JP2007305783A - Light emitting element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a light emitting element which has a good emission efficiency and has a stable luminescent color. <P>SOLUTION: The light emitting element consists of a light emission layer containing a plurality of phosphorescence materials having different triplet energies between a first electrode and a second electrode. Out of the plurality of phosphorescence materials used for forming the light emission layer, the emission life of the high-energy phosphorescence materials having a high triplet energy is longer than that of the low-energy phosphorescence materials having a low triplet energy. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a light-emitting element using a plurality of phosphorescent materials.

  Organic electroluminescence (hereinafter referred to as organic EL) elements are expected to be applied to display devices such as televisions, personal computers and portable terminals, backlights used in these display devices, and lighting equipment. For this reason, the development of a technique for driving the organic EL element at a low voltage and various techniques for improving the light emission efficiency of the organic EL element are in progress.

  The organic EL element has a structure in which an organic layer is formed between a transparent electrode (anode) such as ITO (indium-tin oxide) and a metal electrode (cathode) such as magnesium. The organic layer includes a light emitting material (light emitting layer), and a voltage is applied to the light emitting material via a transparent electrode and a metal electrode.

  When a voltage is applied between the transparent electrode and the metal electrode, holes are injected from the transparent electrode and electrons are injected from the metal electrode, and the electrons and holes are recombined with the light emitting material to emit light. In this case, in order to improve the luminous efficiency, an electron transport material (electron transport layer), a hole transport material (hole transport layer), or the like may be provided between the transparent electrode and the metal electrode. .

  When the organic EL element is used in, for example, a backlight or a lighting device, the emission color is preferably white. In this case, in order to obtain white light emission, the light emitting element is configured so that the light emission covers the visible light region as widely as possible. For example, as an example of a method for forming an organic layer of a light-emitting element having a white emission color, there is a method in which a material exhibiting red, green, and blue emission colors is stacked.

  However, when the organic layer has a structure having multiple light emitting layers as described above, a trial and error process of repeating the light emission experiment by changing the thickness of each light emitting layer is required before obtaining white light. There was a problem of cost and time for development to obtain.

  In addition, in a light-emitting element having a multi-layered light-emitting layer, when the driving current is changed, the recombination region of electrons and holes moves, so that the emission color changes greatly and white light emission cannot be obtained. There was a problem. As described above, the method of obtaining white light emission by laminating light emitting layers having different light emission colors has various problems.

  Therefore, a method for obtaining white light emission by mixing a plurality of light emitting materials to form a light emitting layer has been proposed. For example, the emission spectrum can be changed by a guest mixed with the light emitting layer host, and the emission color can be controlled using a plurality of host materials.

  The light emitting layer formed by such a method is composed of a single layer having substantially uniform light emission characteristics as long as the mixed concentration of the guest to the host is uniform throughout the light emitting layer. For this reason, even if the recombination region of electrons and holes moves in response to changes in the drive current, the change in the emission spectrum is smaller than in the multilayer emission layer, and the emission color tends to be stable (for example Reference 1).

  In recent years, phosphorescent materials have been used in place of conventional fluorescent materials in order to improve the luminous efficiency of organic EL elements. Since conventional fluorescent materials utilize fluorescence, which is a radiative transition from a singlet excited state to a singlet ground state of an organic compound, the theoretical external quantum efficiency (the number of emitted photons to the outside relative to the number of injected electrons). As a percentage), 5% is the limit.

On the other hand, the phosphorescent material has a feature that the emission efficiency is good because phosphorescence, which is a radiation transition from a triplet excited state to a singlet ground state, is used (for example, see Patent Document 2).
J. Kido, H. Shiyonoya, and K. Nagai, Appl. Phys. Lett. 67, 2281 (1995) MABaldo, S. Lamansky, PEBurrows, METhompson, and SRForrest, Appl. Phys. Lett. 75, 4 (1999)

  However, when phosphorescent materials having different triplet energies (different emission spectra) are mixed, energy transfer occurs between the materials, and this energy transfer affects the emission color. Furthermore, when phosphorescent materials are mixed, there is a problem that triplet-triplet annihilation occurs at the same time when the drive current is increased in addition to the above energy transfer. In this case, since the degree of decrease in luminous efficiency due to triplet-triplet annihilation varies depending on the phosphorescent material, it is difficult to control the emission color in the light emitting layer in which the phosphorescent material is mixed. It was happening.

  As a result, for example, in the above-described light emitting element, there arises a new problem that the emission color changes when the drive current is changed. As described above, it has been difficult to obtain a stable emission color (for example, white) by mixing phosphorescent materials.

  Therefore, in the present invention, it is a general object to provide a new and useful light-emitting element that solves the above problems.

  A specific problem of the present invention is to provide a light-emitting element that has good luminous efficiency and stable emission color.

  The present invention provides a light-emitting element in which an organic layer including a plurality of phosphorescent materials having different triplet energies is formed between a first electrode and a second electrode. The light emission lifetime of a high energy phosphorescent material having a high triplet energy among a plurality of phosphorescent materials used for formation is solved by a light emitting element characterized by being longer than that of a low energy phosphorescent material having a low triplet energy. .

