JP2014187130A - Organic electroluminescent element, display device and illuminating device, evaluation method of hole transport material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an organic EL element including a luminous layer using a both charge-transport or electron transport host material, and a hole transport layer, and having a long life and high luminous efficiency, and to provide an evaluation method of a hole transport material for use in an organic EL element.SOLUTION: In an organic electroluminescent element 1 having a luminous layer 6 and a hole transport layer 7 arranged in contact therewith, between a pair of electrodes, the luminous layer 6 is composed of a host material and a gest material of luminescent material. The host material is an electron transport material or a both charge-transport material of hole and electron, and the hole transport layer 7 is the organic electroluminescent element 1 containing a hole transport material composed of a dibenzothiophene derivative.

Description

  The present invention relates to an organic electroluminescence (hereinafter, electroluminescence (electroluminescence) may be referred to as “EL”) element that is suitably used for a display device or a lighting device, a display device and a lighting device including the same, an organic The present invention relates to a method for evaluating a hole transport material used for an EL element.

The organic EL element is thin, flexible and flexible. In addition, a display device using an organic EL element can display with high brightness and high definition as compared with a liquid crystal display device and a plasma display device which are currently mainstream. Furthermore, a display device using an organic EL element has a wider viewing angle than a liquid crystal display device. For this reason, the use of organic EL elements is expected in the future as displays for televisions and mobile phones.
The organic EL element is also expected to be used as a lighting device.

  The organic EL element has a structure in which a plurality of layers such as an electron transport layer, a light emitting layer, and a hole transport layer are laminated between a cathode and an anode. Currently, research and development have been conducted on materials suitable for constituting each layer of the organic EL element.

As a light emitting layer, there is one composed of a host material that plays a role of charge transport and recombination and a light emitting material (guest material) that emits light. In such a light emitting layer, holes supplied from the anode and electrons supplied from the cathode are recombined mainly by the host material to generate an excited state. At this time, a singlet excited state (S ₁ ) and a triplet excited state (T ₁ ) are generated at a ratio of 25:75. Light emission from the singlet excited state (S ₁ ) is called fluorescence emission. In addition, light emission from the triplet excited state (T ₁ ) is called phosphorescence emission.

In general, in an organic EL element utilizing fluorescence, only energy in a singlet excited state (S ₁ ) can be extracted as light emission. On the other hand, in an organic EL element using phosphorescence emission, inter-system crossing (ISC) occurs from S ₁ of the guest material to T ₁ of the guest material, thereby extracting all excited state energy as light emission. be able to. Therefore, the organic EL element using phosphorescence emission can be expected to improve the light emission efficiency as compared with the organic EL element using fluorescence emission. For this reason, in recent years, research on organic EL elements using phosphorescence has been actively conducted (for example, see Non-Patent Document 1 and Non-Patent Document 2).

As a host material used for a light emitting layer that emits phosphorescence, for example, many materials having carbazole as a substituent have been reported (for example, see Non-Patent Document 3). Since carbazole has a large triplet excited state (T ₁ ) energy, it is preferable as a material for obtaining an organic EL element having high emission efficiency utilizing phosphorescence emission.

In the organic EL element using phosphorescence emission, when a material having carbazole as a substituent is used as the host material, the energy of the triplet excited state (T ₁ ) of the host material is changed from the viewpoint of energy confinement to the guest material. It is necessary to make it larger than the energy of the triplet excited state (T ₁ ) (see, for example, Non-Patent Document 4).

  Carbazole has a hole transporting property. In a light emitting layer using a material having carbazole as a substituent as a host material, holes are excessive with respect to electrons. Excess holes supplied to the light emitting layer escape from the light emitting layer without recombining with electrons. For this reason, as a material for the electron transport layer disposed in contact with the cathode side of the light emitting layer, a material that has a high hole blocking property at the cathode side interface of the light emitting layer is used to prevent holes that escape from the light emitting layer. ing.

Furthermore, in the light emitting layer using a material having carbazole as a substituent as a host material, the light emitting region extends to the cathode side interface of the light emitting layer, and therefore the electron transport layer material disposed in contact with the cathode side of the light emitting layer In addition to high electron transport properties, large triplet excited state (T ₁ ) energy is required. In recent years, a large number of materials have been reported as materials for the electron transport layer satisfying such conditions (see, for example, Non-Patent Document 5 and Non-Patent Document 6).

  On the other hand, in recent years, a host material having both charge transporting properties capable of transporting both holes and electrons and a host material having electron transporting properties have been reported (for example, see Non-Patent Documents 7 to 10). .

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  In a light-emitting layer using a both-charge-transporting host material that can transport both holes and electrons and an electron-transporting host material, electrons supplied to the light-emitting layer are excessive. Excess electrons supplied to the light emitting layer escape from the light emitting layer without recombining with holes. In order to prevent electrons that escape from the light emitting layer and improve the light emission efficiency, as a material of the hole transport layer disposed in contact with the anode side of the light emitting layer, it has high hole transportability, It is necessary to use a material capable of obtaining a high electron blocking property at the anode side interface of the light emitting layer.

In the light emitting layer using such a host material, the light emitting region extends to the anode side (hole transport layer side) interface of the light emitting layer. For this reason, when the light emitting material is a phosphorescent material, as a material of the hole transport layer disposed in contact with the anode side of the light emitting layer, in addition to high hole transportability, a large triplet excited state (T ₁ ) It is preferable to use one having energy. When the hole transport layer has a large triplet excited state (T ₁ ) energy, the triplet excited state (T ₁ ) energy transfer from the light emitting layer to the hole transport layer can be prevented. Since the effect of confining the energy of the triplet excited state (T ₁ ) inside is obtained, luminous efficiency is improved.

  Further, when the light emitting region extends to the anode side interface of the light emitting layer, the stability of the material used for the hole transport layer disposed in contact with the anode side of the light emitting layer affects the life of the organic EL element. For this reason, it is required to use a material having excellent stability as the material of the hole transport layer disposed in contact with the anode side of the light emitting layer.

  However, there has been no report on a material for a hole transport layer that can realize an organic EL device having a long lifetime and high light emission efficiency in combination with a light emitting layer using a host material having both charge transport properties or electron transport properties. .

The present invention solves the above-described problems, and has a long-life organic EL element and an organic EL element having a light-emitting layer using a host material having both charge transporting properties or electron transporting properties and a hole transporting layer. It is an object of the present invention to provide a method for evaluating a hole transporting material used in the present invention.
It is another object of the present invention to provide a display device and a lighting device including an organic EL element having a long lifetime and high luminous efficiency.

In view of the above problems, the present inventor has repeatedly studied as described below.
When a plurality of organic EL devices including a light-emitting layer containing an electron-transporting host material and a hole transport layer are manufactured by changing only the material of the hole transport layer, the external quantum efficiency of each organic EL device is positive. It differs depending on the material of the hole transport layer.
However, the difference in the external quantum efficiency of the organic EL element in this case is due to the fact that the drive voltage of the organic EL element changes according to the material of the hole transport layer, and the charge balance in the organic EL element changes. The impact is included.

