JP2011258373A - Organic el element and organic el display - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an organic EL element capable of transporting holes and electrons in good balance without decreasing the transportability, even when a host material in a light emitting layer is doped with a phosphorescent material.SOLUTION: The organic EL element 20 constitutes a laminate structure where a first light emitting layer 22 and a second light emitting layer 23 are laminated between a pair of electrodes 21 and 24. The first light emitting layer 22 includes a first material obtained by doping a first host material with a first phosphorescent material as a dopant, and the second light emitting layer 23 includes a second material obtained by doping a second host material with a second phosphorescent material as a dopant. The first material has a ratio of a hole mobility of the first material to the hole mobility of the first host material greater than a first predetermined value, and the second material has a ratio of an electron mobility of the second material to the electron mobility of the second host material greater than a second predetermined value.

Description

  The present invention relates to an organic EL element and an organic EL display having a stacked structure in which a first light emitting layer and a second light emitting layer are stacked between a pair of electrodes.

  An organic electroluminescent element (hereinafter referred to as “organic EL element”) has a laminated structure in which a light emitting layer made of an organic compound is laminated between a pair of electrodes made of an anode and a cathode. The organic EL element applies a voltage between the anode and the cathode, thereby injecting electrons and holes from the respective electrodes to the light emitting layer to recombine. The recombination causes the light emitting layer to be electrically connected. A light-emitting element that emits light when excited. The organic EL element is expected to be applied as a next-generation display element because it can emit light with high luminance even when driven at a low voltage, that is, it can emit light with high efficiency.

  From the viewpoint of practical use, the organic EL element is required to further reduce the drive voltage and further increase the light emission efficiency. One method for improving the luminous efficiency of the organic EL element is to use triplet excitons. In an organic EL device, holes and electrons are recombined in the light emitting layer, and singlet excitons and triplet excitons are generated in a statistical ratio of 1: 3, and then their energy is deactivated. It is thought to emit light. Therefore, the light emission efficiency can be improved by using triplet excitons for light emission. Fluorescence emits light by radiation deactivation from singlet excitons, and phosphorescence emits light by radiation deactivation from triplet excitons.

Therefore, research and development of organic EL elements using phosphorescent light emitting materials for the light emitting layer are being actively conducted. As a phosphorescent material suitable for an organic EL element, a complex mainly having a heavy metal such as iridium or platinum can be given. Typical green phosphorescent materials include Ir (ppy) ₃ (Tris (2-phenylpyridine) iridium (III); Tris (2-phenylpyridine) iridium), Ir (ppy) ₂ acac (Bis (2-phenylpyridine) (acetylacetonate) iridium (III); bis (2-phenylpyridine) iridium acetylacetonate) has been reported.

  In order to further improve the light emission efficiency of an organic EL device using such a phosphorescent material as a light emitting layer, the light emitting layer is divided into two light emitting layers, and one light emitting layer is injected with holes injected from the anode in the light emitting layer. An organic EL element having a light emitting layer functioning as a block layer for confinement in a light emitting layer is disclosed (for example, see Patent Document 1). In the example shown in Patent Document 1, it is described that a first light-emitting layer in which a host material is doped with a phosphorescent material and a second light-emitting layer in which a hole block material is doped with a phosphorescent material are provided.

  In addition, in order to efficiently emit light from the light-emitting layer, including when a phosphorescent material is used for the light-emitting layer, a light-emitting layer containing a material that can transport electrons and holes in a well-balanced manner, that is, a material and an electron excellent in hole transportability An organic EL element having a light emitting layer containing a material having excellent transportability is disclosed (for example, see Patent Document 2). In the example shown in Patent Document 2, the light-emitting layer is a layer containing a first compound and a second compound, and the first compound has a hole mobility higher than that of the second compound, and the second compound Describes that the electron mobility is higher than that of the first compound.

JP 2010-34484 A JP 2002-313583 A

  However, the organic EL element described above has the following problems.

  In the organic EL device described in Patent Document 1, when a light emitting layer doped with a phosphorescent material is used, the host material in which the host material doped with the phosphorescent material has no charge transporting property. In some cases, the charge transport property may be greatly reduced. Therefore, for example, an organic EL element having a light emitting layer containing a host material doped with a phosphorescent light emitting material has a higher driving voltage than an organic EL element having a light emitting layer made of a host material not doped with a phosphorescent light emitting material. There is a problem due to the charge transport property of the.

  Moreover, in the organic EL element described in Patent Document 2, the host material of the light emitting layer has a bipolar transport property that conducts both holes and electrons. However, when the host material has bipolar transport properties, doping the host material with a phosphorescent material may reduce the transport properties of one of the holes and electrons, and balance the holes and electrons. There is a problem that it cannot be transported well.

  The present invention has been made in view of the above points, and even if the host material in the light emitting layer is doped with a phosphorescent light emitting material, the hole and electron transport properties are not lowered, and the holes and electrons are balanced. An organic EL element that can be transported is provided.

  In order to solve the above problems, the present invention is characterized by the following measures.

  According to an embodiment of the present invention, in the organic EL element having a stacked structure in which a first light emitting layer and a second light emitting layer are stacked between a pair of electrodes, the first light emitting layer includes the first light emitting layer and the first light emitting layer. The host material includes a first material doped with a first phosphorescent material as a dopant, and the second light emitting layer includes a second host material doped with a second phosphorescent material as a dopant. The first material has a ratio of the hole mobility of the first material to the hole mobility of the first host material greater than a first predetermined value; The second material is provided with an organic EL element in which a ratio of the electron mobility of the second material to the electron mobility of the second host material is larger than a second predetermined value.