  According to the present invention, it is possible to provide a light emitting device that has good light emission efficiency and stable emission color.

  Further, when the plurality of phosphorescent materials are mixed and diffused in the organic layer, the emission color becomes more stable.

  In the organic layer, energy transfer from the high energy phosphorescent material to the low energy phosphorescent material occurs, and the emission lifetimes of light emission corresponding to the plurality of phosphorescent materials in the organic layer are the same. The emission color becomes more stable.

  In addition, when the organic layer includes an organic material involved in energy transfer from the high energy phosphorescent material to the low energy phosphorescent material, the emission color can be easily controlled.

  Further, when the triplet energy of the organic material is larger than the low energy phosphorescent material and smaller than the high energy phosphorescent material, the emission color can be easily controlled.

  Further, when the organic material is made of an electron transport material for transporting electrons or a hole transport material for transporting holes, the emission color can be easily controlled and the light emission efficiency is improved.

  Further, when the organic material is mixed with each of the plurality of phosphorescent materials and diffused in the organic layer, the emission color is further stabilized.

  According to the present invention, it is possible to provide a light emitting device that has good light emission efficiency and stable emission color.

  Phosphorescent materials used as light-emitting materials for light-emitting elements use phosphorescence, which is a radiative transition from a triplet excited state to a singlet ground state, and therefore have better luminous efficiency than conventional fluorescent materials. It has certain characteristics. On the other hand, when a plurality of phosphorescent materials are mixed to obtain a desired emission color, energy transfer occurs between the plurality of phosphorescent materials, and triplet-triplet annihilation occurs. Since the degree of the decrease in luminous efficiency is different for each of a plurality of materials, there is a problem that the control of the emission color is complicated and difficult.

  Therefore, as a result of intensive studies by the inventors of the present invention, by using the effect of energy transfer between a plurality of phosphorescent materials, the influence of the variation in emission color due to the difference in triplet-triplet annihilation of the phosphorescent material can be considered. It was found that it is possible to form a light-emitting element that suppresses light emission and has good emission efficiency and stable emission color.

  Hereinafter, the principle for constructing a light-emitting element that has good luminous efficiency and can obtain a stable emission color will be described.

The dependence of the external quantum efficiency η _{TT on} the current density J due to triplet-triplet annihilation in phosphorescent materials is shown by the following formula (1) (MABaldo, C. Adachi, and SRForrest, Phys. Rev. B62, 10967). (2000)).

ここで、η₀は、三重項−三重項消滅が無い場合の外部量子効率（初期効率）、Ｊ₀は外部量子効率が初期量子効率の１／２となる電流密度であり、この値は外部量子効率の低下を示す指標となる。また、Ｊ₀は、以下の式（２）で示される。

Here, η ₀ is the external quantum efficiency (initial efficiency) when there is no triplet-triplet annihilation, and J ₀ is the current density at which the external quantum efficiency is ½ of the initial quantum efficiency. It becomes an index indicating a decrease in quantum efficiency. J ₀ is expressed by the following equation (2).

上記の式（２）においては、ｑが電荷、ｄが励起子形成領域の幅、τが発光寿命、ｋ_TTは、三重項−三重項消滅に関する定数である。

In the above formula (2), q is the charge, d is the width of the exciton formation region, τ is the emission lifetime, and k _TT is a constant related to triplet-triplet annihilation.

For example, when two or more kinds of phosphorescent materials are added to the same host material, the triplet-triple between phosphorescent materials can be obtained by making the above-mentioned J ₀ values corresponding to the respective phosphorescent materials equal. The change in emission color due to the difference in term annihilation (change accompanying the change in current density) does not occur.

For example, when the phosphorescent material is mixed substantially uniformly diffused in the light-emitting layer, a large difference does not occur in the d and k _TT between a plurality of phosphorescent materials. Therefore, if the phosphorescent materials are mixed so that the emission lifetimes τ of light emission corresponding to the respective phosphorescent materials to be mixed are substantially equal, the change accompanying the change in the current density of the emission color is suppressed.

  The emission lifetime of a plurality of mixed phosphorescent materials depends largely on the triplet energy transfer rate between the plurality of phosphorescent materials. The triplet energy transfer rate can be controlled by changing the distance between phosphorescent materials, that is, the concentration of the phosphorescent material.

  Therefore, when a plurality of phosphorescent materials are mixed, the emission color becomes stable if the phosphorescent materials are mixed at such a concentration that the emission lifetimes corresponding to the respective phosphorescent materials become equal.

  In addition, when a plurality of phosphorescent materials are mixed, energy transfer occurs between the plurality of phosphorescent materials, so that the emission lifetime corresponding to each phosphorescent material changes in the same direction due to this energy transfer. In addition, it is important to select and mix phosphorescent materials.

