Light-emitting element and light-emitting device

Abstract

To provide a light-emitting element having high external quantum efficiency.SOLUTION: A light-emitting element includes a light-emitting layer containing a guest, an n-type host and a p-type host between a pair of electrodes. The value obtained by subtracting an energy difference between the triplet excited state and the ground state of the n-type host (or p-type host) from an energy difference between the triplet excited state and the ground state of the guest is 0.15 eV or more. Since the light-emitting element is unlikely to cause transition from the guest in the triplet excited state to the triplet excited state of the n-type host (or the p-type host), the light emission from the guest in the triplet excited state is efficiently performed. Alternatively, in the light-emitting element, the LUMO level of the n-type host is higher than the LUMO level of the guest by 0.1 eV or more, or the HOMO level of the p-type host is lower than the HOMO level of the guest by 0.1 eV or more. The light-emitting element allows efficient electron-hole recombination in the guest and therefore can increase the luminous efficiency or external quantum efficiency.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a light-emitting device using an organic electroluminescence (EL) phenomenon. (Hereinafter, also referred to as an organic EL device).

Background Art

[0002] Research and development of organic EL devices are actively carried out (see Patent Document 1, Non-Patent Document 1, and Non-Patent Document 2). The basic structure of an organic EL device is one in which a layer containing a light-emitting organic compound (hereinafter, also referred to as a light-emitting layer) is sandwiched between a pair of electrodes, and it can be made thin and lightweight, respond quickly to an input signal, and be driven at a low DC voltage. Due to such characteristics, it is attracting attention as a next-generation flat panel display device. In addition, a display using such a light-emitting device also has characteristics such as excellent contrast and image quality, and a wide viewing angle. Furthermore, since the organic EL device is a surface light source, its application as a backlight for a liquid crystal display or a light source for lighting is also being considered. The light-emitting mechanism of an organic EL device is a carrier injection type. That is, by applying a voltage across the electrodes with the light-emitting layer sandwiched therebetween, electrons and holes injected from the electrodes recombine, causing the light-emitting substance to enter an excited state, and light is emitted when the excited state returns to the ground state. The excited state includes a singlet excited state and a triplet excited state. Also, the statistical generation ratio in the light-emitting device is considered to be 1 / 3 of the former to the latter. In this specification, the singlet excited state (triplet excited state) refers to the one with the lowest energy level among the singlet excited state (triplet excited state), unless otherwise specified.

[0003]

[0004] Luminescent organic compounds usually have a singlet ground state. Therefore, luminescence from the singlet excited state is called fluorescence because it is an electronic transition between the same spin multiplicities. On the other hand, luminescence from the triplet excited state is called phosphorescence because it is an electronic transition between different spin multiplicities. Here, a compound that emits fluorescence (hereinafter referred to as a fluorescent compound) usually shows only fluorescence and no phosphorescence is observed at room temperature. Therefore, the theoretical limit of the internal quantum efficiency (the ratio of photons generated to the injected carriers) in a light-emitting device using a fluorescent compound is set at 25% based on the ratio of the singlet excited state to the triplet excited state described above.

[0005] On the other hand, if a compound that emits phosphorescence (hereinafter referred to as a phosphorescent compound) is used, the internal quantum efficiency can theoretically be increased to 100%. That is, higher luminous efficiency can be obtained compared to fluorescent compounds. For this reason, in order to realize a highly efficient light-emitting device, the development of light-emitting devices using phosphorescent compounds has been actively carried out in recent years.

[0006] In particular, due to the high phosphorescence quantum efficiency, organometallic complexes having iridium or the like as a central metal as phosphorescent compounds have attracted attention. For example, Patent Document 1 discloses an organometallic complex having iridium as a central metal as a phosphorescent material.

[0007] When forming the light-emitting layer of a light-emitting device using the above-described phosphorescent compound, in order to suppress concentration quenching of the phosphorescent compound and quenching due to triplet-triplet annihilation, the phosphorescent compound is often formed so as to be dispersed in a matrix composed of other compounds. At this time, the compound serving as the matrix is called the host, and the compound dispersed in the matrix like the phosphorescent compound is called the guest. ​​​​​​ It occurs.

[0008] In such a light-emitting element using a phosphorescent compound as a guest, there are several general elementary processes of light emission, which will be described below. Although there are several, they will be described below.

[0009] (1) When an electron and a hole recombine in a guest molecule and the guest molecule becomes an excited state ( direct recombination process). (1-1) When the excited state of the guest molecule is a triplet excited state, the guest molecule emits phosphorescence. (1-2) When the excited state of the guest molecule is a singlet excited state, the singlet excited state guest molecule undergoes intersystem crossing to a triplet excited state and emits phosphorescence.

[0010] That is, in the direct recombination process of (1) above, if the intersystem crossing efficiency of the guest molecule and even the phosphorescence quantum efficiency are high, a high luminous efficiency can be obtained.

[0011] (2) When an electron and a hole recombine in a host molecule and the host molecule becomes an excited state ( energy transfer process).