It is difficult to evaluate the influence caused by the change in the charge balance in the organic EL element separately from the influence of the characteristics of the material of the hole transport layer itself included in the difference in the external quantum efficiency of the organic EL element. In addition, the influence caused by the change in the charge balance in the organic EL element is that the composition of the materials used for the hole transport layer and the light emitting layer, the band gap energy, and the triplet excited state (T ₁ ) energy magnitude. It could not be easily evaluated based on the above.
Therefore, based on the difference in external quantum efficiency among the plurality of organic EL elements, the characteristics of the material of the hole transport layer disposed in contact with the light emitting layer using the electron transporting host material are strictly evaluated. I couldn't.

  Therefore, the present inventor has repeatedly studied focusing on the relationship between the charge balance in the organic EL element and the external quantum efficiency. An organic EL device including a light emitting layer using a material that can obtain high light emission efficiency without depending on the charge balance in the organic EL device as an electron transporting host material, and a hole transport layer is used as a hole transport. It has been found that it is only necessary to manufacture a plurality of layers with different layer materials and measure the external quantum efficiency. In this case, the influence due to the charge balance in the organic EL element included in the difference in external quantum efficiency of each organic EL element is small. Therefore, the characteristics of the material of the hole transport layer itself can be evaluated based on the difference in external quantum efficiency of each organic EL element.

  Therefore, the present inventor has repeatedly studied an electron transporting host material capable of obtaining high luminous efficiency without depending on the charge balance in the organic EL element. As a result, a plurality of organic EL elements each including a light-emitting layer using Bepp2 as a host material and a hole transport layer disposed in contact with the light-emitting layer are manufactured by changing only the material of the hole transport layer. When measuring efficiency, it discovered that the characteristic of the material itself of a positive hole transport layer itself could be strictly evaluated based on the difference of the external quantum efficiency of each organic EL element.

  Then, the present inventor uses a hole transport material used for a high external quantum efficiency among the plurality of organic EL elements manufactured by changing only the material of the hole transport layer, and the host material has an electron transport property. It was evaluated that the material or the hole and electron charge transporting material had good characteristics. Based on the evaluation results, the inventor of the present invention has arranged an organic EL in which a hole transport layer including a hole transport material composed of a dibenzothiophene derivative is disposed in contact with a light emitting layer using a host material having both charge transport properties or electron transport properties. It has been found that an organic EL device having high luminous efficiency can be obtained by using the device. Furthermore, since the dibenzothiophene derivative has excellent stability, the use of the dibenzothiophene derivative as the hole transport material can cause a long time when the light emitting region extends to the hole transport layer side interface of the light emitting layer. It was confirmed that a long-life organic EL element was obtained, and the present invention was completed.

That is, the present invention relates to the following inventions.
(1) An organic electroluminescence device having a light emitting layer between a pair of electrodes and a hole transport layer disposed in contact with the light emitting layer, wherein the light emitting layer is a guest material made of a host material and a light emitting material. The host material is an electron transport material or a charge transport material of both holes and electrons, and the hole transport layer contains a hole transport material made of a dibenzothiophene derivative. Organic electroluminescence device.

(2) The organic electroluminescence device according to (1), wherein the dibenzothiophene derivative is a compound represented by the following general formula (1).

(3) The organic electroluminescence device according to (1) or (2), wherein the band gap energy of the hole transport material is not less than the band gap energy of the host material.
(4) The organic electroluminescent element according to any one of (1) to (3), wherein the guest material is a phosphorescent material or a material that generates thermally activated delayed fluorescence.
(5) The organic electroluminescence element according to any one of (1) to (4), wherein the host material is a metal complex.
(6) The organic electroluminescence device according to any one of (1) to (5), wherein the content of the guest material in the light emitting layer is 1 wt% or more and less than 15 wt%. .

(7) A display device comprising: a display panel using the organic electroluminescence element according to any one of (1) to (6); and a drive circuit for driving the display panel.
(8) An illuminating device formed using the organic electroluminescence element according to any one of (1) to (6).

(9) a host material comprising a light-emitting layer between a pair of electrodes and a hole transport layer disposed in contact with the light-emitting layer, wherein the light-emitting layer is made of Bepp2 represented by the following general formula (2): A step of producing a plurality of organic electroluminescent devices for evaluation comprising a guest material made of a light emitting material, wherein only the hole transport material of the hole transport layer is different, and measuring the external quantum efficiency of each organic electroluminescent device for evaluation; When the host material is an electron transporting material or a hole and electron charge transporting material, the hole transporting material used for the high external quantum efficiency among the plurality of organic electroluminescent elements for evaluation The method for evaluating a hole transport material of an organic electroluminescence device, comprising: a step of evaluating that the material has good characteristics.

(10) Each of the organic electroluminescent elements for evaluation has an electron blocking layer between the hole transport layer and the light emitting layer. The positive electrode of the organic electroluminescent element according to (9) Evaluation method of pore transport material.

  The organic EL device of the present invention has a light emitting layer between a pair of electrodes and a hole transport layer disposed in contact with the light emitting layer, and the light emitting layer is composed of a host material and a guest material made of the light emitting material. The host material is an electron transporting material or a hole and electron charge transporting material, and the hole transporting layer includes a hole transporting material composed of a dibenzothiophene derivative, so that the light emitting region is the positive electrode of the light emitting layer. It spreads to the pore transport layer side interface. In the organic EL device of the present invention, when the light emitting region extends to the hole transport layer side interface of the light emitting layer, the hole transport layer that affects the lifetime is a hole transport composed of a dibenzothiophene derivative having excellent stability. Since it contains materials, it has a long life.

In the organic EL device of the present invention, the host material is an electron transporting material or a charge transporting material for both holes and electrons, and the hole transporting layer includes a hole transporting material composed of a dibenzothiophene derivative. Therefore, high luminous efficiency can be obtained.
Thus, since the organic EL element of the present invention has a long lifetime and high luminous efficiency, it can be suitably used for a display device or a lighting device.

The method for evaluating a hole transport material of the present invention includes a light emitting layer between a pair of electrodes and a hole transport layer disposed in contact with the light emitting layer, and the light emitting layer is a host made of Bepp2. A step of producing a plurality of organic EL elements for evaluation comprising a material and a guest material comprising a light emitting material, wherein only the hole transport material of the hole transport layer is different, and measuring the external quantum efficiency of each organic EL element for evaluation; The hole transport material used for the high external quantum efficiency among the plurality of organic EL elements for evaluation when the host material is an electron transport material or a hole and electron charge transport material. And a step of evaluating that it has good characteristics, and since the influence due to the charge balance included in the difference in external quantum efficiency of each organic EL element for evaluation is very small, the material of the hole transport layer Strict evaluation of its own characteristics Kill.
Therefore, by using the method for evaluating a hole transporting material of the present invention and combining with a light emitting layer using a host material having both charge transporting properties or electron transporting properties, an organic EL device having a long lifetime and high luminous efficiency is realized. The material of the hole transport layer that can be searched can be searched.