  According to the present invention, in the organic EL device, even if the host material in the light emitting layer is doped with a phosphorescent material, the hole and electron transport properties are not deteriorated, and the holes and electrons can be transported in a balanced manner. it can.

It is sectional drawing which shows schematic structure of the organic EL element which concerns on 1st Embodiment. It is sectional drawing which shows schematic structure of another example of the organic EL element which concerns on 1st Embodiment. It is a figure explaining the typical transient photocurrent waveform observed by measuring by TOF method. It is a graph which shows typically the electric field dependence of the mobility calculated | required from the transient photocurrent waveform about the host material which concerns on 1st Embodiment. It is an energy diagram which shows the level of the molecular orbital in a positive hole transport light emitting layer and an electron transport light emitting layer. It is sectional drawing which shows schematic structure of the organic electroluminescent display which concerns on 2nd Embodiment. It is a figure which shows the example of the data of the transient photocurrent waveform for measuring a hole mobility and an electron mobility about the element produced by the comparative example 1. FIG. It is a graph which shows the electric field dependence of the mobility calculated | required from the transient photocurrent waveform about the element produced by the comparative example 1. FIG. FIG. 4 is a diagram showing an example of transient photocurrent waveform data for measuring hole mobility and electron mobility with respect to the device manufactured according to Example 1. It is a graph which shows the electric field dependence of the mobility calculated | required from the transient photocurrent waveform about the element produced by Example 1. FIG. It is a figure which shows the example of the data of the transient photocurrent waveform for measuring a hole mobility and an electron mobility about the element produced by Example 2. FIG. It is a graph which shows the electric field dependence of the mobility calculated | required from the transient photocurrent waveform about the element produced by Example 2. FIG. 2 is an energy diagram showing molecular orbital levels in the light emitting layer of Example 1 and the light emitting layer of Example 2. FIG.

Next, a mode for carrying out the present invention will be described with reference to the drawings.
(First embodiment)
First, the configuration of the organic EL element according to the first embodiment will be described with reference to FIGS. 1 and 2. FIG. 1 is a cross-sectional view showing a schematic configuration of the organic EL element 20. FIG. 2 is a cross-sectional view showing a schematic configuration of another example (20a) of the organic EL element.

  As shown in FIG. 1, the organic EL element 20 includes an anode 21, a hole transporting light emitting layer 22, an electron transporting light emitting layer 23, and a cathode 24 on a substrate 11. The organic EL element 20 has a stacked structure in which a hole transporting light emitting layer 22 and an electron transporting light emitting layer 23 are stacked between an anode 21 and a cathode 24 which are a pair of electrodes.

  The anode 21 and the cathode 24 correspond to a pair of electrodes in the present invention. The hole transporting light emitting layer 22 corresponds to the first light emitting layer in the present invention. The electron transporting light emitting layer 23 corresponds to the second light emitting layer in the present invention.

  By applying a voltage between the anode 21 and the cathode 24, holes are injected from the anode 21 and electrons are injected from the cathode 24, and either one of the hole transporting light emitting layer 22 and the electron transporting light emitting layer 23. Recombination in the middle or both layers causes electroluminescence.

  A layer made of an organic film having a hole transporting property such as a hole injection layer, a hole transporting layer, or an electron blocking layer may be laminated between the anode 21 and the hole transporting light emitting layer 22. . Further, a layer made of an organic film having electron transport properties such as an electron injection layer, an electron transport layer, and a hole blocking layer may be laminated between the cathode 24 and the electron transport light emitting layer 23.

  A transparent substrate such as glass, quartz, or plastic film can be used as the material of the substrate 11.

  The anode 21 is provided with a pair of the cathode 24 and the light emitting layer 23 sandwiched from above and below so that a voltage can be applied. When the organic EL element 20 emits light, the anode 21 has a positive potential relative to the cathode 24. Is given. In the present embodiment, as shown in FIG. 1, the anode 21 is provided below the light emitting layer 23. The material of the anode 21 is preferably a transparent and highly conductive material, and a transparent electrode material such as ITO (Indium Tin Oxide) or IZO (registered trademark; Indium Zinc Oxide) can be used.

  The hole transporting light emitting layer 22 and the electron transporting light emitting layer 23 constitute a light emitting layer. That is, the light emitting layer in the present embodiment has a stacked structure of a hole transporting light emitting layer 22 formed on the anode 21 side and an electron transporting light emitting layer 23 formed on the cathode 24 side.

  The hole-transporting light-emitting layer 22 and the electron-transporting light-emitting layer 23 are made of a material obtained by doping a common host material with a different phosphorescent material as a dopant. That is, the hole transporting light emitting layer 22 includes a first material doped with a first phosphorescent light emitting material as a dopant in a common host material, and the electron transporting light emitting layer 23 is formed in a common host material. And a second material doped with a second phosphorescent material as a dopant.

  As will be described later, the ratio of the hole mobility of the first material to the hole mobility of the common host material is greater than the first predetermined value, and the second material relative to the electron mobility of the common host material. The ratio of the electron mobility is greater than the second predetermined value.

  In general, when a light-emitting layer doped with a phosphorescent material is used, the charge transport property of the host material doped with the phosphorescent material is higher than the charge transport property of the host material not doped with the phosphorescent material. Decrease significantly. However, in the present embodiment, the hole-transporting light-emitting layer 22 has a structure in which the decrease in hole-transporting property is relatively small when doped with the first phosphorescent material, and the electron-transporting light-emitting layer 23 Has a structure in which the decrease in the electron transport property is relatively small when the second phosphorescent material is doped.

  A common host material for the hole transporting light emitting layer 22 and the electron transporting light emitting layer 23 is composed of a hole transporting component and an electron transporting component. That is, the common host material for the hole transporting light emitting layer 22 and the electron transporting light emitting layer 23 includes a hole transporting material and an electron transporting material. Thus, the common host material is a bipolar transport material that exhibits both hole transport properties and electron transport properties.