[0012] (2-1) When the excited state of the host molecule is a triplet excited state, and the energy level (T1 level) of the triplet excited state of the host molecule is higher than the T1 level of the guest molecule, the excitation energy moves from the host molecule to the guest molecule, and the guest molecule becomes a triplet excited state. The guest molecule in the triplet excited state emits phosphorescence. Although energy transfer to the singlet excited state (S1 level) of the guest molecule is formally possible, in many cases the S 1 level of the guest molecule is located on the higher energy side than the T1 level of the host molecule, and it is difficult to become the main energy transfer process, so it is omitted here.

[0013] (When the excited state of the host molecule is a singlet excited state, the singlet excited state of the host molecule (S1 level) is higher than the S1 level and the T1 level of the guest molecule, excitation energy moves from the host molecule to the guest molecule, and the guest molecule becomes a singlet excited state or a triplet excited state. The guest molecule in the triplet excited state emits phosphorescence. Also, the singlet excited guest molecule undergoes intersystem crossing to the triplet excited state and emits phosphorescence.

[0014] That is, in the energy transfer process of (2) above, it is important how both the triplet excitation energy and the singlet excitation energy of the host molecule can efficiently move to the guest molecule.

[0015] In view of this energy transfer process, before the excitation energy moves from the host molecule to the guest molecule, if the host molecule itself releases and deactivates its excitation energy as light or heat, the luminescence efficiency will decrease.

[0016] <Energy Transfer Process> Hereinafter, the energy transfer process between molecules will be described in detail.

[0017] First, as mechanisms of energy transfer between molecules, the following two mechanisms have been proposed. Here, the molecule that gives the excitation energy is referred to as the host molecule, and the molecule that receives the excitation energy is referred to as the guest molecule.

[0018] ≪Förster Mechanism (Dipole-Dipole Interaction)≫ The Förster mechanism does not require direct contact between molecules for energy transfer. Energy transfer occurs through the resonance phenomenon of dipole vibrations between the host molecule and the guest molecule. Dipole vibrations ​​The host molecule transfers energy to the guest molecule through the resonance phenomenon of the vibration, and the host molecule The guest molecule enters the excited state. The rate constant of the Förster mechanism is k h * →g of This is shown in equation (1).

[0019]

number

[0020] In formula (1), ν represents the frequency, and f' h (ν) is the normalized Emission spectrum (fluorescence spectrum when discussing energy transfer from a singlet excited state, When discussing energy transfer from a triplet excited state, it represents the phosphorescence spectrum, and ε g (ν ) represents the molar extinction coefficient of the guest molecule, N represents Avogadro's number, and n represents the refractive index of the medium. represents the rate of excitation, R represents the intermolecular distance between the host molecule and the guest molecule, and τ represents the excitation state represents the lifetime of the photoelectron state (fluorescence lifetime or phosphorescence lifetime), c represents the speed of light, and φ represents the luminescence quantum efficiency (single When discussing energy transfer from single excited states, the fluorescence quantum efficiency is used. When discussing energy transfer from triplet excited states, the fluorescence quantum efficiency is used. When discussing energy transfer, it stands for phosphorescence quantum efficiency, and K 2 is the host molecule and guest molecule This is a coefficient (0 to 4) that represents the orientation of the transition dipole moment. In the case of random orientation, K 2 =2 / 3.

[0021] <Dexter mechanism (electron exchange interaction)> The Dexter mechanism occurs when the host and guest molecules approach the effective contact distance where orbitals overlap. The energy is transferred through the exchange of electrons of the excited host molecule and the ground state guest molecule. - transfer occurs. The rate constant of the Dexter mechanism is k h * →g is shown in formula (2).

[0022]

number

[0023] In formula (2), h is the Planck constant, and K is a constant with the dimension of energy. where ν is the frequency and f' is the h (ν) is the normalized emission spectrum of the host molecule (When discussing energy transfer from a singlet excited state, use the fluorescence spectrum, and when discussing energy transfer from a triplet excited state, use the When discussing energy transfer from g (ν) is a guest represents the normalized absorption spectrum of the molecule, L represents the effective molecular radius, and R represents the effective molecular radius of the host molecule. Represents the intermolecular distance between the child and guest molecules.

[0024] Here, the efficiency of energy transfer from the host molecule to the guest molecule Φ ET is expressed by the formula (3). It is believed that. r is the emission process of the host molecule (from the singlet excited state of the host molecule). When discussing energy transfer, it is energy transfer from the triplet excited state of the fluorescence or host molecule. represents the rate constant of phosphorescence, and k n is the rate of non-radiative processes (thermal deactivation and intersystem crossing) represents the degree constant, and τ represents the measured lifetime of the excited state of the host molecule.