FIG. 1 is a schematic cross-sectional view for explaining an example of the organic EL element of the present invention. FIG. 2 is a schematic cross-sectional view for explaining another example of the organic EL element of the present invention. 3A is a graph showing the relationship between luminance and applied power, FIG. 3B is a graph showing the relationship between current density and applied power, and FIG. 3C is a graph showing luminance and applied power. It is the graph which showed the relationship with electric power.

Hereinafter, the present invention will be described in detail with reference to examples.
(First embodiment)
FIG. 1 is a schematic cross-sectional view for explaining an example of the organic EL element of the present invention. The organic EL element 1 of this embodiment shown in FIG. 1 includes a first electrode 9, a hole injection layer 8, a hole transport layer 7, a light emitting layer 6, an electron transport layer 5, and an electron on a substrate 2. The injection layer 4 and the second electrode 3 have a laminated structure formed in this order, and are in contact with the light emitting layer 6 and the light emitting layer 6 between a pair of electrodes (first electrode 9 and second electrode 3). And a hole transport layer 7 disposed in a row.

The organic EL element 1 shown in FIG. 1 is one in which the entire laminated structure constituting the organic EL element formed on the substrate 2 is made of an organic compound.
Note that the organic EL element 1 shown in FIG. 1 may be a HOILED element in which a layered structure of the organic EL element 1 formed on the substrate 2 includes a layer made of an inorganic compound. In this case, for example, in the organic EL element 1 shown in FIG. 1, an electron injection layer 4 made of an inorganic oxide and a hole injection layer 8 made of an inorganic oxide are provided as layers made of an inorganic compound. Can be. Since inorganic compounds are more stable than organic compounds, HOILED elements are preferable because they have higher resistance to oxygen and water than organic EL elements that do not include layers made of inorganic compounds.

  In the organic EL element 1 shown in FIG. 1, the case where the electron injection layer 4 and the hole injection layer 8 are provided will be described as an example. The injection layer 8 may be omitted. Further, in the organic EL element 1 shown in FIG. 1, an electron injection layer made of an inorganic compound may be provided instead of the electron injection layer 4 made of an organic compound, or instead of the hole injection layer 8 made of an organic compound. A hole injection layer made of an inorganic compound may be provided.

The organic EL element 1 shown in FIG. 1 may be a top emission type that extracts light to the side opposite to the substrate 2 side, or may be a bottom emission type that extracts light to the substrate 2 side.
The organic EL element 1 shown in FIG. 1 has a forward structure in which a first electrode 9 that functions as an anode is disposed between a substrate 2 and a light emitting layer 6.

"substrate"
As the material of the substrate 2, resin materials such as polyethylene terephthalate, polyethylene naphthalate, polypropylene, cycloolefin polymer, polyamide, polyethersulfone, polymethyl methacrylate, polycarbonate, polyarylate, quartz glass, soda glass, etc. A glass material etc. are mentioned, These 1 type (s) or 2 or more types can be used.

When the organic EL element 1 is a bottom emission type, a transparent material is used as the material of the substrate 2.
When the organic EL element 1 is a top emission type, not only a transparent material but also an opaque material can 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, an oxide film (insulating film) formed on the surface of a metal substrate such as stainless steel, and a substrate made of a resin material. It is done.

  The average thickness of the substrate 2 is preferably 0.1 to 30 mm, and more preferably 0.1 to 10 mm. The average thickness of the substrate 2 can be measured with a digital multimeter or a caliper.

"First electrode"
The first electrode 9 in the organic EL element 1 shown in FIG. 1 functions as an anode. Examples of the material of the first electrode 9 include oxides such as ITO (indium tin oxide), IZO (indium zinc oxide), FTO (fluorine tin oxide), In ₃ O ₃ , SnO ₂ , Sb-containing SnO ₂ , and Al-containing ZnO. Etc. Among these, it is preferable to use ITO, IZO, or FTO as the material of the first electrode 9.
The average thickness of the first electrode 9 is not particularly limited, but is preferably 10 to 500 nm, and more preferably 100 to 200 nm.
The average thickness of the first electrode 9 can be measured by a stylus profilometer or spectroscopic ellipsometry.

"Hole injection layer"
For the hole injection layer 8 in the present embodiment, any compound that can be normally used as the material of the hole injection layer 8 may be used, or a mixture thereof may be used. Specifically, examples of the material for the hole injection layer 8 include metal or metal-free phthalocyanine compounds such as PEDOT: PSS, phthalocyanine (H ₂ Pc), and copper phthalocyanine represented by the following general formula (3). 1 type, or 2 or more types of these can be used.

Further, when the hole injection layer 8 is made of an inorganic compound, the hole injection layer 8 is preferably a layer made of an inorganic oxide such as a metal oxide layer. As the metal oxide layer is not particularly limited, for example, vanadium oxide _{(V 2 O} 5), molybdenum oxide _(MoO 3), can be used alone or in combination, such as ruthenium oxide (RuO _2). Among these, the hole injection layer 8 is preferably composed mainly of vanadium oxide or molybdenum oxide. When the hole injection layer 8 is composed mainly of vanadium oxide and / or molybdenum oxide, positive holes are injected from the first electrode 9 (anode) and transported to the light emitting layer 6 or the hole transport layer 7. The function as the hole injection layer 8 is more excellent. Further, since vanadium oxide and molybdenum oxide itself have high hole transport properties, it is possible to suitably prevent the efficiency of hole injection from the first electrode 9 to the light emitting layer 6 or the hole transport layer 7 from being lowered. .

Although the average thickness of the positive hole injection layer 8 is not specifically limited, It is preferable that it is 1-1000 nm, and it is more preferable that it is 5-50 nm.
The average thickness of the hole injection layer 8 can be measured at the time of film formation with a crystal oscillator thickness meter.

`` Hole transport layer ''
The hole transport layer 7 is disposed in contact with the light emitting layer 6. In the light emitting layer 6 of the organic EL element 1 shown in FIG. 1, the light emitting region extends to the hole transport layer 7 side (anode side) interface of the light emitting layer 6 as described later.
A dibenzothiophene derivative is used as a hole transport material used for the hole transport layer 7. As the dibenzothiophene derivative, DBTPB which is a compound represented by the above general formula (1) is preferably used. Among the dibenzothiophene derivatives, DBTPB has a large band gap energy and a high triplet excited state (T ₁ ) energy, and is preferable as a hole transport material. DBTPB is preferable because it can realize an organic EL element having a long lifetime and high luminous efficiency when Bepp2 represented by the general formula (2) is used as a host material of the light emitting layer 6.

