JPH09171892A - Organic thin film el element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a device with low power consumption by making the light emitting side of a base into lens structure, and setting the flat part thickness of the base, the lens part thickness, and the curvature radius to specified ratios. SOLUTION: The diameter Al of an organic EL light emitting part 2 is set to 1mm, the thickness d1 of a base flat part to 1.1mm, the thickness d2 of a base lens part to 0.4mm, the curvature radius R2 of the base surface lens to 4mm, and the output of the organic EL emitting part to 1mW. The light emitting side of a base 3 adjacent to such an organic EL emitting part 2 is made into lens structure, whereby the emitted light can be efficiently converged. Since the ratio in which the incident angle on the base surface becomes less than a critical angle is increased, the brightness is improved, even when seen from the angle other than the optical axis. Further, d1+d2=1.5mm is set smaller than R2=4mm, whereby the field angle can be increased. The light emitting side of the base is made into lens structure, and d1+d is set to R2 or less, whereby a device with low power consumption can be provided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an organic thin film EL device used as a light source for displaying segments or dots.

[0002]

2. Description of the Related Art A new type of organic thin film EL device reported by Tang et al. (Applied Physics Letters).
Letters), 51, 913, 1987.
Year) is composed of a flat substrate, an anode, a hole transport layer, a light emitting layer, and a cathode. As the anode, an indium tin oxide alloy (IT) formed on a flat glass substrate
O), the hole transport layer is formed of 1,1'-bis (4-N, N'-ditolylaminophenyl) cyclohexane, the light emitting layer is formed of tris (8-hydroxyquinolinol aluminum), and the cathode is formed of a magnesium-silver alloy. ing.

The principle of light emission operation of the organic thin film EL device of Tang et al. Is briefly described. First, holes injected from the anode into the hole transport layer move toward the interface of the light emitting layer, and electrons are emitted from the cathode. It is injected into the layer and moves through the light emitting layer. Then, the holes and electrons injected into the light emitting layer recombine in the light emitting layer, and emit light through an excited state. At this time, the external quantum efficiency of the organic thin film EL device is represented by the product of the EL quantum efficiency and the light extraction efficiency. The EL quantum efficiency greatly depends on the light emitting material and the element structure, and the light extraction efficiency on the other hand largely depends on the refractive index of light of the substrate or the organic thin film.

[0004] Basically, the organic thin-film EL devices currently being developed have basically been improved based on the device structure and material concepts reported by Tongue et al., With improvements in organic materials and electrodes, as well as improvements in device structures. The purpose of the present invention is to increase the light emission intensity by adding a small amount of a quinacridone derivative to the light emitting layer to obtain 4.1% (photon).
/ ELECTRON ELECTRONIC DEVICE WITH EXTERNAL QUANTITY EFFICIENCY ("Practical use technology of organic semiconductor", Science Forum, New Trigger Series, 95)
-116 (1993)) has been reported by Wakimoto et al.

[0005]

In addition to the fact that the generation probability of singlet excitons is 40% at the maximum, light generated in the organic thin film → radiated light is not effectively extracted to the outside, that is, light Due to the low extraction efficiency,
It has been reported that the external quantum efficiency of an organic thin film EL that emits planar light is theoretically only about 8% at a maximum (The Organic Electronics Materials Research Group (The Japanes).
e Research Association fo
r Organic Electronics Mat
erials) WORKSHOP 95, pp. 1-6, 19
1995).

[0006] The cause of the low light extraction efficiency is considered as follows. Normal organic thin film EL
In the element, as shown in FIG. 6, the light emitting section 2 of the organic thin film EL element is directly connected to the back surface of the flat substrate 4. The output light from the light emitting section 2 passes through the inside of the flat substrate 4 from the back surface and is extracted from the front surface. However, in the organic thin-film EL element having such a structure, the light from the organic thin-film layer, which is the isotropic light-emitting source, has a critical angle of incidence on the substrate surface or the transparent electrode surface on the light extraction side as shown in FIG. If the angle is exceeded, the light is totally reflected and cannot be taken out of the substrate. For example, when the diameter of the organic EL light emitting portion 2 is A ₁ and the thickness of the substrate is d ₁ , and A ₁ is 1 mm and the thickness d ₁ of the flat substrate 4 is 1.1 mm, the light extraction efficiency is approximately 25%. Become.