[0025]

number

[0026] First, from equation (3), the energy transfer efficiency Φ ETTo increase it, the energy transfer rate constant k h * →g should be made much larger than other competing rate constants k r +k n (=1 / τ). It can be seen that the rate constant k of the energy transfer should be increased. To increase the rate constant k of the energy transfer h * →g from equations (1) and (2), in either the Förster mechanism or the Dexter mechanism , it can be seen that the overlap between the emission spectrum of the host molecule (fluorescence spectrum when discussing energy transfer from the singlet excited state, phosphorescence spectrum when discussing energy transfer from the triplet excited state) and the absorption spectrum of the guest molecule (usually phosphorescence, so the energy difference between the triplet excited state and the ground state) should be large. For example, in a material where the energy difference between the triplet excited state and the ground state of the host molecule is selected to overlap with the energy difference between the triplet excited state and the ground state of the guest molecule, more efficient energy transfer from the host to the guest occurs. When discussing energy transfer, the fluorescence spectrum is used for energy transfer from the singlet excited state, and the phosphorescence spectrum is used for energy transfer from the triplet excited state. When discussing energy transfer from the triplet excited state of the host molecule to the ground state of the guest molecule, it can be seen that the overlap between the emission spectrum of the host molecule (fluorescence spectrum when discussing energy transfer from the singlet excited state, phosphorescence spectrum when discussing energy transfer from the triplet excited state) and the absorption spectrum of the guest molecule (usually phosphorescence, so the energy difference between the triplet excited state and the ground state) should be large. For example, in a material where the energy difference between the triplet excited state and the ground state of the host molecule is selected to overlap with the energy difference between the triplet excited state and the ground state of the guest molecule, more efficient energy transfer from the host to the guest occurs.

[0027] However, the above energy transfer also occurs in exactly the same way from the triplet excited state of the guest molecule to the ground state of the host molecule. In a material where the energy difference between the triplet excited state and the ground state of the host molecule is selected to overlap with the energy difference between the triplet excited state and the ground state of the guest molecule, it also means that the triplet excited state of the guest molecule easily transfers energy to the triplet excited state of the host molecule. This results in a decrease in the emission efficiency.

[0028] However, the above energy transfer also occurs in exactly the same way from the triplet excited state of the guest molecule to the ground state of the host molecule. In a material where the energy difference between the triplet excited state and the ground state of the host molecule is selected to overlap with the energy difference between the triplet excited state and the ground state of the guest molecule, it also means that the triplet excited state of the guest molecule easily transfers energy to the triplet excited state of the host molecule. This results in a decrease in the emission efficiency. In a material where the energy difference between the triplet excited state and the ground state of the host molecule is selected to overlap with the energy difference between the triplet excited state and the ground state of the guest molecule, it also means that the triplet excited state of the guest molecule easily transfers energy to the triplet excited state of the host molecule. This results in a decrease in the emission efficiency.

[0029] Regarding such problems, for example, as described in Non-Patent Document 1, a host molecule has a larger energy difference between its triplet excited state and the ground state than the energy difference between the triplet excited state and the ground state of the guest molecule is proposed to be overcome by making it larger.

[0030] In Non-Patent Document 1, the energy difference between the triplet excited state and the ground state of the host molecule is made 0.3 electron volts (currently corrected to 0.15 electron volts) larger than the energy difference between the triplet excited state and the ground state of the guest molecule with respect to the host molecule of the guest molecule, so as not to cause a transition from the triplet excited state of the guest molecule to the triplet excited state of the host molecule. That is, by making the energy difference between the triplet excited state and the ground state of the host molecule 0.15 electron volts or more larger than the energy difference between the triplet excited state and the ground state of the host molecule of the guest molecule, the transition from the triplet excited state of the guest molecule to the triplet excited state of the host molecule can be sufficiently prevented.

[0031] That is, when the energy difference between the triplet excited state and the ground state of the host molecule is made 0.15 electron volts or more larger than the energy difference between the triplet excited state and the ground state of the host molecule of the guest molecule, the transition from the triplet excited state of the guest molecule to the triplet excited state of the host molecule can be sufficiently blocked.

Prior Art Documents

Patent Documents

[0032]

Patent Document 1

Non-Patent Documents

[0033]

Non-Patent Document 1

Non-Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0034] However, the fact that the energy difference between the host molecule and the guest molecule is different in this way means that the above Förster mechanism and Dexter mechanism are less likely to occur, and the resulting decrease in luminescence efficiency becomes a problem. One aspect of the present invention provides a light-emitting device based on a new principle that overcomes such a contradiction.

[0035] Also, as described above, there are various excitation processes, but the excitation process with less deactivation is a direct recombination process, and improving its ratio is preferable for improving the luminescence efficiency or the external quantum efficiency. One aspect of the present invention aims to provide a method for efficiently generating a direct recombination process. Another aspect of the present invention aims to provide a light-emitting device with a high external quantum efficiency.

Means for Solving the Problems

[0036] ​​​One aspect of the present invention is a light-emitting layer including a phosphorescent compound (guest), a first organic compound, and a second organic compound between a pair of electrodes, and the energy difference between the triplet excited state and the ground state of the first organic compound and the second organic compound is 0.15 electron volts or more greater than the energy difference between the triplet excited state and the ground state of the guest characterized by a light-emitting device

[0037] In the above, the first organic compound and the second organic compound may be a combination that forms an exciplex Also, the first organic compound may be more excellent in electron transporting property than hole transporting property and the second organic compound may be more excellent in hole transporting property than electron transporting property. In such a case the first organic compound and the second organic compound are respectively referred to as an n-type host and a p-type host

[0038] Also, one aspect of the present invention is a light-emitting layer including a guest, an n-type host, and a p-type host between a pair of electrodes and the LUMO (Lowest Unoccupied Molecular Orbital) level of the n-type host is 0.1 electron volts or more higher than the LUMO level of the guest characterized by a light-emitting device

[0039] Note that if the LUMO level of the guest is too lower than the LUMO level of the n-type host, it is not preferable in terms of electrical conduction characteristics Therefore, the value obtained by subtracting the LUMO level Ea of the guest from the LUMO level En of the n-type host, (En - Ea), is preferably 0.1 electron volts or more and 0.5 electron volts or less

[0040] Also, one aspect of the present invention is a light-emitting layer including a guest, an n-type host, and a p-type host between a pair of electrodes ​​​​It has between electrodes, and the HOMO (Highest Occupied Molecular Orbital) level of the p-type host is 0.1 electron volt or more lower than the HOMO level of the guest. This is a light-emitting device characterized by this.