  In the organic EL element 1 shown in FIG. 1, as will be described later, an electron transporting material or a hole and electron charge transporting material is used as a host material of the light emitting layer 6. Therefore, in the light emitting layer 6, the charge recombination region is in contact with the hole transport layer / light emitting layer interface. In the present embodiment, as the hole transport material used for the hole transport layer 7, a dibenzothiophene derivative having a large band gap energy and high electron blocking properties is used. Therefore, in the organic EL element 1 shown in FIG. 1, it is possible to prevent excess electrons supplied to the light emitting layer 6 from escaping from the light emitting layer 6 to the hole transporting layer 7 without recombining with holes. . Therefore, a decrease in light emission efficiency due to the escape of electrons from the light emitting layer 6 to the hole transport layer 7 is suppressed, and high light emission efficiency is obtained.

  The band gap energy of the hole transport layer 7 is preferably not less than the band gap energy of the host material of the light emitting layer 6. When the band gap energy of the hole transport layer 7 is equal to or higher than the band gap energy of the host material of the light emitting layer 6, the electron blocking property of the hole transport layer 7 becomes much higher, and the excess supplied to the light emitting layer 6 is increased. It is possible to more effectively prevent such electrons from escaping from the light emitting layer 6 to the hole transporting layer 7, and higher luminous efficiency can be obtained.

In the organic EL device 1 shown in FIG. 1, the hole transport layer 7 includes a hole transport material made of a benzothiophene derivative that is a material having a high triplet excited state (T ₁ ) energy. It is possible to prevent the triplet excited state (T ₁ ) energy from being transferred from 6 to the hole transport layer 7, and the effect of confining the triplet excited state (T ₁ ) energy in the light emitting layer 6 can be obtained. Therefore, when the light emitting material described later is a phosphorescent material, the energy of the excited state of the host material can be efficiently extracted as phosphorescent light emission.

Although the average thickness of the positive hole transport layer 7 is not specifically limited, It is preferable that it is 10-150 nm, and it is more preferable that it is 20-100 nm.
The average thickness of the hole transport layer 7 can be measured by, for example, a stylus profilometer or spectroscopic ellipsometry.

  In the present embodiment, the hole transport material used for the hole transport layer 7 uses the following evaluation method, and when the host material used for the light emitting layer 6 is a material having both charge transport properties or electron transport properties, This is determined based on a result of searching for a material from which the organic EL element 1 having high luminous efficiency can be obtained.

  In the hole transport material evaluation method of this embodiment, first, a plurality of evaluation organic EL elements are manufactured. The plurality of organic EL elements for evaluation in the present embodiment have a light emitting layer between a pair of electrodes and a hole transport layer disposed in contact with the light emitting layer, and the light emitting layer has an electron transporting property. It consists of a host material composed of Bepp2 (see, for example, Non-patent Document 11 and Non-patent Document 12) shown in the general formula (2) and a guest material composed of a light emitting material, and only the hole transport material of the hole transport layer is different. Is.

In this embodiment, the organic EL element for evaluation has the same structure as the organic EL element 1 shown in FIG. 1, uses Bepp2 as the host material of the light emitting layer, and differs only in the hole transport material of the hole transport layer. A plurality of products are manufactured.
In addition, the structure of the organic EL element for evaluation can be appropriately determined according to the structure of the organic EL element that actually uses the evaluated hole transport material. The structure of the organic EL element for evaluation is preferably a structure similar to the structure of the organic EL element using the evaluated hole transport material in order to improve the reliability of the evaluation result, and the evaluated hole transport material It is more preferable that the structure is the same as that of the organic EL device using the above.

Next, the external quantum efficiency of each organic EL element for evaluation is measured. As a method for measuring the external quantum efficiency, a known method can be used.
Thereafter, the hole transport material used for the organic EL element for evaluation having a high external quantum efficiency is high when the host material is an electron transport material or a hole and electron charge transport material. The organic EL device 1 having luminous efficiency is evaluated as having good characteristics, and is used as a hole transport material for the hole transport layer 7 in the organic EL device 1 of the present embodiment.

  In this embodiment, since the hole transport material used for the hole transport layer 7 is evaluated using the above evaluation method, the influence caused by the charge balance included in the difference in the external quantum efficiency of each organic EL element for evaluation. Therefore, the characteristics of the material of the hole transport layer itself can be strictly evaluated. Therefore, the hole transport material 7 evaluated as having good characteristics in the present embodiment, the hole transport layer 7 is used in combination with a light emitting layer using a host material having both charge transport properties or electron transport properties, The organic EL element having a high luminous efficiency can be realized with high reliability.

Moreover, each organic EL element for evaluation used in the present embodiment may have an electron block layer between the hole transport layer and the light emitting layer. When the hole transport material is evaluated using such an organic EL element for evaluation, the influence on the evaluation due to the escape of electrons from the light emitting layer to the hole transport layer without recombination with holes is suppressed. Therefore, the characteristics of the material of the hole transport layer itself can be evaluated more strictly.
As a material used for an electronic block layer, the material which can be used as an electronic block layer of an organic EL element can be used, for example.

As shown in FIG. 1, the hole transport layer 7 may consist of only one layer, or the hole injection layer of the hole transport layer 7 disposed in contact with the light emitting layer 6 described above. It may have one or more second hole transport layers (not shown) on the 8 side.
As the hole transporting organic material used for the second hole transporting layer, various p-type polymer materials (organic polymers) and various p-type low molecular materials can be used alone or in combination.

  Specifically, as a material of the second hole transport layer, for example, N, N′-di (1-naphthyl) -N, N′-diphenyl-1,1′-biphenyl-4,4′-diamine ( α-NPD), polyarylamine, fluorene-arylamine copolymer, fluorene-bithiophene copolymer, poly (N-vinylcarbazole), polyvinylpyrene, polyvinylanthracene, polythiophene, polyalkylthiophene, polyhexylthiophene, poly ( p-phenylene vinylene), polytinylene vinylene, pyrene formaldehyde resin, ethylcarbazole formaldehyde resin or derivatives thereof. These materials for the second hole transport layer can also be used as a mixture with other compounds. For example, poly (3,4-ethylenedioxythiophene / styrenesulfonic acid) is used as a mixture containing polythiophene. (PEDOT / PSS).

Although the average thickness of a 2nd positive hole transport layer is not specifically limited, It is preferable that it is 10-150 nm, and it is more preferable that it is 20-100 nm.
The average thickness of the second hole transport layer can be measured, for example, by a stylus profilometer or spectroscopic ellipsometry.

"Light emitting layer"
The light emitting layer 6 includes a host material that plays a role of charge transport and recombination, and a guest material made of a light emitting material.
As the host material, an electron transporting material or a hole and electron charge transporting material is used. Specifically, examples of the electron transporting host material include Bepp2 represented by the above general formula (2), compounds represented by the following general formulas (4) to (9), and the like. It is preferable that it is Bepp2 shown by General formula (2).

  By using a metal complex such as Bepp2 as the host material, energy transfer from the host material to the guest material occurs efficiently, and the organic EL element 1 having high luminous efficiency can be obtained.

  Examples of the host material having both charge transporting properties of holes and electrons include compounds represented by the following general formulas (10) to (33).