For this reason, in the conventional configuration, since light from the organic thin-film EL element cannot be effectively extracted, if a higher luminance is required, a higher voltage must be applied, resulting in an increase in power consumption. The result was.

An object of the present invention is to provide an organic thin film EL device which can be driven with low power consumption by increasing the external quantum efficiency of the device by increasing the light extraction efficiency of the organic thin film EL device.

[0009]

In the organic thin film EL element of the present invention, an organic thin film EL element having a light transmitting portion is
The light extraction side of the substrate has a lens structure. Further, the lens structure has a curvature centered on the center of the light emitting portion.

Further, the light transmitting portion is such that d ₁ + d ₂ ≤R ₂ is established between the thickness d _{1 of} the substrate, the distance d ₂ from the substrate surface to the apex of the lens structure, and the radius of curvature R ₂ of the lens. Characterize. Further, the lens structure is a substrate having a distributed refractive index.

In the present invention, a lens structure having a radius of curvature is provided on a surface of a transparent substrate or a transparent electrode from which output light is extracted, in an organic thin film EL element having an organic EL light emitting portion formed on a substrate surface. With this lens structure, light emitted in directions other than the optical axis is collected in the optical axis direction,
The illuminance seen from the optical axis direction is improved. In addition, the rate at which the angle of incidence on the substrate surface where light radiated from the light source is viewed is less than the critical angle increases, that is, light radiated at a large solid angle can be extracted, so that light viewed from a direction other than the optical axis can be obtained. In this case, the illuminance will be improved.

Further, when the lens structure has a curvature centered on the center of the light emitting portion of the organic thin film EL element, the light extraction efficiency can be further increased.

[0013]

Embodiments of the present invention will be described below in detail with reference to the drawings.

FIG. 1 is a sectional view showing an organic thin film EL device according to a first embodiment of the present invention. In the first embodiment, the substrate 3 in contact with the light emitting portion 2 of the organic thin film EL element
Has a lens-like structure on the light emission side. Here, only the organic EL light emitting portion 2 of the organic thin film EL element is shown in the figure, and other structures are omitted.

In the first embodiment, the light from the organic EL light emitting section 2 can be efficiently collected by forming the light emitting side of the substrate in contact with the light emitting section with a lens-like structure.
Since the ratio of the incident angle on the substrate surface being equal to or less than the critical angle increases, the illuminance is improved even when viewed from an angle other than the optical axis.

FIG. 2 shows the case where the output of the organic EL light emitting portion is 1 mW and the angle θ from the center of the light emitting portion of the organic thin film EL element to the outside θ.
It is a figure which shows the light intensity distribution when it sees from a direction.

Example 1 of the first embodiment shown in FIG.
Will be described. Let A _{1 be} the diameter of the organic EL light emitting portion 2, d _{1 be} the thickness of the flat portion of the substrate, d _{2 be} the thickness of the lens portion of the substrate, and R ₂ be the radius of curvature of the lens structure on the substrate surface.
A ₁ = 1 mm, d ₁ = 1.1 mm, d ₂ = 0.4 mm, R ₂ =
4 mm, and the output of the organic EL light emitting unit was 1 mW. At this time, the light intensity distribution of Example 1 is as shown in 2 in FIG. 2, and it can be seen that the illuminance and the uniformity are improved.

Here, when the thickness of the flat portion of the substrate is d ₁ , the thickness of the lens portion of the substrate is d _2, and the radius of curvature of the lens structure on the substrate surface is R ₂ , the light intensity distribution and the viewing angle are changed. FIG. 5 shows the relationship. In the figure, the light intensity distribution indicated by the solid line is the organic thin film EL device of Example 1, and the light intensity distribution indicated by the broken line is d ₁ + d ₂ (= 6.0 mm)> R ₂ (= 2.3 mm).
It is what it was. In the example of the broken line, the brightness is slightly improved, but the viewing angle is greatly reduced as compared with the first embodiment. Therefore, it is necessary to satisfy d ₁ + d ₂ ≦ R ₂ in order to increase the viewing angle.