[0041] Note that if the HOMO level of the guest is too higher than the HOMO level of the p-type host, it is not preferable in terms of electrical conduction characteristics. Therefore, the value obtained by subtracting the HOMO level Eb of the guest from the HOMO level Ep of the p-type host, (Ep - Eb), is preferably -0.5 electron volt or more and -0.1 electron volt or less.

[0042] In the above light-emitting device, the guest is preferably an organometallic complex. In the above light-emitting device, at least one of the n-type host and the p-type host may be a fluorescent compound. The light-emitting device according to one aspect of the present invention can be applied to a light-emitting device, an electronic device, and an illumination device.

[0043] In one aspect of the present invention, the light-emitting layer has an n-type host molecule, a p-type host molecule, and a guest molecule. Of course, the molecules do not necessarily have to be regularly arranged, and they may be in a state with extremely little regularity. Particularly when the light-emitting layer is a thin film of 50 nm or less, it is preferably in an amorphous state. Therefore, it is preferable to select a combination of materials that are difficult to crystallize.

[0044] Also, as shown in FIG. 1(A), in one aspect of the present invention, a light-emitting device in which a first electrode 103, a light-emitting layer 102 having the above configuration, and a second electrode 104 are stacked on a substrate 101 may be used. Here, the first electrode 103 is one of the anode and the cathode, and the second electrode 104 is the other of the anode and the cathode. ​​​​​​​​​​​​​

[0045] Further, as shown in FIG. 1(B), in one aspect of the present invention, in addition to the first electrode 103, the light-emitting layer 102 , the first carrier injection layer 105, the first carrier transport layer 106, the second carrier injection layer 107, and the second carrier transport layer 108 may be provided in a stacked manner. Here, the first carrier is one of electrons and holes, and the second carrier is the other of electrons and holes. Further, if the first electrode is an anode, the first carrier is a hole , and if the first electrode is a cathode, the first carrier is an electron.

Advantages of the Invention

[0046] In one aspect of the present invention, by making the energy difference between the triplet excited state and the ground state of the host (n-type host and p-type host) molecules 0.15 electron volts or more higher than the energy difference between the triplet excited state and the ground state of the guest molecules, the transition from the triplet excited state of the guest molecules to the triplet excited state of the host (n -type host and p-type host) molecules can be sufficiently prevented, and a light-emitting device with high external quantum efficiency can be provided. -type host and p-type host) molecules can be sufficiently prevented, and a light-emitting device with high external quantum efficiency can be provided.

[0047] On the other hand, regarding the energy transfer process using the Förster mechanism or the Dexter mechanism , the process of energy transfer from the excited complex of the n-type host molecule and the p-type host molecule to the guest molecule can be passed through. When the energy has moved to the guest molecule, the excited complex splits into the n-type host molecule and the p-type host molecule, and the energy difference between the triplet excited state and the ground state of the n-type host molecule (or p-type host molecule) is 0.15 electron volts or more higher than the energy difference between the triplet excited state and the ground state of the guest molecule. Therefore, the triplet excited state of the guest molecule is n-type host molecule and the p-type host molecule, and the energy difference between the triplet excited state and the ground state of the n-type host molecule (or p-type host molecule) is 0.15 electron volts or more higher than the energy difference between the triplet excited state and the ground state of the guest molecule. Therefore, the triplet excited state of the guest molecule is n-type host It does not transfer energy to the triplet excited state of the host molecule (or p-type host molecule). .

[0048] In addition, in one aspect of the present invention, for example, since the LUMO level of the n-type host molecule is 0.1 electron volts or more higher than the LU MO level of the guest molecule, the electrons that have conducted through the n-type host molecule preferentially enter the LUMO level of the guest molecule. As a result, the guest molecule becomes an anion and attracts holes, and holes and electrons recombine in the guest molecule.

[0049] In addition, in one aspect of the present invention, for example, since the HOMO level of the p-type host molecule is 0.1 electron volts or more lower than the HO MO level of the guest molecule, the holes that have conducted through the p-type host molecule preferentially enter the HOMO level of the guest molecule. As a result, the guest molecule becomes a cation and attracts electrons, and holes and electrons recombine in the guest molecule.

[0050] In this way, by using one aspect of the present invention, carriers can be efficiently injected into the guest molecule and the ratio of the direct recombination process can be increased. In particular, in one aspect of the present invention, since an n-type host and a p-type host are mixed and used in the light-emitting layer, electrons tend to conduct through the n-type host molecule and holes tend to conduct through the p-type host molecule. As a result, electrons are injected into the LUMO level of the guest molecule from the n-type host molecule, and holes are injected into the HOMO level of the guest molecule from the p-type host molecule.