As the guest material, a phosphorescent material is preferably used.
Examples of the phosphorescent material include Ir (mppy) ₃ which is a green phosphorescent material represented by the following general formula (34). By using a phosphorescent material as a guest material, an organic EL element using phosphorescence emission is obtained, and light emission efficiency can be improved as compared with an organic EL element using a fluorescent material as a guest material.

  The content of the guest material in the light emitting layer 6 is preferably 1% by weight or more and less than 15% by weight. By setting the content of the guest material in the light emitting layer 6 in the above range, a further excellent luminous efficiency can be obtained. When the content of the guest material in the light emitting layer 6 is less than the above range, energy transfer from the host material to the guest material does not occur sufficiently, and the host material itself emits light, which may reduce the light emission efficiency. In addition, when the content of the guest material in the light emitting layer 6 exceeds the above range, the electron transport property of the host material becomes insufficient, and the guest material carries charge, so that the light emitting region is a hole transport layer of the light emitting layer 6. There is a possibility that the luminous efficiency is lowered due to separation from the 7-side interface.

Although the average thickness of the light emitting layer 6 is not specifically limited, It is preferable that it is 10-150 nm, and it is more preferable that it is 20-100 nm.
The average thickness of the light emitting layer 6 may be measured by a stylus type step meter, or may be measured at the time of forming the light emitting layer 6 by a crystal resonator film thickness meter.

"Electron transport layer"
Examples of materials used for the electron transport layer 5 include phosphine oxide derivatives such as phenyl-dipyrenylphosphine oxide (POPy ₂ ), tris-1,3,5- (3 ′-(pyridin-3 ″ -yl) phenyl. ) Pyridine derivatives such as benzene (TmPyPhB), quinoline derivatives such as (2- (3- (9-carbazolyl) phenyl) quinoline (mCQ)), 2-phenyl-4,6-bis (3,5-dipyridyl) Pyrimidine derivatives such as phenyl) pyrimidine (BPyPPM), pyrazine derivatives, phenanthroline derivatives such as bathophenanthroline (BPhen), 2,4-bis (4-biphenyl) -6- (4 '-(2-pyridinyl) -4 -Biphenyl)-[1,3,5] triazine (MPT) and other triazine derivatives, 3-phenyl Triazole derivatives such as 4- (1′-naphthyl) -5-phenyl-1,2,4-triazole (TAZ), oxazole derivatives, 2- (4-biphenylyl) -5- (4-tert-butylphenyl- Oxadiazole derivatives such as 1,3,4-oxadiazole) (PBD), 2,2 ′, 2 ″-(1,3,5-bentriyl) -tris (1-phenyl-1-H— Imidazole derivatives such as benzimidazole (TPBI), aromatic tetracarboxylic anhydrides such as naphthalene and perylene, bis [2- (2-hydroxyphenyl) benzothiazolate] zinc (Zn (BTZ) ₂ ), tris (8- Various metal complexes represented by hydroxyquinolinato) aluminum (Alq3), 2,5-bis (6 ′-(2 ′, 2 ″ -bipyridyl))-1,1- Organic silane derivatives typified by silole derivatives such as methyl-3,4-diphenyl silole (PyPySPyPy) and the like, can be used alone or in combination of two or more thereof. Among these electron transport layer material, phosphine oxide derivatives such as Popy _2, metal complexes such as Alq _3, it is preferable to use pyridine derivatives such as TmPyPhB.

Although the average thickness of the electron carrying layer 5 is not specifically limited, It is preferable that it is 10-150 nm, and it is more preferable that it is 20-100 nm.
The average thickness of the electron transport layer 5 can be measured by a stylus profilometer or spectroscopic ellipsometry.

"Electron injection layer"
In the case of providing an electron injection layer 4 made of an organic compound, any compound that can be normally used as a material for the electron injection layer may be used, or a mixture thereof may be used. Specifically, for example, polyethyleneimine, poly (9,9-bis (6 ″ -trimethylammonium hexyl) fluorene-co-alt-phenylene) with bromide counties (FPQ-Br), poly (9,9-bis) (3′-dimethylamino) propyl) 2,7-fluorene-alt-2,7- (9,9-dioctylfluorene) (PFNR ² ), polyalkylene oxide and the like.

Moreover, when the electron injection layer 4 consists of an inorganic compound, it is preferable that it is a layer consisting of inorganic oxides, such as a metal oxide layer. The metal oxide layer is not particularly limited. For example, titanium oxide (TiO ₂ ), zinc oxide (ZnO), tungsten oxide (WO ₃ ), niobium oxide (Nb ₂ O ₅ ), iron oxide (Fe ₂ O) ₃ ), one or more selected from tin oxide (SnO ₂ ), magnesium oxide (MgO), hafnium oxide (HfO ₂ ), zirconium oxide (ZrO ₂ ), and indium gallium zinc oxide (IGZO) may be used. it can. The electron injection layer 4 made of these metal oxide layers has a function as an electron injection layer and a function as an electrode (cathode).
Further, the material of the electron injection layer 4 is not limited to the above material, and for example, LiF may be used.

Although the average thickness of the electron injection layer 4 is not specifically limited, It is preferable that it is 1-1000 nm, and it is more preferable that it is 2-100 nm.
The average thickness of the electron injection layer 4 can be measured by a stylus profilometer or spectroscopic ellipsometry.

"Second electrode"
The second electrode 3 in the organic EL element 1 shown in FIG. 1 functions as a cathode. Examples of the second electrode 3 include Au, Pt, Ag, Cu, Al, and alloys containing these. Among these, it is preferable to use Au, Ag, or Al as the second electrode 3.
Although the average thickness of the 2nd electrode 3 is not specifically limited, It is preferable that it is 10-1000 nm, and it is more preferable that it is 30-150 nm. Even when an opaque material is used for the second electrode 3, for example, by setting the average thickness to about 10 to 30 nm, it can be used as a top emission type or transparent type anode.
The average thickness of the second electrode 3 can be measured at the time of film formation of the second electrode 3 by using a quartz vibrator thickness meter.

  The organic EL element 1 of the present embodiment shown in FIG. 1 can be manufactured using a known method as appropriate.

  The organic EL element 1 according to the present embodiment includes a light emitting layer 6 and a hole transport layer 7 disposed in contact with the light emitting layer 6 between the first electrode 9 and the second electrode 3. Is composed of a host material and a guest material made of a light emitting material, the host material is an electron transporting material or a charge transporting material for both holes and electrons, and the hole transporting layer 7 is a positive material made of a dibenzothiophene derivative. Since the hole transport material is included, the light emitting region extends to the hole transport layer 7 side interface of the light emitting layer 6. In the present embodiment, when the light emitting region extends to the hole transport layer 7 side interface of the light emitting layer 6, the hole transport layer 7 that affects the lifetime of the organic EL element is a hole having excellent stability. Since it includes a transport material, it has a long life.