Next, an example of Example 2 of the first embodiment shown in FIG. 1 will be described. In the second embodiment, the center of curvature of the lens structure is centered on the center of the organic EL light emitting unit. In this case, the light extraction efficiency can be increased to 50%. At this time, the organic thin-film EL element has A ₁ = 1 mm, d
_{1 = 1.1mm, d 2 = 1.2mm} , was R ₂ = 2.3 mm. The light intensity distribution when the output of the organic EL light emitting unit is 1 mW in Example 2 is shown in FIG. in this case,
Under the same conditions, it is possible to achieve about twice the light extraction efficiency as compared with the case where a flat substrate is used.

FIG. 3 shows a cross-sectional structure of the organic thin film EL device used in the first embodiment of the present invention. In the first embodiment, as shown in FIG. 3, an anode 21 and an anode interface layer 22 are formed on a glass substrate 3 having a lens-shaped structure 1, and a hole transport layer 23, a light emitting layer 24, and an electron transport layer 25, cathode 2
6. A structure in which the cathode protection layer 27 is formed.

The procedure for fabricating the organic thin film EL device according to the first embodiment will be described below as Example 3.
First, the glass substrate 3 with the lens-like structure 1 is NC
A mold cut by processing is produced, and a lens structure is formed by molding using the mold as a replica. The thickness d ₁ of the glass substrate 3 is 0.3 mm. The refractive index of light is 1.5.
In addition, the radius of curvature R ₂ = 2.5 mm, and the thickness d _{2 of the} lens is 2.
5 mm.

Next, ITO is formed on the glass substrate 3 on the side opposite to the lens structure 1 by sputtering so that the sheet resistance becomes 15 Ω / □, and the film is etched by etching.
An anode 21 having a width of mm was formed. Then, copper phthalocyanine as an anode interface layer 22 was 7.0 × 1 by MBE.
5 nm was formed under 0 ^-9 Torr vacuum.

Next, α-NPD was used as a hole transport layer in MB.
The film was formed to a thickness of 60 nm under a vacuum of 5.8 × 10 ⁻⁹ Torr using the E method. Next, as the light emitting layer 24, tris (8-hydroxyquinolinol aluminum) and quinacridone were MBE
Under 5.6 × 10 ⁻⁹ Torr vacuum by simultaneous evaporation from separate evaporation sources (K-cells) by MBE so that quinacridone contains 0.5 mol% of the light-emitting layer 24 by the method 20.
nm formed.

Further, as the electron transport layer 25, tris (8
-Hydroxyquinolinol aluminum) was formed to a thickness of 40 nm by a MBE method under a vacuum of 4.3 × 10 ⁻⁹ Torr. Note that the organic thin films from the anode interface layer 21 to the electron transport layer 25 were formed on the entire surface of the substrate except for the terminal extraction portion of the anode 21.

Next, the substrate 3 on which the organic film is formed is placed on a stainless steel shadow mask using a load lock mechanism without breaking the vacuum, and an aluminum alloy containing 1 mol% of scandium is used as a cathode with an argon gas. In this method, lithium was evaporated by an RF sputtering method, and lithium was evaporated from another evaporation source, so that lithium was formed to a thickness of 20 nm so as to occupy 0.3 mol% of the cathode.

Further, as the protective layer 27 of the cathode, an aluminum alloy containing 1 mol% of scandium was formed to a thickness of 300 nm by RF sputtering in argon gas. Thus, an organic thin-film EL element having a light-emitting area of 2 mm × 2 mm and a center of curvature of the lens structure centered on the center of the organic EL light-emitting portion is completed.

The thus prepared organic thin film E of Example 3
FIG. 4 shows the results of measuring the optical output of the L element when driven at a constant current using a power meter. The vertical axis represents the light output (μW), and the horizontal axis represents the current (μA). The figure shows a comparison between the organic thin film EL device of Example 3 and a conventional organic thin film EL device using a flat substrate. Here, the conventional organic thin film EL element was a glass substrate having no lens structure (radius of curvature R ₂ = ∞, d ₁ = 0.3 mm).

