JPH053081A - Flat luminous device - Google Patents

Abstract

PURPOSE:To reduce the visual angle dependent property of the brightness by using a material in which the half width of the luminous intensity component in the thickness direction is small is used as a luminous layer, and setting the membrane thickness of the luminous layer or a transparent layer within a specific scope. CONSTITUTION:Between a pair of metallic cathode 1 and transparent anode 2, an EL layer 3 and a hole transport layer 4 are laminated as membranes, to form a membranous two-layer structure. As the cathode 1, a metal of a small work function such as aluminum, magnesium, or indium, with the thickness 1000 to 5000Angstrom level is used. As the anode 2, a conductive material of a large work function such as an indium-tin oxide, with the thickness 1000 to 3000Angstrom level is used. The EL layer 3 is formed of an organic phosphor compound, and as the layer 4, a triphenyl diamine derivative is used. As a result, the brightness in the oblique side is made larger sufficiently than the brightness in the front side, and the dependent property of the brightness to the visual angle can be reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface emitting device in which a decrease in brightness due to a viewing angle is reduced.

[0002]

2. Description of the Related Art As a surface emitting device, for example, an LED or an illuminating display using an incandescent lamp is known.

[0003]

Since these surface emitting devices have a finite area and have a radiation pattern of light by a substantially perfect diffusing surface, the brightness is substantially constant even in the front or diagonal direction. Therefore, the brightness T at the viewing angle θ
Is

[Equation 1] It is represented by.

Here, S is the area of the surface emitting device, and L is the brightness. Therefore, the brightness T depends on the viewing angle θ, and when viewed from an oblique direction, the visibility as a display decreases and the illumination becomes dark.

The present invention has been made in view of such circumstances, and an object thereof is to provide a surface emitting device in which the dependence of the brightness on the viewing angle is suppressed.

[0006]

In order to achieve the above-mentioned object, the present invention comprises a reflective layer laminated on the light emitting layer directly or via a transparent layer, and the light emitting layer has a predetermined thickness direction. And a film thickness between a reflection layer and a region that gives a peak of the emission intensity distribution, including a film thickness that produces one minimum value of the film thickness luminance attenuation characteristic. It is characterized in that the amplitude is set to a value within a range in which brightness is equal to or less than the convergent brightness value and is equal to or more than a film thickness value in which the convergent brightness value is generated and is equal to or less than a film thickness value in which the one minimum value is generated. ..

[0007]

In the surface emitting device of the present invention, a light emitting layer having a very small full width at half maximum of the emission intensity distribution in the thickness direction is used, and a reflective layer or a structure in which a reflective layer is laminated via a transparent layer is used. The thickness of the light-emitting layer or the transparent layer is set to a specific range by focusing on the fact that the brightness changes due to the light interference effect depending on the viewing angle depending on the thickness of the light-emitting layer or the light-emitting layer and the transparent layer. It is possible to make the luminance in the oblique direction sufficiently larger than the luminance in the direction and reduce the dependence of the brightness on the viewing angle.

[0008]

Embodiments of the present invention using an organic EL element as a surface emitting device will be described below with reference to the drawings.

As an organic EL element, as shown in FIG.
Between the metal cathode 1 and the transparent anode 2, an EL layer 3 made of light emitting thin films made of organic compounds and laminated on each other.
And a two-layer structure in which the hole transport layer 4 is arranged, as shown in FIG. 2, an electron transport layer 5 and an EL layer made of an organic compound laminated between the metal cathode 1 and the transparent anode 2 as shown in FIG. A three-layer structure in which 3 and the hole transport layer 4 are arranged is known. Here, the hole transport layer 4 has a function of facilitating the injection of holes from the anode and a function of blocking electrons, and the electron transport layer 5 has a function of facilitating the injection of electrons from the cathode and the function of blocking holes. It has the function and.

