JP2012243983A - Organic electroluminescent element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an organic electroluminescent element which can be reliably driven at a low voltage and has high luminous efficiency with little deterioration, and a display device using the same.SOLUTION: The organic electroluminescent element of the present invention including a light-emitting layer between a pair of electrodes is characterized in that: the light-emitting layer consists only of a host material comprising a metal complex, and a guest material comprising a phosphorescent material; and energy of an excited state where the host material emits light is higher than energy of a singlet excited state of the guest material.

Description

  The present invention relates to an organic electroluminescence element, and particularly relates to an organic electroluminescence element having a light emitting layer between a pair of electrodes.

  Conventionally, in an organic light emitting device having a light emitting layer composed of two or more components between opposed electrodes, the main constituent material of the light emitting layer is a metal complex, and the sub constituent material is a phosphorescent light emitting material. A light emitting element is known (see, for example, Patent Document 1).

  In Patent Document 1, an object of the present invention is to obtain a highly efficient light emitting element that can be driven at a low voltage and has little light emission deterioration in a light emitting element using a phosphorescent light emitting material.

WO 2005/112520

  However, in the configuration described in Patent Document 1, the content of the phosphorescent light-emitting material as a sub-constituent material is 6% by weight, and the phosphorescent light-emitting material is included at a general level of mixing ratio.

  In an organic light-emitting element including a phosphorescent light-emitting material, a guest material (sub-component material) traps charges directly in the light-emitting layer and emits light by recombination. Therefore, the guest material hinders the charge transfer in the device, and the drive voltage increases as the guest concentration increases. On the other hand, when the guest concentration is low, efficient energy transfer between the host and the guest does not occur, so that high light emission efficiency cannot be obtained. Therefore, the optimum guest concentration of a normal phosphorescent light-emitting material is about 6 to 10% by weight.

  Also in the organic light emitting device described in Patent Document 1 described above, the guest concentration is 6% by weight. Therefore, depending on the configuration of the device, the driving voltage is approximately the same as that of a normal device, and the driving voltage cannot be sufficiently reduced. There was a problem that there was a case. Further, when the guest concentration is less than 6% by weight, there is a possibility that the driving voltage can be lowered, but this time there is a problem that high luminous efficiency may not be obtained.

  Furthermore, in the organic light emitting element described in Patent Document 1, no element other than the red element is considered, and there is a problem that the configuration that can display each color is unknown.

  Therefore, an object of the present invention is to provide an organic electroluminescence element that has high luminous efficiency, can be reliably driven at a low voltage, and can display each color, and a display device using the organic electroluminescence element.

In order to achieve the above object, an organic electroluminescence element according to one embodiment of the present invention is an organic electroluminescence element having a light emitting layer between a pair of electrodes,
The light emitting layer is composed only of a host material made of a metal complex and a guest material made of a phosphorescent material,
An energy of an excited state in which the host material emits light is higher than an energy of a singlet excited state of the guest material.

  Thereby, the energy of the excited state of the host material generated by current excitation is efficiently transferred to the guest material, and high light emission efficiency can be obtained.

The host material is made of a phosphorescent material,
The excited state energy is triplet excited state energy.

  As a result, even when a phosphorescent material is used as the host material, energy transfer to the guest material can be efficiently performed and light emission efficiency can be increased.

  The host material is an iridium complex.

  Thereby, highly efficient light emission is realizable, utilizing the phosphor material generally used.

The host material is made of a fluorescent material,
The excited state energy is singlet excited state energy.

  As a result, it is possible to use a material other than the expensive phosphorescent material as the host, and the cost can be reduced.

  The host material is a beryllium complex or a zinc complex.

  The mixing ratio of the guest material is 1% by weight or more and less than 6% by weight.

  Accordingly, by reducing the mixing ratio of the guest material contained in the light emitting layer, the cost can be reduced, and the guest material can be prevented from becoming a charge trap, and a low voltage can be expected.

  The emission wavelength of the guest material is longer than the emission wavelength of the host material.

  Thereby, it is possible to emit light with a desired emission color, and it is possible to perform color emission.

A display device according to another aspect of the present invention includes a display panel using the organic electroluminescence element,
And a driving circuit for driving the display panel.

  Accordingly, a display device that can be driven at a low voltage, has high light emission efficiency, and has a long lifetime can be realized.

  ADVANTAGE OF THE INVENTION According to this invention, the luminous efficiency of an organic electroluminescent element and a display apparatus using the same can be improved.

It is a section lineblock diagram showing an example of an organic EL device concerning an embodiment of the present invention. It is the table | surface which showed the characteristic of the organic EL element which concerns on Examples 1-5 and Comparative Examples 1-3. It is the figure which showed the emission spectrum of the host material of the organic EL element concerning Examples 1-3 and the comparative example 1. FIG. It is the figure which showed the light emission mechanism of the organic EL element which concerns on Examples 1-3 and the comparative example 1. FIG. FIG. 4A is a diagram illustrating the energy relationship between the host material and the guest material of the light emitting layer of the organic EL element according to Example 2. FIG. 4B is a diagram illustrating the energy relationship between the host material and the guest material of the organic EL element according to Example 3. FIG. 4C is a diagram illustrating the energy relationship between the host material and the guest material of the organic EL element according to Example 1. FIG. 4D is a diagram showing the energy relationship between the host material and the guest material of the organic EL element according to Comparative Example 1. It is the figure which showed the absorption spectrum of the guest material of the light emitting layer of the organic EL element which concerns on Examples 1-5 and the comparative example 1, and the emission spectrum of host material. It is a measurement result of the organic EL element which concerns on the comparative examples 2 and 3. FIG. 6A is a diagram showing voltage-luminance characteristics of the organic EL elements according to Comparative Examples 2 and 3, and FIG. 6B is a voltage-current of the organic EL element according to Comparative Examples 2 and 3. It is the figure which showed the density characteristic. 6C is a diagram showing current density-external quantum efficiency characteristics of the organic EL elements according to Comparative Examples 2 and 3, and FIG. 6D is an organic EL according to Comparative Examples 2 and 3. It is the figure which showed the current density-power efficiency characteristic of an element. It is the figure which showed the wavelength-electroluminescence intensity | strength characteristic of the organic EL element which concerns on the comparative examples 1 and 2. FIG. It is the table | surface which showed the characteristic of the organic EL element which concerns on Example 6 and Comparative Examples 4-6. It is the figure which showed the emission spectrum of the organic EL element which concerns on Example 6 and Comparative Examples 4-6. It is the figure which showed the light emission mechanism of the organic EL element which concerns on Example 6 and Comparative Examples 4 and 5. FIG. FIG. 10A is a diagram illustrating the energy relationship between the host material and the guest material of the light-emitting layer of the organic EL element according to Example 6. FIG. 10B is a diagram showing the energy relationship between the host material and the guest material of the organic EL element according to Comparative Example 4. FIG. 10C is a diagram showing the energy relationship between the host material and the guest material of the organic EL element according to Comparative Example 5.

  DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments for carrying out the present invention will be described with reference to the drawings.

  FIG. 1 is a cross-sectional configuration diagram showing an example of an organic electroluminescence element (hereinafter referred to as “organic EL element”) according to an embodiment of the present invention. In FIG. 1, the organic EL device according to this embodiment includes a substrate 10, an ITO (Indium Tin Oxide) electrode 20, a hole injection layer 30, a hole transport layer 40, and a light emitting layer 50. The electron transport layer 60 and the electrode layer 70 are included.

  In the organic EL device according to this embodiment, an ITO electrode 20, a hole injection layer 30, a hole transport layer 40, a light emitting layer 50, an electron transport layer 60, and an electrode layer 70 are sequentially stacked on the surface of the substrate 10. It has a configuration. Since the hole injection layer 30, the hole transport layer 40, the light emitting layer 50, and the electron transport layer 60 are all configured as an organic layer made of an organic material, the hole injection layer 30 is formed as the first organic layer 30, The hole transport layer 40 may be called the second organic layer 40, the light emitting layer 50 may be called the third organic layer 50, and the electron transport layer 60 may be called the fourth organic layer 60. Further, the ITO electrode 20 and the electrode layer 70 are arranged to face each other with a predetermined interval, and a hole injection layer 30, a hole transport layer 40, a light emitting layer 50, and an electron transport layer 60 are provided therebetween. It becomes the composition. That is, the organic EL element according to this embodiment has a configuration in which the first to fourth organic layers 30 to 60 are sandwiched between the pair of ITO electrodes 20 and the electrode layer 70.

  The substrate 10 is a transparent substrate that transmits light, and for example, a glass substrate or a plastic substrate may be used. If the board | substrate 10 is a transparent material, it can be comprised from various materials according to a use.

  The ITO electrode 20 is made of a transparent metal thin film that transmits light, and is configured as a transparent electrode. The ITO electrode 20 functions as an anode, and a positive voltage is applied during driving to inject holes. In the present embodiment, an example in which the ITO electrode 20 is used as an anode is given. However, other materials may be used as long as a transparent electrode can be formed.

  The hole injection layer 30 has a work function intermediate between the ITO electrode 20 and the hole transport layer 40, and is a buffer layer that bridges the holes injected into the ITO electrode 20 into the hole transport layer 40. . As described above, the hole injection layer 30 is made of an organic material. The hole injection layer 30 may be composed of various organic materials having a function of bridging holes injected by the ITO electrode 20 to the hole transport layer 40. For example, the hole injection layer 30 is represented by the following formula (1). It may be composed of a compound (hereinafter referred to as “PEDOT: PSS”).

正孔輸送層４０は、正孔注入層３０から注入された正孔を発光層５０へと輸送する層である。正孔輸送層４０も、有機材料で構成される。正孔輸送層４０は、正孔を輸送する機能を有すれば、種々の有機材料を用いることができるが、例えば、下記の式（２）に示す化合物（以下、「α−ＮＰＤ」と呼ぶ。）を用いるようにしてもよい。

The hole transport layer 40 is a layer that transports holes injected from the hole injection layer 30 to the light emitting layer 50. The hole transport layer 40 is also made of an organic material. The hole transport layer 40 can use various organic materials as long as it has a function of transporting holes. For example, a compound represented by the following formula (2) (hereinafter referred to as “α-NPD”). .) May be used.

電極層７０は、駆動時に陰極として機能し、負電圧が印加される。電極層７０は、金属薄膜で構成されてよく、例えば、アルミニウム、銀−マグネシウム合金、カルシウム等の金属薄膜として構成されてよい。

The electrode layer 70 functions as a cathode during driving, and a negative voltage is applied. The electrode layer 70 may be formed of a metal thin film, for example, a metal thin film of aluminum, silver-magnesium alloy, calcium, or the like.

  The electron transport layer 60 is a layer for transporting electrons injected from the electrode layer 70 to the light emitting layer 50. The electron transport layer 60 may be composed of an organic material. As the organic material, various organic materials can be used as long as they can function to transport electrons. For example, 2,2 ′, 2 ″-(1,3,5-benzenetriyl) -tris ( 1-phenyl-1-H-benzimidazole) (or 2, 2 ', 2 "-(1, 3, 5-benzenetriyl) -tris (1-phenyl-1-H-benzimidazole)) (hereinafter referred to as" TPBI May also be used. The chemical formula of TPBI is shown in the following formula (3).

発光層５０は、電子と正孔の結合により発光する層である。発光層５０は、電子と正孔の結合により励起され、励起状態から基底状態に戻る際に光を発光する。本実施形態に係る有機ＥＬ素子においては、励起状態（一重項状態）から直接基底状態に戻る際に発光する蛍光ではなく、一重項状態から、それよりもややエネルギー状態の低い三重項状態を介して基底状態に戻る際に発光する燐光を用いる。

The light emitting layer 50 is a layer that emits light by the combination of electrons and holes. The light emitting layer 50 is excited by the combination of electrons and holes, and emits light when returning from the excited state to the ground state. In the organic EL device according to the present embodiment, the fluorescence is not emitted when returning directly from the excited state (singlet state) to the ground state, but from the singlet state via the triplet state having a slightly lower energy state. Thus, phosphorescence emitted when returning to the ground state is used.