  Further, in the hole transporting light emitting layer 22, the triplet energy of the hole transporting material and the electron transporting material constituting the common host material is larger than the triplet energy of the first phosphorescent light emitting material to be doped. Larger is preferred. Further, in the electron transporting light emitting layer 23, the triplet energy of the hole transporting material constituting the common host material and the electron transporting material is larger than the triplet energy of the doped second phosphorescent light emitting material. It is preferable.

  Note that the triplet energy of one material is larger than the triplet energy of the other material because the energy difference between the ground state and the excited triplet state of one material is different from that of the other material. It means larger than the energy difference from the term state.

  As a hole transporting material, for example, the formula (1)

で表されるＨＭＴＰＤを用いることができる。また、電子輸送性材料として、例えば式（２）

HMTPD represented by can be used. Moreover, as an electron transport material, for example, the formula (2)

で表されるＴＢＰｈＢを用いることができる。共通のホスト材料として、例えばＨＭＴＰＤを正孔輸送性材料とし、例えばＴＢＰｈＢを電子輸送性材料とするとき、例えば式（３）

TBPhB represented by can be used. As a common host material, for example, when HMTPD is a hole transporting material and TBPhB is an electron transporting material, for example, the formula (3)

で表されるＳＤＧＥ−１１を用いることができる。

SDGE-11 represented by can be used.

  As the first phosphorescent material, for example, the formula (4)

で表される、Ｉｒ（ｐｐｙ）_２ａｃａｃ構造を含むイリジウム錯体ＩｒＳＴを用いることができる。そして、ホスト材料に第１の燐光発光材料をドーパントとしてドープした材料を第１の材料とするとき、第１の材料として、例えば式（５）

An iridium complex IrST containing an Ir (ppy) ₂ acac structure represented by the following formula can be used. Then, when the first material is a material obtained by doping the host material with the first phosphorescent material as a dopant, the first material is, for example, the formula (5)

で表されるＳＴＧＥＲ−１１を用いることができる。

STGER-11 represented by can be used.

As will be described later, in addition to the iridium complex IrST containing the Ir (ppy) ₂ acac structure as the first phosphorescent material, the highest occupied molecular orbital level lower than the highest occupied molecular orbital level of the hole transporting material. By having molecular orbital levels, a material in which the ratio of the hole mobility of the first material to the hole mobility of the host material is larger than a predetermined value can be used.

  As the second phosphorescent material, for example, the formula (6)

で表されるＩｒＧ−３を用いることができる。そして、ホスト材料に第２の燐光発光材料をドーパントとしてドープした材料を第２の材料とするとき、第２の材料として、例えば式（７）

IrG-3 represented by the formula can be used. When the second material is a material in which the host material is doped with the second phosphorescent material as a dopant, the second material is, for example, the formula (7)

で表されるＳＴＧＥＴ−１１を用いることができる。

STGET-11 represented by can be used.

  Further, as will be described later, in addition to IrG-3, the second phosphorescent material has a lowest unoccupied molecular orbital level higher than the lowest unoccupied molecular orbital level of the electron transporting material. A material in which the ratio of the electron mobility of the second material to the electron mobility is larger than a predetermined value can be used.

  The hole transporting light emitting layer 22 is formed by co-evaporating a low molecular weight hole transporting material, a low molecular weight electron transporting material, and a low molecular weight phosphorescent light emitting material on the anode 21. Form. The hole transporting light emitting layer 22 may be formed by applying a liquid containing the material of the hole transporting light emitting layer 22 on the anode 21 by a method such as an ink jet method.

  The electron transporting light emitting layer 23 is formed by co-evaporating a low molecular weight hole transporting material, a low molecular weight electron transporting material, and a low molecular weight phosphorescent light emitting material on the hole transporting light emitting layer 22. To form. Further, the electron transporting light emitting layer 23 may be formed by applying a liquid containing the material of the electron transporting light emitting layer 23 onto the hole transporting light emitting layer 22 by a method such as an inkjet method.

  In the present embodiment, the holes injected from the anode 21 side and the electrons injected from the cathode 24 side recombine mainly at the interface between the hole transporting light emitting layer 22 and the electron transporting light emitting layer 23. Thus, excitons (excitons) are generated, and the generated excitons return to the ground state to emit fluorescence or phosphorescence.

  The cathode 24 is provided so as to be able to apply a voltage with a pair of the anode 21 and the light emitting layer 23 sandwiched from the upper and lower sides. When the organic EL element 20 emits light, the cathode 24 has a relatively negative potential. Is given. In the present embodiment, the cathode 24 is provided on the upper side of the light emitting layer 23 as shown in FIG. As the material of the cathode 24, an electrode material made of a metal having a relatively small work function such as Ca or Al can be used. By using a metal having a small work function, a barrier for injecting electrons from the cathode to the light emitting layer can be lowered, so that electrons can be easily injected from the cathode to the light emitting layer. However, a metal having a small work function easily reacts with oxygen and moisture in the air. Therefore, when the cathode 24 is made of a metal having a small work function, it is preferable to form the cathode 24 by laminating a metal such as Au on these metals.

  In the present embodiment, an example in which the hole transporting light emitting layer 22 is directly formed on the anode 21 will be described. However, various functional layers such as a hole injection layer may be formed between the anode 21 and the hole transporting light emitting layer 22. In FIG. 2, as the hole injection layer 21a between the anode 21 and the hole transporting light-emitting layer 22, for example, polyethylene dioxythiophene / polystyrene sulfonate (Poly (3,4-ethylenedioxythiophene) / Polystyrenesulfonate; PEDOT / An example in which PSS) is formed is shown. As a result, the voltage of the organic EL element 20a can be reduced, the light emission efficiency can be increased, the life can be extended, and the performance of the organic EL element 20a can be further improved.

