JP2843924B2 - Surface emitting device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface light 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 light emitting device, for example, an illuminated display using LEDs or incandescent light bulbs is known.

[0003]

Since these surface light emitting devices have a finite area and have a light emission pattern based on a substantially perfect diffusion surface, the luminance is substantially constant even in the front or oblique 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 light emitting device, and L is the luminance. Therefore, the brightness T depends on the viewing angle θ, and when viewed from an oblique direction, the visibility of the display decreases, and the brightness of the display decreases.

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

[0006]

In order to achieve the above object, the present invention comprises a reflective layer laminated directly or via a transparent layer on a light emitting layer, wherein the light emitting layer has a predetermined thickness in the thickness direction. A surface light emitting device having a light emission intensity distribution of, the film thickness between the region giving the peak of the light emission intensity distribution and the reflective layer, including a film thickness that produces one minimum value of the film thickness luminance attenuation characteristics, and The amplitude is set to one value within a range in which the luminance is equal to or less than the convergent luminance value and is equal to or greater than the film thickness value in which the convergent luminance value is generated and equal to or less than the film thickness value in which the one minimum value is generated. .

[0007]

According to the surface light emitting device of the present invention, a light emitting layer having an extremely small half width of the light emission intensity distribution in the thickness direction is used, and a reflective layer or a reflective layer is laminated via a transparent layer. Paying attention to the fact that the luminance changes due to the light interference effect depending on the viewing angle according to the thickness of the layer or the light emitting layer and the transparent layer, the thickness of the light emitting layer or the transparent layer is set to a specific range, thereby The luminance in the oblique direction can be made sufficiently larger than the luminance in the direction, and the dependence of the brightness on the viewing angle can be reduced.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment according to 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 device, as shown in FIG.
An EL layer 3 composed of a luminous thin film made of an organic compound and laminated on each other between a metal cathode 1 and a transparent anode 2
And an electron transport layer 5 made of an organic compound laminated between a metal cathode 1 and a transparent anode 2, and an EL layer, as shown in FIG. 2. One having a three-layer structure in which a hole transport layer 3 and a hole transport layer 4 are arranged 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 functions of facilitating injection of electrons from the cathode and blocks holes. Function.

In these organic EL devices, the transparent anode 2
A glass substrate 6 is arranged outside the substrate, 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 to generate holes in the EL layer. The exciton emits light in the process of radiation deactivation near the interface with the transport layer, and this light is transmitted to the transparent anode 2 and the glass substrate 6.
(See JP-A-59-194393).
And JP-A-63-295695).

The organic EL device of this embodiment has a structure similar to that shown in FIG.
L layer 3 and hole transport layer 4 were laminated and formed as a thin film 2
It has a layer 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 thereof, and has a thickness of 1000 to 500.
Those having a thickness of about 0 Å 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 a thickness of about 800 to 1500 angstroms of gold. 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 Al oxine chelate)
Alq3), tetraphenylbutadiene derivatives and the like. The hole transport layer 4 has a triphenyldiamine derivative N, N'-diphenyl-N, N'-bis (3-methylphenyl) -1,1'-biphenyl-4,4
'-Diamine (hereinafter referred to as TPD) is preferably used, and CTM (Carrier Transport) is further used.
Compounds known as ing Materials) can be used alone or as a mixture.

The inventor of the present invention has proposed an organic EL device having a two-layer structure.
As a result of studying the layer thickness, the emission spectrum, the luminance, and the viewing angle, it was found that the luminance has a dependency on the EL layer thickness and a viewing angle dependence. That is, as shown in FIG. 3, the emission spectrum and the luminance 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, light emitted from one point of the light emitting source P in the EL layer includes:
The path A directly to the substrate 6 in FIG.
And the two lights on the path B toward the substrate 6 after being reflected by the light source.
Since the lights of these two paths hold the optical path difference L shown in the following equation 2 and the phase difference ηy shown in the following equation 3, they interfere with each other (in both equations, n represents the refractive index of the EL layer 3; y indicates the distance from the light emitting source P to the metal electrode 1, θ indicates the viewing angle deviating from the normal of the display surface in the EL layer, and λ indicates the wavelength.

[0014]

(Equation 2)

[0015]

(Equation 3) Therefore, the intensity I (y, λ) as an interference effect is given by the following equation (4).
Can be expressed as

[0016]

(Equation 4)

The distribution of the emission intensity f (y) in the EL layer is as follows:
As shown in FIG. 4, at the boundary surface of the hole transport layer 4, the intensity decreases strongly toward the metal electrode 1, and can be expressed as an exponential function distribution relating to the film thickness as shown in Equation 5, and the entire EL layer is normalized as shown in Equation 6. (In both formulas, d is the distance from the light source to the metal electrode, ε is the emission intensity distribution parameter, k
Indicates a constant. 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 specific to the illuminant. Therefore, the emission intensity T (λ, θ, d) of the EL element actually observed by the viewer can be expressed as in Expression 7.

[0021]

(Equation 7)

Here, the emission intensity T (λ, θ,
In order to confirm d), organic E including an EL layer made of Alq ₃ having a thickness (y = d) of 6000 Å was used.
An L element was prepared, and the light 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 substantially coincides with the emission intensity T (λ, θ, d) in Equation (7). As is apparent from the drawing, the viewer sees the colors sequentially different from the viewing angle of 0 ° to 75 ° depending on the viewing direction of the EL element display surface.