  For example, FIG. 1 is a diagram schematically showing energy transfer when a blue phosphorescent material and an orange phosphorescent material are mixed. As described above, the phosphorescent material emits light with a radiation transition from the triplet excited state to the singlet ground state. The triplet energy of the phosphorescent material (the energy corresponding to the emission peak on the shortest wavelength side of the phosphorescent material) is such that the phosphorescent material that emits light on the short wavelength side emits light on the long wavelength side. It is known to be big. That is, when a blue phosphorescent material and an orange phosphorescent material are compared, the triplet energy of the blue phosphorescent material is larger than that of the orange phosphorescent material.

  In this way, when phosphorescent materials having different triplet energies are mixed, the phosphorescent material having a high triplet energy (for example, a blue phosphorescent material) is moved to the phosphorescent material having a low triplet energy (for example, an orange phosphorescent material). Energy transfer occurs.

  Due to the above-described energy transfer, a phenomenon occurs in which the light emission lifetime corresponding to each phosphorescent material changes. For example, light emission corresponding to a phosphorescent material having a large triplet energy changes in a direction in which the light emission lifetime is shortened.

  In view of the above, in order to stabilize the emission color of the light-emitting element, among the plurality of phosphorescent materials used for forming the light-emitting layer, the emission lifetime of a high-energy phosphorescent material with a high triplet energy is low energy with a low triplet energy. It can be seen that it is preferable to use a material longer than the emission lifetime of the phosphorescent material.

  Next, a specific example in which phosphorescent materials having different triplet energies are mixed to control the emission lifetime corresponding to each phosphorescent material and a light emitting element is formed will be described below.

  FIG. 2 is a diagram schematically showing the light emitting device 10 according to Example 1 of the invention. Referring to FIG. 2, the light emitting element 10 is formed on a substrate 1 made of transparent glass, and an organic layer 3 containing a light emitting material is formed between an electrode 2 (anode) and an electrode 4 (cathode). It has the structure which becomes. The electrode 2 formed on the substrate 1 side is made of ITO, and the electrode 4 facing the substrate 1 is formed by laminating Ba (film thickness: 3 nm) and Al (film thickness: 150 nm).

  The organic layer 3 contains a phosphorescent material composed of the following two types of iridium complexes as a light emitting material, and each is mixed and diffused into the organic layer. One of the two types of phosphorescent materials is the following FIrTp (hereinafter phosphorescent material 1),

もう１つは、下記のｍ−ＰＦ−ＰＹ（以下燐光材料２）である。

The other is the following m-PF-PY (hereinafter, phosphorescent material 2).

また、前記有機層３には、正孔を輸送する性質を有する正孔輸送材料として、下記のポリ（Ｎ−ビニルカルバゾール）（ＰＶＫ）（以下正孔輸送材料１）と、

The organic layer 3 has the following poly (N-vinylcarbazole) (PVK) (hereinafter referred to as hole transport material 1) as a hole transport material having a property of transporting holes,

さらに、電子を輸送する性質を有する電子輸送材料として、下記のＮＡＢ２（以下電子輸送材料１）とが加えられている。

Furthermore, the following NAB2 (hereinafter referred to as electron transport material 1) is added as an electron transport material having a property of transporting electrons.

前記有機層３は以下のようにして形成した。まず、上記の燐光材料１，燐光材料２、正孔輸送材料１、および電子輸送材料１を所定の溶液に対して均一となるように混合して、スピンコート法により、前記電極２上に形成した。この場合、燐光材料１，燐光材料２、正孔輸送材料１、および電子輸送材料１の重量濃度比は、燐光材料１：燐光材料２：正孔輸送材料１：電子輸送材料２が、２０：０．１：５０：３０となるように構成した。

The organic layer 3 was formed as follows. First, the phosphorescent material 1, the phosphorescent material 2, the hole transporting material 1, and the electron transporting material 1 are mixed so as to be uniform with respect to a predetermined solution, and formed on the electrode 2 by spin coating. did. In this case, the weight concentration ratio of phosphorescent material 1, phosphorescent material 2, hole transporting material 1, and electron transporting material 1 is as follows: phosphorescent material 1: phosphorescent material 2: hole transporting material 1: electron transporting material 2 is 20: It comprised so that it might be set to 0.1: 50: 30.

  FIG. 3 shows a photoluminescence (PL) spectrum (excitation wavelength: 355 nm) of a film (thickness: 100 nm) formed by mixing the phosphorescent material 1, phosphorescent material 2, hole transporting material 1, and electron transporting material 1 described above. It is shown.

  Referring to FIG. 3, blue light emission having peaks at 460 nm and 490 nm is light emission corresponding to phosphorescent material 1, and orange light emission having a peak at 560 nm is light emission corresponding to phosphorescent material 2. In the light-emitting element described above, light emission in a wide wavelength region can be obtained by the color mixture (additive color mixture) of the light emission by the phosphorescent material 1 and the light emission by the phosphorescent material 2, and substantially white light emission can be obtained. ing.

  The triplet energy generally corresponds to the energy of the shortest wavelength peak in the PL (phosphorescence) spectrum. Therefore, the triplet energy of the phosphorescent material 1 is 2.7 eV (460 nm), and the triplet energy of the phosphorescent material 2 is The energy is estimated at 2.2 eV (560 nm).

