminance attenuation curve.

Description

[Detailed description of the invention]

0001

TECHNICAL FIELD The present invention relates to an EL device that utilizes electroluminescence (hereinafter referred to as EL) of a substance that emits light when an electric current is injected, and has an EL layer formed of such a substance in a thin film. The present invention relates to an organic EL element that is a compound.

0002

BACKGROUND ART As shown in FIG. 1, this type of organic EL device has an EL layer 3 consisting of a luminescent thin film made of an organic compound and laminated on each other, and an anode layer 3 between a metal cathode 1 and a transparent anode 2. There may be a two-layer structure in which a hole transport layer 4 is arranged, or as shown in FIG.
A three-layer structure including an L layer 3 and a hole transport layer 4 is known. Here, the hole transport layer 4 has a function of facilitating injection of holes from the anode and a function of blocking electrons, and the electron transport layer 5 has a function of facilitating injection of electrons from the cathode.

[0003] In these organic EL devices, a transparent anode 2
A glass substrate 6 is disposed on the outside of the EL layer, and excitons are generated by recombination of electrons injected from the metal cathode 1 and holes injected from the transparent anode 2 into the EL layer 3, and holes in the EL layer In the process of radiation deactivation of excitons near the interface with the transport layer, light is emitted, and this light is transmitted to the transparent anode 2 and the glass substrate 6.
is released to the outside through the
(See Japanese Patent Application Laid-Open No. 63-295695).

[0004] However, although the conventional organic EL element having the above-mentioned structure consumes energy within the EL layer and emits light at a low voltage, it generally has a short lifespan when the EL layer has a thin film thickness of 500 Å or less. For example, an organic EL element having a two-layer structure and a 300 Å thick EL layer shown in FIG. 1 has an initial luminance of 4
When continuously emitting light at a rate of 00 cd/m2, the device's brightness decreases by half and deteriorates in 100 hours or less.

On the other hand, when the thickness of the EL layer is increased, its brightness decreases as the thickness increases even when driven at a constant voltage. Considering the light emission principle of an EL element, it is thought that the brightness is proportional to the applied current, but this is actually different.

[0006]

OBJECTS OF THE INVENTION An object of the present invention is to provide an organic EL element that emits light with high brightness stably for a long period of time.

[0007]

[Structure of the Invention] The organic EL device according to the present invention includes an electron transport layer, an EL layer, and a hole transport layer made of organic compounds and stacked on each other and arranged between a cathode and an anode, and the electron transport layer is , the film thickness is within a range that includes a film thickness that produces a second-order maximum value of the film thickness luminance attenuation characteristic, and that produces a luminance whose amplitude exceeds its convergent luminance value.

[0008]

Embodiments Examples of the present invention will be described below with reference to the drawings. The organic EL device of this example is similar to that shown in FIG. 2, in which an electron transport layer 5, an EL layer 3, and a hole transport layer 4 are laminated as thin films between a pair of metal cathodes 1 and a transparent anode 2. It has a three-layer structure. For example, for cathode 1,
Made of a metal with a small work function such as aluminum, magnesium, indium, silver, or an alloy of each, and has a thickness of about 10
A thickness of about 0 to 5000 Å can be used. Further, for example, the anode 2 is made of a conductive material with a large work function such as indium tin oxide (hereinafter referred to as ITO) and has a thickness of 10
00 to 3000 Å or gold with a thickness of 800 to 1
A thickness of about 500 Å can be used.

Electron transport layer 5 of organic EL device according to the present invention
As Bu-PBD, which is an oxadiazole derivative
[2-(4'-tert-Butylphenyl)-
5-(biphenyl)-1,3,4-oxadia
zole] (hereinafter referred to as PBD) can be preferably used. A specific example of the organic fluorescent compound forming the EL layer 3 of the organic EL device according to the present invention is an aluminum quinolinol complex, that is, Al oxine chelate (hereinafter referred to as Alq
3), tetraphenylbutadiene derivatives, etc. may be used. The thickness of the EL layer 3 is preferably 200 Å or less, which is at least the limit for emitting light. The hole transport layer 4 contains N,N′-diphenyl-N, which is a triphenyldiamine derivative.
N'-bis(3methylphenyl)-1,1'-biphenyl-4,4'-diamine (hereinafter referred to as TPD) is preferably used, and CTM (Carrier Tran
Compounds known as sporting materials may be used alone or in mixtures.