  In the organic EL device 1 of the present embodiment, the host material is an electron transporting material or a hole and electron charge transporting material, and the hole transporting layer 7 is a hole transporting material made of a dibenzothiophene derivative. Therefore, high luminous efficiency can be obtained.

(Second Embodiment)
FIG. 2 is a schematic cross-sectional view for explaining another example of the organic EL element of the present invention. An organic EL element 1A of the present embodiment shown in FIG. 2 includes a first electrode 9, an electron injection layer 4, an electron transport layer 5, a light emitting layer 6, a hole transport layer 7, and a hole on a substrate 2. The injection layer 8 and the second electrode 3 have a laminated structure formed in this order.

The first electrode 9 in the organic EL element 1A shown in FIG. 2 functions as a cathode, and the second electrode 3 functions as an anode.
The organic EL element 1 </ b> A shown in FIG. 2 has a reverse structure in which a first electrode 9 that functions as a cathode is formed on a substrate 2, and the first electrode 9 is disposed between the substrate 2 and the light emitting layer 6. .

  The difference between the organic EL element 1A of this embodiment shown in FIG. 2 and the organic EL element 1 of this embodiment shown in FIG. 1 is that an electron injection layer 4, an electron transport layer 5, a light emitting layer 6, and a positive This is only where the stacking order of the hole transport layer 7 and the hole injection layer 8 is reversed. Therefore, in the organic EL element 1A of the present embodiment shown in FIG. 2, the same members as those of the organic EL element 1 shown in FIG.

In addition, the organic EL element of this invention is not limited to the organic EL element shown in FIG.1 and FIG.2. That is, the organic EL element of the present invention only needs to have the light emitting layer 6 and the hole transport layer 7 disposed in contact with the light emitting layer 6 between the first electrode 9 and the second electrode 3. The electron injection layer 4, the electron transport layer 5, the second hole transport layer, and the hole injection layer 8 may be formed as necessary.
Moreover, the organic EL element of this invention may have another layer between each layer shown in FIG.

The organic EL element of the present invention may have, for example, a hole blocking layer, an electronic element layer, and the like as necessary for the purpose of further improving the characteristics of the organic EL element.
As a material for forming these layers, a material usually used for forming these layers can be used. Moreover, as a method for forming these layers, a method usually used for forming these layers can be used.

Further, the organic EL device of the present invention may be one using thermally activated delayed fluorescence (TADF), which has recently been accelerated.
In this case, as the guest material, a material that generates thermally activated delayed fluorescence is used. Examples of the material that generates thermally activated delayed fluorescence include compounds represented by the following general formulas (35) to (38).

The organic EL device of the present invention can change the emission color by appropriately selecting the material of the organic compound layer, and can also obtain a desired emission color by using a color filter or the like in combination. Therefore, it can be suitably used as a light emitting part of a display device or a lighting device.
The display device of the present invention has a display panel using the organic EL element of the present invention having a long lifetime and high light emission efficiency, and a drive circuit for driving the display panel. For this reason, it is preferable as a display device.
Moreover, the illuminating device of this invention is equipped with the organic EL element of this invention which has a long lifetime and high luminous efficiency. For this reason, it becomes a preferable thing as an illuminating device.

The following two organic EL elements for evaluation were manufactured and evaluated.
"Evaluation organic EL element A"
On a substrate, a first electrode (anode) made of ITO (indium tin oxide), a hole injection layer with a thickness of 30 nm made of PEDOT: PSS, and a thickness of 40 nm made of α-NPD represented by the following general formula (39) A hole transport layer, a 10 nm thick electron blocking layer made of Ir (ppz) _{3 represented} by the following general formula (40), Ir (mppy) ₃ as a guest material, and Bepp ₂ as a host material. 35 nm thick light emitting layer with a guest material content of 6% by weight in layer 6, 40 nm thick electron transport layer made of TPBI represented by the following general formula (41), and 1 nm thick electron injection made of LiF film A layer and a second electrode (cathode) made of an Al film were sequentially formed by a known method.

"Evaluation organic EL element B"
Evaluation organic EL element B was formed in the same manner as evaluation organic EL element A, except that the material of the electron transport layer was changed to ETM-143 (trade name: manufactured by Hodogaya Chemical Co., Ltd.).

With respect to the obtained organic EL element for evaluation A and organic EL element for evaluation B, the relationship between luminance and applied power, the relationship between current density and applied power, and the relationship between current density and external quantum efficiency were examined. The results are shown in FIGS. 3 (a) to 3 (c).
3A is a graph showing the relationship between luminance and applied power, FIG. 3B is a graph showing the relationship between current density and applied power, and FIG. 3C is a graph showing luminance and applied power. It is the graph which showed the relationship with electric power.

In addition, Ir (ppz) ₃ used for the material of the electronic block layer of the evaluation organic EL element A and the evaluation organic EL element B is a material having a high electron blocking property. However, an organic EL element having an electron block layer using Ir (ppz) ₃ has a short lifetime. For this reason, the evaluation of the lifetime of the evaluation organic EL element A and the evaluation organic EL element B is not performed.

  As shown in FIG. 1A and FIG. 1B, it can be seen that the evaluation organic EL element B is operated at a lower drive voltage than the evaluation organic EL element A. Moreover, as shown in FIG.1 (c), both the organic EL element for evaluation A and the organic EL element for evaluation B have a high external quantum efficiency of 20% or more.

Usually, in an organic EL device using phosphorescence emission, the external quantum efficiency largely depends on the charge balance of the device (see, for example, Non-Patent Document 13).
However, as shown in FIG. 1C, in the evaluation organic EL element A and the evaluation organic EL element B using Bepp2 as the host material, the charge balance is different by changing the material of the both electron transport layers. In spite of this, high luminous efficiency was obtained regardless of the charge balance. The reason for this is considered to be that energy transfer from the host material to the guest material occurred efficiently by using Bepp2 as the electron transporting host material.

From this, based on the result of manufacturing a plurality of organic EL elements each having a light-emitting layer using Bepp2 as a host material and a hole transport layer, differing only in the material of the hole transport layer, and measuring the external quantum efficiency Thus, it was confirmed that the characteristics of the material of the hole transport layer itself can be evaluated.
In addition, from the results shown in FIG. 1C, Bepp2 is used as the host material, and a material having a high electron blocking property and capable of confining the energy of the triplet excited state (T ₁ ) is used as the material of the hole transport layer. Thus, it can be estimated that an organic EL element with high luminous efficiency can be obtained.

The organic EL elements of Experimental Examples 1 to 8 shown below were manufactured and evaluated.
"Experiment 1"
On the substrate 2, a first electrode 9 (anode) made of ITO (indium tin oxide), a hole injection layer 8 made of PEDOT: PSS with a thickness of 30 nm, and a second hole transport made of α-NPD with a thickness of 10 nm. A layer (not shown), a 10 nm thick hole transport layer 7 made of α-NPD, Ir (mppy) ₃ as a guest material, Bepp ₂ as a host material, and the content of the guest material in the light emitting layer 6 A light emitting layer 6 having a thickness of 6% by weight, an electron transport layer 5 having a thickness of 40 nm made of TPBI, an electron injection layer 4 having a thickness of 1 nm made of a LiF film, and a second electrode 3 (cathode) made of an Al film. Were sequentially formed by a known method to form the organic EL element 1 shown in FIG.

