JP4216681B2 - Organic EL device - Google Patents

Description

  The present invention relates to an organic EL element.

  An organic electroluminescence (EL) element is a self-luminous element utilizing the principle that a fluorescent substance emits light by recombination energy between holes injected from an anode and electrons injected from a cathode by applying an electric field. is there. C. W. Tang et al. Reported a low-voltage driven organic EL element using a stacked element (CW Tang, SA VanSlyke, Applied Physics Letters, 51, 913, 1987, etc.). Since then, researches on organic EL elements using organic materials as constituent materials have been actively conducted. Tang et al. Use tris (8-quinolinol) aluminum for the light emitting layer and a triphenyldiamine derivative for the hole transporting layer. The advantages of the stacked structure are that it increases the efficiency of hole injection into the light-emitting layer, blocks the electrons injected from the cathode, increases the generation efficiency of excitons generated by recombination, and generates in the light-emitting layer For example, confining excitons. As in this example, the element structure of the organic EL element includes a hole transport (injection) layer, a two-layer type of an electron transporting light emitting layer, or a hole transport (injection) layer, a light emitting layer, an electron transport (injection) layer. The three-layer type is well known. In such a stacked structure element, the element structure and the formation method are devised in order to increase the recombination efficiency of injected holes and electrons.

  However, in the organic EL element, the probability of singlet generation is limited due to the dependence of spin statistics upon carrier recombination, and thus the light emission probability has an upper limit. This upper limit is known to be approximately 25%. Furthermore, in an organic EL element, light having an emission angle greater than the critical angle is totally reflected and cannot be extracted outside due to the influence of the refractive index of the light emitter. For this reason, when the refractive index of the light emitter is 1.6, only about 20% of the total light emission can be effectively used, and as a limit of energy conversion efficiency, the combined efficiency of singlet generation is about 5% as a whole. (Tetsuo Tsutsui “Current Status and Trends of Organic Electroluminescence”, Monthly Display, vol. 1, No. 3, p11, September 1995). In an organic EL element in which the light emission probability is severely limited, the light extraction efficiency causes a decrease in efficiency which can be regarded as fatal.

  As a method for improving the light extraction efficiency, a light emitting device having an equivalent structure such as an inorganic electroluminescence device has been conventionally studied. For example, a method for improving the efficiency by giving the substrate a light condensing property (Japanese Patent Laid-Open No. Sho 63-314795) and a method for forming a reflective surface on the side surface of the element (Japanese Patent Laid-Open No. 1-220394) have been proposed. . However, these methods are effective for an element with a large light emitting area, but in a minute element with a small pixel area such as a dot matrix display, a lens for condensing light, a reflective surface on a side surface, or the like is formed. Processing is difficult. Furthermore, in the organic EL element, since the film thickness of the light emitting layer is several μm or less, it is difficult to form a reflecting mirror on the side surface of the element by applying a taper process, which requires a significant cost. Bring up. Also, there is a method (Japanese Patent Laid-Open No. 62-172691) in which a flat layer having an intermediate refractive index is introduced between the substrate glass and the light emitter to form an antireflection film. Although there is an effect of improving the extraction efficiency, total reflection cannot be prevented. Therefore, even if it is effective for inorganic electroluminescence having a large refractive index, a large improvement effect cannot be achieved for an organic EL element that is a light emitter having a relatively low refractive index.

  Therefore, the light extraction method useful for the organic EL element is still insufficient, and the development of this light extraction method is indispensable for improving the efficiency of the organic EL element. Thus, an organic EL element having a diffraction grating as a constituent element in order to improve the light extraction efficiency is disclosed in Japanese Patent Application Laid-Open No. 11-283951. By this method, the light extraction efficiency of the organic EL element is improved, and the light emission efficiency of the element is improved. However, even in this case, the light extraction efficiency is not sufficiently high.

  The present invention has been made in view of the above-described circumstances, and an object thereof is to provide a highly efficient organic EL element.

  As a result of intensive studies, the present inventor has obtained the present invention. That is, this invention is the organic EL element of (1)-(7) shown next.

(1) In an organic EL element in which one or more organic thin film layers are sandwiched between a pair of electrodes, at least one of which is a transparent electrode,
A high refractive index layer having a higher refractive index than the organic thin film layer;
An organic EL element, further comprising a layer in which microspheres made of materials having different refractive indexes are dispersed in a medium in contact with the high refractive index layer.