  The light emitting layer 50 is composed of only a metal complex as a host material and a phosphorescent material as a guest material. The metal complex becomes a main component of the light emitting layer 50, and the phosphorescent material becomes a minor component of the light emitting layer 50. The host material and the guest material exist in the light emitting layer 50 in a mixed state, and the light emitting layer 50 is configured in a state where a small amount of guest material is mixed in the host material as the main component. In the light emitting layer 50, the guest material plays a role of light emission. The light emitting layer 50 of the organic EL device according to the present embodiment is composed only of a metal complex and a phosphorescent material, and does not include other materials.

  Various metal complexes can be used as the metal complex serving as the host material. If it is a metal complex, a phosphorescent material such as an iridium complex may be used. Further, a fluorescent material may be used. The metal complex includes both a phosphorescent material such as iridium (Ir) and a fluorescent material such as zinc (Zn) and beryllium (Be). Both the phosphorescent material and the fluorescent material can be used as a host material. .

  Note that when these metal complexes are used as the host material, it is preferable to use a metal complex material that emits light having an emission wavelength similar to the wavelength of the absorption spectrum derived from the band gap of the guest material. This enables efficient energy transfer from the host metal complex to the guest, and light can be generated from the light emitting layer 50 in the emission color of the guest material.

  More specifically, it is preferable to select the metal complex so that the energy of the excited state emitted from the host material is higher than the energy of the singlet excited state emitted from the guest material. Specifically, when the host material is a phosphorescent material, the excited state in which the host material emits light is a triplet excited state. Therefore, the energy of the triplet excited state of the host material is the singlet excited state of the guest material. It needs to be higher than energy. On the other hand, when the host material is a fluorescent material, the excited state in which the host material emits light is a singlet excited state. Therefore, the energy of the singlet excited state of the host material is higher than the energy of the singlet excited state of the guest material. High is needed.

  By having such an energy relationship between the host material and the guest material, the guest material can reliably absorb the energy to be emitted generated by recombination of charges in the host material, and light emission by the guest material The function can be fully exhibited. In addition, the energy said here means the energy on the basis of a ground state. In addition, the energy relationship between the host material and the guest material will be described in more detail and specifically later using examples.

  Since energy and wavelength have a correlation, hereinafter, the relationship between the host material and the guest material may be expressed using energy or may be expressed using wavelength.

  In a normal phosphorescent light emitting element, the guest material directly traps charges in the light emitting layer and recombines to emit light. Therefore, the guest hinders the charge movement in the device, and the drive voltage increases with an increase in the guest concentration. On the other hand, when the guest concentration is low, efficient energy transfer between the host and the guest does not occur, and thus high light emission efficiency cannot be obtained. The optimum guest concentration of a normal phosphorescent device is about 6 to 10% by weight.

  On the other hand, even when the mixing ratio of the guest material is low, efficient energy transfer occurs by appropriately selecting the absorption wavelength of the guest material and the emission wavelength of the host material shown in this embodiment. Therefore, high luminous efficiency can be obtained even with a mixing ratio of about 1% by weight. In addition, since the mixing ratio of the guest material is low, the influence of charge trapping by the guest material is small, and the element can be driven with a low driving voltage.

  In the organic EL device according to the present embodiment, the mixing ratio of the guest material in the light emitting layer 50, that is, the ratio of the guest material in the light emitting layer 50 made of the host material and the guest material is 1 wt% or more and less than 6 wt%. It is preferable to do. As described above, in the organic EL element according to this embodiment, high luminous efficiency can be obtained even when the mixing ratio of the guest material is about 1% by weight. In addition, if the mixing ratio of the guest material is 6% by weight or more, the driving voltage may increase. Therefore, the range of the mixing ratio of the guest material that provides high luminous efficiency without increasing the driving voltage is 1% by weight. It is preferable to set it to less than 6% by weight.

  Note that when a phosphorescent material is used for the metal complex of the host material, a phosphorescent material that emits light having a shorter wavelength than that of the phosphorescent material used for the guest material is used. Thereby, the emission from the phosphorescent material of the guest material having a long wavelength is given priority, and light can be generated from the light emitting layer 50 with the emission color of the guest material.

For example, a phosphorescent material having an emission spectrum rise at a wavelength of about 500 nm is used for the metal complex of the host material, and a phosphorescent material that emits red light for the phosphorescent material of the guest material, and the absorption spectrum derived from the band gap has a peak at about 500 nm If there is used, the emission spectrum of the host material and the absorption spectrum of the guest material overlap, and efficient energy transfer occurs. A phosphorescent material having an absorption spectrum peak at a wavelength of about 500 nm is a material that emits red light. Therefore, when these two types of materials are used for the light emitting layer, the phosphorescent material is a guest material rather than the light emission of the host material. The light emission in the red wavelength region 610 to 750 nm is given priority, and the light emitting layer 50 emits red light. In this case, for example, a compound represented by the following formula (4) (hereinafter referred to as “Ir (ppy) ₂ acac”) may be used as the metal complex material of the host material. Further, as the red phosphorescent material of the guest material, for example, a compound represented by the following formula (5) (hereinafter referred to as “Ir (piq) ₃ ”) may be used.

一方、ホスト材料の金属錯体に燐光材料を用いると、希少金属を含むため材料自体のコストが懸念される。これについては、下記の式（６）、（７）で示されるＢｅやＺｎ等の安価な金属で構成される金属錯体（以下「Ｂｅｂｑ_２」、「Ｚｎ（ＢＴＺ）_２」という。）を用いることも可能である。

On the other hand, when a phosphorescent material is used for the metal complex of the host material, since the rare metal is included, there is a concern about the cost of the material itself. For this, the following equation (6), using a metal complex composed of inexpensive metal of Be and Zn or the like represented by (7) (hereinafter "Bebq _2", referred to as "Zn _{(BTZ) 2".)} It is also possible.

