JP3065704B2 - Organic electroluminescence device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an EL device having an EL layer in which a thin film is formed by utilizing electroluminescence (hereinafter, referred to as EL) of a material which emits light by current injection. The present invention relates to an organic EL device which is a compound.

[0002]

2. Description of the Related Art As this type of organic EL device, as shown in FIG. 1, between a metal cathode 1 and a transparent anode 2, an EL layer 3 composed of a luminous thin film made of an organic compound and laminated on each other, and A two-layer structure in which a hole transport layer 4 is disposed, or an electron transport layer 5 made of an organic compound laminated between a metal cathode 1 and a transparent anode 2 as shown in FIG.
A three-layer structure having an EL 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.

In these organic EL devices, a 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).

However, the conventional organic EL device having the above-described structure consumes energy in the EL layer and emits light at a low voltage. However, when the EL layer has a small thickness of 500 ° or less, the life is generally short. For example, when continuous emission is allowed to be an organic EL element having an EL layer of thickness 300Å by a two-layer structure shown in FIG. 1 the initial luminance 400 cd / m ^2, the element is deteriorated by half brightness below 100 hours .

On the other hand, when the thickness of the EL layer is increased, the luminance decreases as the film thickness increases even in the case of constant voltage driving. Considering the light emitting principle of the EL element, the luminance is considered to be proportional to the applied current, but is actually different.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to provide an organic EL device which emits light with high luminance stably for a long period of time.

[0007]

The organic EL device according to the present invention is an organic EL device comprising an organic compound and an EL layer and a hole transporting layer laminated on each other between a cathode and an anode. It is characterized in that it has a film thickness that includes a film thickness that produces a secondary maximum value and that has an amplitude whose luminance exceeds the convergent luminance value.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment according to the present invention will be described below with reference to the drawings. The organic EL device of the present embodiment has an EL element between a pair of metal cathodes 1 and a transparent anode 2 similar to that shown in FIG.
It has a two-layer structure in which the layer 3 and the hole transport layer 4 are laminated and formed as a thin film. For example, the cathode 1 has aluminum,
A metal having a small work function, such as magnesium, indium, silver or an alloy of each, and having a thickness of about 100 to 5000 ° 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 mm, or gold and has a thickness of about 800 to 1500 mm. 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)
Alq ₃ ), a tetraphenylbutadiene derivative, or the like. In the hole transport layer 4, N, N'-diphenyl-N, N'-bis (3-methylphenyl) -1,1'-biphenyl-4, which is a triphenyldiamine derivative,
4'-diamine (hereinafter referred to as TPD) is preferably used, and CTM (Carrier Transporting Materials) is further used.
) May 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 research on the layer thickness, the emission spectrum, the luminance, and the viewing angle, it was found that the luminance has a thickness dependence and the luminance has a viewing angle dependence between the luminance and the EL layer thickness. That is, FIG.
As shown in (1), the emission spectrum and 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 the viewer, the light emitted from one point of the light emitting source P in the EL layer includes two lights, a path A direct to the substrate 6 in the drawing and a path B reflected by the metal electrode 1 on the rear surface and directed to the substrate 6. included. The lights of these two paths hold the optical path difference L shown in the following equation 1 and the phase difference ηy shown in the following equation 2, and thus 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 of the display surface in the EL layer,
λ indicates a wavelength. same as below).

[0011]

(Equation 1)

[0012]

(Equation 2)

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

[0014]

(Equation 3)

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 with respect to the film thickness as in Expression 4, and the entire EL layer is normalized as in Expression 5. In both formulas, d indicates the thickness of the EL layer 3 , ε indicates the emission intensity distribution parameter, and k indicates a constant. The same applies hereinafter.

[0016]

(Equation 4)

[0017]

(Equation 5)

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 Equation 6.

[0019]

(Equation 6)

Here, the emission intensity T (λ, θ,
In order to confirm d), the film thickness (y = d) was set to 6000 °, and the emission intensity distribution parameter ε was kept constant at 200 °.
Create an organic EL element including an EL layer composed of q _3, the test of the emission intensity were carried out while varying the viewing angle θ from 0 ° to 75 °. FIG. 5 shows the emission intensity distribution with respect to the emission wavelength. The luminous intensity distribution and the luminous intensity T of the above equation (6)
It was confirmed that (λ, θ, d) substantially matched. As is apparent from the figure, the viewing angle is 0 ° to 75 ° for the viewer.
The color appears to be different sequentially depending on the viewing direction of the EL element display surface up to °.

Further, a luminosity characteristic E (λ) of a viewer or a photodetector which responds to a specific value with respect to the wavelength λ 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 in the sensitivity characteristic can be expressed as a function of d as shown in Expression 7 (K is a constant).

