JP6803727B2 - Organic electroluminescence element - Google Patents

Description

The present invention relates to an organic electroluminescence device (hereinafter, electroluminescence (electroluminescence) may be referred to as "EL").

Organic EL elements are self-luminous, have a wide viewing angle, have excellent visibility, can be driven at a low voltage, can be made thinner and lighter by surface emission, and can display in multiple colors. doing. Therefore, the organic EL element can be suitably used for an image display device such as a display or a lighting device.

An organic EL element is usually configured by laminating an anode, a hole injection layer, a hole transport layer, a light emitting layer, an electron transport layer, an electron injection layer, and a cathode in this order on a transparent substrate. The light emission of the organic EL element is generated through the steps (i) to (v) shown below.
(I) Holes and electrons are injected from the electrodes.
(Ii) The injected holes and electrons are transported.
(Iii) Holes and electrons recombine in the light emitting layer.
(Iv) The luminescent material forms an electronically excited state.
(V) The light emitting material emits light from an electronically excited state.

In an organic EL device, it has been proposed to use a phosphorescent material as a light emitting material of a light emitting layer in order to improve efficiency. When the luminescent material gains energy and becomes an electronically excited state, it generates a singlet excited state (S ₁ ) and a triplet excited state (T ₁ ) with a probability of 1: 3. Then, when the luminescent material returns from the electronically excited state to the ground state, it emits energy as light.
When a fluorescent material is used as the light emitting material, only the energy from S ₁ is converted into light. On the other hand, when a phosphorescent material is used, not only the energy from S ₁ but also the energy from T ₁ is converted into light. Therefore, higher efficiency can be expected in the organic EL element using the phosphorescent material than in the organic EL element using the fluorescent material as the light emitting material (see, for example, Non-Patent Document 1 and Non-Patent Document 2). ).

Phosphorescent materials are typically used with host materials as guest materials. In an organic EL device having a light emitting layer containing a host material and a phosphorescent material (guest material), the energy of the host material excited by the recombination of holes and electrons is transferred to the phosphorescent material. The phosphorescent material is excited by the energy and emitted as light energy. In order to enable efficient energy transfer from the host material to the phosphorescent material, the energy of the triplet excited state (T ₁ ) of the host material is made larger than the T ₁ energy of the phosphorescent material which is the guest material. (See, for example, Non-Patent Document 3). If the T ₁ energy of the guest material is larger than the T ₁ energy of the host material, reverse energy transfer from the guest material to the host material may occur, which may hinder the high efficiency of phosphorescence emission.

Many host materials used for the light emitting layer have been reported so far. For example, examples of the host material include carbazole compounds (see, for example, Non-Patent Document 4). Carbazole compounds have a relatively large T ₁ energy. Examples of the carbazole-based compound generally used as a host material include CBP represented by the following general formula (10) (see, for example, Non-Patent Document 5).

Nature, 395, 151 (1998) Nature, 403,750 (2000) Apple. Phys. Lett. , 83,569 (2003) Apple. Phys. Lett. , 69, 2160 (1996) Monthly display separate volume "Organic EL display" pp. 51 (1998)

In an organic EL element, it is required to lower the drive voltage while ensuring the luminous efficiency. However, in the conventional organic EL element, the drive voltage cannot be sufficiently lowered because the energy barrier in the movement of holes and electrons from the electrode to the light emitting layer is large.
The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an organic EL element having high luminous efficiency and low driving voltage.

The present inventor has made extensive studies in order to solve the above problems. As a result, the laminated structure including the light emitting layer formed between the two electrodes of the organic EL device has both an indolocarbazole group contributing to hole transportability and a sulfonyl group contributing to electron transportability. The present invention was conceived by finding that it is sufficient to have a layer containing the compound shown in (1).

That is, the present invention relates to the following invention.
[1] An organic electroluminescence device characterized in that a laminated structure including a light emitting layer is formed between two electrodes, and the laminated structure has a layer containing a compound represented by the following general formula (1).

（一般式（１）中のＲは、それぞれ独立に水素、又は炭素数１〜２０のアルキル基、炭素数１〜２０のアルコキシ基、炭素数１〜２０のアルキルアミノ基からなる群から選択される置換基である。）

(R in the general formula (1) is independently selected from the group consisting of hydrogen, an alkyl group having 1 to 20 carbon atoms, an alkoxy group having 1 to 20 carbon atoms, and an alkylamino group having 1 to 20 carbon atoms. Substituent.)