[0010] As a result of research on the total film thickness, emission spectrum, brightness, and viewing angle of the electron transport layer and EL layer of a three-layer organic EL element, the inventor found that there is a difference between the brightness and the electron transport layer film thickness. We found that the film thickness dependence and the luminance dependence on the viewing angle. That is, as shown in FIG. 3, the emission spectrum and brightness change depending on the angle at which a viewer views the surface of the organic EL element on the glass substrate 6 side. For the viewer, the light emitted from one point of the light emitting source P in the EL layer has a direct contact with the substrate 6 in the figure.
Two types of light are included: a path A that goes toward the substrate 6, and a path B that is reflected by the metal electrode 1 on the back surface and goes toward the substrate 6. Since the lights in these two paths have an optical path difference L shown in the following equation 1 and a phase difference ηy shown in equation 2, they interfere with each other. (In both formulas, n is the refractive index of the EL layer 3, y is the distance from the light emitting source P to the metal electrode 1, θ is the viewing angle deviating from the normal to the display surface in the EL layer, and λ is the wavelength. (The same applies below).

[0011]

[Math 1]

0012

[Math 2]

Therefore, the intensity I(y,
λ) can be expressed as in Equation 3.

[0014]

[Math 3]

The distribution of emission intensity f(y) in the EL layer is as follows:
As shown in FIG. 4, at the boundary of the hole transport layer 4, the decrease is stronger toward the metal electrode 1, and can be expressed as an exponential distribution with respect to the film thickness as shown in Equation 4, and for the entire EL layer, it is normalized as shown in Equation 5. (In both formulas, d is the distance from the emission source to the metal electrode, ε is the emission intensity distribution parameter, and k
represent constants, respectively. same as below).

[0016]

[Math 4]

[0017]

[Math 5]

Intensity distribution F of the emission spectrum of the light source itself
(λ) can be expressed as a function of the wavelength λ specific to the light emitter. Therefore, the emission intensity T(λ, θ, d) of the EL element actually observed by a viewer can be expressed as shown in Equation 6.

[0019]

[Math 6]

Here, the emission intensity T(λ, θ,
To confirm d), the total film thickness (y = d) was 6000 Å.
An organic EL device including an electron transport layer made of PBD and an EL layer made of Alq3 with a constant emission intensity distribution parameter ε of 200 Å was fabricated, and the viewing angle θ was varied from 0° to 75°.
The luminescence intensity was tested with various changes. Figure 5
shows the emission intensity distribution with respect to the emission wavelength. It was confirmed that this emission intensity distribution and the emission intensity T (λ, θ, d) of Equation 6 above substantially matched. As is clear from the figure,
To the viewer, the colors appear to vary sequentially depending on the direction in which the EL element display surface is viewed from a viewing angle of 0° to 75°.

Furthermore, in order to be practical, consideration is given to the visibility characteristic E(λ) of a viewer or a photodetector that is sensitive to a wavelength λ at a specific value. For example, if the visibility characteristic E(λ) is a normal distribution, the luminance characteristic L(d) of the electron transport layer and the EL element within the sensitivity characteristic can be expressed as a function of d as shown in Equation 7 (K indicates a constant .)

[0022]

[Math 7]

FIG. 6 shows an electron transport layer made of PBD and a
Of these total film thicknesses for the EL layer (θ=0, n=1.7) consisting of lq3, the electron transport layer is 0 to 8000 Å.
It shows the film thickness brightness attenuation (characteristic) curve of the brightness/current characteristics against the film thickness when calculated by varying the film thickness over Å.
This attenuation curve shows the film thickness dependence of brightness in the organic EL element.

When an organic EL element is prepared and tested to confirm the dependence of luminance on film thickness of such an organic EL element, results of attenuation characteristics similar to those shown in FIG. 6 are obtained. Such organic EL
As shown in Fig. 6, the element exhibits the minimum film thickness and maximum brightness, and this is taken as the first maximum value, and the order increases sequentially (film thickness increases).
As the brightness increases, the maximum value of the brightness appears periodically, and this maximum value decreases, indicating the characteristic of the film thickness brightness attenuation curve, that is, the dependence of the brightness on the film thickness of the electron transport layer. The actually measured film thickness luminance decay curve is based on a TP film with a film thickness of 500 Å in the organic EL element.
Due to the use of the D hole transport layer, the entire characteristic curve shifts compared to the calculated one in FIG.

As is clear from FIG. 6, in a preferred embodiment of such an organic EL device, the thickness of the EL layer of Alq3 is 20%.
This is an organic EL device in which the thickness of the electron transport layer is fixed at 0 Å, and the thickness of the electron transport layer is set to 2000 ű300 Å, which corresponds to the secondary maximum value C. By forming the electron transport layer within this thickness range, the EL layer can be protected from high applied current while ensuring brightness. In other words, in this film thickness range, the second maximum amplitude of the second highest luminance of the film thickness luminance attenuation curve of the luminance/current characteristics with respect to the film thickness according to the material of the electron transport layer and EL layer shown in FIG. 6 converges. The range D exceeds the luminance value (convergent luminance value) of And a high brightness organic EL element can be obtained,
preferable.