  In addition, FIG. 4 shows the attenuation characteristics of the PL emission of each of the phosphorescent material 1 and the phosphorescent material 2 before being mixed alone. The light emission characteristics shown in FIG. 4 show the PL attenuation characteristics (excitation wavelength: 355 nm) of a film formed by adding phosphorescent material 1 and phosphorescent material 2 individually to polycarbonate, which is a chemically inert medium. is there. In this case, the weight concentration ratio between the phosphorescent material 1 and the phosphorescent material 2 was 5% by weight.

  Referring to FIG. 4, the time during which the emission intensity becomes 1 / e (defined as the emission lifetime) is 3.5 μs in the case of phosphorescent material 1 (blue phosphorescent material) and in the case of phosphorescent material 2 (orange phosphorescent material). 1.1 μs. That is, it can be seen that the phosphorescent material 1 has a light emission lifetime approximately three times as long as that of the phosphorescent material 2 in a state before the respective phosphorescent materials are mixed.

  On the other hand, FIG. 5 shows a predetermined wavelength (460 nm, 560 nm) of the organic layer 3 (a film formed by mixing phosphorescent material 1, phosphorescent material 2, hole transporting material 1, and electron transporting material 1). It is the figure which investigated PL attenuation characteristic of. Referring to FIG. 5, the intensity of light emission at 460 nm (corresponding to the light emission of phosphorescent material 1) and the intensity of light emission at 560 nm (corresponding to the light emission of phosphorescent material 2) show similar attenuation characteristics. The light emission lifetimes at which 1 / e is 1 / e are both 2.3 μs, which are substantially equal.

  This is probably because triplet energy transfer from phosphorescent material 1 (blue phosphorescent material) to phosphorescent material 2 (orange phosphorescent material) occurs as described above with reference to FIG. That is, energy is transferred from the phosphorescent material 1 having high triplet energy (having a light emission peak on the short wavelength side) to the phosphorescent material 2 having low triplet energy (having a light emission peak on the long wavelength side). It is considered that the emission lifetime of the phosphorescent material 1 is remarkably shortened.

  Further, in the organic layer 3, although the concentration of the phosphorescent material 1 is remarkably higher than the concentration of the phosphorescent material 2, the emission intensity corresponding to each material is the same level. It is inferred that energy transfer to the phosphorescent material 2 occurs.

  In the case of forming the organic layer according to this embodiment, the combination of the phosphorescent material having a high triplet energy and the phosphorescent material having a low triplet energy is preferably as follows. That is, the emission lifetimes of the phosphorescent materials having a high triplet energy are longer than the emission lifetimes of the phosphorescent materials having a low triplet energy, by comparing the actual emission lifetimes of the respective phosphorescent materials before mixing. Such a combination is preferable.

  In the above combination, when the phosphorescent material is mixed and energy transfer occurs, the phosphorescent material having a high triplet energy changes in a direction in which the emission lifetime is shortened, and the emission lifetimes of a plurality of phosphorescent materials are approached. Energy transfer will occur.

  In addition, the emission lifetime of the phosphorescent material 2 is 1.1 μs before mixing, whereas it is 2.3 μs after mixing because the concentration of the phosphorescent material 2 is extremely small. This is considered to be because the interaction between the materials 2 is reduced.

  In the organic layer 3, the hole transport material 1 and the electron transport material 1 are not directly involved in the energy transfer between the phosphorescent material 1 and the phosphorescent material 2. This is because the triplet energies of the hole transport material 1 and the electron transport material 1 are 2.9 eV and 2.8 eV, respectively, and are larger than the triplet energies of the phosphorescent material 1 and the phosphorescent material 2. . Therefore, the triplet energy of the phosphorescent material 1 and the phosphorescent material 2 is confined to the triplet energy of the hole transport material 1 and the electron transport material 1.

FIG. 6 is a diagram showing an EL spectrum when a voltage is applied to the light emitting element 10 described above. Referring to FIG. 6, the EL spectrum shows no significant change in shape even when the current density is changed to 0.1 mA / cm ² , 1.0 mA / cm ² , or 10 mA / cm ² . Further, when the above light emission is shown in chromaticity coordinates, when the current density is 0.1 mA / cm ² (0.33, 0.40), when the current density is 10 mA / cm ² (0.32, 0.40) (shown later in FIG. 14), it can be seen that a stable emission color (white emission) is exhibited even when the drive current is changed.

In the light emitting device according to this example, a luminance exceeding 2000 cd / m ² at the maximum was obtained. For example, considering that the luminance of a cathode ray tube is about 100 cd / m ² , it was confirmed that the light-emitting element has sufficient brightness for practical use. Further, in the above light emitting device, a high external quantum efficiency of 5% was obtained.

  In the light-emitting element 10 in this embodiment, the hole transport material may be a tertiary amine carbazole, and the electron transport material may include an imidazole or triazole structure. The hole transport material may be a thiophene, phenylene, p-phenylene vinylene, or fluorene structure, which is a main chain conjugated polymer that is a fluorescent polymer and has a hole transport capability.

  In addition, the film forming method for forming the organic layer 3 is not limited to the spin coating method, and for example, a wet method such as a printing method or an ink jet method can be applied. A large-screen display device can be configured.