(2) The organic EL element according to (1), wherein the high refractive index layer is provided between the transparent electrode and the substrate.

(3) The organic material according to (1) or (2), wherein the high refractive index layer is formed by solidifying after applying a coating liquid in which a precursor containing a metal compound is dispersed. EL element.

(4) In an organic EL element in which one or more organic thin film layers are sandwiched between a pair of electrodes, at least one of which is a transparent electrode,
The transparent electrode is a high refractive index layer having a higher refractive index than the organic thin film layer, and a layer in which microspheres made of materials having different refractive indexes are dispersed in a medium is further provided in contact with the high refractive index layer. An organic EL element characterized by comprising:

(5) The organic EL element of (1) to (4), wherein the diameter of the microsphere is 50 nm to 5 μm.

(6) The pair of electrodes includes a transparent electrode and a metal electrode, and a recombination light-emitting region of holes and electrons in the organic thin film layer is separated from the metal electrode by 100 nm or more (1) Organic EL element of (5).

(7) The organic EL device according to any one of (1) to (5), wherein a light emitting layer in the organic thin film layer is separated from the metal electrode by 100 nm or more.

That is, when a high refractive index layer having a higher refractive index than that of the organic layer is provided, the EL emission propagating in the lateral direction is concentrated on the high refractive index layer, so that the influence of propagation loss from the electrode is reduced.
Furthermore, when a periodic structure in a direction parallel to the substrate surface is provided, the light extraction efficiency is improved. As this periodic structure, a layer having a periodic structure in which microspheres are dispersed is inserted in contact with the high refractive index layer. A periodic structure with a difference in refractive index can be formed by appropriately selecting a material system so that the refractive index of the medium and the microsphere can be different. In this case, either the medium or the microsphere may have a higher refractive index. The diameter of the microsphere is preferably 50 nm to 5 μm in order to efficiently extract EL light emission.

  The following conditions are required for the high refractive index layer. The thickness of the high refractive index layer is preferably 50 nm or more, and more preferably 200 nm or more because EL emission cannot be effectively confined if it is too thin. Moreover, although the refractive index of a high refractive index layer needs to be higher than an organic layer, specifically, 1.7 or more are preferable. Moreover, in order to reduce the loss by absorption, it needs to be transparent. When film formation under such conditions is performed by a normal vapor deposition method such as sputtering, it is difficult to achieve both film thickness and transparency. That is, in the vapor deposition method, there is a problem that as the film thickness is increased, the uniformity of the film is lowered and the transparency is lowered. Further, the vapor deposition method has a problem that the equipment becomes large and the manufacturing cost becomes very high.

  Therefore, when the high refractive index layer is provided between the transparent electrode and the substrate, it is provided by solidifying after applying a coating liquid in which a precursor containing a metal compound is dispersed, or the transparent electrode itself has a high refractive index. The aforementioned problem can be solved by selecting a layer having a refractive index larger than that of the organic thin film layer so as to form a layer.

  In the former case, by applying a coating liquid in which a precursor containing a metal compound is dispersed and then solidifying to form a high refractive index layer, it becomes possible to achieve both a film thickness and transparency that have been difficult in the past, In addition, the manufacturing cost can be reduced. Specific examples include, but are not limited to, a sol-gel method, a coating pyrolysis method, and an organic acid salt method. By such a method, the merit that a thick film can be formed uniformly and the manufacturing cost is low is obtained.

  Known precursors can be used as the precursor containing the metal compound, and examples thereof include metal alkoxides, organic acid salts, metal complex salts, and oxides.

  Further, when the counter electrode of the transparent electrode is a metal electrode, it is effective to take out the light emitting place from the metal electrode efficiently by separating the EL light emitting region from the metal electrode. For this reason, the light emission efficiency is improved by efficiently extracting the light emission component having a particularly large emission angle to the outside.