As can be seen from the figure, the current-light output characteristics in the third embodiment are such that the light output is about 2.6 times larger at the same current as compared with the conventional organic thin film EL device. This indicates that the use of the organic thin film EL device of the present invention improves the light extraction efficiency by about 2.6 times.

An organic thin film EL having a lens structure in the same manner as in Example 3 except that the thickness of the substrate in the flat portion was changed.
An element was prepared, and the same evaluation as the current-light output characteristic was performed.
When d ₁ is set to 0.8mm as in Example 4 this time the light output is improved by about 2 times compared with the conventional organic thin film EL element. Further, as Example 5, when d ₁ was 1.3 mm, the light output was improved by about 1.6 times.

Although one example of an organic thin-film EL element has been described above, when the organic thin-film EL elements are arranged in an array in order to apply the organic thin-film EL element to a display device,
The pitch p of the diameter A ₁ and the element of the light emitting portion of the organic thin film EL element, it is necessary to number 1 relationship as denoted below.

[0031]

[Equation 1]

Examples of the light transmitting portion having the lens structure which constitutes the organic thin film EL element of the present invention include a substrate or a lens-like structure in contact with these. As a lens structure provided on the substrate, a buried type three-dimensional distributed refractive index is formed by patterning a flat substrate glass containing low-refractive-index ions and immersing it in a high-refractive-index ion molten salt to selectively perform ion exchange. There are a lens, a glass or plastics lens formed by molding using a mold cut by NC processing as a replica, and a plastics lenslet array.

The organic thin-film layer of the organic thin-film EL element applicable to the present invention is not particularly limited, but has a single-layer structure of only a light-emitting layer, a hole transport layer, an electron transport layer, an anode interface layer,
Any thin film structure such as one having a cathode interface layer or the like is applicable. Further, the thin film layer other than the light emitting layer is not limited to an organic substance, but is effective even if it is a thin film using an inorganic substance or a thin film of a mixture of an organic substance and a metal.

The organic thin film layer of the organic thin film EL device of the present invention may be formed by vacuum evaporation, molecular beam evaporation (MBE), dipping of a solution dissolved in a solvent, spin coating, casting, bar coating, roll coating, or the like. It can be formed by a known method such as a coating method such as a coating method.

In the present invention, the material of the hole transporting layer is not particularly limited, but for example, triphenyldiamine derivative, oxadiazole derivative, porphyrin derivative, stilbene derivative, arylamine derivative and the like can be used. .

Further, the hole transporting compound can be used as a layer dispersed in a known polymer as a medium.

The polymer is preferably one that does not extremely impair the hole transporting property. Examples thereof include poly- (N-vinylcarbazole), polycarbonate, polymethyl acrylate, polymethyl methacrylate, polystyrene-based polymer, and polysilylene-based polymer. A polymer, polythiophene, polyaniline, polyphenylene pinylene, or the like can be used.

The anode interface layer is introduced in order to achieve stable hole injection, but it must play a role of maintaining the adhesiveness between the organic thin film layer and the anode. Unnecessarily increasing the film thickness may increase the driving voltage for light emission or cause unevenness on the surface of the thin film to cause uneven light emission. The anode interface layer preferably has a film thickness of 30 nm or less.

The anode interface layer applicable in the present invention is, for example, a spiro compound, an azo compound, a quinone compound, an indigo compound, a diphenylmethane compound, a quinacridone compound, a polymethine compound, an acridine compound described in "Dye Handbook: Kodansha '86". , Porphyrin compounds and the like, and bonded polycyclic dyes can be applied.

In addition, "organic" such as aromatic amine
Semiconductors: Ferrac Chemie, 1974 (ORGANIC SEMICONDUCTORS:
VERLAG CHEMIE '74) ".