  The electrode 2 (anode) is generally formed using a transparent material that can be formed on a glass substrate. For example, as the electrode 2, it is preferable to use ITO, indium oxide, tin oxide, or an indium zinc oxide alloy. The electrode 2 may be a thin film of metal such as gold, platinum, or silver magnesium. Alternatively, a conductive polymer including polyaniline, polythiophene, polypyrrole, and derivatives thereof may be used.

  Further, as the electrode 4 (cathode), it is preferable from the viewpoint of ease of electron injection to use an alkali metal such as Li or K having a low work function or an alkaline earth metal such as Mg, Ca, or Ba. . Moreover, you may use stable Al as a substance. Further, in order to achieve both stability as a substance and ease of electron injection, a layer containing two or more kinds of materials may be used. For example, the electrode 4 may be formed by combining (stacking) an alkali metal or alkaline earth metal layer (thickness of about 0.01 to 10 nm) such as cesium, barium, calcium, or strontium with Al. .

  The electrode 2 and the electrode 4 can be formed by a known method such as a vacuum deposition method, a sputtering method, or an ion plating method. The patterning of the electrode 2 or the electrode 4 can be performed by chemical etching using photolithography or physical etching using a laser or the like. Alternatively, the electrodes may be patterned by overlapping the mask and performing vacuum deposition, sputtering, or the like.

  As the substrate 1, for example, a plastic substrate can be used in addition to a normal glass substrate. The plastic used as the substrate is, for example, excellent in heat resistance, dimensional stability, solvent resistance, electrical insulation, workability, and the like, and preferably has low air permeability and low moisture absorption.

  Examples of the plastic having the above properties include polyethylene terephthalate, polyethylene naphthalate, polystyrene, polycarbonate, polyethersulfone, polyarylate, and polyimide. Moreover, it becomes possible to comprise a flexible polymer organic electroluminescent element by using the board | substrate which has said flexible property.

  Further, it is preferable to form a moisture permeation preventing layer (gas barrier layer) on the side of the substrate 1 facing the electrode 2, on the opposite side, or on both sides of the substrate 1. The moisture permeation preventing layer can be formed by, for example, a high frequency sputtering method. Moreover, you may provide a hard-coat layer and an undercoat layer in a board | substrate as needed.

  The substrate 1 may be formed with a transistor (for example, a TFT) that controls the light emitting element for each pixel. By driving the transistor, a so-called active type display device can be formed. Further, the transistor may be formed using an organic material.

  In the light emitting device 10 according to the first embodiment, an organic material (hereinafter referred to as energy transfer material) involved in energy transfer from the phosphorescent material 1 to the phosphorescent material 2 may be added to the organic layer 3.

  FIG. 7 is a diagram schematically showing the action of the energy transfer material. Referring to FIG. 7, the energy transfer material has a triplet energy smaller than phosphorescent material 1 (indicated as blue phosphorescent material in the drawing) and larger than phosphorescent material 2 (indicated as orange phosphorescent material in the drawing). preferable. In this case, the energy of the phosphorescent material 1 once moves to the energy transfer material, and the energy transferred to the energy transfer material further moves to the phosphorescent material 2. The energy transfer material may be a material that does not directly participate in light emission, or may be a material that also functions as a hole transport material or an electron transport material.

  For example, by adding the above-described energy transfer material to the organic layer, energy transfer between a plurality of phosphorescent materials is facilitated, and the degree of freedom in combining phosphorescent materials is improved. For example, depending on the combination of phosphorescent materials, the difference in triplet energy may become too large, and it may be difficult to set a concentration difference for controlling the light emission lifetime.

  Therefore, by using the energy transfer material described above, it is possible to reduce the required concentration difference of the phosphorescent material and substantially widen the range of control of light emission.

  For example, in the light-emitting element 10 of Example 1, in place of the electron transport material 1 (NAB2), OXD-, which is an oxadiazole derivative shown below, which has both a function as an electron transport material and a function as an energy transfer material 7 (hereinafter referred to as energy transfer material 1) can be used.

上記のエネルギー移動材料１の三重項エネルギーは２．６ｅＶであり、燐光材料１の三重項エネルギー（２．７ｅＶ）と、燐光材料２の三重項エネルギー（２．２ｅＶ）の間に位置している。

The triplet energy of the energy transfer material 1 is 2.6 eV, and is located between the triplet energy of the phosphorescent material 1 (2.7 eV) and the triplet energy of the phosphorescent material 2 (2.2 eV). .

  In the case of this example, in the organic layer 3, the weight concentration ratio of phosphorescent material 1, phosphorescent material 2, hole transporting material 1, and energy transfer material 1 is defined as phosphorescent material 1: phosphorescent material 2: hole transporting material 1. The energy transfer material 1 was set to 10: 0.2: 60: 30, and other configurations were performed in the same manner as in Example 1 to form a light emitting element.