  In the present embodiment, an example in which the cathode 24 is directly formed on the electron-transporting light-emitting layer 23 will be described. However, various functional layers such as an electron injection layer may be formed between the electron transporting light emitting layer 23 and the cathode 24. As a result, the voltage of the organic EL element can be reduced, the light emission efficiency can be increased, the life can be extended, and the performance of the organic EL element can be further improved.

  Next, referring to FIGS. 3 to 5, even when the organic EL element according to the present embodiment is doped with a phosphorescent material in the host material in the light emitting layer, the hole and electron transport properties are not lowered, The effect of transporting holes and electrons in a balanced manner will be described.

  FIG. 3 is a diagram for explaining a typical transient photocurrent waveform observed by measurement by the Time-of-Flight method (hereinafter referred to as “TOF method”). FIG. 4 is a graph schematically showing the electric field dependence of the mobility obtained from the transient photocurrent waveform for the host material according to the present embodiment. FIG. 5 is an energy diagram showing the level of molecular orbitals in the hole transporting light emitting layer 22 and the electron transporting light emitting layer 23. FIG. 5A shows the hole-transporting light-emitting layer 22, and FIG. 5B shows the electron-transporting light-emitting layer 23.

  The charge transport property of each material such as the light emitting layer of the organic EL element is evaluated by measuring the mobility. Mobility is a value indicating the rate of movement of electric charge with respect to electric field strength, and is an important characteristic for estimating the conduction mechanism and an important characteristic for determining the driving voltage of the organic EL element. As a method for measuring mobility, there is a TOF method.

In the TOF method, a circuit capable of applying a voltage to a pair of electrodes is formed in a state where a thin film as a test substance is sandwiched between the pair of electrodes. Then, an electric field is applied to the thin film by applying a voltage between the pair of electrodes by a power source included in the circuit. Then, by irradiating a light pulse with a voltage applied to the thin film, a pair consisting of holes and electrons is generated in the thin film, and the holes and electrons generated by the electric field are separated. The separated charge of one of the hole and electron immediately disappears at the neighboring electrode, but the other charge of the hole and electron moves toward the opposite electrode. While the charge is moving toward the electrode, a constant transient photocurrent flows through the circuit, and the current value decreases as the charge reaches the electrode. Therefore, the mobility can be obtained by measuring the time required for the electric charge to move through the thin film from the time when the current flows, and dividing the film thickness of the thin film by the product of the time and the electric field strength. That is, when the mobility is μ, the time required for the charge to move through the thin film is t, the thin film thickness is d, and the voltage applied to the thin film is V, the following formula (8)
μ = d ² / (t · V) (8)
Can be obtained.

  In addition, the mobility measurement by the TOF method is greatly influenced by whether or not a charge trap that traps the moving charge exists in the thin film. When there is no charge trap in the thin film, the observed transient photocurrent has a waveform called a non-dispersion type as shown in FIG. 3A, and a time during which a constant transient photocurrent flows (for example, FIG. 3 ( The mobility can be obtained based on the time t1) of a). On the other hand, when a charge trap exists in the thin film, the charge is trapped or moved while being scattered, so that the observed transient photocurrent has a waveform called a dispersion type as shown in FIG. The mobility cannot be obtained easily.

  In the present embodiment, regarding the mobility measured by such a TOF method, the hole-transporting light-emitting layer 22 has a structure in which the decrease in hole-transporting property is relatively small when doped with the first phosphorescent material. Thus, the electron-transporting light-emitting layer 23 has a relatively small decrease in electron-transporting property when doped with the second phosphorescent material.

  4A, in the hole-transporting light-emitting layer 22, the electron mobility of the material (first material) in which the host material is doped with the phosphorescent material is doped with the phosphorescent material. It is lower than the electron mobility of the non-host material. However, as indicated by the solid line in FIG. 4A, the hole mobility of the material in which the host material is doped with the phosphorescent material (first material) is that of the host material not doped with the phosphorescent material. It does not decrease much with respect to hole mobility. That is, the ratio of the hole mobility of the first material to the hole mobility of the first host material is larger than the first predetermined value.

  4B, in the electron-transporting light-emitting layer 23, the hole mobility of the material (second material) in which the host material is doped with the phosphorescent material is the phosphorescent material. It is lower than the hole mobility of the host material not doped with. However, as indicated by the dotted line in FIG. 4B, the electron mobility of the material (second material) in which the host material is doped with the phosphorescent material is the electron mobility of the host material not doped with the phosphorescent material. It does not decrease much with respect to mobility. That is, the ratio of the electron mobility of the second material to the electron mobility of the second host material is larger than the second predetermined value.

  Here, when the host material is doped with a material different from the host material as a dopant, the hole mobility and electron mobility of the doped material are usually smaller than the hole mobility and electron mobility of the host material, respectively. Become. However, when the host material is doped with a material different from the host material as a dopant, the hole mobility and electron mobility of the doped material are 1 to 1 for the hole mobility and electron mobility of the host material, respectively. When the voltage drops below / 10, the drive voltage must be increased, and the drive voltage cannot be reduced. Therefore, it is preferable to prevent the hole mobility and electron mobility of the doped material from decreasing to 1/10 of the hole mobility and electron mobility of the host material, respectively. Accordingly, the ratio of the hole mobility of the first material to the hole mobility of the first host material is preferably greater than 0.1 and less than 1, and the first mobility relative to the electron mobility of the second host material is The ratio of the electron mobility of the material 2 is preferably larger than 0.1 and smaller than 1.