In these organic EL devices, the transparent anode 2
A glass substrate 6 is disposed on the outer side of, and excitons are generated by recombination of electrons injected from the metal cathode 1 and holes injected from the transparent anode 2 to the EL layer 3, and holes in the EL layer are generated. Excitons emit light in the process of radiation deactivation in the vicinity of the interface with the transport layer, and this light emits light, which is transparent anode 2 and glass substrate 6
Is released to the outside via a light source (Japanese Patent Laid-Open No. 59-194393).
(See Japanese Patent Laid-Open No. 63-295695).

The organic EL device of this embodiment has an electrode E between the pair of metal cathode 1 and transparent anode 2 similar to that shown in FIG.
The L layer 3 and the hole transport layer 4 were laminated as a thin film to form a film 2
It has a layered structure. For example, the cathode 1 is made of a metal having a small work function, such as aluminum, magnesium, indium, silver, or an alloy of each, and has a thickness of 1000 to 500.
Those having a thickness of about 0 angstrom can be used. Further, for example, the anode 2 is made of a conductive material having a large work function such as indium tin oxide (hereinafter referred to as ITO) and has a thickness of about 1000 to 3000 angstroms, or gold with a thickness of about 800 to 1500 angstroms. Can be used.

As a specific example of the organic fluorescent compound forming the EL layer 3 of the organic EL device according to the present invention, an aluminum quinolinol complex, that is, an Al oxine chelate (hereinafter, referred to as
Alq3), tetraphenyl butadiene derivative and the like can be used. The hole transport layer 4 includes N, N′-diphenyl-N, N′-bis (3methylphenyl) -1,1′-biphenyl-4,4, which is a triphenyldiamine derivative.
'-Diamine (hereinafter referred to as TPD) is preferably used, and further CTM (Carrier Transport) is used.
Compounds known as ing Materials) may be used alone or as a mixture.

The inventor has found that the EL of an organic EL device having a two-layer structure
As a result of research on the layer thickness, the emission spectrum and the luminance, and the viewing angle, it was found that the luminance depends on the EL layer thickness and the viewing angle. That is, as shown in FIG. 3, the emission spectrum and the brightness change depending on the angle at which the viewer views the surface of the organic EL element on the glass substrate 6 side. For a viewer, the light emitted from one point of the light emission source P in the EL layer is
The path A directly to the substrate 6 in the figure and the metal electrode 1 on the back surface
Two lights of the path B reflected by and traveling toward the substrate 6 are included.
The light of these two paths holds the optical path difference L shown in the following expression 2 and the phase difference ηy shown in the following expression 3 and therefore interferes with each other (wherein, 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.

[0014]

[Equation 2]

[0015]

[Equation 3] Therefore, as the interference effect, the intensity I (y, λ) is
Can be expressed as

[0016]

[Equation 4]

The distribution of the emission intensity f (y) in the EL layer is
As shown in FIG. 4, the boundary surface of the hole transport layer 4 strongly decreases toward the metal electrode 1 and can be expressed as an exponential function distribution with respect to the film thickness as in Expression 5, and the EL layer as a whole is normalized as in Expression 6. (In both equations, d is the distance from the light emitting source to the metal electrode, ε is the emission intensity distribution parameter, k
Indicates constants, respectively. same as below).

[0018]

[Equation 5]

[0019]

[Equation 6]

The intensity distribution F of the emission spectrum of the emission source itself
(Λ) can be expressed as a function of the wavelength λ peculiar to the light emitter. Therefore, the light emission intensity T (λ, θ, d) of the EL element that is actually observed by the viewer can be expressed by Equation 7.

[0021]

[Equation 7]

Here, the emission intensity T (λ, θ,
In order to confirm d), an organic E including an EL layer made of Alq ₃ having a film thickness (y = d) of 6000 angstroms.
An L element was prepared, and the emission intensity was tested by changing the viewing angle θ from 0 ° to 75 °. FIG. 5 shows the emission intensity distribution with respect to the emission wavelength. It was confirmed that the emission intensity distribution and the emission intensity T (λ, θ, d) of the above formula 7 substantially match. As is apparent from the figure, to the viewer, the colors seem to be sequentially different from the viewing angle of 0 ° to 75 ° depending on the viewing direction of the EL element display surface.