  FIG. 8 shows an organic layer (a film formed by mixing phosphorescent material 1, phosphorescent material 2, hole transporting material 1, and energy transfer material 1) in the same manner as in Example 1. It is the figure which investigated PL attenuation characteristic of a predetermined wavelength (460 nm, 560 nm). Referring to FIG. 8, the intensity of 460 nm light emission (corresponding to the light emission of phosphorescent material 1) and the intensity of light emission of 560 nm (corresponding to the light emission of phosphorescent material 2) show similar attenuation characteristics, and the light emission lifetime. Are both substantially equal at 6.2 μs.

  In the case of the present embodiment, the emission lifetimes of the phosphorescent material 1 and the phosphorescent material 2 are longer than those in the case of the embodiment 1, and the mixing ratio of the phosphorescent material 1 and the phosphorescent material 2 when the emission lifetime is the same. Is different. This is because the energy transfer material 1 is involved in the transfer of energy from the phosphorescent material 1 to the phosphorescent material 2.

  Further, the concentration difference between the phosphorescent material 1 and the phosphorescent material 2 for making the emission lifetime the same is smaller than that in the case of Example 1, and the use of the energy transfer material allows the emission lifetime (emission color). It can be seen that the control of) is easy.

FIG. 9 is a diagram showing an EL spectrum when a voltage is applied to the light emitting element according to this example. Referring to FIG. 9, the EL spectrum shows almost no change in shape even when the current density is changed to 0.1 mA / cm ² , 1.0 mA / cm ² , or 10 mA / cm ² . Further, when the above light emission is shown in chromaticity coordinates, when the current density is 0.1 to 10 mA / cm ² , the coordinates are almost the same as (0.32, 0.43) (described later in FIG. 14), and the driving is performed. It was found that even when the current was changed, a stable emission color (white light emission) was exhibited.

  Further, the case where a polymer material is used as the hole transport material of the light emitting device according to the above embodiment has been described as an example, but the present invention is not limited thereto. For example, a hole transport layer is formed between an organic layer and an electrode (anode) using a low molecular material as a material corresponding to the hole transport material 1 used in Examples 1 and 2. Also good.

  FIG. 10 is a cross-sectional view schematically showing a light emitting device 10A according to Example 3 of the present invention. However, the parts described above are denoted by the same reference numerals, and description thereof is omitted. Further, parts not specifically described are the same as those in the first embodiment.

  Referring to FIG. 10, in the light emitting device 10A according to the present example, as a configuration corresponding to the organic layer 3 of Example 1, a hole transport layer 3A, a light emitting layer 3B, And the structure where 3 C of electron carrying layers were laminated | stacked is formed. The hole transport layer 3A corresponds to the hole transport material 1 used in Example 1, and the electron transport layer 3C corresponds to the electron transport material 1 used in Example 1.

  When the above structure is formed, the hole transport layer 3A, the light emitting layer 3B, and the electron transport layer 3C can be formed by, for example, a vapor deposition method. The light emitting layer 3B is formed in the same manner as the organic layer 3 in the case of Example 1, and is formed so as to contain a plurality of phosphorescent materials.

  Moreover, as a material which comprises 3 A of said hole transport layers, TPD, (alpha) -NPD, a triphenylamine multimer etc. can be used, for example. The light emitting layer 3B can be formed by doping a phosphorescent material in a host having a hole transporting property or an electron transporting property. Moreover, as a material which comprises the said electron carrying layer 3C, an oxadiazole derivative, a triazole derivative, a triazine derivative, and an imidazole derivative can be used.

  In order to further increase the exciton confinement effect, a hole blocking layer made of, for example, BCP may be provided between the light emitting layer and the electron transporting layer.

  In the light emitting element 10A according to this example, the light emitting layer 3B was formed by doping 10 wt% of the phosphorescent material 1 and 0.2 wt% of the phosphorescent material 2 using the fluorescent material CDBP shown below as a host.

当該発光層３Ｂの所定の波長（４６０ｎｍ、５６０ｎｍ）のＰＬ減衰特性を調べたところ、４６０ｎｍの発光（燐光材料１の発光に対応）の発光寿命は２．１μｓ、５６０ｎｍの発光（燐光材料２の発光に対応）の発光寿命は２．２μｓとなり、発光寿命は実質的に等しくなったことが確認された。

When the PL attenuation characteristics of the light emitting layer 3B at a predetermined wavelength (460 nm, 560 nm) were examined, the light emission lifetime of light emission of 460 nm (corresponding to the light emission of the phosphorescent material 1) was 2.1 μs, and light emission of 560 nm (the phosphorescent material 2 (Corresponding to light emission) was 2.2 μs, and it was confirmed that the light emission lifetimes were substantially equal.

FIG. 11 is a diagram showing an EL spectrum when a voltage is applied to the light emitting element 10A according to this example. Referring to FIG. 11, the EL spectrum shows almost no change in shape even when the current density is changed to 0.1 mA / cm ² , 1.0 mA / cm ² , and 10 mA / cm ² . Further, when the above light emission is shown in chromaticity coordinates, when the current density is 0.1 to 10 mA / cm ² , the coordinates are almost the same as (0.41, 0.47) (described later in FIG. 14), and driving is performed. It was found that even when the current was changed, a stable emission color (white light emission) was exhibited.