波長は、青色＜緑色＜黄色＜橙色＜赤色であるので、ホスト材料の金属錯体材料と、ゲスト材料に用いられる燐光材料の波長を適切に選択し、ゲスト材料の発光波長がホスト材料の発光波長より長くなるような組み合わせとすれば、赤色以外の発光を得ることも可能である。

Since the wavelength is blue <green <yellow <orange <red, the wavelength of the host material metal complex material and the phosphorescent material used for the guest material is appropriately selected, and the emission wavelength of the guest material is the emission wavelength of the host material. If the combination is longer, light emission other than red can be obtained.

For example, a metal complex material having a light emission rise at a wavelength of about 420 nm is used as a host material metal complex, and an absorption spectrum derived from a band gap of a phosphorescent material that emits orange light has a peak at about 420 nm. If the material is used, the emission spectrum of the host material and the absorption spectrum of the guest material overlap, and efficient energy transfer occurs. A phosphorescent material having an absorption spectrum peak at a wavelength of about 420 nm is a material that emits orange light. Therefore, when these two types of materials are used in the light emitting layer, the phosphorescent material is a guest material rather than the light emission of the host material. Priority is given to light emission in the red wavelength region of 560 to 710 nm, and the light emitting layer 50 emits orange light. In this case, for example, Zn (BTZ) ₂ represented by the formula (7) may be used as the metal complex material of the host material. Further, as the orange phosphorescent material of the guest material, for example, a compound represented by the following formula (8) (hereinafter referred to as “m-PF-py”) may be used.

このように、ホストとなる金属錯体の発光波長とゲストとなる燐光材料の吸収スペクトルの相関から適切な材料選択を行うことができれば、緑や青色の発光を得ることも可能である。

As described above, if an appropriate material can be selected from the correlation between the emission wavelength of the metal complex serving as the host and the absorption spectrum of the phosphorescent material serving as the guest, it is possible to obtain green or blue light emission.

  Note that the organic EL element according to the present embodiment can be manufactured by using a film forming method such as a spin coating method or a vacuum evaporation method. The ITO electrode 20, the first to fourth organic layers 30 to 60, and the electrode layer 70 formed on the substrate 10 can be formed by film formation. Therefore, according to the present embodiment by various film formation methods. An organic EL element can be manufactured.

  Moreover, since the light emitting layer 50 is formed in a state in which the host material and the guest material are mixed, the host material and the guest material may be simultaneously formed by co-evaporation or the like, for example.

  Next, the organic EL element of the present invention will be described in more detail with reference to specific examples.

  The organic EL element according to the example of the present invention was configured in the same manner as the organic EL element according to this embodiment described in FIG. Specifically, the organic EL device according to the example was manufactured as follows. Note that the same reference numerals are assigned to the components corresponding to the components described so far, and the description thereof is omitted.

First, PEDOT: PPS to be the hole injection layer 30 was formed on the substrate 10 on which the thin film of the ITO electrode 20 was formed by spin coating. Next, an organic layer that becomes the hole transport layer 40, the light emitting layer 50, and the electron transport layer 60, and the electrode layer 70 were vacuum-deposited by resistance heating in a vacuum chamber of 10 ⁻⁴ Pa and continuously formed. . Specifically, the organic EL element according to Example 1 was configured by forming a film with the following materials and thickness.

Hole injection layer 30 (35 nm): PEDOT: PSS
Hole transport layer 40 (40 nm): α-NPD
Light emitting layer 50 (35 nm): Predetermined host / predetermined guest Electron transport layer 60 (40 nm): TPBI
Electrode layer 70 (0.5 nm / 100 nm): LiF / Al
The structure of the light emitting layer 50 is a predetermined host and a predetermined guest because the material changes according to a specific embodiment.

  Next, voltage was applied with the ITO electrode 20 side of the produced organic EL element according to this example as an anode and the electrode layer 70 side as a cathode, and current, luminance, and emission spectrum were measured. In such a measurement, oxygen and water cause problems as a cause of device deterioration. To remove the cause, take out the dry box from the vacuum chamber without touching the air, and seal it with a glass cap. Measurements were made after

  Next, organic EL elements according to specific comparative examples and examples will be described. In addition, the organic EL element which concerns on a comparative example means the organic EL element which has the conventional structure produced as a comparison object, in order to confirm the effect of the organic EL element which concerns on a present Example.

[Example 1]
The compound (Ir (piq) ₃ ) represented by the formula (5) is a guest material, the compound (Ir (ppy) ₂ acac) represented by the formula (4) is a host material, and the guest material in the light emitting layer 50 The light emitting layer 50 was formed by doping by co-evaporation so that the mixing ratio was 1% by weight, and the organic EL device according to Example 1 was configured.

[Example 2]
1 wt% in which the compound (Ir (piq) ₃ ) represented by the formula (5) is a guest material, the compound Bebq2 represented by the formula (7) is a host material, and the mixing ratio of the guest material in the light emitting layer 50 is extremely low. The light emitting layer 50 was formed by doping by co-evaporation so that the organic EL element according to Example 2 was formed.

Example 3
The compound (Ir (piq) ₃ ) represented by the formula (5) is a guest material, the compound Zn (BTZ) ₂ represented by the formula (6) is a host material, and the mixing ratio of the guest material in the light emitting layer 50 is The light emitting layer 50 was formed by doping by co-evaporation so as to be very low 1% by weight, and the organic EL device according to Example 3 was configured.