Brief Description of the Drawings

[0051]

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Mode for Carrying Out the Invention

[0052] The embodiments will be described in detail with reference to the drawings. However, the present invention is not limited to the following description, and it will be easily understood by those skilled in the art that the form and details can be variously changed without departing from the spirit and scope of the present invention. Therefore, the present invention should not be construed as being limited to the description of the embodiments shown below. In the configuration of the invention described below, the same reference numerals are commonly used for the same parts or parts having the same function in different drawings, and the repeated description thereof will be omitted. ​​​​​

[0053] (Embodiment 1) In this embodiment, the principle of the light-emitting element according to one aspect of the present invention will be described with reference to FIG. 2. FIG 2(A) shows the energy states of these molecules when two n-type host molecules (H_n_1, H_n_2), one guest molecule (G) and two p-type host molecules (H_p_1, H_p_2) are arranged linearly . Each molecule has a HOMO and a LUMO respectively

[0054] Here, for simplicity of explanation, the LUMO level En of the n-type host molecule and the L UMO level Ea of the guest molecule are made equal, and the HOMO level Ep of the p-type host molecule and the HO MO level Eb of the guest molecule are made equal, but it is not limited to such a case, -0.3 [electron volts] <Ea - En < +0.3 [electron volts], -0.3 [electron volts]) < Eb - Ep < +0 .3 [electron volts] is sufficient. Also, the difference between the LUMO level and the HOMO level of the n-type host molecule (or p-type host molecule) is preferably more than 0.5 electron volts greater than the difference between the LUMO level and the HOMO level of the guest molecule .

[0055] In the ground state, all of the n-type host molecule, p-type host molecule, and guest molecule have two electrons in the HOMO and no electrons in the LUMO. For example, the n-type host molecule H_n_2, the guest molecule G, and the p-type host molecule H_p_2 have two electrons in the HOMO and no electrons in the LUMO .

[0056] On the other hand, since holes are injected from the anode (right side of the figure) and electrons are injected from the cathode (left side of the figure), the n-type host molecule H_n_1 has an electron in the LUMO, and the p-type host molecule H_p_1 has a hole in the HOMO There is only one electron (one hole). That is, the n-type host molecule H _n_1 is an anion, and the p-type host molecule H_p_1 is a cation.

[0057] Electrons and holes conduct while hopping between such n-type host molecules and p-type host molecules. And as shown in Fig. 2(B), electrons are injected into the LUMO of the guest molecule and holes are injected into the HOMO (direct recombination process), and the guest molecule becomes an excited state (intramolecular exciton, exciton). Thus, even in the direct excitation recombination process, the phenomenon in which carriers are directly injected from the n-type host and p-type host into the guest, in particular, is called Guest Coupled with Co mplementary Hosts (GCCH).

[0058] By the way, as is also clear from Fig. 2, the difference between the LUMO level and the HOMO level of the n-type host molecule and the difference between the LUMO level and the HOMO level of the p-type host molecule are both much larger than the difference between the LU MO level and the HOMO level of the guest molecule. Therefore, the probability that the triplet excited state of the guest transfers to the triplet excited state of the n-type host or p-type host by the Förster mechanism or the Dexter mechanism is quite small.

[0059] That is, as shown in Fig. 2(C), when the ground states of the guest molecule G and the n-type host molecule H_n_1 (S0_G and S0_H_n_1, respectively) are used as a reference, the energy level T1_H_n_1 of the triplet excited state of the n-type host molecule H _n_1 is higher than the energy level T1_G of the triplet excited state of the guest molecule G by ΔEt (≧0.15 electron volts). Therefore, this transition is unlikely to occur at room temperature. In Fig. 2(C), S1_G and S1_H_n_1 ​​​​are, respectively, the energy of the singlet excited state of the guest molecule G and the n-type host molecule H_n_1 level.

[0060] In Fig. 2(C), the energy state of the n-type host molecule was described. However, for the p-type host molecule as well, if the energy level of its triplet excited state is higher than the energy level of the triplet excited state of the guest molecule, the same effect can be obtained.

[0061] Strictly speaking, the difference between the LUMO level and the HOMO level of a molecule is not the same as the energy difference between its triplet excited state and the ground state, but there is a certain correlation. For example, (acetylacetonato)bis(4,6-diphenylpyrimidinato)iridium(III) (abbreviation: [Ir(dppm)2(acac)]) described later is used as a guest. Its difference between the HOMO level and the LUMO level is 2.58 electron volts, while the energy difference between its triplet excited state and the ground state is 2.22 electron volts. Also, 2-[3-(dibenzothiophen-4-yl)phenyl]dibenz[f,h]quinoxaline (abbreviation: 2mDBTPDBq-II) used as an n-type host has values of 3.10 electron volts and 2.5 4 electron volts, respectively, and 4,4'-di(1-naphthyl)-4' '-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PC BNBB) used as a p-type host has values of 3.15 electron volts and 2.40 electron volts, respectively. By the way, when using the above-mentioned [Ir(dppm)2(acac)] as a guest, 2mDBTPDBq-II as an n-type host, and PCBNBB as a p-type host, 2mDBTP 4,4'-di(1-naphthyl)-4' '-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PC BNBB) has values of 3.15 electron volts and 2.40 electron volts, respectively.