  Further, as shown in Example 2, an energy transfer material may be added to the light emitting element according to this example. In this case, by doping the light-emitting layer 3B with the energy transfer material, the energy transfer between the plurality of phosphorescent materials is facilitated and the degree of freedom of the combination of the phosphorescent materials is improved as in the second embodiment. Can be made.

  Moreover, you may use the phosphorescent material which is heavy metal complexes, such as iridium, platinum, palladium, and rhodium, for the host of the said light emitting layer 3B.

  In the light-emitting elements described in Examples 1 to 3, the light-emitting material is a heavy metal complex such as iridium, platinum, palladium, or rhodium, and three or more types of phosphorescence including red, green, and blue light-emitting components as light-emitting components. A material may be used. As a result, the light emission wavelength region of the light emitting element can be substantially widened, and the color rendering properties (color reproducibility) necessary for the illumination light source can be improved.

  In this case, energy transfer as described in the case of Example 1 occurs between three or more types of phosphorescent materials. In this case, the combination in the case of mixing a phosphorescent material having a high triplet energy and a phosphorescent material having a low triplet energy may be the same as the case shown in Example 1. That is, the emission lifetimes of the phosphorescent materials having a high triplet energy are longer than the emission lifetimes of the phosphorescent materials having a low triplet energy, by comparing the actual emission lifetimes of the respective phosphorescent materials before mixing. Such a combination is preferable.

  In the above combination, when the phosphorescent material is mixed and energy transfer occurs, the phosphorescent material having a high triplet energy changes in a direction in which the emission lifetime is shortened, and the emission lifetimes of a plurality of phosphorescent materials are approached. Energy transfer occurs.

  In addition, a simple full color display can be configured by combining the light emitting element described in the first to third embodiments and a color filter. The above color filter can be disposed at various locations, for example, on the surface of the substrate (the side opposite to the side on which the electrode of the substrate is formed) or between the substrate and the electrode. Also, different types of color filters may be used in combination to modulate the emission color.

In addition, the color filter is configured to have a structure (pattern) that combines, for example, the three primary colors of red, green, and blue, and can easily form a fine structure by a method such as a printing method. Is possible.
[Comparative example]
For comparison with the above-described example, the phosphorescent material 1 and the phosphorescent material 2 in the light-emitting element described in Example 1 were changed to the phosphorescent material 3 and the phosphorescent material 4 described below, respectively, to form a light-emitting element.

  As phosphorescent material 3, the following FIrpic having an emission peak at 470 nm was used.

また、燐光材料４として、６１０ｎｍに発光のピークを有する、以下に示すｂｔｐ_２Ｉｒ（ａｃａｃ）を用いた。

As the phosphorescent material 4, btp ₂ Ir (acac) shown below having a light emission peak at 610 nm was used.

この場合、燐光材料３，燐光材料４、正孔輸送材料１、および電子輸送材料１の重量濃度比は、燐光材料３：燐光材料４：正孔輸送材料１：電子輸送材料１が、２０：０．２：５０：３０となるように構成した。上記の変更以外は実施例１に記載した場合と同様の構成（方法）により、発光素子を形成した。

In this case, the weight concentration ratio of phosphorescent material 3, phosphorescent material 4, hole transport material 1 and electron transport material 1 is as follows: phosphorescent material 3: phosphorescent material 4: hole transport material 1: electron transport material 1 is 20: The configuration was 0.2: 50: 30. A light emitting element was formed by the same configuration (method) as described in Example 1 except for the above changes.

  In the above comparative example, contrary to the cases of Examples 1 to 3, when the attenuation characteristics of light emission before mixing of the phosphorescent material 3 and the phosphorescent material 4 are compared, the phosphorescent material (phosphorescence having a high triplet energy) is compared. The emission lifetime of the material 3) is a combination shorter than that of the phosphorescent material (phosphorescent material 4) having a low triplet energy.

  In the above combination, when phosphorescent materials are mixed and energy transfer occurs, energy transfer occurs in a direction in which the difference in emission lifetimes of the plurality of phosphorescent materials is increased.

  FIG. 12 shows a predetermined wavelength (470 nm, 470 nm, organic film (film formed by mixing phosphorescent material 3, phosphorescent material 4, hole transporting material 1, and electron transporting material 1) of the light emitting device according to this comparative example. It is the figure which investigated PL attenuation | damping property of 610 nm). Referring to FIG. 12, it can be seen that the attenuation characteristics of the intensity of 470 nm emission (corresponding to the emission of phosphorescent material 3) and the intensity of 610 nm emission (corresponding to the emission of phosphorescent material 4) are clearly different. . In this case, since the phosphorescent material 3 has a light emission peak at 470 nm, the triplet energy was 2.6 eV, and the corresponding light emission lifetime was 0.6 μs. Further, since the phosphorescent material 4 has a light emission peak at 610 nm, the triplet energy was 2.0 eV and the corresponding light emission lifetime was 5.7 μs.

  That is, in the case of this comparative example, it can be seen that the emission lifetimes of the emission corresponding to the plurality of phosphorescent materials in the formed organic layer are greatly different.