Further, in consideration of the practical use, the visual sensitivity characteristic E (λ) of the viewer or the photodetector sensitive to the wavelength λ with a specific value is considered. For example, when the luminosity characteristic E (λ) is a normal distribution, the luminance characteristic L (d) of the EL element within the sensitivity characteristic can be expressed as a function of d as in Expression 8 (K indicates a constant).

[0024]

[Equation 8]

FIG. 6 shows an EL layer made of Alq ₃ (θ =
0, n = 1.7), the film thickness-brightness decay (characteristic) curve of the brightness / current characteristic with respect to the film thickness is shown when the film thickness is calculated by changing the film thickness from approximately 0 angstrom to 8000 angstrom. The film thickness dependence of the brightness in the EL element is shown.

When an organic EL element for confirming the film thickness dependence of the brightness of the organic EL element is prepared and tested, the result of the attenuation characteristic as shown in FIG. 7 is obtained. The plurality of organic EL elements tested were TP with a film thickness of 500 Å.
D hole transport layer and film thickness from 1150 Å to 7
It has a two-layer structure with an EL layer of Alq ₃ of 725 Å. As shown, such organic E
Similar to FIG. 6, the L element shows the minimum film thickness and the maximum brightness, and the maximum value and the minimum value of the brightness periodically appear as the order increases (film thickness increases) with this as the primary maximum value. This shows the characteristic of the film thickness luminance decay curve in which the maximum value decreases and the minimum value increases, that is, the film thickness dependence of the luminance.

In FIG. 6, the period T of each maximum value or minimum value is represented by λm / 2n. Where λm is F
The wavelength that gives the maximum intensity at (λ) (peak wavelength),
n is the average refractive index from the metal cathode to the position where the peak of the emission intensity distribution is given (the interface between the hole transport layer and the EL layer).

Further, in the film thickness luminance decay curve of FIG. 7, the horizontal axis represents the film thickness of the entire organic EL layer including the TPD hole transport layer having a film thickness of 500 angstroms. It shifts to the right in the figure compared to the ones. Figure 8
Is 2 of the above-mentioned TPD hole transport layer and Alq ₃ EL layer.
The results of measuring the relative values of luminance / current with respect to the viewing angle are shown for each of the organic EL devices having a layered structure.

As is apparent from the figure, the primary, secondary and tertiary
Organic EL elements having a film thickness of 1150 angstroms to 2500 angstroms and 4565 angstroms corresponding to the next maximum value tend to decrease in brightness as the viewing angle increases.
It can be seen that the organic EL elements having the film thicknesses of 2050 angstroms, 3550 angstroms and 5275 angstroms corresponding to the second and third minimum values tend to increase in luminance as the viewing angle increases.

Among such organic EL devices, the preferred embodiment is an organic EL device in which the thickness of the Alq ₃ EL layer is, for example, 2400 angstroms to 2700 angstroms, as is apparent from FIG.

That is, the thickness range of this EL layer is as shown in FIG.
Film thickness according to the EL layer material shown in (the interface between the hole transport layer and the EL layer, that is, a region giving a peak of the emission intensity distribution,
Film corresponding to the minimum value of the brightness from the film thickness values l _{1 1} , l _{2 1} , l _{3 1} corresponding to the converged brightness value of the film thickness brightness decay curve of the brightness / current characteristics with respect to the distance to the metal cathode, that is, the reflection layer) thickness values _{l 1 2, l 2 2,} l 3 up to ₂ range _{l (l 1 1 ~l 1 2} , l 2 1 ~l
A _{2 2, l 3 1 ~l 3} 2). It should be noted that l _{1 1} , l _{2 1} , and l _{3 1} are respectively l _{1 2} -0.25T and l _{2 2 when the} above-mentioned period T is used.
-0.25T, expressed as l _{3 2} -0.25T.