Further, a luminous sensitivity characteristic E (λ) of a viewer or a photodetector which responds to the wavelength λ with a specific value is considered so as to be suitable for practical use. For example, assuming that the visibility 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 is a constant).

[0024]

(Equation 8)

FIG. 6 shows an EL layer made of Alq ₃ (θ =
(0, n = 1.7), the thickness / luminance decay (characteristic) curve of the luminance / current characteristic with respect to the film thickness when the film thickness is changed from approximately 0 Å to 8000 Å is shown. 9 shows the dependency of luminance on the film thickness in the EL element.

When an organic EL device for confirming the dependence of the luminance of the organic EL device on the film thickness is prepared and tested, the results of the attenuation characteristics shown in FIG. 7 are obtained. The organic EL devices tested were TP having a thickness of 500 Å.
D from the hole transport layer of 1150 Å to 7
It has a two-layer structure with a 725 Å Alq ₃ EL layer. As shown, the organic E
The L element exhibits a minimum film thickness and a maximum luminance as shown in FIG. 6, and the maximum value and the minimum value of the luminance appear periodically as the order sequentially increases (thickness increase) using this as the primary maximum value. It 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 luminance.

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

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 is shifted to the right of the figure compared to the one. FIG.
Is the TPD hole transport layer and the Alq ₃ EL layer described above.
The result of measuring the relative value of the luminance / current with respect to the viewing angle for each of the organic EL elements having the layer structure is shown.

As is apparent from the figure, the first, second and third order
The organic EL device having a film thickness of 1150 angstroms to 2500 angstroms and 4565 angstroms corresponding to the next maximum value tends to decrease in luminance as the viewing angle increases.
It can be seen that the luminance of the organic EL element having the film thickness of 2050 Å, 3550 Å, and 5275 Å corresponding to the next and third minimum values tends to increase as the viewing angle increases.

Among these organic EL elements, a preferred embodiment is an organic EL element in which the thickness of the Alq ₃ EL layer is, for example, 2400 Å to 2700 Å, as is apparent from FIG.

That is, the thickness range of the EL layer is as shown in FIG.
(The interface between the hole transport layer and the EL layer, that is, the region giving the peak of the emission intensity distribution,
Thickness values l _{1 1} corresponding to converge the luminance value of the film thickness luminance decay curve of the luminance / current characteristics with respect to distance) to the metal cathode i.e. reflective layer, l _{2 1,} l ₃ film corresponding to ₁ to the minimum value of luminance 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). _{Incidentally, l 1 1, l 2 1} , l 3 1 each With period T described _{above, l 1 2 -0.25T, l 2} 2
-0.25T, expressed as l _{3 2} -0.25T.

If the EL layer thickness is selected within the above range, increasing the viewing angle has the same effect as decreasing the film thickness. Therefore, the brightness increases as the viewing angle increases. In the case of a display having a finite size, the organic E is less dependent on the viewing angle of brightness
An L element is 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 luminance / current decay curve of the luminance / current characteristic with respect to the film thickness shown in FIG. 6 according to the material of the EL layer. EL layer thickness value can be obtained.

The present invention is not limited to the two-layer structure of the above-described embodiment, but also has a three-layer structure shown in FIG. 2 with respect to the thickness of the electron transport layer (transparent layer) and the EL layer (light-emitting layer). Luminance/
A film thickness luminance decay curve of the current characteristic 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 device of the above embodiment, but the half 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 the peak value). For example, a light emitting layer having a width of 500 Å or less may be used, and the light emitting layer and the reflective layer may be laminated, or the light emitting layer, the transparent layer, and the reflective layer may be laminated.
Also in the case of the surface light emitting device, the same attenuation characteristics as in FIG. 6 can be obtained, and the film thickness of the light emitting layer or the light emitting layer and the transparent layer corresponding to the minimum amplitude can be obtained.

[0036]

As described above, in the surface emitting device according to the present invention, the 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 light emission intensity distribution to the reflection layer) is determined by the film thickness luminance. A film thickness that includes a film thickness that produces one minimum value of the attenuation characteristic and whose amplitude is within a range that produces a luminance that is less than or equal to the convergence luminance value and that is greater than or equal to a film thickness value that produces a convergence luminance value. With one of the following values, the viewing angle dependency of brightness can be reduced.

[Brief description of the drawings]

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

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

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

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

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

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Metal cathode 2 Transparent anode 3 EL layer 4 Hole transport layer 5 Electron transport layer 6 Glass substrate

Claims (1)

(57) [Claims] 1. A surface light-emitting device comprising a reflective layer laminated directly or via a transparent layer on a light-emitting layer, wherein the light-emitting layer has a predetermined light-emission intensity distribution in a thickness direction of the light-emitting layer. The film thickness between the region giving the distribution peak and the reflective layer is converged within a range that includes the film thickness that produces one minimum value of the film thickness luminance attenuation characteristic and whose amplitude produces a luminance that is not more than the convergent luminance value. A surface light emitting device characterized in that it is one value within a range not less than a film thickness value that produces a luminance value and not more than a film thickness value that produces the one minimum value.