FIG. 13 shows an EL spectrum when voltage is applied to the light-emitting element. Referring to FIG. 13, when the current density is changed to 0.1 mA / cm ² , 1.0 mA / cm ² , and 10 mA / cm ² , the spectral shape on the long wavelength side changes greatly. It can be seen that the emission intensity is significantly attenuated on the long wavelength side.

This phenomenon is considered to be due to the long emission lifetime of the phosphorescent material (phosphorescent material 4) having a small triplet energy in this comparative example. According to the equation (2) described above, J ₀ becomes small in the phosphorescent material 4 (long wavelength side) having a long emission lifetime (τ). In this case, it is considered that the effect of lowering the light emission efficiency due to triplet-triplet annihilation is greater on the longer wavelength side than on the shorter wavelength side.

In the case of this comparative example, when the light emission is shown in chromaticity coordinates, when the current density is 0.1 mA / cm ² (0.41, 0.39), the current density is 10 mA / cm ² . (0.36, 0.38) (described later in FIG. 14), it can be seen that a change in chromaticity is caused by a change in drive current.

  FIG. 14 shows a comparison of the luminescent colors of the light emitting elements according to Examples 1 to 3 and the comparative example shown in chromaticity coordinates.

  Referring to FIG. 14, in the case of the comparative example in which the emission lifetime of light emission corresponding to a plurality of mixed phosphorescent materials is greatly different, the chromaticity change occurs when the drive current is changed, and stable light emission is achieved. It can be seen that no color is obtained.

  On the other hand, the light emission colors of the light emitting elements according to Examples 1 to 3 in which the light emission lifetimes corresponding to the plurality of mixed phosphorescent materials are substantially the same are obtained when the drive current is changed. It can be seen that the chromaticity change occurs and is stable. For this reason, it turns out that the light emitting element which concerns on said Example is a light emitting element from which stable white light emission is obtained.

  In the light emitting element, a plurality of light emitting materials (phosphorescent materials) are mixed and diffused into the organic layer (light emitting layer). For this reason, for example, when compared with a light-emitting element having a structure in which a plurality of different light-emitting layers are stacked, there is a feature that the emission color varies little with respect to a change in drive current. In addition, the light emitting device according to the above embodiment has a simple light emitting device structure and is easy to manufacture, and has a smaller viewing angle dependency of emitted light than a light emitting device in which a plurality of light emitting layers are stacked. It has characteristics.

  Although the present invention has been described with reference to the preferred embodiments, the present invention is not limited to the specific embodiments described above, and various modifications and changes can be made within the scope described in the claims.

It is the figure (the 1) which showed the energy transfer between phosphorescent materials typically. 6 is a diagram showing a light emitting device according to Example 1. FIG. It is a figure which shows PL spectrum of the light emitting element of FIG. FIG. 2 is a diagram (part 1) illustrating a PL attenuation characteristic of a phosphorescent material. It is the figure (the 1) which investigated the PL attenuation characteristic of the organic layer. It is the emission spectrum (the 1) of a light emitting element. It is the figure (the 2) which showed the energy transfer between phosphorescent materials typically. It is the figure (the 2) which investigated the PL attenuation | damping characteristic of the organic layer. It is the emission spectrum (the 2) of a light emitting element. 6 is a view showing a light emitting device according to Example 3. FIG. It is the emission spectrum (the 3) of a light emitting element. It is the figure (the 3) which investigated the PL attenuation | damping characteristic of the organic layer. It is the emission spectrum (the 4) of a light emitting element. It is the figure which compared the chromaticity of light emission of a light emitting element.

Explanation of symbols

10, 10A Light emitting element 1 Substrate 2, 4 Electrode 3 Organic layer 5 Power source 3A Hole transport layer 3B Light emitting layer 3C Electron transport layer

Claims (7)

A light-emitting element in which an organic layer containing a plurality of phosphorescent materials having different triplet energies is formed between a first electrode and a second electrode,
Among the plurality of phosphorescent materials used for forming the organic layer, a light-emitting element characterized in that a light-emitting lifetime of a high-energy phosphorescent material having a high triplet energy is longer than a light-emitting lifetime of a low-energy phosphorescent material having a low triplet energy .   The light emitting device according to claim 1, wherein the plurality of phosphorescent materials are mixed and diffused in the organic layer.   In the organic layer, energy transfer from the high energy phosphorescent material to the low energy phosphorescent material occurs, and emission lifetimes of light emission corresponding to the plurality of phosphorescent materials in the organic layer are the same. The light emitting device according to claim 1 or 2.   4. The light-emitting element according to claim 1, wherein the organic layer includes an organic material involved in energy transfer from the high-energy phosphor material to the low-energy phosphor material.   The light emitting device according to claim 4, wherein the triplet energy of the organic material is larger than the low energy phosphorescent material and smaller than the high energy phosphorescent material.   The light emitting device according to claim 4, wherein the organic material is composed of an electron transport material for transporting electrons or a hole transport material for transporting holes.   The light-emitting element according to claim 4, wherein the organic material is mixed with the plurality of phosphorescent materials and diffused into the organic layer.

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