[0022]

(Equation 7)

FIG. 6 shows an EL layer made of Alq ₃ (θ =
0, n = 1.7), the film thickness is changed from 0 ° to 8000.
4 shows a film thickness decay (characteristic) curve of the luminance / current characteristic with respect to the film thickness when calculated by changing over Å, and this decay curve shows the film thickness dependence of the luminance in the organic EL element. When an organic EL element for confirming the dependence of the luminance of the organic EL element on the film thickness is prepared and tested, the results of the attenuation characteristics shown in FIG. 7 are obtained. The plurality of organic EL devices tested consisted of a TPD hole transport layer having a thickness of 500 ° and an Alq ₃ EL layer having a thickness of 1150 ° to 7725 °.
It has a layered structure. As shown in the figure, the organic EL element exhibits a minimum film thickness and a maximum luminance as shown in FIG. 6, and uses this as a primary maximum value, and periodically increases the luminance maximum value as the order increases (film thickness increase). Appears, indicating the characteristic of the film thickness luminance decay curve in which the maximum value decreases, that is, the film thickness dependence of luminance. Note that the film thickness luminance decay curve of FIG. 7 shows that the entire characteristic curve shifts to the right in FIG. 6 as compared with that of FIG. doing.

Among these organic EL devices, a preferred embodiment is an organic EL device in which the thickness of the Alq ₃ EL layer is set to 2000 ° ± 300 ° corresponding to the secondary maximum value C, as apparent from FIG. . By setting the EL layer in this thickness range, an EL layer that can withstand a high applied current while maintaining luminance can be obtained. That is, the thickness range of this EL layer is determined by the luminance / luminance / film thickness according to the EL layer material shown in FIG.
The secondary maximum amplitude of the second highest luminance in the film thickness luminance decay curve of the current characteristic is a range D exceeding the converged luminance value (converged luminance value). By setting the film thickness to correspond to the vicinity of the secondary maximum value showing high brightness, a highly reliable and high brightness organic EL element can be obtained, which is preferable.

Further, since a change in the viewing angle is equivalent to a change in the film thickness, the viewing angle is slightly changed by setting the EL layer to a film thickness corresponding to the vicinity of the secondary maximum value in the film thickness decay curve. High-brightness organic EL with little change in brightness
An element is obtained. FIG. 8 shows the results of measuring the relative values of the luminance / current with respect to the viewing angle for each of the two-layered organic EL devices of the TPD hole transport layer and the Alq ₃ EL layer. As is apparent from the figure, the film thicknesses 1150, 2500 and 45 corresponding to the primary, secondary and tertiary maxima.
It can be seen that the organic EL element having an angle of 65 ° tends to be less dependent on the viewing angle at which the luminance increases as the viewing angle increases. Therefore, if the EL layer film thickness corresponds to the maximum value of each amplitude in the film thickness luminance decay curve, an organic EL element in which the viewing angle dependence of the luminance and emission spectrum is small and the change in color depending on the viewing angle is small. The above-described embodiment in which the EL layer has a film thickness corresponding to the vicinity of the second-order maximum value showing the second highest luminance in the film thickness luminance decay curve also has a small viewing angle dependency of the luminance.

Here, as an embodiment, an organic EL element having a film thickness corresponding to the vicinity of the secondary maximum value in the film thickness luminance decay curve is preferable. This is because the life of the organic EL element is short because it is thin, and the luminance / current becomes low when the organic EL element has a tertiary maximum value or more. That is, the above-mentioned embodiment having the film thickness of the secondary maximum value is a highly reliable and high luminance organic EL element in which the life and luminance of the organic EL element are balanced.

Further, the present invention relates to the Alq ₃
The EL layer thickness value corresponding to the secondary maximum amplitude can be obtained from the thickness / luminance decay curve of the luminance / current characteristics with respect to the film thickness shown in FIG.

[0028]

As described above, in the organic EL device according to the present invention, the EL layer has a thickness within the range where the amplitude of the secondary maximum value of the second highest luminance on the luminance decay curve exceeds the converged luminance value. , It is possible to efficiently emit light with high luminance at a low voltage while improving durability.
Further, according to the present invention, a highly reliable and high-brightness organic EL element having a small change in color due to the viewing angle can be obtained because the viewing angle dependence of the spectrum is small.

[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 a light emission intensity distribution of a film thickness of an EL layer in an organic EL element having a two-layer structure.

FIG. 5 is a graph illustrating a wavelength emission 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

──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Yukio Tanaka 465 Osato-cho, Kosato-cho, Yamanashi Prefecture Pioneer Video Co., Ltd. (72) Inventor Yoshinobu Yonemoto 465 Osato-cho, Kofu-shi, Yamanashi Pioneer Video Co., Ltd. Inside a semiconductor factory (56) References 51st Annual Meeting of the Japan Society of Applied Physics 28a-PB-11 (1990) "Control of photon emission probability in organic EL devices" (58) Fields investigated (Int. Cl. ⁷ , DB name) H05B 33/14 H01L 51/00 C09K 11/00 C09K 11/06 602

Claims (3)

(57) [Claims] 1. An organic electroluminescent device comprising an organic compound and an electroluminescent layer and a hole transporting layer laminated on each other and disposed between a cathode and an anode, wherein the electroluminescent layer has a film thickness and luminance decay characteristic. An organic electroluminescent device comprising a film thickness that includes a secondary maximum value and a film thickness whose amplitude exceeds a convergence luminance value within a range that generates luminance. 2. The organic electroluminescence device according to claim 1, wherein the electroluminescence layer has only a film thickness that causes a secondary maximum of the film thickness luminance attenuation characteristic. 3. The method according to claim 1, wherein the hole transport layer is made of a triphenyldiamine derivative, and the electroluminescent layer is made of an aluminum quinolinol complex, and a range in which a luminance exceeding the convergent luminance value is generated is 2,000 ± 300 °. The organic electroluminescence device according to claim 1, wherein