[2] The organic electroluminescence device according to [1], wherein the layer containing the compound is any one selected from a hole transport layer, a light emitting layer, and an electron transport layer.

[3] Described in [1], wherein the light emitting layer contains a host material composed of a compound represented by the general formula (1) and a guest material represented by the following general formula (6-2). Organic electroluminescence element.

In the organic EL device of the present invention, a laminated structure including a light emitting layer is formed between two electrodes, and the laminated structure has a layer containing a compound represented by the general formula (1). Therefore, the organic EL element of the present invention has high luminous efficiency and low drive voltage.

It is sectional drawing for demonstrating an example of the organic EL element of this embodiment. It is a graph which shows the result (spectrum) of ^{1 1} H-NMR measurement of the compound used for the thin film formed in Experiment 1. It is a graph which showed the result of having measured the emission spectrum at 300K and 77K of the thin film formed in Experiment 1. It is a graph which showed the result of having measured the emission spectrum at 300K and 77K of the thin film formed in Experiment 2. It is a graph which showed the relationship between the applied voltage and the brightness in the organic EL element of an Example and a comparative example. It is a graph which showed the relationship between the current density and the power efficiency with respect to the organic EL element of an Example and a comparative example.

Hereinafter, the present invention will be described in detail.
The present inventor has focused on the host material of the light emitting layer and repeated diligent studies in order to solve the above problems and realize an organic EL device having high luminous efficiency and low driving voltage.
In a conventional organic EL device, CBP represented by the general formula (10), which is a carbazole-based compound, is generally used as a host material for a light emitting layer. CBP is a material that exhibits only hole transportability and has a large difference between S ₁ (singlet excited state) energy and T ₁ (triplet excited state) energy, and has a large energy gap. Therefore, in the organic EL device having a light emitting layer using CBP, the energy barrier in the movement of holes and electrons from the electrode to the light emitting layer is large, and the driving voltage is high.

To lower the drive voltage, use a compound that has the same T ₁ energy as when CBP is used as the host material of the light emitting layer, and the difference between S ₁ energy and T ₁ energy is smaller than that of CBP. It may be used. This makes it possible to reduce the energy gap of the host material as compared to CBP.

Therefore, the present inventor has made extensive studies and found that the compound represented by the general formula (1) may be used as the host material for the light emitting layer. The compound represented by the general formula (1) has both an indolocarbazole group that contributes to hole transportability and a sulfonyl group that contributes to electron transportability, and the difference between S ₁ energy and T ₁ energy is small. .. Therefore, in an organic EL device having a light emitting layer containing the compound represented by the general formula (1) as a host material, the energy gap of the host material becomes small. As a result, the energy barrier in the hole transfer from the electrode to the light emitting layer and / or the electron transfer from the electrode to the light emitting layer becomes small, and the driving voltage of the organic EL element becomes low. Moreover, in an organic EL device having a light emitting layer containing the compound represented by the general formula (1) as a host material, high luminous efficiency equivalent to that when CBP is used as the host material can be obtained.

Further, the present inventor has made extensive studies, and any one selected from the hole transport layer, the light emitting layer, and the electron transport layer contained in the laminated structure including the light emitting layer formed between the two electrodes is the general formula (1). ) Is included, the energy barrier in hole transfer and / or electron transfer from the electrode to the light emitting layer can be reduced, and it has been confirmed that the above effect can be obtained, and the present invention has been conceived.

"Organic EL element"
FIG. 1 is a schematic cross-sectional view for explaining an example of the organic EL element of the present embodiment. The organic EL element 1 of the present embodiment shown in FIG. 1 has a laminated structure including a light emitting layer 6 formed between an anode 9 (electrode) and a cathode 3 (electrode).
In the laminated structure of the organic EL element 1 of the present embodiment, the hole injection layer 8, the hole transport layer 7, the light emitting layer 6, the electron transport layer 5, and the electron injection layer 4 are formed in this order. Is.

The organic EL element 1 shown in FIG. 1 may be a top emission type that extracts light on the side opposite to the substrate 2, or may be a bottom emission type that extracts light on the substrate 2 side.