Next, the first phosphorescent material is the highest occupied molecular orbital level (Highest) of the hole transporting material.
By having the highest occupied molecular orbital level lower than Occupied Molecular Orbital (HOMO), the ratio of the hole mobility of the first material to the hole mobility of the host material is larger than the first predetermined value. This will be explained. In addition, since the second phosphorescent material has a lowest unoccupied molecular orbital level (LUMO) higher than the lowest unoccupied molecular orbital level (LUMO) of the electron-transporting material, The fact that the ratio of the electron mobility of the second material becomes larger than a predetermined value will be described.

  The holes conduct HOMO of the hole transporting material, and the electrons conduct LUMO of the electron transporting material. Therefore, when the HOMO of the phosphorescent light emitting material has higher energy than the HOMO of the hole transporting material (when it is above in FIG. 5), the holes are trapped by the HOMO of the phosphorescent light emitting material, so that the hole transportability is high. It is thought to decline. On the other hand, when the LUMO of the phosphorescent material is lower in energy than the LUMO of the electron transporting material (when it is below in FIG. 5), the electrons are trapped in the LUMO of the phosphorescent material, and thus the electron transporting property is considered to decrease. It is done.

  In the present embodiment, in the material of the hole transporting light emitting layer 22, as shown in FIG. 5A, the LUMO of the phosphorescent light emitting material has lower energy than the LUMO of the electron transporting material. It is considered that the electron transporting property is lowered by being trapped by the LUMO of the light emitting material. However, since the HOMO of the phosphorescent material is not higher in energy than the HOMO of the hole transporting material, the holes are not trapped by the HOMO of the phosphorescent material, and the hole transporting property is considered not to deteriorate so much.

  In the present embodiment, in the material of the electron transporting light emitting layer 23, as shown in FIG. 5B, the HOMO of the phosphorescent light emitting material has higher energy than the HOMO of the hole transporting material. It is considered that the holes are trapped by HOMO of the phosphorescent material and the hole transport property is lowered. However, since the LUMO of the phosphorescent material is not lower in energy than the LUMO of the electron transporting material, the electrons are not trapped by the LUMO of the phosphorescent material, and the electron transporting property is considered not to deteriorate much.

  In the present embodiment, the hole transporting light emitting layer 22 and the electron transporting light emitting layer 23 use a host material having bipolar transportability. Thereby, holes injected from the anode 21 can move in the hole transporting light emitting layer 22 with high mobility, and electrons injected from the cathode 24 can move in the electron transporting light emitting layer 23 with high mobility. . In addition to the interface between the hole-transporting light-emitting layer 22 and the electron-transporting light-emitting layer 23, holes and electrons are recombined in the hole-transporting light-emitting layer 22 or the electron-transporting light-emitting layer 23. Can emit light.

  When triplet excitons are present at a high density, the concentration of triplet excitons may increase, so that phosphorescence may be quenched, so-called concentration quenching may occur. However, in the present embodiment, triplet excitons are not limited to the interface between the hole-transporting light-emitting layer 22 and the electron-transporting light-emitting layer 23, but also in the hole-transporting light-emitting layer 22 or the electron-transporting light-emitting layer 23. It is also present inside and can emit phosphorescence, so that concentration quenching hardly occurs and the light emission efficiency can be improved.

  Further, by adjusting the amount of the phosphorescent light emitting material that does not decrease the hole transporting property and the electron transporting property to the common host material, the positive transport property in each of the hole transporting light emitting layer 22 and the electron transporting light emitting layer 23 is adjusted. The hole mobility and electron mobility can be freely adjusted. Thereby, the freedom degree of design of an organic EL element, such as the film thickness of the positive hole transport light emitting layer 22 and the electron transport light emitting layer 23, can be improved, for example.

  Further, by using a common host material doped with a phosphorescent material that does not decrease the hole transporting property and the electron transporting property as the host material for the hole transporting light emitting layer 22 and the electron transporting light emitting layer 23, respectively. Degradation of characteristics at the interface between the hole-transporting light-emitting layer 22 and the electron-transporting light-emitting layer 23 can be reduced. This is because, for example, an interface state that traps charges at the interface is less likely to occur, and accumulation of charges at the interface can be prevented.

  As described above, according to the present embodiment, a common host material having a bipolar transport property is used, and the common host material is doped with a phosphorescent material that does not decrease the hole transport property and the electron transport property. The hole transporting light emitting layer 22 and the electron transporting light emitting layer 23 are formed. Thereby, even if the host material in the light emitting layer is doped with a phosphorescent light emitting material, the hole and electron transport properties are not lowered, and the holes and electrons can be transported in a balanced manner.

In the present embodiment, the example in which the hole-transporting light-emitting layer 22 and the electron-transporting light-emitting layer 23 have a common host material has been described. However, the host material of the hole transporting light emitting layer 22 and the host material of the electron transporting light emitting layer 23 may be different. At this time, the host material of the hole transporting light emitting layer 22 and the host material of the electron transporting light emitting layer 23 correspond to the first host material and the second host material in the present invention, respectively. The ratio of the hole mobility of the first material to the hole mobility of the first host material is greater than the first predetermined value, and the ratio of the second material to the electron mobility of the second host material is The ratio of electron mobility is larger than the second predetermined value. Further, when the first host material is a material doped with the first phosphorescent light emitting material as a dopant, the highest occupied molecular orbital level of the hole transporting material is used as the first phosphorescent light emitting material. By having the highest occupied molecular orbital level lower than the position, the ratio of the hole mobility of the first material to the hole mobility of the first host material can be larger than a predetermined value. . Further, when the second host material is a material doped with the second phosphorescent light emitting material as a dopant, the second phosphorescent light emitting material is used as the second phosphorescent light emitting material from the lowest unoccupied molecular orbital level of the electron transport material In addition, since the lowest unoccupied molecular orbital level is higher, the ratio of the electron mobility of the second material to the electron mobility of the second host material can be larger than a predetermined value.
(Second Embodiment)
Next, the configuration of the organic EL display according to the second embodiment will be described with reference to FIG. FIG. 6 is a cross-sectional view illustrating a schematic configuration of the organic EL display 10.