[0062] Incidentally, when using the above [Ir(dppm)2(acac)] as a guest, 2mDBTPDBq-II as an n-type host, and PCBNBB as a p-type host, 2mDBTP 2mDBTPDBq-II, and PCBNBB, respectively, Energy difference between the triplet excited state and the ground state of DBq-II and the triplet state of PCBNBB The energy difference between the excited state and the ground state (optical measurements show that it is 2.54 electrons each) t, 2.40 eV) is the energy difference between the triplet excited state of the guest and the ground state ( The optical measurement results show that the electron density is 0.18 electron volts higher than 2.22 electron volts, so The triplet excited state of the cation is hardly transferred to the host.

[0063] In addition, (dipivaloylmethanato)bis(3,5-dimethyl-2-phenylpyridinium) Radinato)iridium(III) (abbreviation: [Ir(mppr-Me)2(dpm)]) The triplet excited state and ground state of [Ir(mppr-Me)2(dpm)] can also be used. The energy difference between this state and the valence state is 2.24 eV, as determined by optical measurements.

[0064] Therefore, 2mDBTPDBq-II was used as the n-type host and PCBNB was used as the p-type host. When B is used, the energy difference between the triplet excited state and the ground state is [Ir(mp The energy difference between the triplet excited state and the ground state of )] is 0.16 Since the energy is more than 10 electron volts higher, the triplet excited state of the guest is hardly transferred to the host.

[0065] The above is a direct recombination process in which electrons and holes are injected into the guest. The host molecule forms an exciplex, which transfers energy to the guest molecule, The guest molecule can also be in an excited state. In this case, the energy transfer involves Use the Star or Dexter mechanism.

[0066] An exciplex is formed by the interaction between different molecules in the excited state. It is generally known that an exciplex is likely to be formed between a material with a relatively deep LUMO level and a material with a shallow HOMO level. For example, a p-type host can be used for the former and an n-type host for the latter. Here, the HOMO and LUMO levels of the n-type host and the p-type host are different from each other, and the HOMO level of the n-type

[0067] host < the HOMO level of the p-type host < the LUMO level of the n-type host < the LUMO level of the p-type host in this order. When an exciplex is formed by this n-type host and p-type host, the LUMO

[0068] level of the exciplex is derived from the n-type host, and the HOMO level is derived from the p-type host. Therefore , the energy difference of the exciplex is smaller than the energy difference of the n-type host and the energy difference of the p-type host. That is, compared with the emission wavelengths of the n-type host and the p-type host respectively, the emission wavelength of the exciplex becomes longer. The formation process of the exciplex can be roughly divided into the following two processes.

[0069] <<Electroplex>> In this specification, electroplex refers to the direct formation of an exciplex from the ground-state n-type host and the ground-state p type host. As described above, in the Förster mechanism and the Dexter mechanism, when electrons and holes recombine in the host,

[0070] the excitation energy moves from the excited-state host to the guest, and the guest reaches the excited state and emits light.

[0071] ​​ Here, before the excitation energy is transferred from the host to the guest, the host itself emits light, or the excitation energy is converted into thermal energy, resulting in a partial loss of the excitation energy. In particular, when the host is in the singlet excited state, the excitation lifetime is shorter than when it is in the triplet excited state, so the deactivation of singlet excitons is likely to occur. The deactivation of excitons is one of the factors contributing to the reduction of the lifetime of the light-emitting device.

[0072] On the other hand, in one aspect of the present invention, since the n-type host and the p-type host are present in the same light-emitting layer, the n-type host molecules and the p-type host molecules are often in a state of carrying carriers (anions and cations), and thus an exciplex is often formed. Therefore, it is difficult to form singlet excitons of n-type host molecules with a short excitation lifetime or singlet excitons of p-type host molecules. That is, in most cases, an exciplex is directly formed without forming singlet excitons of individual molecules. As a result, the deactivation of the above singlet excitons can also be suppressed. Then,

[0073] energy transfer occurs from the generated exciplex to the guest, and a light-emitting device with high luminous efficiency can be obtained.

[0074] ≪Formation of exciplex by excitons≫ As another process, after one of the n-type host molecules and the p-type host molecules, which are the hosts, forms a singlet exciton, a elementary process of interacting with the other in the ground state to form an exciplex can be considered. Different from the exciplex, in this case, once a singlet exciton of the n-type host molecule or the p-type host molecule is generated, if this can be quickly converted into an exciplex, the deactivation of singlet excitons can still be suppressed. As described above, as for the n-type host, ​ When the n-type host and the p-type host are present in the same light-emitting layer, this process is less likely to occur.

[0075] For example, the n-type host is an electron-trapping compound, while the p-type host is a hole-trapping compound. When the difference in the HOMO levels and the difference in the LUMO levels of these compounds are large (specifically, the difference is 0.3 eV or more), electrons preferentially enter the n-type host molecules, and holes preferentially enter the p-type host molecules. In this case, it is considered that the process of forming an electrocomplex is preferred over the process of forming an exciplex through a singlet exciton.