(substrate)
Examples of the material of the substrate 2 include a resin material and a glass material. As the material of the substrate 2, only one kind may be used, or two or more kinds may be used in combination.
Examples of the resin material used for the substrate 2 include polyethylene terephthalate, polyethylene naphthalate, polycarbonate, polyarylate and the like. When a resin material is used as the material of the substrate 2, the organic EL element 1 having excellent flexibility can be obtained, which is preferable.
Examples of the glass material used for the substrate 2 include quartz glass, soda glass, Pyrex (registered trademark) and the like.

When the organic EL element 1 is a bottom emission type, a transparent substrate is used as the material of the substrate 2.
When the organic EL element 1 is a top emission type, not only a transparent substrate but also an opaque substrate may be used as the material of the substrate 2. Examples of the opaque substrate include a substrate made of a ceramic material such as alumina, a substrate having an oxide film (insulating film) formed on the surface of a metal plate such as stainless steel, and a substrate made of a resin material.

(anode)
The anode 9 injects holes into the hole injection layer 8 or the hole transport layer 7. Therefore, as the material of the anode 9, various metal materials having a relatively large work function, various alloys, and the like are used. Examples of the material of the anode 9 include gold, copper iodide, tin oxide, aluminum-doped zinc oxide (ZnO: Al), indium tin oxide (ITO), indium zinc oxide (IZO), tin fluorine oxide (FTO), and the like. Can be mentioned. Among these, ITO, IZO, and FTO are preferable as the material of the anode 9 from the viewpoint of transparency and work function.

When the organic EL element 1 is of the bottom emission type, a transparent conductive material is used as the material of the anode 9.
When the organic EL element 1 is of the top emission type, not only a transparent conductive material but also an opaque material may be used or a reflective material may be used as the material of the anode 9.

(Hole injection layer)
The material used for the hole injection layer 8 is selected according to the relationship between the work function of the anode and the ionization potential (IP) of the hole transport layer 7, the charge transport characteristics, and the like. The material of the hole injection layer 8 may be any compound having appropriate IP and charge transport characteristics, and various organic compounds and inorganic compounds can be selected and used regardless of whether they are small molecules or polymers. The material of the hole injection layer 8 may be only one kind or may be used in combination of two or more kinds.

As the inorganic compound used for the hole injection layer 8, for example, molybdenum oxide (MoOx), vanadium oxide _(V 2 _{O 5)} or the like. Since the inorganic compound is more stable than the organic compound, when the inorganic compound is used for the hole injection layer 8, high resistance to oxygen and water can be easily obtained as compared with the case where the organic compound is used.

Examples of the organic compound used in the hole injection layer 8 include compounds represented by the following general formulas (8-1) to (8-19). Among the compounds represented by the general formulas (8-1) to (8-19), the poly (3,4-ethylenedioxythiophene) represented by the general formula (8-11): poly (styrene sulfonate) (PEDOT: PSS), poly (3,4-ethylenedioxythiophene) (PEDOT) represented by the general formula (8-19), and copper phthalocyanine (CuPc) represented by the general formula (8-4) are preferable, and in particular, the general formula. PEDOT represented by (8-19) is preferable.

(Hole transport layer)
Examples of the material used for the hole transport layer 7 include compounds represented by the following general formulas (7-1) to (7-36). Among the compounds represented by the general formulas (7-1) to (7-36), the α-NPD represented by the general formula (7-1) has a large bandgap, and is electrically and thermally stable. It is preferable to use it in combination with a compound represented by the general formula (7-36), which is excellent in the above.
The material of the hole transport layer 7 may be only one type or may be used in combination of two or more types. Further, the hole transport layer 7 may be formed by only one layer, or may be formed by laminating two or more layers. For example, the hole transport layer 7 is represented by a layer composed of a compound represented by the general formula (7-36) arranged on the light emitting layer 6 side and a general formula (7-1) arranged on the hole injection layer 8 side. The layer made of α-NPD can be laminated.

(Light emitting layer)
The light emitting layer 6 included in the organic EL element 1 of the present embodiment includes a host material that performs charge transport and charge recombination, and a guest material that is a light emitting material.
"Host material"
In this embodiment, a compound represented by the general formula (1) is used as the host material.

（一般式（１）中のＲは、それぞれ独立に水素、又は炭素数１〜２０のアルキル基、炭素数１〜２０のアルコキシ基、炭素数１〜２０のアルキルアミノ基からなる群から選択される置換基である。）

(R in the general formula (1) is independently selected from the group consisting of hydrogen, an alkyl group having 1 to 20 carbon atoms, an alkoxy group having 1 to 20 carbon atoms, and an alkylamino group having 1 to 20 carbon atoms. Substituent.)