  As shown in FIG. 6, the organic EL display 10 has a plurality of pixels 20 made of organic EL elements on a display substrate 11. The pixel 20 is the organic EL element 20 according to the first embodiment. The pixel 20 is a pair of electrodes. That is, the pixel 20 includes an anode 21, a hole transporting light emitting layer 22, an electron transporting light emitting layer 23, and a cathode 24. And it has the laminated structure by which the positive hole transport light emitting layer 22 and the electron transport light emitting layer 23 were laminated | stacked between the anode 21 and the cathode 24. FIG.

  Further, the pixels 20 are formed in a pixel region 20 b defined by the partition walls 12 on the display substrate 11. Therefore, the anode 21, the hole transporting light emitting layer 22, the electron transporting light emitting layer 23, and the cathode 24 in each pixel 20 are separated from each other by the partition 12, and the anode 21, the hole transporting light emitting layer 22, and the electron transport in the adjacent pixel 20. The light emitting layer 23 and the cathode 24 are separated from each other.

  As a material for the display substrate 11, a transparent substrate such as glass or plastic film can be used. In addition, an active matrix circuit (not shown) using a transistor element made of, for example, a thin film transistor (TFT) is formed in the lower portion of the anode 21 or adjacent to the anode 21 in the display substrate 11. May be. When the active matrix circuit is formed, each pixel 20 can be actively driven, and can display a high-definition image with excellent response characteristics.

  As described above, the partition 12 partitions the pixel region 20b where the pixels 20 are formed. The material of the partition wall 12 is not particularly limited as long as it is an insulating material and does not dissolve in the solvent in the ink applied by the inkjet method or the like.

  The anode 21 is provided so that a voltage can be applied with a pair of the cathode 24 and the hole transporting light emitting layer 22 and the electron transporting light emitting layer 23 sandwiched from above and below, and when the organic EL element 20 emits light, A relatively positive potential is applied to the cathode 24.

  The organic EL display 10 according to the present embodiment is capable of multicolor display by coating the pixel regions 20b with different materials for the hole transporting light emitting layer 22 and the electron transporting light emitting layer 23 that emit different colors. is there.

  The organic EL display 10 according to the present embodiment includes the organic EL element 20 according to the first embodiment as a pixel 20. Also in this embodiment, the organic EL element 20 constituting the pixel 20 uses a common host material having a bipolar transport property, and does not lower the hole transport property and the electron transport property in the common host material, respectively. The hole-transporting light-emitting layer 22 and the electron-transporting light-emitting layer 23 are made of a material doped with a phosphorescent material. Thereby, even if the host material in the light emitting layer is doped with a phosphorescent light emitting material, the hole and electron transport properties are not lowered, and the holes and electrons can be transported in a balanced manner.

  In this embodiment as well, as in the first embodiment, the host material of the hole-transporting light-emitting layer 22 and the host material of the electron-transporting light-emitting layer 23 may be the same or different. It may be a thing.

Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not construed as being limited to the examples.
Example 1
First, as Example 1, for measuring the mobility of the material constituting the hole-transporting light-emitting layer in the organic EL device according to the first embodiment (first material in the present invention) by the TOF method. The device was fabricated.

  An anode was formed by forming a conductive film made of ITO having a thickness of 150 nm on a glass substrate having a thickness of 0.7 mm by sputtering.

Next, on the anode, HMTPD which is a hole transporting material, TBPhB which is an electron transporting material, and an iridium complex IrST which includes an Ir (ppy) ₂ acac structure which is a first phosphorescent material are formed by co-evaporation. As a result, a 50 nm-thick hole transporting light-emitting layer made of STGER-11 was formed. Further, HMTPD: TBPhB: IrST = 47.5: 47.5: 5 in weight ratio.

  Next, a conductive film made of Cs having a thickness of 1 nm is formed on the hole-transporting light emitting layer by vapor deposition, and further a conductive film made of Al having a thickness of 100 nm is formed thereon by vapor deposition. A cathode was formed. Thereby, an element according to Example 1 was manufactured.

  Further, as Comparative Example 1, an element for measuring mobility by a TOF method was prepared for a common host material in the organic EL element according to the first embodiment.

  Similarly to Example 1, an anode was formed by forming a conductive film made of ITO having a thickness of 150 nm on a glass substrate having a thickness of 0.7 mm by sputtering.

  Next, HMTPD as a hole transporting material and TBPhB as an electron transporting material were formed on the anode by co-evaporation to form a light emitting layer having a thickness of 50 nm made of SDGE-11. Moreover, it was set as HMTPD: TBPhB = 50: 50 by weight ratio.

  Next, a conductive film made of Cs having a thickness of 1 nm was formed on the light emitting layer by vapor deposition, and a conductive film made of Al having a thickness of 100 nm was formed thereon by vapor deposition, thereby forming a cathode. . Thereby, an element according to Comparative Example 1 was produced.

  About each of the element produced by Example 1 and Comparative Example 1, the mobility was measured by TOF method.