Furthermore, since a change in viewing angle is equivalent to a change in film thickness, by setting the electron transport layer to a film thickness corresponding to the vicinity of the second-order maximum value in the film thickness brightness attenuation curve,
A high-luminance organic EL element whose luminance changes little even when the viewing angle changes somewhat can be obtained. FIG. 7 shows the brightness/brightness versus viewing angle for each of the three-layer organic EL elements of the above-mentioned PBD electron transport layer, Alq3EL layer, and TPD hole transport layer.
The results of measuring the relative value of current are shown. The organic EL devices tested had a three-layer structure: a hole transport layer of TPD with a thickness of 500 Å, an EL layer of Alq3 with a thickness of 200 Å, and an electron transport layer of PBD with a thickness of 650 Å to 7225 Å. be. As is clear from the figure, primary, secondary and tertiary
Electron transport layer thickness 650 Å, 2000 Å corresponding to the next maximum value
Å and 4065 Å, the brightness tends to be less dependent on the viewing angle, where the brightness increases as the viewing angle increases. Therefore, if the film thickness of the EL layer and electron transport layer corresponds to the maximum value of each amplitude in the film thickness brightness attenuation curve, the viewing angle dependence of the brightness and emission spectrum will be small, and the organic EL element will have a small change in color depending on the viewing angle. is obtained. The above embodiment in which the electron transport layer has a film thickness corresponding to the vicinity of the second maximum value indicating the second highest luminance in the film thickness luminance attenuation curve also has a small viewing angle dependence of luminance.

[0027] Here, as an example, it is preferable to use an organic EL element in which the electron transport layer has a thickness corresponding to the vicinity of the second maximum value in the film thickness brightness attenuation curve. Because it is thin, it is not possible to ensure a sufficient thickness to protect the EL layer, which shortens the life of the organic EL element, and the brightness/current value becomes low when the 3rd maximum value or more is exceeded. . That is, the above-mentioned example having the film thickness of the second-order maximum value is a highly reliable and high-brightness organic EL device with a balance between the life span and brightness of the organic EL device. The reason why the electron transport layer thickness is changed is that the electron transport layer material has a lower specific resistance than the EL layer material, that is, it has better conductivity, so there is no need to change the EL layer thickness and increase the drive current. It is from.

Furthermore, the present invention is not limited to the electron transport layer materials and EL layer materials of the above embodiments, but also applies to electron transport layer materials and EL layer materials.
The electron transport layer thickness value corresponding to the second maximum amplitude can be obtained from the film thickness luminance attenuation curve of the luminance/current characteristic with respect to film thickness shown in FIG. 6 according to the material of the L layer.

[0029]

Effects of the Invention As explained above, in the organic EL device according to the present invention consisting of an electron transport layer, an EL layer, and a hole transport layer which are laminated together, the electron transport layer is located at the second position in the film thickness brightness decay curve. Since the film thickness is within a range in which the amplitude of the secondary maximum value of high brightness exceeds the convergent brightness value, it is possible to efficiently emit light with high brightness at low voltage while improving durability. According to the present invention, a highly reliable and high-brightness organic EL element can be obtained in which the viewing angle dependence of the emission spectrum is small and the change in color due to the viewing angle is also small.

[Brief explanation of drawings]

FIG. 1 is a structural diagram showing an organic EL element with a two-layer structure.

FIG. 2 is a structural diagram showing a three-layer organic EL element.

FIG. 3 is a partially enlarged cross-sectional view illustrating light interference in a two-layer organic EL element.

FIG. 4 is a graph illustrating the film thickness and emission intensity distribution of an EL layer in an organic EL element with a two-layer structure.

FIG. 5 is a graph illustrating the wavelength emission intensity distribution of an EL layer in an organic EL element with a two-layer structure.

FIG. 6 is a graph illustrating a film thickness luminance attenuation curve of a single EL layer in a two-layer organic EL element. It is.

FIG. 7 is a graph showing an actually measured viewing angle luminance characteristic curve of an organic EL element having a two-layer structure of an EL layer and a hole transport layer.

[Explanation of symbols]

1...metal cathode 2...Transparent anode 3...EL layer 4...Hole transport layer 5...Electron transport layer 6...Glass substrate

Claims (3)

[Claims] 1. An organic electroluminescent element in which an electron transport layer, an electroluminescence layer, and a hole transport layer made of organic compounds and stacked on each other are arranged between a cathode and an anode, the electron transport layer having a film thickness of 1. An organic electroluminescent element having a film thickness within a range that includes a film thickness that produces a second-order maximum value of luminance attenuation characteristics and that produces a luminance whose amplitude exceeds its convergent luminance value. 2. The organic electroluminescent device according to claim 1, wherein the electron transport layer has only a thickness that produces a second-order maximum value of the thickness luminance attenuation characteristic. 3. The electroluminescent layer is made of an aluminum quinolinol complex, the hole transport layer is made of a triphenyldiamine derivative, and the electron transport layer is made of an oxadiazole derivative, and the electroluminescent layer has a luminance exceeding the convergent luminance value. 2. The organic electroluminescent device according to claim 1, wherein the resulting range is 2000 ű300 Å.