R in the general formula (1) is hydrogen or a substituent selected from the group consisting of an alkyl group having 1 to 20 carbon atoms, an alkoxy group having 1 to 20 carbon atoms, and an alkylamino group having 1 to 20 carbon atoms. is there. The Rs in the general formula (1) may be all different, or some or all may be the same.
R in the general formula (1) is particularly selected from the group consisting of hydrogen, an alkyl group having 1 to 5 carbon atoms, an alkoxy group having 1 to 5 carbon atoms, and an alkylamino group having 1 to 5 carbon atoms. It is preferably a substituent, and most preferably hydrogen.

(Identification of compound)
The compound represented by the general formula (1) can be identified by using the result of nuclear magnetic resonance (NMR) measurement. When NMR measurement is performed, a spectrum peculiar to the compound can be obtained. From this spectrum, the compound can be identified by knowing the bonding state, chemical shift, and coupling information of the atoms in the compound. NMR measurement is performed by dissolving a small amount of sample in various deuterated solvents.

(Emission spectrum of compound)
By measuring the fluorescence spectrum of the compound, the S ₁ (singlet excited state) energy of the compound can be obtained. Further, the T ₁ (triplet excited state) energy of the compound can be obtained by measuring the phosphorescence spectrum of the compound.

"Guest material"
As the guest material, a fluorescent material and / or a phosphorescent material is used. The guest material preferably has an absorption wavelength that overlaps the emission wavelength of the host material in order to effectively transfer energy from the host material.

(Phosphorescent material)
If the guest material is a phosphorescent material, the T ₁ energy of the guest material is preferably smaller than the T ₁ energy of the host material.
Examples of the phosphorescent material used as the guest material include compounds represented by the following general formulas (6-1) to (6-29). In this embodiment, since the compound represented by the general formula (1) is used as the host material, among the phosphorescent materials represented by the general formulas (6-1) to (6-29), the general formula (6--) is particularly used. Green light emission such as Ir (mppy) ₃ represented by 2) is preferable.

Ir (mppy) ₃ has a smaller T ₁ energy than the compound represented by the general formula (1). Therefore, when a compound represented by the general formula (1) is used as the host material and Ir (mppy) ₃ is used as the guest material, efficient energy transfer from the host material to the guest material occurs. As a result, the organic EL element 1 having a low drive voltage is obtained. Further, since Ir (mppy) ₃ has an absorption wavelength that overlaps with the emission wavelength of the compound represented by the general formula (1), the organic EL element 1 has high luminous efficiency.

When a compound represented by the general formula (1) is used as the host material and Ir (mppy) ₃ is used as the guest material, the content of the guest material in the host material is preferably 1 to 6% by weight. When the content of the guest material is within the above range, energy transfer from the host material to the guest material occurs efficiently, and the efficiency decrease due to triplet-triplet annihilation (TTA) due to the increase in guest concentration can be prevented. Therefore, the luminous efficiency of the organic EL element 1 becomes good.

(Fluorescent material)
Examples of the fluorescent material used as the guest material include compounds represented by the following general formulas (6-30) to (6-51).

(Electronic transport layer)
When an electron transport layer 5 having an appropriate minimum unoccupied molecular orbital (LUMO) level is provided between the cathode 3 or the electron injection layer 4 and the light emitting layer 6, the cathode 3 or the electron injection layer 4 is transferred to the electron transport layer 5. The electron injection barrier is relaxed, and the electron injection barrier from the electron transport layer 5 to the light emitting layer 6 is relaxed. Also, if the material used for the electron transport layer 5 has an appropriate highest occupied molecular orbital (HOMO) level, holes that flow out to the counter electrode without recombination in the light emitting layer 6 are blocked. As a result, holes are confined in the light emitting layer 6, and the recombination efficiency in the light emitting layer 6 is enhanced.
The electron transport layer 5 may be omitted if the electron injection barrier does not matter and the electron transport capacity of the light emitting layer 6 is sufficiently high.

Examples of the material used for the electron transport layer 5 include compounds represented by the following general formulas (5-1) to (5-28). Among the compounds represented by the general formulas (5-1) to (5-28), TPBi represented by the general formula (5-4) is particularly preferable.