  First, with reference to FIGS. 7 and 8, the results of the element manufactured according to Comparative Example 1 will be described. FIGS. 7A and 7B are diagrams showing examples of transient photocurrent waveform data for measuring the hole mobility and the electron mobility, respectively, for the device manufactured according to Comparative Example 1. FIG. FIG. 8 is a graph showing the electric field dependence of the mobility obtained from the transient photocurrent waveform for the element manufactured according to Comparative Example 1.

As shown in FIGS. 7 (a) and 7 (b), the transient photocurrent waveform in the device according to Comparative Example 1 that does not include a phosphorescent material is good in both hole mobility measurement and electron mobility measurement. Non-distributed type is shown. And as shown in FIG. 8, hole mobility shows the value of a 10 ^<-4 > cm < ² > / Vs order, and electron mobility shows the value of about 10 ^<-3 > cm < ² > / Vs. Therefore, the host material (SDGE-11) made of HMTPD and TBPhB has a bipolar transport property capable of conducting both holes and electrons with a high transport property.

  Next, with reference to FIGS. 9 and 10, the results of the element manufactured according to Example 1 will be described. FIG. 9A and FIG. 9B are diagrams showing examples of transient photocurrent waveform data for measuring hole mobility and electron mobility, respectively, for the device manufactured according to Example 1. FIG. FIG. 10 is a graph showing the electric field dependence of the mobility obtained from the transient photocurrent waveform for the element manufactured according to Example 1.

As shown in FIG. 9A, the transient photocurrent waveform in the device according to Example 1 including the phosphorescent material shows a good non-dispersion type in the hole mobility measurement. And as shown in FIG. 10, hole mobility shows the value of a 10 ^<-4 > cm < ² > / Vs order.

  On the other hand, as shown in FIG. 9B, the transient photocurrent waveform shows a dispersion type in electron mobility measurement, and it is considered that the electron transport property is greatly disturbed. For this reason, the value of the electron mobility could not be obtained.

Comparing FIG. 8 with FIG. 10, the electron mobility (STGER-11) of a material (STGER-11) in which a host material made of HMTPD and TBPhB is doped with a phosphorescent material made of an iridium complex IrST containing an Ir (ppy) ₂ acac structure In Example 1), the value cannot be obtained, and it is considered that the electron transport property is greatly disturbed. On the other hand, the hole mobility (Example 1) for a material (STGER-11) doped with a phosphorescent material made of an iridium complex IrST containing an Ir (ppy) ₂ acac structure is a host material that is not doped with a phosphorescent material (Example 1). It is only reduced to about 1/10 of the hole mobility for SDGE-11) (Comparative Example 1). Therefore, a material (STGER-11) in which a host material made of HMTPD and TBPhB is doped with a phosphorescent light-emitting material made of an iridium complex IrST containing an Ir (ppy) ₂ acac structure (STGER-11) has a higher electron content than an undoped material (SDGE-11). Although the transportability is greatly lowered, the hole transportability is hardly lowered, and it is confirmed that the high transportability is maintained.
(Example 2)
Next, as Example 2, for the material (second material in the present invention) constituting the electron transporting light emitting layer in the organic EL device according to the first embodiment, the mobility is measured by the TOF method. An element was produced.

  Similarly to Example 1, an anode was formed by forming a conductive film made of ITO having a thickness of 150 nm on a glass substrate having a thickness of 0.7 mm by sputtering.

  Next, on the anode, HMTPD which is a hole transport material, TBPhB which is an electron transport material, and IrG-3 which is a second phosphorescent material are formed by co-evaporation to form STGET-11. An electron transporting light emitting layer having a thickness of 50 nm was formed. Further, HMTPD: TBPhB: IrG-3 = 47.5: 47.5: 5 in weight ratio.

  Next, a conductive film made of Cs having a thickness of 1 nm is formed on the electron-transporting light-emitting layer by vapor deposition, and further a conductive film made of Al having a thickness of 100 nm is formed thereon by vapor deposition. Formed. Thereby, an element according to Example 2 was manufactured.

  About the element produced by Example 2, the mobility was measured by TOF method.

  With reference to FIG. 11 and FIG. 12, the result of the element manufactured by Example 2 will be described. FIG. 11A and FIG. 11B are diagrams showing examples of transient photocurrent waveform data for measuring hole mobility and electron mobility, respectively, for the device manufactured according to Example 2. FIG. FIG. 12 is a graph showing the electric field dependence of the mobility obtained from the transient photocurrent waveform for the element manufactured according to Example 2.

As shown in FIG. 11 (a) and FIG. 11 (b), the transient photocurrent waveform in the element according to Example 2 containing the phosphorescent material is good in both hole mobility measurement and electron mobility measurement. Decentralized type is shown. Then, as shown in FIG. 12, the hole mobility has a value of the order of 10 ⁻⁶ cm ² / Vs, and the electron mobility has a value of the order of 10 ⁻⁴ cm ² / Vs.

  Comparing FIG. 8 with FIG. 12, the hole mobility (Example 2) for the material (STGET-11) in which the host material made of HMTPD and TBPhB is doped with the phosphorescent light emitting material made of IrG-3 is phosphorescent. It decreases to about 1/100 of the hole mobility (Comparative Example 1) for the host material (SDGE-11) not doped with the material. On the other hand, the electron mobility (Example 2) about the material (STGET-11) doped with the phosphorescent material made of IrG-3 is the electron mobility about the host material (SDGE-11) not doped with the phosphorescent material (SDGE-11). Compared to Comparative Example 1), it is only reduced to about 1/10. Therefore, although the material (STGET-11) in which the host material made of HMTPD and TBPhB is doped with the phosphorescent light emitting material made of IrG-3 is significantly less in hole transportability than the material not doped (SDGE-11). As a result, it was confirmed that the electron transporting property hardly deteriorated and the high transporting property was maintained.