"Experimental example 2"
Organic EL element 1 of Experimental Example 2 was formed in the same manner as Experimental Example 1 except that the material of the hole transport layer 7 was changed to HMTPD represented by the following general formula (42).

"Experiment 3"
The organic EL element 1 of Experimental Example 3 was formed in the same manner as Experimental Example 1 except that the material of the hole transport layer 7 was changed to TPD15 represented by the following general formula (43).

"Experimental example 4"
The material of the hole transport layer 7 was replaced with ST1411: N, N′-Bis- (phenyl) -N, N′-bis- (4-triphenylmethyl-phenyl) -benzidine represented by the following general formula (44). Except for this, the organic EL element 1 of Experimental Example 4 was formed in the same manner as Experimental Example 1.

“Experimental Example 5”
The organic EL element 1 of Experimental Example 5 was formed in the same manner as Experimental Example 1 except that the material of the hole transport layer 7 was replaced with DBTPB.
"Experimental example 6"
The organic EL element 1 of Experimental Example 6 was formed in the same manner as Experimental Example 1, except that the material of the hole transport layer 7 was changed to 3BTPD represented by the following general formula (45).

"Experimental example 7"
The organic EL element 1 of Experimental Example 7 was formed in the same manner as Experimental Example 1, except that the material of the hole transport layer 7 was changed to NPAPF represented by the following general formula (46).

"Experimental example 8"
The organic EL element 1 of Experimental Example 8 was formed in the same manner as Experimental Example 1 except that the material of the hole transport layer 7 was changed to CzTP represented by the following general formula (47).

Table 1 shows the material of the hole transport layer used in Experimental Examples 1 to 8, and the highest occupied orbital (HOMO) and lowest unoccupied orbital (LUMO) energy and band gap energy of the host material Bepp2.
Table 1 shows the material of the hole transport layer used in Experimental Examples 1 to 8 and the energy of the triplet excited state (T ₁ ) of Ir (mppy) ₃ that is the guest material.

The external quantum efficiency (EQE), drive voltage, presence / absence of light emission from the hole transport layer, and device lifetime of the organic EL devices of Experimental Examples 1 to 8 thus obtained were measured. The results are shown in Table 1.
In addition, the element lifetime of an organic EL element is a lifetime (time) until a brightness | luminance is reduced by half when an organic EL element is made to light-emit with the drive voltage shown in Table 1. FIG.
Further, the presence or absence of light emission of the hole transport layer is obtained by observing light emission from the hole transport material on the short wavelength side of the light emission of Ir (mppy) _{3 as} the guest material in the emission spectrum of the organic EL element. “No” was evaluated, and those that were not observed were evaluated as “None”.

  The organic EL elements of Experimental Examples 1 to 8 are provided with a light emitting layer using Bepp2 as a host material and a hole transport layer, and only the material of the hole transport layer is different. For this reason, the hole transport material used for the organic EL elements of Experimental Examples 1 to 8 having a high result of measuring the external quantum efficiency, the host material is an electron transport material or both charge transport of holes and electrons. It can be evaluated that it has good characteristics when it is an adhesive material.

As shown in Table 1, in Experimental Example 1, the energy position of LUMO between Bepp2 as the host material and α-NPD as the material of the hole transport layer is relatively close, and the triplet energy of α-NPD is the guest material. Is smaller than that of Ir (mppy) ₃ . For this reason, in the organic EL element of Experimental Example 1 using α-NPD as the material for the hole transport layer, the external quantum efficiency was low, and a sufficiently high light emission efficiency was not obtained.

  In addition, the organic EL elements of Experimental Example 1, Experimental Example 3, and Experimental Example 7 using α-NPD / TPD15 / NPAPPF having a band gap energy smaller than that of the host material Bepp2 as the material of the hole transport layer are positive. Fluorescence emission was observed in the hole transport layer. This is presumably because excess electrons supplied to the light emitting layer escaped from the light emitting layer to the hole transport layer, and the excited state of the host material was transferred to the material of the hole transport layer. Thus, in the organic EL elements of Experimental Example 1, Experimental Example 3, and Experimental Example 7, it is considered that the emission efficiency of the phosphorescent element is reduced.

  On the other hand, organic materials of Experimental Example 2, Experimental Example 4, and Experimental Example 5 using HMTPD, ST1411, and DBTPB, which are similar in structure to α-NPD and different in the substituent structure of the naphthalene portion as the material of the hole transport layer. In the EL element, the band gap energy of the hole transport layer is equal to or higher than Bepp2, which is a host material. Therefore, in the organic EL elements of Experimental Example 2, Experimental Example 4, and Experimental Example 5, the energy of the excited state of the host material can be extracted as phosphorescence. For this reason, the luminous efficiency higher than Experimental Example 1 is obtained.

In addition, in the organic EL device of Experimental Example 6 using 3BTPD having a triphenylamine skeleton as a material of the hole transport layer and having a three-dimensional molecular structure, the positive band gap energy is large and high electron blocking property is obtained. Since it has a hole transport layer, high luminous efficiency is obtained.
On the other hand, in the organic EL device of Experimental Example 8, although the carbazole skeleton, which is the material of the hole transport layer, has a large triplet energy and a band gap energy, it was not suitable as an electron blocking layer, so high luminous efficiency was obtained. Probably not.

As shown in Table 1, the organic EL elements of Experimental Examples 2 to 6 and 8 had higher luminous efficiency than the organic EL element of Experimental Example 1. Therefore, the hole transport material used in the organic EL elements of Experimental Examples 2 to 6 and 8 has good characteristics when the host material is an electron transport material or a hole and electron charge transport material. It is.
However, as shown in Table 1, with respect to the element lifetime, only Experimental Example 5 out of Experimental Examples 2 to 6 and 8 had better characteristics than Experimental Example 1.
This indicates that when an electron transporting material such as Bepp2 is used as the host material, a material made of a dibenzothiophene derivative such as DBTPB is suitable as the material for the hole transporting layer. This is because the material of the hole transport layer used in the organic EL elements of Experimental Examples 2 to 5 has a similar structure to that of DBTPB except for the terminal, but only Experimental Example 5 has good life characteristics. This is because.

"Experimental example 9"
On the substrate 2, a first electrode 9 (anode) made of ITO (indium tin oxide), a hole injection layer 8 made of PEDOT: PSS with a thickness of 30 nm, and a second hole transport made of α-NPD with a thickness of 10 nm. A light-emitting layer 6 using a layer (not shown), a 10 nm-thick hole transport layer 7 made of DBTPB, Ir (mppy) ₃ as a guest material, and CBP represented by the following general formula (48) as a host material A light emitting layer 6 having a thickness of 35 nm with a guest material content of 6% by weight, an electron transport layer 5 having a thickness of 40 nm made of TPBI, an electron injection layer 4 having a thickness of 1 nm made of LiF film, and a first layer made of an Al film. Two electrodes 3 (cathode) were sequentially formed by a known method to form the organic EL element 1 shown in FIG.