[0076] By the way, the energy transfer from the exciplex formed as described above to the guest molecule is due to the Förster mechanism or the Dexter mechanism. However, as described above, in these mechanisms, for example, it is preferable that the energy difference between the triplet excited state and the ground state of the host molecule is smaller than the energy difference between the triplet excited state and the ground state of the guest molecule.

[0077] In this case, the energy difference between the triplet excited state and the ground state of the exciplex corresponds to the difference between the LUMO level of the n-type host molecule and the HOMO level of the p-type host molecule. When these are equal to or close to the difference between the LUMO level and the HOMO level of the guest molecule, energy can be transferred efficiently and the guest molecule can be brought into the triplet excited state, and the exciplex itself returns to the ground state.

[0078] However, since the exciplex is stable only in the excited state, when it returns to the ground state, it separates into the n-type host molecule and the p-type host molecule. And, as described above, the energy difference between these triplet excited states and the ground state is smaller than the energy difference between the triplet excited state and the ground state of the guest molecule. Because it is large, it is extremely unlikely for the triplet excited state of the guest molecule to transfer energy to any host molecule at room temperature. This is extremely unlikely to occur at room temperature.

[0079] (Embodiment 2) In this embodiment, the principle of the light-emitting element according to one aspect of the present invention will be described with reference to FIG. 3. FIG. 3(A) shows the energy states of these molecules when two n-type host molecules (H_n_1, H_n_2), one guest molecule (G), and two p-type host molecules (H_p_1, H_p_2) are arranged linearly. Each molecule has a HOMO and a LUMO, respectively. Here, the LUMO level En of the n-type host molecule is 0.

[0080] 1 electron volt or more higher than the LUMO level Ea of the guest molecule, and the HOMO level Ep of the p-type host molecule is higher than the HOM O level Eb of the guest molecule. Also, the difference between the LUMO level and the HOMO level of the n-type host molecule and the difference between the LUMO level and the HOMO level of the p-type host molecule are both preferably 0.5 electron volts or more larger than the difference between the LUM O level and the HOMO level of the guest molecule. As shown in FIG. 3(A), holes are injected from the anode (right side of the figure), and electrons are injected from the cathode (left side of the figure).

[0081] Therefore, the n-type host molecule H_n_1 has electrons in its LUMO, and the p-type host molecule H _p_1 is in a state where there is only one electron in its HOMO (there is one hole). Therefore, the n-type host molecule H_n_1 is an anion, and the p-type host molecule H_p_1 is a cation. That is, The electrons and holes conduct while hopping between such n-type host molecules and p-type host molecules.

[0082] The electrons and holes conduct while hopping between such n-type host molecules and p-type host molecules. As shown in the figure, since the LUMO level of the p-type host molecule is higher than that of the n-type host molecule, electrons are conducted through the n-type host molecule. Also, since the HOMO level of the n-type host molecule is lower than that of the p-type host molecule, holes are conducted through the p-type host molecule. And, as shown in Fig. 3(B), electrons are injected into the LUMO of the guest molecule, and the guest molecule becomes an anion. Here, the LUMO level of the n-type host molecule is 0.1 electron volt or more higher than that of the guest molecule, and of course, the LUMO level of the p-type host molecule is even higher. Then, the electrons that enter the LUMO of the guest molecule become in a metastable state, so to speak, trapped by the guest molecule. As a result, since the guest molecule becomes an anion with a negative charge, it attracts the surrounding holes by Coulomb interaction (denoted as F in the figure). Therefore, as shown in Fig. 3(C), the holes in the p-type host molecule H_p_2 are injected into the guest molecule G. Since the Coulomb interaction extends relatively far, electrons and holes efficiently gather in the guest molecule.

[0083] At this time, the electrons in the LUMO of the guest molecule G recombine with the holes in the HOMO of the p-type host molecule H_p_2 (that is, the electrons in the LUMO of the guest molecule G move to the HOMO of the p-type host molecule H_p_2, or the holes in the HOMO of the p-type host molecule H_p_2 move to the LUMO of the guest molecule G), and light emission occurs at that stage. Also, if the above electron transfer is prohibited, the holes in the HOMO of the p-type host molecule H_p_2

[0084]

[0084]

[0085]

[0085]

[0086]

[0086] moves to the HOMO of the guest molecule G, and the guest molecule G becomes excited. Subsequently, the guest molecule G transitions to the ground state, and light emission occurs during this process. The guest molecule G transitions to the ground state, and light emission occurs during this process.

[0087] To attract holes to the guest by Coulomb interaction, when (the HOMO level of the p-type host) - (the HOMO level of the guest) is ΔEp, and (the LUMO level of the n-type host) - (the LUMO level of the guest) is ΔEn, then ΔEp < ΔEn + 0.2 [electron volts], preferably, Δ Ep < ΔEn is advisable. Through the above actions, holes and electrons recombine within the guest molecule. .

[0088] The above process occurs because the guest molecule becomes an anion. If the charge of the guest molecule is neutral, the HOMO level of the guest molecule is lower than that of the p-type host molecule, and the possibility of holes being injected into the guest molecule is low.