Example 4
The compound (Ir (piq) ₃ ) represented by the formula (5) is a guest material, the compound Bebq ₂ represented by the formula (7) is a host material, and the mixing ratio of the guest material in the light emitting layer 50 is normal phosphorescence. The light emitting layer 50 was formed by doping by co-evaporation so that the optimum mixing ratio of the organic EL element was 6% by weight, and the organic EL element according to Example 4 was configured. In addition, although the organic EL element which concerns on the comparative example 1 is the same as that of the organic EL element which concerns on the kind of host material and guest material in Example 2, the mixing ratio of guest material differs.

Example 5
The compound (Ir (piq) ₃ ) represented by the formula (5) is a guest material, the compound Zn (BTZ) ₂ represented by the formula (6) is a host material, and the mixing ratio of the guest material in the light emitting layer 50 is The light emitting layer 50 was formed by doping by co-evaporation so that the optimum mixing ratio in an ordinary phosphorescent organic EL element was 6% by weight, and the organic EL element according to Example 5 was configured. In the organic EL device according to Example 5, the types of the host material and the guest material are the same as those of the organic EL device according to Example 3, but the mixing ratio of the guest material is different.

[Comparative Example 1]
The compound (Ir (piq) ₃ ) represented by the formula (5) is a guest material, the compound m-PF-py represented by the formula (8) is a host material, and the mixing ratio of the guest material in the light emitting layer 50 is The light emitting layer 50 was formed by doping by co-evaporation so as to be very low 1 wt%, and the organic EL device according to Comparative Example 1 was configured.

[Comparative Example 2]
The compound (Ir (piq) ₃ ) represented by the formula (5) is used as a guest material, TPBI represented by the formula (3) is used as a host material, and the mixing ratio of the guest material in the light emitting layer 50 is 1% by weight. Then, doping by co-evaporation was performed to form the light emitting layer 50, and an organic EL device according to Comparative Example 2 was configured.

[Comparative Example 3]
As in Comparative Example 1, the compound (Ir (piq) ₃ ) represented by the formula (5) is used as the guest material, TPBI represented by the formula (3) is used as the host material, and the mixing ratio of the guest material is set to normal phosphorescence. A light emitting layer 50 was formed by doping by co-evaporation with an optimum mixing ratio of 6 wt% in the organic EL element, and an organic EL element according to Comparative Example 3 was configured.

FIG. 2 is a table showing the characteristics of the organic EL elements according to Examples 1 to 5 and Comparative Examples 1 to 3. In the table of FIG. 2, the host material and guest concentration of the light emitting layer 50 of the organic EL elements according to Examples 1 to 5 and Comparative Examples 1 to 3 are shown, and driving when the luminous intensity is 100 cd and 1000 cd as characteristic items. The drive voltage when the voltage and the current density are 1 mA / cm ² , the external quantum efficiency EQE, the light emission efficiency (power efficiency) PE when the light intensity is 100 cd, the CIE chromaticity, and the presence or absence of host light emission.

In the table of FIG. 2, in Examples 1 to 3, a higher external quantum efficiency is obtained even when the guest concentration is 1% by weight than the TPBI host system of Comparative Examples 2 and 3. In addition, since the drive voltage can be driven at a relatively low value, and the host does not emit much light, good red light emission is obtained. In other words, it is considered that even at a guest concentration of 1% by weight, high luminous efficiency was obtained by efficient energy transfer between the host and the guest. The reason why the driving voltage at the current density of 1 mA / cm ² is the same in Comparative Example 3 and Example 3 is considered to be due to the difference in charge mobility of the host material.

  In the table of FIG. 2, Comparative Example 1 in which a metal complex of a phosphorescent material is used as the host material has lower luminous efficiency than the organic EL elements according to Examples 1 to 3.

  FIG. 3 is a diagram showing emission spectra of host materials of organic EL elements according to Examples 1 to 3 and Comparative Example 1. In FIG. 3, the characteristics indicated by the white dots are the emission spectrum of the organic EL according to Example 2, the characteristics indicated by the triangular dots are the emission spectra of the organic EL element according to Comparative Example 1, and the characteristics indicated by the square dots. Shows the emission spectrum of the organic EL device according to Example 3, and the characteristics indicated by the diamond dots indicate the emission spectrum of the organic EL device according to Example 1.

  As shown in FIG. 3, only the organic EL element according to Comparative Example 1 indicated by the triangular dots, the light emission of the host material is observed in the vicinity of 550 nm to 580 nm, and the chromaticity is light emission slightly deviated from the red light emission. ing.

  Next, the difference in the structure of the light emitting layer 50 of the organic EL element which concerns on Examples 1-3 and the organic EL element which concerns on the comparative example 1 is demonstrated using FIG.4 and FIG.5. FIG. 4 is a diagram showing a light emission (energy transfer) mechanism of the organic EL elements according to Examples 1 to 3 and Comparative Example 1. FIG. 4A is a diagram showing the energy relationship between the host material and the guest material of the light emitting layer 50 of the organic EL element according to Example 2. FIG. 4B is a diagram illustrating the energy relationship between the host material and the guest material of the light emitting layer 50 of the organic EL element according to Example 3. FIG. 4C is a diagram illustrating the energy relationship between the host material and the guest material of the light emitting layer 50 of the organic EL element according to Example 1. FIG. 4D is a diagram illustrating the energy relationship between the host material and the guest material of the light emitting layer 50 of the organic EL element according to Comparative Example 1.

  FIG. 5 is a diagram showing the absorption spectrum of the guest material and the emission spectrum of the host material of the light emitting layer 50 of the organic EL elements according to Examples 1 to 5 and Comparative Example 1.

4A to 4D, the singlet excited state S ₁ , the triplet excited state T _1, and the ground state S 0 of the host material and guest material constituting the light emitting layer 50 are shown, respectively. Note that the energy relationship between the singlet excited state S ₁ and the triplet excited state T ₁ was determined from the absorption spectra and emission spectra of various materials shown in FIG.

As shown in FIGS. 4A to 4D, holes injected from the ITO electrode 20 serving as the anode and electrons injected from the electrode layer 70 serving as the cathode are recombined in the host material. excited state of the two host materials ₁ and T ₁ are produced.