"Experimental example 10"
The organic EL element 1 of Experimental Example 10 was formed in the same manner as Experimental Example 9 except that the hole transport layer 7 was not formed.

  The external quantum efficiency (EQE), drive voltage, and device lifetime of the organic EL devices of Experimental Examples 9 and 10 obtained in this manner were measured in the same manner as Experimental Example 1. The results are shown in Table 2.

As shown in Table 2, the external quantum efficiency of Experimental Example 9 having the hole transport layer is higher than that of Experimental Example 10 having no hole transport layer. This is presumably because, in Experimental Example 9, the effect of confining the energy of the triplet excited state (T ₁ ) by the hole transport layer is obtained.
On the other hand, Experimental Example 10 is longer than Experimental Example 9 in terms of life. This is because the organic EL elements of Experimental Example 9 and Experimental Example 10 use CBP, which is a hole transporting material, as the host material. In this case, the light emitting region extends to the electron transport layer side (cathode side) interface of the light emitting layer. Therefore, it is considered that the influence of the stability of the material used for the hole transport layer on the lifetime of the organic EL element is reduced.

  Further, from the results of Experimental Example 5 and Experimental Example 9, the hole transport layer 7 made of a dibenzothiophene derivative does not have a sufficient lifetime when a hole transporting material is used as the host material, and the host It has been found that an organic EL device having a long lifetime and high luminous efficiency can be obtained when a material of an electron transporting material such as Bepp2 or a charge transporting material of both holes and electrons is combined.

"Experimental example 11"
On the substrate 2, a first electrode 9 (anode) made of ITO (indium tin oxide), a hole injection layer 8 made of PEDOT: PSS with a thickness of 30 nm, and a second hole transport made of α-NPD with a thickness of 20 nm. A layer (not shown), a 10 nm-thick hole transport layer 7 made of HMTPD, 3BTPD, DBTPB, and α-NPD, Ir (mppy) ₃ as a guest material, and the above general formula (33) as a host material The charge transport material BUPH1 (see Non-Patent Document 10), the light-emitting layer 6 has a thickness of 25 nm of the light-emitting layer 6 having a guest material content of 6% by weight, and the electron transport of TPBI having a thickness of 40 nm A layer 5, a 1 nm-thick electron injection layer 4 made of a LiF film, and a second electrode 3 (cathode) made of an Al film are sequentially formed by a known method to form the organic EL element 1 shown in FIG. did.

  The external quantum efficiency (EQE) and device lifetime of the organic EL device of Experimental Example 11 obtained in this manner were measured in the same manner as in Experimental Example 1. The results are shown in Table 3.

As shown in Table 3, the external quantum efficiency of Experimental Example 11 having the hole transport layer made of a material other than α-NPD is higher than that of the element having the hole transport layer made of α-NPD.
Further, the external quantum efficiency of Experimental Example 11 is almost the same as the case of using Bepp2, which is an electron transporting material, as the host material shown in Table 1 (3BTPD>HMTPD>DBTPB> α-NPD). From this, even when BUPH1 is used as the host material, as in the case of using Bepp2, only a material for the hole transport layer is made different to produce a plurality of organic EL elements and the external quantum efficiency is measured. It can be seen that the hole transporting material used for a material with high quantum efficiency can be evaluated as having good characteristics when the host material is an electron transporting material or a dual charge transporting material.

  As shown in Tables 1 and 3, the difference in external quantum efficiency is smaller when BUPH1 is used as the host material than when Bepp2 is used. This is because the emission region is more localized at the hole transport layer side interface of the light emitting layer when Bepp2 is used as the electron transporting material than when using BUPH1 which is both charge transporting materials. It is thought that it is to do.

Further, when attention is paid to the lifetime, in Experimental Example 11 shown in Table 3, as in the case of using Bep2 shown in Table 1, 3BTPD and HMTPD are compared with the device having the hole transport layer made of DBTPB and α-NPD. The lifetime of a device having a hole transport layer made of is shortened.
From this, it was found that when a material having both charge transport properties such as BUPH1 is used as the host material, a material comprising a dibenzothiophene derivative such as DBTPB is suitable as the material of the hole transport layer.

  As shown in Tables 1 and 3, the difference in lifetime is smaller when BUPH1 is used as the host material than when Bepp2 is used. The reason for this is considered that DUPH1 has lower durability of the material itself than Bepp2.

1: Organic EL element (organic electroluminescence element), 2: substrate, 3: second electrode, 4: electron injection layer, 5: electron transport layer, 6: light emitting layer, 7: hole transport layer, 8: hole Injection layer, 9: first electrode.

Claims (10)

An organic electroluminescence device having a light emitting layer between a pair of electrodes and a hole transport layer disposed in contact with the light emitting layer,
The light emitting layer is composed of a host material and a guest material made of a light emitting material,
The host material is an electron transporting material or a hole and electron charge transporting material,
The organic electroluminescent device, wherein the hole transport layer includes a hole transport material made of a dibenzothiophene derivative. The organic electroluminescence device according to claim 1, wherein the dibenzothiophene derivative is a compound represented by the following general formula (1).
  3. The organic electroluminescence device according to claim 1, wherein a band gap energy of the hole transport material is equal to or higher than a band gap energy of the host material.   The organic electroluminescence device according to claim 1, wherein the guest material is a phosphorescent material or a material that generates thermally activated delayed fluorescence.   The organic electroluminescence device according to any one of claims 1 to 4, wherein the host material is a metal complex.   6. The organic electroluminescence element according to claim 1, wherein the content of the guest material in the light emitting layer is 1% by weight or more and less than 15% by weight.   A display device comprising: a display panel using the organic electroluminescence element according to claim 1; and a drive circuit for driving the display panel.   It forms using the organic electroluminescent element as described in any one of Claims 1-6, The illuminating device characterized by the above-mentioned. A light emitting layer between a pair of electrodes and a hole transport layer disposed in contact with the light emitting layer, wherein the light emitting layer is composed of a host material and a light emitting material made of Bepp2 represented by the following general formula (2). A plurality of organic electroluminescent elements for evaluation, which differ only in the hole transport material of the hole transport layer, and measuring the external quantum efficiency of each organic electroluminescent element for evaluation,
When the host material is an electron transporting material or a hole and electron charge transporting material, the hole transporting material used for the high external quantum efficiency among the plurality of organic electroluminescence elements for evaluation A method for evaluating a hole transport material of an organic electroluminescence element, comprising: a step of evaluating that it has good characteristics.
  Each organic electroluminescent element for evaluation has an electron block layer between the said positive hole transport layer and the said light emitting layer, The hole transport material of the organic electroluminescent element of Claim 9 characterized by the above-mentioned. Evaluation method.