When the EL layer film thickness is selected within the above range, the effect of increasing the viewing angle is equal to that of decreasing the film thickness. Therefore, the brightness increases as the viewing angle increases. In a display with a finite size, there is little dependence of brightness on the viewing angle.
An L element will be obtained.

The present invention is not limited to the Alq ₃ EL layer of the above-described embodiment, but corresponds to the minimum value amplitude from the film thickness brightness decay curve of the brightness / current characteristic with respect to the film thickness shown in FIG. 6 depending on the material of the EL layer. It is possible to obtain the EL layer film thickness value.

Further, the present invention is not limited to the two-layer structure of the above-mentioned embodiment, but also in the case of the three-layer structure shown in FIG. 2, similarly with respect to the film thickness of the electron transport layer (transparent layer) and the EL layer (light emitting layer). Luminance/
A film thickness luminance decay curve of current characteristics is obtained, and a film thickness value corresponding to the minimum value amplitude can be obtained.

Further, the present invention is not limited to the organic EL devices of the above-mentioned embodiments, but the half value width of the emission intensity distribution in the thickness direction of the emission layer (the thickness of the emission region when the intensity distribution curve becomes half of the peak value). The width in the direction) may be, for example, a light emitting layer having a thickness of 500 angstroms or less, and the light emitting layer and the reflective layer are laminated, or the light emitting layer, the transparent layer, and the reflective layer are laminated.
Also in the case of the surface emitting device, the same attenuation characteristic as in FIG. 6 is obtained, and the film thickness value of the light emitting layer or the light emitting layer and the transparent layer corresponding to the minimum value amplitude can be obtained.

[0036]

As described above, in the surface emitting device according to the present invention, the film thickness of the light emitting layer or the light emitting layer and the transparent layer (the distance from the region giving the peak of the emission intensity distribution to the reflective layer) is the film thickness luminance. A film thickness value that includes one of the minimum values of the attenuation characteristics, and the amplitude is within a range in which the brightness produces a brightness equal to or less than the convergent brightness value, and the film thickness value that produces the one minimum value is equal to or more than the film thickness value that generates the convergent brightness value. By adopting one of the following values, the viewing angle dependency of brightness can be reduced.

[Brief description of drawings]

FIG. 1 is a structural diagram showing an organic EL device having a two-layer structure.

FIG. 2 is a structural diagram showing an organic EL device having a three-layer structure.

FIG. 3 is a partial enlarged cross-sectional view illustrating light interference in an organic EL element having a two-layer structure.

FIG. 4 is a graph illustrating a film thickness luminous intensity distribution of an EL layer in an organic EL device having a two-layer structure.

FIG. 5 is a graph illustrating a wavelength luminous intensity distribution of an EL layer in an organic EL device having a two-layer structure.

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

FIG. 7 is a graph showing an actually measured film thickness luminance decay curve in an organic EL device having a two-layer structure of an EL layer and a hole transport layer.

FIG. 8 is a graph showing a measured visual angle luminance characteristic curve in an organic EL device 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 (1)

Claim: What is claimed is: 1. A surface emitting device comprising a reflective layer laminated directly on a light emitting layer or via a transparent layer, wherein the light emitting layer has a predetermined light emission intensity distribution in its thickness direction. Then, the film thickness between the region that gives the peak of the emission intensity distribution and the reflective layer includes the film thickness that produces one minimum value of the film thickness brightness attenuation characteristic, and the amplitude of the brightness is equal to or less than the convergent brightness value. A surface emitting device, wherein the surface emitting device has a value within the range of occurrence and at least equal to or less than the film thickness value at which the converged luminance value is generated and at or below the film thickness value at which the one minimum value is generated.