(Electron injection layer)
The material used for the electron injection layer 4 is selected from the viewpoint of the work function of the cathode 3 and the LUMO level of the electron transport layer 5. The material used for the electron injection layer 4 is selected in consideration of the LUMO level of the guest material and the host material of the light emitting layer 6 when the electron transport layer 5 is not provided.
The material used for the electron injection layer 4 may be an organic compound or an inorganic compound. When the electron injection layer 4 is made of an inorganic compound, for example, in addition to alkali metal and alkaline earth metal, lithium fluoride, sodium fluoride, potassium fluoride, cesium fluoride, cesium carbonate and the like can be used. It can be used, and it is preferable to use lithium fluoride.

(cathode)
The cathode 3 injects electrons into the electron injection layer 4 or the electron transport layer 5. Therefore, as the material of the cathode 3, various metal materials having a relatively small work function, various alloys, and the like are used. Examples of the material of the cathode 3 include aluminum, silver, magnesium, calcium, gold, indium tin oxide (ITO), indium zinc oxide (IZO), magnesium indium alloy (MgIn), and silver alloy.

When the organic EL element 1 is of the bottom emission type, an opaque electrode made of metal can be used as the material of the cathode 3, and a reflective material may be used.
When the organic EL element 1 is of the top emission type, a transparent conductive material is used as the material of the cathode 3. When ITO is used as the material of the cathode 3, electron injection becomes difficult because the work function of ITO is large. Further, since the ITO film is formed by using a sputtering method or an ion beam vapor deposition method, the electron injection layer 4 and the like may be damaged during the film formation. Therefore, when ITO is used as the material of the cathode 3, it is preferable to provide a magnesium layer or a copper phthalocyanine layer between the electron injection layer 4 and the ITO.

The organic EL element 1 shown in FIG. 1 has an anode 9, a hole injection layer 8, a hole transport layer 7, a light emitting layer 6, an electron transport layer 5, an electron injection layer 4, and an electron injection layer 4 on a substrate 2. It can be manufactured by forming the cathode 3 in this order. The method for forming each of the anode 9, the hole injection layer 8, the hole transport layer 7, the light emitting layer 6, the electron transport layer 5, the electron injection layer 4, and the cathode 3 is not particularly limited, and the characteristics of the material used for each layer are not particularly limited. It can be formed by appropriately using various conventionally known forming methods.

Specifically, for example, as a method for forming the cathode 3 and the anode 9, a sputtering method, a vacuum vapor deposition method, a sol-gel method, a spray pyrolysis (SPD) method, an atomic layer deposition (ALD) method, a vapor phase deposition method, etc. Examples thereof include a liquid phase deposition method.
Further, as a method of forming each layer of the electron injection layer 4, the electron transport layer 5, the light emitting layer 6, the hole transport layer 7, and the hole injection layer 8, a coating method of applying an organic compound solution containing an organic compound to be each layer is applied. , Vacuum vapor deposition method, ESDUS (Evaportive Spray Depositionation from Ultra-dilution Solution) method and the like. Among these forming methods, it is particularly preferable to use the coating method.

When any one of the electron injection layer 4, the electron transport layer 5, the hole transport layer 7, and the hole injection layer 8 is made of an inorganic material, the layer made of the inorganic material is, for example, a sputtering method. , Can be formed by using a method such as vacuum deposition.

The organic EL element 1 shown in FIG. 1 has a laminated structure including a light emitting layer 6 formed between an anode 9 and a cathode 3, and the light emitting layer 6 contains a compound represented by the general formula (1). Therefore, the organic EL element 1 of the present embodiment has high luminous efficiency. Further, the organic EL element 1 of the present embodiment has a low drive voltage and low power consumption.

"Other examples"
The organic EL device of the present invention is not limited to the organic EL device described in the above-described embodiment.
Specifically, in the above-described embodiment, the case where the layer containing the compound represented by the general formula (1) is the light emitting layer 6 has been described as an example, but the compound represented by the general formula (1) is used. The containing layer may be included in the laminated structure formed between the two electrodes, and is not limited to the light emitting layer 6.
For example, the layer containing the compound represented by the general formula (1) may be a hole transport layer 7 or an electron transport layer 5.