In the present invention, the light emitting layer material of the organic thin film EL element is not particularly limited, and known light emitting materials can be applied. For example, a metal complex of 8-hydroxyquinolinol and a derivative thereof, a tetraphenylbutadiene derivative, a distyrylaryl derivative, a coumarin derivative, a quinacridone derivative, a perylene derivative, a polymethine derivative, an anthracene derivative, and polyvinyl carbazole can be given. The light emitting layer may be a single component or a system doped with another light emitting material.

In the present invention, an electron transport layer may be provided between the light emitting layer and the cathode, if necessary. The electron transport material is not particularly limited, but 8-hydroxyquinolinol and its derivatives, oxadiazole derivatives, diphenylquinone derivatives and the like can be applied.

The anode of the organic thin film EL element plays a role of injecting holes into the hole transporting layer, and it is effective that the anode has a work function of 4.5 eV or more. Organic thin film E
Specific examples of the anode material used for the L element include indium tin oxide (ITO), tin oxide (NESA), gold, silver,
Platinum, copper, etc. can be applied. As the cathode material, a material having a small work function is preferable for the purpose of injecting electrons into the electron transport band or the light-emitting layer, and specifically, indium, aluminum, magnesium, a magnesium-indium alloy, a magnesium-aluminum alloy, an aluminum-
Metals containing a lithium alloy, an aluminum-scandium alloy, or the like as a main component can be used. In order to protect the element from oxygen and moisture, metal oxides, metal sulfides, metal sulfides,
It is also effective to provide a sealing layer formed of a known sealing material or the like made of an organic compound.

[0044]

As described above, since the structure of the organic thin film EL element in the present invention improves the light extraction efficiency,
A small amount of current is required to obtain the required light output, and an organic thin-film EL device with low power consumption can be obtained. Further, by applying the organic thin-film EL element of the present invention to a segment display, a matrix-type organic thin-film EL display, or the like, an optical display or optical output device with low power consumption can be obtained.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram illustrating an organic thin-film EL device according to a first embodiment of the present invention.

FIG. 2 is a diagram showing a light intensity distribution of an organic thin film EL device of Example 2 according to the first embodiment of the present invention.

FIG. 3 is a sectional view of the organic thin film EL device according to the first embodiment of the present invention.

FIG. 4 is a graph showing current-light output characteristics of Examples 3 to 5 of the present invention and a conventional organic thin film EL element.

FIG. 5 shows the relationship between the light intensity distribution and the viewing angle when the thickness of the flat portion of the substrate is d ₁ , the thickness of the lens portion of the substrate is d _2, and the radius of curvature of the lens structure on the substrate surface is R _2. FIG.

FIG. 6 is a structural view showing a conventional organic thin film EL element.

[Explanation of symbols]

1 lens-like structure 2 organic EL light emitting part 3 substrate 4 flat substrate A ₁ diameter of organic EL light emitting part R ₂ radius of curvature of lens-like structure d ₁ thickness of substrate d ₂ lens thickness θ observation direction and optical axis Angle formed by n1 Substrate refractive index 21 ITO transparent electrode 22 Anode interface layer 23 Hole transport layer 24 Light emitting layer 25 Electron transport layer 26 Cathode 27 Cathode protection layer

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Kenichi Kasahara 5-7-1, Shiba, Minato-ku, Tokyo Inside NEC Corporation (72) Inventor Yoshimasa Sugimoto 5-7-1, Shiba, Minato-ku, Tokyo Japan Electric stock company

Claims (4)

[Claims] 1. An organic thin film EL element having a light transmitting portion, wherein the light extraction side of the substrate has a lens structure. 2. The organic thin film EL element according to claim 1, wherein the lens structure has a curvature centered on the center of the light emitting portion. 3. The light-transmitting portion is a substrate, the thickness d _{1 of} the substrate, the distance d ₂ from the substrate surface to the apex of the lens structure, and the radius of curvature R ₂ of the lens are d ₁ + d ₂ ≦ The organic thin film EL element according to claim 1, wherein R ₂ is satisfied. 4. The organic thin film EL according to claim 1, wherein the lens structure is a substrate having a distributed refractive index.
element.