As shown in FIG. 4C, when the host material is an Ir complex, since it is a phosphorescent material, the singlet excited state S ₁ generated by the intersystem crossing instantaneously shifts to the triplet excited state T ₁ . Therefore, the excited state in which light emission occurs is only the triplet excited state T ₁ when Ir (ppy) ₂ acac is the host material.

Here, in order for the guest material made of a phosphorescent material to emit light, it is necessary to supply energy higher than that of the singlet excited state S ₁ or the triplet excited state T ₁ . Therefore, the energy of the triplet excited state T ₁ generated in the host material is higher than the singlet excited state S1 of the guest material, the energy to be emission generated in the host material, a singlet excited all guest material can be moved to the state S ₁ or triplet excited state T _1, efficient energy transfer is performed.

In Example 3, the case where the host material is Ir (ppy) ₂ acac is described as an example, but the energy relationship between the host material and the guest material is the case where the host material is a phosphorescent material. All can be applied.

On the other hand, as shown in FIGS. 4A and 4B, when Bebq ₂ and Zn (BTZ) _{2 that} are fluorescent materials are host materials, the excited state in which light emission occurs is a singlet excited state S _1. . Even fluorescent material, when cooled to a low temperature, it is possible to emit light by triplet excitation state T _1, here, consider the behavior at room temperature.

If the host material is a fluorescent material, the energy of the singlet excited state S1, emits light, if combinations higher than the singlet energy of the excited state S ₁ of the guest material is a phosphorescent material, occurs in the host material All of the energy to be emitted can move to the singlet excited state S ₁ or the triplet excited state T ₁ of the guest material, and energy transfer from the host material to the guest material can be performed efficiently.

  In the case of a fluorescent material, the excited state emitting light at room temperature is only a singlet excited state, but a triplet excited state is also generated as energy. Since the energy of the triplet excited state can be transferred to the triplet excited state of the guest material, energy transfer is efficiently performed. Since the guest material is also a kind of metal complex like the host material, the guest material has a similar energy structure, and the energy level relationship of the singlet excited state also applies to the energy relationship of the triplet excited state. That is, if there is a relationship of guest material <host material in the energy relationship in the singlet excited state, the guest material <host material also in the energy relationship in the triplet excited state.

  Therefore, when the host material is a fluorescent material, efficient energy transfer can be achieved by combining the host material so that the singlet excited state energy is higher than the singlet excited state energy of the guest material. Efficiency can be increased.

In Examples 2 and 3, the case where the host material is Bebq ₂ and Zn (BTZ) ₂ has been described as an example. However, even if the host material is another fluorescent material, the energy described above is used. Relationships can be applied as well.

FIG. 4D shows the energy relationship of the organic EL element according to Comparative Example 1 in the case where the host material is a phosphorescent material m-PF-py. The triplet excited state in which the host material emits light is shown. The energy is higher than the energy of the triplet excited state T ₁ of the guest material, but is lower than the energy of the singlet excited state S ₁ . In such state, the energy of the triplet excited state T ₁ generated in the host material, but can be moved to the triplet excited state T _1, it can not be moved to the singlet excited state S _1. Therefore, it is considered that energy transfer between the host and the guest does not occur efficiently, light emission of the host is observed, and the light emission efficiency is lowered.

  Note that the description using the energy relationship in FIG. 4 can also be described using the emission spectrum of the host material and the absorption spectrum of the guest material shown in FIG.

5, the absorption spectrum Ir _(piq) 3 Abs of Ir _{(piq) 3} which is a guest material of a light-emitting layer 50 of the organic EL device according to Example 1-3 and Comparative Example 1, the host material of Example 3 Zn (BTZ) ₂ singlet excited state emission spectrum Zn (BTZ) ₂ Fluor and triplet excited state emission spectrum Zn (BTZ) ₂ Phos and singlet excited state emission spectrum of host material Bebq ₂ of Example 2 Bebq ₂ Fluoro and the emission spectrum Bebq ₂ Phos triplet excited state, and light emission spectrum Ir _(ppy) 2 acac triplet excited state of the host material Ir _(ppy) 2 acac example 1, Comparative example 1 host material The emission spectra of m-PF-py Phos are shown respectively.

5, the singlet excited state emission spectrum Zn (BTZ) ₂ Fluor of the host material of Example 3, the singlet excited state emission spectrum Bebq ₂ Fluoro of the host material Bebq ₂ of Example 2, and the host of Example 1 The emission spectrum Ir (ppy) ₂ acac in the triplet excited state of the material Ir (ppy) ₂ acac has many regions overlapping the rising part of the absorption spectrum derived from the S ₁ of the absorption spectrum Ir (piq) ₃ Abs of the guest material. And it turns out that it has many wavelength regions to share. Thereby, energy transfer is efficiently performed in the shared wavelength region. In the case of an absorption spectrum, since absorption starts from the long wavelength side, the long wavelength side becomes a rising portion.

On the other hand, the emission spectrum of the host material m-PF-py Phos of Comparative Example 1 overlaps with the absorption spectrum derived from T ₁ of the guest material's absorption spectrum Ir (piq) ₃ Abs, but the spectrum derived from S ₁ There are not many overlapping regions, and it is shown that there is less sharing of the wavelength region as compared with Examples 1-3. Therefore, energy transfer between the host and the guest is not performed very efficiently.

  As described above, by selecting the host material and the guest material in consideration of the energy relationship and wavelength in which efficient energy transfer occurs, an element capable of obtaining high luminous efficiency with a low driving voltage can be realized. The organic EL element according to this example can reduce the amount of phosphorescent material used as a guest material, and is considered to be effective for cost reduction.

  Next, returning to the table of FIG. 2, Examples 4 and 5 will be described.