  That is, the organic EL element has a configuration in which an organic layer is sandwiched between a pair of counter electrodes. Usually, a transparent electrode such as ITO is used as one electrode for extracting light, and the other electrode is generally a metal electrode. In this configuration, when EL emitted light is guided through the organic layer, it receives a propagation loss from the metal electrode (“Optical Integrated Circuit”, written by Hiroshi Nishihara, Ohm). This is because the metal generally behaves as a dielectric having a negative dielectric constant and a large loss due to the inertial effect of electric charge in the metal in the region of the optical wavelength. The light emission component with an emission angle close to 0 ° is less affected by propagation loss because the distance passing through the vicinity of the metal electrode is short. However, the component with a larger emission angle is affected by propagation loss because the distance through the vicinity of the metal electrode is long. Is large (FIG. 1). In FIG. 1, 100 indicates an ITO substrate, 101 indicates an organic film, and 102 indicates a cathode. When the emission angle becomes larger than a certain level, the light is totally reflected at the interface with the air, so that it is confined, and the influence of the propagation loss of the metal electrode becomes larger.

As described above, in the organic EL element, efficiently extracting a light emitting component having a large emission angle to the outside leads to an improvement in light extraction efficiency. As a result of intensive studies, the present inventors have found that the luminous efficiency is improved by separating the light emitting region and the metal electrode by 100 nm or more.
By using this configuration for an organic EL element having a high refractive index layer and a layer in which microspheres are dispersed, an organic EL element with improved luminous efficiency can be obtained.

  In the organic EL device of the present invention, a light emitting component having a large emission angle can be efficiently extracted to the outside, and thus an organic EL device with improved light extraction efficiency is obtained.

  The element structure of the organic EL element according to the present invention is a structure in which one or two or more organic layers are laminated between electrodes. Examples of the structure include an anode / light emitting layer / cathode structure, and an anode / hole transport layer. Examples of the structure include: / light emitting layer / electron transport layer / cathode structure, anode / hole transport layer / light emitting layer / cathode structure, and anode / light emitting layer / electron transport layer / cathode structure.

  In general, EL emission is taken out from the anode side using the anode as a transparent electrode and the cathode as a metal electrode. In this case, the distance between the light emitting region and the cathode is increased. In the case where an electron transport layer is inserted between the light emitting layer and the cathode, the distance between the light emitting region and the cathode can be increased by setting the thickness of the electron transport layer to 100 nm or more. Further, the electron transporting layer can be composed of two or more layers of a hole and exciton blocking layer and an electron transporting spacer layer. By using a highly conductive material for the spacer layer, the distance between the light emitting region and the cathode can be increased without increasing the driving voltage of the element. In the case of an anode / hole transport layer / light-emitting layer / cathode device structure, a material having a light-emitting property and an electron transport property is generally used for the light-emitting layer. In this case, the vicinity of the hole transport layer in the light-emitting layer is positive. Since the hole-electron recombination light-emitting region is formed, the light-emitting region is separated from the cathode by increasing the thickness of the light-emitting layer.

  When the anode is a metal electrode, the light emitting region and the anode are separated. In the case where a hole transport layer is inserted between the light emitting layer and the anode, the distance between the light emitting region and the anode can be increased by setting the thickness of the hole transport layer to 100 nm or more. Further, the hole transport layer may be composed of two layers or more of a layer for blocking electrons and excitons and a hole transportable spacer layer. By using a highly conductive material for the spacer layer, the distance between the light emitting region and the anode can be increased without increasing the driving voltage of the element. In the case of an anode / light-emitting layer / electron transport layer / cathode device structure, a material having a light-emitting property and a hole-transporting property is generally used for the light-emitting layer. In this case, the vicinity of the electron transport layer in the light-emitting layer is a hole. Therefore, by increasing the thickness of the light emitting layer, the light emitting region is separated from the anode.

  The hole transport material used in the present invention is not particularly limited, and any compound that is usually used as a hole transport material may be used. Specific examples of the hole transport material include, for example, the following bis (di (p-tolyl) aminophenyl) -1,1-cyclohexane [01], N, N′-diphenyl-N, N′-bis (3- Methylphenyl) -1,1'-biphenyl-4,4'-diamine [02], N, N'-diphenyl-NN-bis (1-naphthyl) -1,1'-biphenyl) -4,4 Examples include triphenyldiamines such as' -diamine [03], starburst type molecules ([04] to [06], etc.), conductive polymers such as polyparaphenylene vinylene derivatives, polyaniline derivatives, and polythiophene derivatives. It is done.