  Next, the material for the hole-transporting light-emitting layer (STGER11) in the device manufactured according to Example 1 and the material for the electron-transporting light-emitting layer (STGET11) in the device manufactured according to Example 2 were obtained by atmospheric photoelectron spectroscopy. The result of having measured the level of HOMO and LUMO is shown. FIGS. 13A and 13B are energy diagrams showing molecular orbital levels in the light emitting layer of Example 1 and the light emitting layer of Example 2, respectively. In FIGS. 13A and 13B, the vacuum level is shown as a reference (0 eV).

  Atmospheric photoelectron spectroscopy was performed using an atmospheric photoelectron spectrometer (trade name AC-3) manufactured by Riken Keiki Co., Ltd. LUMO was determined by subtracting the light absorption edge energy from HOMO.

As shown in FIG. 13A, in the hole transporting light emitting layer material (STGER11), LUMO (−3.0 eV) of an iridium complex IrST containing an Ir (ppy) ₂ acac structure, which is a phosphorescent light emitting material. Has lower energy than LUMO (−2.9 eV) of TBPhB which is an electron transporting material. In addition, the HOMO (−5.6 eV) of the iridium complex IrST including the Ir (ppy) ₂ acac structure, which is a phosphorescent material, has energy higher than the HOMO (−5.6 eV) of HMTPD, which is a hole transporting material. not high. Thereby, although electron transport property falls, it is thought that hole transport property does not fall so much.

On the other hand, as shown in FIG. 13B, in the electron transporting light emitting layer material (STGET11), HOMO (−5.4 eV) of IrG-3 which is a phosphorescent light emitting material is a hole transporting material. Energy is higher than HOMO (−5.6 eV) of HMTPD. Further, the LUMO (−2.8 eV) of IrG-3 which is a phosphorescent material is not lower in energy than the LUMO (−2.9 eV) of TBPhB which is an electron transport material. Thereby, although hole transport property falls, it is thought that electron transport property does not fall so much.
(Example 3)
Next, as Example 3, the organic EL element described with reference to FIG. 2 in the first embodiment was manufactured.

  As the substrate 11, a glass substrate having a substrate thickness of 0.7 mm was used. After ultrasonically cleaning the substrate 11 made of a glass substrate in pure water, a conductive film made of ITO having a thickness of 150 nm was formed on the substrate 11 by sputtering. Then, using the resist pattern formed by photolithography as a mask, the conductive film is patterned by wet etching using a transparent conductive film etching solution (trade name ITO-07N) manufactured by Kanto Chemical Co., Inc., thereby forming an anode 21 made of ITO. did.

  Next, PEDOT / PSS (trade name Clevios P VP CH8000) manufactured by HC Starck is applied as a hole injection layer 21a on the anode 21 by a spin coating method, and dried at 180 ° C. for 1 hour in the atmosphere. As a result, a hole injection layer 21a having a thickness of 30 nm was formed.

Next, an iridium complex IrST containing an HMTPD, TBPhB, and Ir (ppy) ₂ acac structure is formed on the hole injection layer 21a by co-evaporation to form a hole transporting light emitting layer 22 having a thickness of 50 nm. did.

  Next, HMTPD, TBPhB, and IrG-3 were deposited on the hole-transporting light-emitting layer 22 by co-evaporation to form an electron-transporting light-emitting layer 23 having a thickness of 50 nm.

  Next, a conductive film made of Cs having a thickness of 1 nm is formed on the electron-transporting light emitting layer 23 by vapor deposition, and a conductive film made of Al having a thickness of 100 nm is further formed thereon by vapor deposition. A cathode 24 was formed. As a result, the organic EL element described with reference to FIG. 2 in the first embodiment could be formed.

  The preferred embodiments of the present invention have been described above. However, the present invention is not limited to such specific embodiments, and various modifications can be made within the scope of the gist of the present invention described in the claims. Can be modified or changed.

DESCRIPTION OF SYMBOLS 10 Organic EL display 11 Display board | substrate 12 Partition 20 Pixel 20, 20a Organic EL element
20b Pixel region 21 Anode 21a Hole injection layer 22 Hole transporting light emitting layer 23 Electron transporting light emitting layer 24 Cathode

Claims (6)

In an organic EL element having a stacked structure in which a first light emitting layer and a second light emitting layer are stacked between a pair of electrodes,
The first light emitting layer includes a first material doped with a first phosphorescent light emitting material as a dopant in a first host material,
The second light-emitting layer includes a second material doped with a second phosphorescent light-emitting material as a dopant in a second host material,
The first material has a ratio of hole mobility of the first material to hole mobility of the first host material larger than a first predetermined value,
The second material is an organic EL element in which a ratio of an electron mobility of the second material to an electron mobility of the second host material is larger than a second predetermined value.   2. The organic EL element according to claim 1, wherein each of the first host material and the second host material includes a hole transporting material and an electron transporting material.   2. The organic EL element according to claim 1, wherein each of the first host material and the second host material includes the same hole transporting material and the same electron transporting material. In the first material, the first phosphorescent material has a highest occupied molecular orbital level lower than the highest occupied molecular orbital level of the hole transporting material contained in the first host material. The ratio of the hole mobility of the first material to the hole mobility of the first host material is greater than the first predetermined value,
In the second material, the second phosphorescent material has a lowest unoccupied molecular orbital level higher than a lowest unoccupied molecular orbital level of the electron transporting material included in the second host material. 4. The organic EL element according to claim 2, wherein a ratio of an electron mobility of the second material to an electron mobility of the second host material is larger than the second predetermined value. 5. . The first predetermined value is 0.1;
The organic EL element according to claim 1, wherein the second predetermined value is 0.1.   An organic EL display comprising the organic EL element according to any one of claims 1 to 5.

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