  In the table of FIG. 2, as shown in Examples 4 and 5, even when the guest concentration is 6 wt%, a relatively good characteristic can be obtained although the voltage is slightly increased. Therefore, if the energy relationship between the host material and the guest material described in FIG. 4 is satisfied, the organic EL device according to this example has an extremely low guest concentration (about 1 wt%) and a lower limit of the optimum guest concentration of a normal phosphorescent device. It can be applied in a low guest concentration range up to a value (about 6 wt%).

  Next, Comparative Examples 2 and 3 will be described. Comparative Examples 2 and 3 have a configuration in which TPBI, which is an organic material, is used as the host material of the light emitting layer 50. Further, Comparative Examples 2 and 3 differ in that the mixing ratio of the guest material is 1% by weight and 6% by weight, respectively.

  FIG. 6 shows the measurement results of the organic EL elements according to Comparative Examples 2 and 3. 6A is a diagram showing voltage-luminance characteristics of the organic EL elements according to Comparative Examples 2 and 3, and FIG. 6B is a voltage-current of the organic EL element according to Comparative Examples 2 and 3. It is the figure which showed the density characteristic. 6C is a diagram showing current density-external quantum efficiency characteristics of the organic EL elements according to Comparative Examples 2 and 3, and FIG. 6D is an organic EL according to Comparative Examples 2 and 3. It is the figure which showed the current density-power efficiency characteristic of an element. 6A to 6D, triangular dots indicate the characteristics of the organic EL element according to Comparative Example 2, and black dots indicate the characteristics of the organic EL according to Comparative Example 3.

  FIG. 7 is a diagram showing the wavelength-electroluminescence intensity characteristics of the organic EL elements according to Comparative Examples 1 and 2. In FIG. 7, the solid line indicates the characteristic of the organic EL element according to Comparative Example 2, and the broken line indicates the characteristic of the organic EL element according to Comparative Example 3.

  As shown in FIG. 6C, the organic EL element according to Comparative Example 3 in which the mixing ratio of the guest material is 6% by weight as compared with the organic EL element according to Comparative Example 2 in which the mixing ratio of the guest material is 1% by weight. The higher the external quantum efficiency (EQE, External Quantum Efficiency) is. As shown in FIG. 7, in the organic EL element according to Comparative Example 2 in which the mixing ratio of the guest material is 1% by weight, light emission of the host material is observed and energy transfer does not occur sufficiently. It is thought to be caused.

  Similarly, also in FIG. 6D, the organic EL element according to Comparative Example 2 shows higher power efficiency than the organic EL element according to Comparative Example 3. In FIG. 6 (D), the power efficiency is shown by the total luminous flux (lm / W) per unit power, which is synonymous with the light emission efficiency. Therefore, the result similar to the external quantum efficiency in FIG. 6 (C) is obtained. Show.

  On the other hand, as shown in FIG. 6B, the driving voltage of the organic EL element that generates current of the same current density is lower in the organic EL element according to Comparative Example 1 in which the mixing ratio of the guest material is 1% by weight. . This reflects that, in the organic EL element having a guest material mixing ratio of 6 wt% according to Comparative Example 3, the charge is directly trapped in the guest material because the guest material mixing ratio is high. In other words, a normal phosphorescent element requires a relatively high guest concentration in order to obtain a high luminous efficiency, which causes a problem that the drive voltage becomes high. Therefore, if efficient energy transfer between the host and the guest occurs at a low guest concentration, it can be expected that high luminous efficiency can be obtained at a low driving voltage.

  Further, as shown in FIG. 6A, in the voltage-luminance characteristics, the organic EL elements according to Comparative Examples 2 and 3 show almost the same characteristics, and it can be seen that there is not much difference in luminance. .

  Next, the usefulness of the organic EL device of the present invention for various emission colors will be described using Example 6 and Comparative Examples 4 to 6.

Example 6
The compound (m-PF-py) represented by the formula (8) is the guest material, Zn (BTZ) ₂ represented by the formula (6) is the host material, and the mixing ratio of the guest material in the light emitting layer 50 is 1 weight. The light emitting layer 50 was formed by doping by co-evaporation so that the organic EL element according to Example 6 was formed. The guest material (m-PF-py) according to Example 6 is a phosphorescent material that emits orange light, and has an emission color different from that of the guest material (Ir (piq) ₃ ) that emits red light according to Examples 1 to 5. Have. In Example 6, an example using a compound having an orange emission color (m-PF-py) will be given to explain that the organic EL device according to this embodiment is also useful for an orange emission color. .

[Comparative Example 4]
The compound (m-PF-py) represented by the formula (8) is a guest material, the host material is Ir (ppy) ₂ acac, and the mixing ratio of the guest material in the light emitting layer 50 is 1% by weight. The light emitting layer 50 was formed by doping by co-evaporation, and the organic EL device according to Comparative Example 4 was configured.

[Comparative Example 5]
The compound (m-PF-py) represented by the formula (8) is the guest material, Bebq ₂ represented by the formula (7) is the host material, and the mixing ratio of the guest material in the light emitting layer 50 is 1% by weight. Thus, the light emitting layer 50 was formed by performing doping by co-evaporation, and the organic EL device according to Comparative Example 5 was configured.

[Comparative Example 6]
The compound (m-PF-py) represented by formula (8) is the guest material, TPBI represented by formula (3) is the host material, and the mixing ratio of the guest material in the light emitting layer 50 is 1% by weight. Thus, the light emitting layer 50 was formed by performing doping by co-evaporation, and the organic EL device according to Comparative Example 6 was configured.

FIG. 8 is a table showing the characteristics of the organic EL elements according to Example 6 and Comparative Examples 4-6. In the table of FIG. 8, as in the table of FIG. 2, the host material and guest concentration of the light emitting layer 50 of the organic EL elements according to Example 6 and Comparative Examples 4 to 6 are shown, and as a characteristic item, the luminous intensity is 100 cd. , Driving voltage at 1000 cd, driving voltage at a current density of 1 mA / cm ² , external quantum efficiency EQE, luminous efficiency (power efficiency) PE at a luminous intensity of 100 cd, CIE chromaticity, and presence / absence of host emission ing.