Since a conductive polymer generally has high conductivity, it is effective as a hole transporting spacer layer. A mixed film of a Lewis acid such as FeCl ₃ and a hole transport material is also applicable.

  The electron transport material used in the present invention is not particularly limited, and any compound that is usually used as an electron transport material may be used. Specific examples of the electron transport material include, for example, 2- (4-biphenylyl) -5- (4-tert-butylphenyl) -1,3,4-oxadiazole [07], bis {2- (4- oxadiazole derivatives such as t-butylphenyl) -1,3,4-oxadiazole} -m-phenylene [08], triazole derivatives ([09], [10], etc.), quinolinol-based metal complexes ([[ 11] to [14]), bathophenanthroline [15], bathocuproine [16], and the like.

  A mixed film of an electron transporting material and a metal is effective as an electron transporting spacer layer because the driving voltage can be kept low even when the film thickness is increased. In this case, the electron transport material can be appropriately selected from known electron transport materials. The metal can be appropriately selected from known metals, but it is preferable to use a metal having a low ionization potential in order to provide electron transport properties. For example, Mg, Ca, Li, Cs, Al, etc. are mentioned.

  The light emitting material used in the present invention is not particularly limited, and any compound that is usually used as a light emitting material may be used. For example, distyrylarylene derivatives (JP-A-2-247278, JP-A-5-17765), coumarin derivatives, dicyanomethylenepyran derivatives, perylene derivatives (JP-A 63-264692), and aromatic ring systems Materials (JP-A-8-298186, JP-A-9-268284) and anthracene compounds (JP-A-9-157743, JP-A-9-268283, JP-A-10-72581), quinacridone derivatives (special Kaihei 5-70773) and the like.

  The anode of the organic EL element plays a role of injecting holes into the hole transport layer, and it is effective to have a work function of 4.5 eV or more. Specific examples of the anode material used in the present invention include indium tin oxide alloy (ITO), tin oxide (NESA), gold and the like. The cathode is preferably a material having a small work function for the purpose of injecting electrons into the electron transport zone or the light emitting layer. The cathode material is not particularly limited, and specifically, indium, aluminum, magnesium, magnesium-indium alloy, magnesium-aluminum alloy, aluminum-lithium alloy, aluminum-scandium-lithium alloy, magnesium-silver alloy and the like can be used.

  The formation method of each layer of the organic EL element of the present invention is not particularly limited, and a known method can be appropriately selected. For example, a vacuum deposition method, a molecular beam deposition method (MBE method), a dipping method of a solution dissolved in a solvent, a spin coating method, a casting method, a bar coating method, a coating method such as a roll coating method, or the like can be given.

  Examples of the present invention will be described in detail below.

A sectional view of the organic EL device according to this example is shown in FIG. Ti (i-OC ₃ H ₇ ) ₄ was diluted with absolute ethanol, a solution obtained by diluting hydrochloric acid with absolute ethanol was added dropwise with stirring to prepare a transparent sol (coating solution), and SiO ₂ microspheres ( An average particle size of 280 nm was dispersed and formed on the glass substrate 900 by dip coating, followed by high temperature treatment to form a titanium oxide layer 901 in which SiO ₂ microspheres were dispersed. Furthermore, Ti (i-OC ₃ H ₇ ) ₄ is diluted with absolute ethanol, and a solution obtained by diluting hydrochloric acid with absolute ethanol is added dropwise with stirring to prepare a transparent sol (coating solution). After forming a film on the glass substrate 900 by dip coating, a titanium oxide film was formed by high-temperature treatment. By repeating this coating process about 10 times, a high refractive index layer 902 made of a titanium oxide layer having a thickness of 0.9 μm and serving as a high refractive index layer was formed.

  Next, ITO was formed into a film by sputtering so that the sheet resistance was 20 Ω / □, and an anode 903 was obtained. The following two layers were formed thereon as the organic layer 904. First, as a hole transport layer, Compound [03] was formed to 50 nm by vacuum deposition. Next, the compound [11] was formed to 60 nm as a light emitting layer by vacuum deposition. Next, a film in which compound [16] and magnesium were co-deposited by a vacuum deposition method at a deposition rate ratio of 2: 1 was formed to 400 nm as an electron transport layer.