Further, in the above-described embodiment, the case where the compound represented by the general formula (1) is used as the host material of the light emitting layer 6 has been described as an example, but the compound represented by the general formula (1) emits light. It can be used as a guest material for the layer, or the light emitting layer can be formed only with the compound represented by the general formula (1).

Further, in the above-described embodiment, the organic EL element 1 having a forward structure in which the anode 9 is arranged between the substrate 2 and the light emitting layer 6 has been described as an example, but the organic EL element of the present invention is the substrate. It may have an inverted structure in which a cathode is arranged between the light emitting layer and the light emitting layer.

Further, in the organic EL element 1 shown in FIG. 1, the electron injection layer 4, the electron transport layer 5, the hole transport layer 7, and the hole injection layer 8 may be formed as needed and are not provided. May be good.
Further, each of the cathode 3, the electron injection layer 4, the electron transport layer 5, the light emitting layer 6, the hole transport layer 7, the hole injection layer 8, and the anode 9 may be formed of one layer. However, it may be composed of two or more layers.

Further, the organic EL element 1 shown in FIG. 1 includes the anode 9, the hole injection layer 8, the hole transport layer 7, the light emitting layer 6, the electron transport layer 5, the electron injection layer 4, and the cathode 3 shown in FIG. It may have another layer in between. Specifically, an electron blocking layer or the like may be provided, if necessary, for the purpose of further improving the characteristics of the organic EL element.

Hereinafter, examples and comparative examples of the present invention will be described. The present invention is not limited to the examples shown below.
"Experiment 1"
A thin film having a thickness of 50 nm made of the following compounds was formed on a substrate made of a glass material by a vacuum vapor deposition method.
FIG. 2 is a graph showing the results (spectrums) of ¹ H-NMR measurement of the compound used in the thin film formed in Experiment 1. The NMR measurement was carried out by dissolving the compound used in the thin film formed in Experiment 1 in deuterated chloroform (CDCl ₃ ) which is a deuterated solvent. From the results shown in FIG. 2, the compound used for the thin film formed in Experiment 1 was identified. As a result, it was confirmed that the compound represented by the general formula (1) (R in the general formula (1) is hydrogen).

"Experiment 2"
A thin film having a thickness of 50 nm made of CBP represented by the general formula (10) was formed on a substrate made of a glass material by a vacuum vapor deposition method.
"Experiment 3"
A thin film having a thickness of 50 nm made of Ir (mppy) ₃ represented by the general formula (6-2) was formed on a substrate made of a glass material by a vacuum vapor deposition method.

For the thin films formed in Experiments 1 and 2, emission spectra at 300K and 77K were measured using a FluoroMax-4 manufactured by HORIBA and an excitation light source having a wavelength of 300 nm. The results are shown in FIGS. 3 and 4. FIG. 3 is a graph showing the measurement results of the thin film formed in Experiment 1. FIG. 4 is a graph showing the measurement results of the thin film formed in Experiment 2.

The emission spectrum at room temperature (300K) shows fluorescence emission. Therefore, from the emission spectrum at room temperature (300K), knowledge corresponding to S ₁ (singlet excited state) energy can be obtained. As shown in FIGS. 3 and 4, the fluorescence emission of the compound represented by the general formula (1) (Experiment 1) is observed on the longer wavelength side than the fluorescence emission of CBP (Experiment 2). Therefore, _{S 1} energy of the compound represented by the general formula (1) is smaller than CBP.

Further, with respect to the thin films formed in Experiments 1 and 2, phosphorescence emission was observed by measuring the emission spectrum at a low temperature (77K), and the energy of the triplet excited state (T ₁ ) of the thin films was determined. The emission spectrum was measured at a low temperature with a delay of 70 ms after the excitation light irradiation in order to remove the fluorescence emission component and observe the phosphorescence spectrum. Table 1 shows the energies of the triplet excited state (T ₁ ) of the thin films formed in Experiments 1 and 2 thus obtained.

As shown in FIGS. 3 and 4, the S ₁ energy of the compound represented by the general formula (1) is smaller than that of CBP. Further, as shown in Table 1, the T ₁ energy of the compound represented by the general formula (1) (Experiment 1) is almost the same as that of CBP (Experiment 2). Therefore, the compound represented by the general formula (1) has a smaller energy gap than CBP. Therefore, the compound represented by the general formula (1) is preferable to CBP as a host material for the light emitting layer.