  FIG. 9 is a diagram showing emission spectra of organic EL elements according to Example 6 and Comparative Examples 4 to 6. In FIG. 9, the characteristics indicated by white dots are the emission spectrum of the organic EL host material according to Comparative Example 5, and the characteristics indicated by the diamond dots are the emission spectrum of the host material of the organic EL element according to Comparative Example 4. The characteristic indicated by the solid line indicates the emission spectrum of the host material of the organic EL element according to Comparative Example 6, and the characteristic indicated by the square dot indicates the emission spectrum of the host material of the organic EL element according to Example 6.

  In the table of FIG. 8, the organic EL elements according to Comparative Examples 4 and 5 have the same external quantum efficiency as the organic EL elements according to Examples 1 and 2, but are shown in FIG. Thus, light emission of the host material was observed in the vicinity of 500 to 540 nm. On the other hand, in Example 6, the host material did not emit light, and high luminous efficiency was obtained. Further, in Comparative Example 6, as shown in FIG. 9, light emission of the host material was observed as in Comparative Example 2, and the light emission efficiency was not high.

  FIG. 10 is a diagram showing a light emission (energy transfer) mechanism of the organic EL element according to Example 6 and Comparative Examples 4 and 5. FIG. 10A is a diagram showing the energy relationship between the host material and the guest material of the light emitting layer 50 of the organic EL element according to Example 6. FIG. 10B is a diagram showing the energy relationship between the host material and the guest material of the light emitting layer 50 of the organic EL element according to Comparative Example 4. FIG. 10C is a diagram showing the energy relationship between the host material and the guest material of the light emitting layer 50 of the organic EL element according to Comparative Example 5.

10A to 10C, the singlet excited state S ₁ , the triplet excited state T _1, and the ground state S _{0 of} the host material and guest material constituting the light emitting layer 50 are shown, respectively. The energy relationship between the singlet excited state S ₁ and the triplet excited state T ₁ was obtained from the absorption spectra and emission spectra of various materials shown in FIG. FIG. 5 also describes an absorption spectrum m-PF-py Abs of m-PF-py, which is a guest material of the organic EL elements according to Example 6 and Comparative Examples 4 to 6.

As shown in FIG. 10B, when Ir (ppy) ₂ acac that is a phosphorescent material is used as a host material, the energy of the singlet excited state S ₁ of the guest material generates light emission of the host material. larger than the energy of the triplet excited state T _1, can not efficient energy transfer, emission of the host material is observed.

As shown in FIG. 10 (B), when using Bebq ₂ is a fluorescent material to the host material, the energy of the singlet excited state S ₁ of the guest material, singlet emitting host material excited state S ₁ Since it is larger than the energy of, efficient energy transfer is not possible and light emission of the host material is observed.

On the other hand, as shown in FIG. 10A, when Zn (BTZ) ₂ that is a fluorescent material is used for the host, the energy of the guest material in the singlet excited state S ₁ emits light from the host material. smaller than the energy of the excited state S _1, it is efficient energy transfer, good orange light can be obtained.

  In Example 6, an orange emission color is obtained, and in Examples 1 to 5, a red emission color is obtained. By selecting various colors of guest materials and using an appropriately combined host material, a desired color can be obtained. Can emit light in color.

Thus, it can be seen that various materials can be used for the metal complex material that is a host and the phosphorescent material that is a guest constituting the light-emitting layer 50 depending on the emission color. Here, Bebq ₂ and Zn (BTZ) ₂ shown in the examples are materials having an electron transporting property, whereas Ir (ppy) ₂ acac is a material having a hole transporting property. Therefore, it can be seen that the selectivity of the metal complex material used as the host material is very wide.

  Various materials can be used as the metal complex material constituting the host material of the light emitting layer 50 depending on the application. In this example, Ir, Be, and Zn complexes were used, but Pt, Os, Ru, Eu, Mg, Ti, V, Cr, Mn, Fe, Co, Ni, and Cu complexes can also be used. .

  Similarly, as the phosphorescent material constituting the guest material of the light emitting layer 50, various materials can be used depending on the application. In this example, an iridium complex and m-PF-py were used, but Pt, Os, Ru, and Eu complexes can also be used.

  In addition, the organic EL element demonstrated in this embodiment and the present Example can be comprised as a display apparatus by producing a display panel using this as a pixel and providing the drive circuit which drives a display panel.

  The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited to the above-described embodiments, and various modifications and substitutions can be made to the above-described embodiments without departing from the scope of the present invention. Can be added.

  The present invention can be used for an organic EL element and a display device using the same.

DESCRIPTION OF SYMBOLS 10 Substrate 20 ITO electrode 30 Hole injection layer 40 Hole transport layer 50 Light emitting layer 60 Electron transport layer 70 Electrode layer

Claims (8)

An organic electroluminescence device having a light emitting layer between a pair of electrodes,
The light emitting layer is composed only of a host material made of a metal complex and a guest material made of a phosphorescent material,
An organic electroluminescence element, wherein energy of an excited state emitted from the host material is higher than energy of a singlet excited state of the guest material. The host material is a phosphorescent material,
The organic electroluminescence device according to claim 1, wherein the energy in the excited state is energy in a triplet excited state.   The organic electroluminescence device according to claim 2, wherein the host material is an iridium complex. The host material is made of a fluorescent material,
The organic electroluminescence device according to claim 1, wherein the energy in the excited state is energy in a singlet excited state.   The organic electroluminescence device according to claim 4, wherein the host material is a beryllium complex or a zinc complex.   5. The organic electroluminescence device according to claim 1, wherein a mixing ratio of the guest material is 1% by weight or more and less than 6% by weight.   The organic electroluminescent element according to claim 1, wherein the emission wavelength of the guest material is longer than the emission wavelength of the host material. A display panel using the organic electroluminescence element according to any one of claims 1 to 7,
And a driving circuit for driving the display panel.

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