Next, a magnesium-silver alloy film co-deposited by a vacuum deposition method at a deposition rate ratio of 10: 1 was formed to 150 nm as the cathode 905 to produce an organic EL element. When a direct current voltage of 5 mA / cm ² was applied to the device, light emission of 391 cd / m ² was obtained.

(Comparative example)
A cross-sectional view of an organic EL device according to this comparative example is shown in FIG. An ITO film was formed on the glass substrate 300 by sputtering so that the sheet resistance was 20Ω / □, and the anode 301 was obtained.

  The following three layers were formed as the organic layer 302 thereon. First, as a hole transport layer, Compound [03] was formed to 50 nm by vacuum deposition. Next, the compound [11] was formed to 60 nm as a light emitting layer by vacuum deposition. Next, a film in which compound [16] and magnesium were co-deposited by a vacuum deposition method at a deposition rate ratio of 2: 1 was formed to 400 nm as an electron transport layer.

Thereafter, a film in which magnesium-silver alloy was co-deposited by a vacuum deposition method at a deposition rate ratio of 10: 1 was formed to 150 nm as the cathode 303 to produce an organic EL element. When a direct current voltage of 5 mA / cm ² was applied to the device, light emission of 143 cd / m ² was obtained.

(Reference Example 1)
A cross-sectional view of the organic EL element according to this reference example is shown in FIG. A colloidal particle composed of indium hydroxide and anhydrous stannic chloride is ultrasonically dispersed in an indium chloride aqueous solution to prepare a coating solution prepared by adding acetic acid and polyvinyl alcohol, and this is formed on the glass substrate 500 by dip coating. After the film formation, an ITO film 501 having a film thickness of 1.3 μm to be a high refractive index layer was formed by high temperature treatment. Then, SiO was formed in a thickness of 50 nm by vacuum deposition, and then reactive gas etching was performed to form a diffraction grating 502 made of SiO. The diffraction grating had a pitch of 700 nm and a line / space = 1: 1.

  The following two layers were formed thereon as the organic layer 503. First, as a hole transport layer, compound [03] was formed to 60 nm by vacuum deposition. Next, Compound [11] was formed to 80 nm as a light emitting layer by a vacuum deposition method.

Next, a magnesium-silver alloy film co-deposited by a vacuum deposition method at a deposition rate ratio of 10: 1 was formed to 150 nm as the cathode 504 to produce an organic EL element. When a direct current voltage of 5 mA / cm ² was applied to the device, light emission of 287 cd / m ² was obtained.

(Reference Example 2)
A cross-sectional view of the organic EL element according to this reference example is shown in FIG. ITO and SiO were formed in the same manner as in Reference Example 1. The following three layers were formed thereon as the organic layer 503. First, as a hole transport layer, Compound [03] was formed to 50 nm by vacuum deposition. Next, the compound [11] was formed to 60 nm as a light emitting layer by vacuum deposition. Next, a film in which compound [16] and magnesium were co-deposited by a vacuum deposition method at a deposition rate ratio of 2: 1 was formed to 400 nm as an electron transport layer.

Next, a magnesium-silver alloy film co-deposited by a vacuum deposition method at a deposition rate ratio of 10: 1 was formed to 150 nm as the cathode 504 to produce an organic EL element. When a direct current voltage of 5 mA / cm ² was applied to the device, light emission of 368 cd / m ² was obtained.

(Reference Example 3)
A cross-sectional view of the organic EL element according to this reference example is shown in FIG. In the same manner as in Reference Example 1, an ITO film having a film thickness of 1.3 μm serving as a high refractive index layer was formed on the glass substrate 500. An ITO film having a thickness of 80 nm was formed thereon by sputtering to form an ITO film 501.

  Then, SiO was formed in a thickness of 50 nm by vacuum deposition, and then reactive gas etching was performed to form a diffraction grating 502 made of SiO. The diffraction grating had a pitch of 700 nm and a line / space = 1: 1.

  The following two layers were formed thereon as the organic layer 503. First, as a hole transport layer, Compound [03] was formed to 50 nm by vacuum deposition. Next, the compound [11] was formed to 60 nm as a light emitting layer by vacuum deposition. Next, a film in which compound [16] and magnesium were co-deposited by a vacuum deposition method at a deposition rate ratio of 2: 1 was formed to 400 nm as an electron transport layer.