Further, for the thin film formed in Experiment 3, the energy of the triplet excited state (T ₁ ) of the thin film was obtained in the same manner as in Experiment 1 and Experiment 2.
Table 1 shows the energy of the triplet excited state (T ₁ ) of the thin film (Ir (mppy) ₃ ) formed in Experiment 3 obtained in this way.

As shown in Table 1, the T ₁ energy of the compound represented by the general formula (1) (Experiment 1) was larger than the T ₁ energy of Ir (mppy) ₃ , which is a general green phosphorescent material.
Therefore, the compound represented by the general formula (1) is suitable as a host material for the light emitting layer.

"Example"
Using the materials shown below, the organic EL device of the example was produced by the method shown below.
On the anode of the substrate, a hole injection layer, a second hole transport layer, a first hole transport layer, a light emitting layer, an electron transport layer, an electron injection layer, and a cathode are formed by a vacuum deposition method. , In this order, the organic EL element of the example was produced.

(Substrate, Anode) A commercially available transparent glass substrate having an electrode made of ITO (indium tin oxide) and having a width of 3 mm and an average thickness of 0.7 mm.
(Hole injection layer) PEDOT (Clevios HIL1.5) (thickness 30 nm)
(Hole transport layer) “First hole transport layer” Compound represented by the general formula (7-36) (thickness 10 nm) “Second hole transport layer” α-NPD (thickness 20 nm)

(Light emitting layer) 1% by weight of Ir (mppy) ₃ which is a guest material is contained in the host material which is a compound represented by the general formula (1) (R in the general formula (1) is hydrogen) ( Thickness 25 nm)
(Electron transport layer) TPBi (thickness 35 nm)
(Electron injection layer) LiF film (thickness 0.8 nm)
(Cathode) Al film (thickness 100 nm)

"Comparison example"
An organic EL device of Comparative Example was produced in the same manner as in Examples except that the compound used as the host material of the light emitting layer was CBP represented by the general formula (10).

The external quantum efficiencies of the organic EL devices of Examples and Comparative Examples thus obtained were measured at a current density of 1 mA / cm ² , respectively. The results are shown in Table 2.
As shown in Table 2, the external quantum efficiency of the organic EL device of the example was about the same as the external quantum efficiency of the organic EL device of the comparative example.

Further, a voltage was applied to the organic EL elements of Examples and Comparative Examples using a "2400 type source meter" manufactured by Keithley, and the brightness was measured using "LS-100" manufactured by Konica Minolta. , The relationship between the applied voltage and the brightness was investigated. The result is shown in FIG.

As shown in FIG. 5, in the organic EL element of the example, higher brightness was obtained when the applied voltage was the same, and the drive voltage was lower than that of the organic EL element of the comparative example. It is presumed that this is because the compound represented by the general formula (1) used as the host material of the light emitting layer in the examples has a small energy gap as compared with the CBP used in the comparative examples.

In addition, the current density and power efficiency of the organic EL elements of Examples and Comparative Examples were examined using a "2400 type source meter" manufactured by Keithley. The result is shown in FIG.

As shown in FIG. 6, in the organic EL element of the example, higher power efficiency is obtained when the current density is the same as compared with the organic EL element of the comparative example. It is presumed that this is because the compound represented by the general formula (1) used as the host material of the light emitting layer in the examples has a small energy gap as compared with the CBP used in the comparative examples.

1 ... organic EL element, 2 ... substrate, 3 ... cathode, 4 ... electron injection layer, 5 ... electron transport layer, 6 ... light emitting layer, 7 ... hole transport layer, 8 ... hole injection layer, 9 ... anode.

Claims (2)

Between the two electrodes, it is formed stacked structure including a light emitting layer comprises the light emitting layer comprises a host material composed of a compound represented by the following general formula (1), a guest material is a phosphorescent material, the guest The material is an organic electroluminescence element characterized in that the energy of the triplet excited state is smaller than that of the host material .

(R in the general formula (1) is independently selected from the group consisting of hydrogen, an alkyl group having 1 to 20 carbon atoms, an alkoxy group having 1 to 20 carbon atoms, and an alkylamino group having 1 to 20 carbon atoms. Substituent.) The organic electrotechnology according to claim 1, wherein the light emitting layer contains a host material composed of a compound represented by the general formula (1) and a guest material represented by the following general formula (6-2). Luminescence element.

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