Next, a magnesium-silver alloy film co-deposited by a vacuum deposition method at a deposition rate ratio of 10: 1 was formed to 150 nm as the cathode 504 to produce an organic EL element. When a direct current voltage of 5 mA / cm ² was applied to the device, light emission of 418 cd / m ² was obtained.

(Reference Example 4)
A cross-sectional view of the organic EL device according to this reference example is shown in FIG. Ti (i-OC ₃ H ₇ ) ₄ is diluted with absolute ethanol, and a solution obtained by diluting hydrochloric acid with absolute ethanol is added dropwise with stirring to prepare a transparent sol (coating solution). After film formation by dip coating, a titanium oxide film was formed by high-temperature treatment. By repeating this coating process about 10 times, a titanium oxide layer 601 having a film thickness of 0.9 μm to be a high refractive index layer was formed.

  Next, ITO was formed into a film by sputtering so that the sheet resistance was 20 Ω / □, and an anode 602 was obtained. Then, SiO was formed with a thickness of 50 nm by vacuum deposition, and then reactive gas etching was performed to form a diffraction grating 603 made of SiO. The diffraction grating had a pitch of 700 nm and a line / space = 1: 1.

  The following two layers were formed as the organic layer 604 thereon. First, as a hole transport layer, Compound [03] was formed to 50 nm by vacuum deposition. Next, the compound [11] was formed to 60 nm as a light emitting layer by vacuum deposition. Next, a film in which compound [16] and magnesium were co-deposited by a vacuum deposition method at a deposition rate ratio of 2: 1 was formed to 400 nm as an electron transport layer.

Next, a magnesium-silver alloy film co-deposited by a vacuum deposition method at a deposition rate ratio of 10: 1 was formed to 150 nm as the cathode 605 to produce an organic EL element. When a direct current voltage of 5 mA / cm ² was applied to the device, light emission of 431 cd / m ² was obtained.

It is sectional drawing of an organic EL element. It is sectional drawing of an example of the organic EL element concerning this invention. It is sectional drawing of the organic EL element of a comparative example. It is sectional drawing of an example of the organic EL element of the reference examples 1-3. It is sectional drawing of an example of the organic EL element of the reference example 4.

Explanation of symbols

100 ITO substrate 101 Organic film 102 Cathode 300 Glass substrate 301 Anode 302 Organic layer 303 Cathode 500 Glass substrate 501 ITO film 502 SiO
503 Organic layer 504 Cathode 600 Glass substrate 601 Titanium oxide layer 602 ITO film 603 SiO
604 Organic layer 605 Cathode 900 Glass substrate 901 Titanium oxide layer 902 in which SiO ₂ microspheres are dispersed High refractive index layer 903 ITO film 904 Organic layer 905 Cathode

Claims (4)

In an organic EL element in which one or more organic thin film layers are sandwiched between a pair of electrodes, at least one of which is a transparent electrode,
Disposed on the opposite side of the organic thin film layer with respect to the transparent electrode, a high refractive index layer having a higher refractive index than the organic thin film layer, and a material having a different refractive index in the medium in contact with the high refractive index layer a layer forms a periodic structure are dispersed more consists microspheres and is further provided,
The diameter of the microsphere is 50 nm to 5 μm,
The organic EL element, wherein the pair of electrodes includes a transparent electrode and a metal electrode, and a light emitting layer in the organic thin film layer is separated from the metal electrode by 100 nm or more. The organic EL element according to claim 1 , wherein the high refractive index layer is provided between the transparent electrode and the substrate. Organic according to the high refractive index layer, any one of claims 1 or 2, characterized in that formed by solidifying after applying the coating solution prepared by dispersing a precursor comprising a metal compound EL element. In an organic EL element in which one or more organic thin film layers are sandwiched between a pair of electrodes, at least one of which is a transparent electrode,
The transparent electrode is a high refractive index layer having a higher refractive index than the organic thin film layer, and the high refractive index layer is in contact with the periodic structure in which microspheres made of materials having different refractive indexes are dispersed in the medium. Is further provided with a layer,
The diameter of the microsphere is 50 nm to 5 μm,
The organic EL element, wherein the pair of electrodes includes a transparent electrode and a metal electrode, and a light emitting layer in the organic thin film layer is separated from the metal electrode by 100 nm or more.

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