[0089] Figure 3 shows that the LUMO level En of the n-type host molecule is higher than the LUMO level Ea of the guest molecule, and also when the HOMO level Ep of the p-type host molecule is higher than the HOMO level Eb of the guest molecule. However, conversely, when the HOMO level Ep of the p-type host molecule is 0.1 electron volts or more lower than the HOMO level Eb of the guest molecule, and the LUMO level En of the n-type host molecule is 0.1 electron volts or more lower than the LUMO level Ea of the guest molecule, by the same principle, holes and electrons recombine efficiently within the guest molecule. In this case, holes are first injected into the HOMO of the guest molecule, and electrons are injected into the guest molecule due to its Coulomb interaction. efficiently recombine. In this case, holes are first injected into the HOMO of the guest molecule, and electrons are injected into the guest molecule due to its Coulomb interaction.

[0090] Note that when the LUMO level En of the n-type host molecule is higher than the LUMO level Ea of the guest molecule, and also In addition, when the HOMO level Ep of the p-type host molecule is lower than the HOMO level Eb of the guest molecule, In this case, the guest can be excited more efficiently by injecting a charge. The LUMO level En of the n-type host molecule is at least 0.1 lower than the LUMO level Ea of the guest molecule. 1 electron volt higher, or the HOMO level Ep of the p-type host molecule is higher than the HOMO level of the guest molecule It is preferably at least 0.1 eV lower than Eb.

[0091] In addition, when an anionized n-type host molecule and a cationized p-type host molecule are adjacent to each other, In this case, the two molecules may be in an excited complex state. To achieve this, the above-mentioned energy transfer process must be carried out. In this case, The energy difference between the excited state and the ground state and the energy difference between the triplet excited state and the ground state of the guest molecule It is better for the energy difference to be as close as possible.

[0092] If the LUMO level of the n-type host molecule is 0.1 electron volts lower than the LUMO level of the guest molecule, If the HOMO level of the p-type host molecule is higher than the HOMO level of the guest molecule by 0. Select a material that is one electron volt lower than the excited state of the exciplex and measure the energy between the excited and ground states. The energy difference between the triplet excited state and the ground state of the guest molecule is as equal as possible. It would be better to make it so.

[0093] Specifically, the LUMO quasi-atomic complex of the guest [Ir(dppm)2(acac)] is The HOMO level is -2.98 eV and -5.56 eV, respectively. In addition, 2mDBTPDBq-II, which is used as an n-type host, has a value of -2.78 The electron volt is -5.88 electron volts, and PCBNBB used as a p-type host is -2.31 electron volts and -5.46 electron volts, respectively.

[0094] In this combination, the LUMO level of the guest is lower than the LU MO levels of the n-type host and the p-type host. In particular, it is 0.2 electron volts lower than the LUMO level of the n-type host. Therefore, the guest molecule tends to trap electrons and become an anion. Also, the HOMO level of the guest molecule is higher than the HOMO level of the n-type host molecule, but 0.1 electron volts lower than the HOMO level of the p-type host molecule.

[0095] Therefore, as shown in Figure 3, electrons are first injected into the LUMO of the guest, and holes are injected into the guest due to its Coulomb interaction, resulting in luminescence.

[0096] Also, the LUMO level of [Ir(mppr-Me)2(dpm)] is -2.77 electron volts which is almost the same as the LUMO level (-2.78 electron volts ) of the n-type host (2mDBTPDBq-II). Also, the HOM O level of [Ir(mppr-Me)2(dpm)] is -5.50 electron volts, which is 0.07 electron volts lower than the HOMO level (- 5.43 electron volts) of the p-type host (PCBNBB).

[0097] These values indicate that when [Ir(mppr-Me)2(dpm)] is used together with the above n-type host and p-type host, its ability to trap electrons and holes is inferior to that of [Ir(dppm )2(acac)].

[0098] (Embodiment 3) In this embodiment, a light-emitting element according to one aspect of the present invention will be described with reference to FIG. 1(B). FIG 1(B) is a diagram showing a light-emitting element having an EL layer 110 between a first electrode 103 and a second electrode 104. The light-emitting element in FIG. 1(B) is sequentially stacked on the first electrode 103 with a first carrier injection layer 105, a first carrier transport layer 106, a light-emitting layer 102, a second carrier transport layer 108, a second carrier injection layer 107, and further provided thereon with a second electrode 104. The EL layer 110 is composed of, in addition to the light-emitting layer 102, a first carrier injection layer 105, a first carrier transport layer 106, a second carrier transport layer 10 8, and a second carrier injection layer 107. Note that the EL layer 110 does not necessarily have all of these layers . Here, the first electrode 103 is one of an anode or a cathode, and the second electrode 104 is the other of an anode or a cathode. Also, the first carrier is one of a hole or an electron, and the second carrier is the other of a hole or an electron. Further, if the first electrode is an anode, the first carrier is a hole, and if the first electrode is a cathode, the first carrier is an electron. Also the first carrier injection layer 105 and the second carrier injection layer 107 are either a hole injection layer or an electron injection layer, and the first carrier transport layer 106 and the second carrier transport layer 108

[0099] are either a hole transport layer or an electron transport layer . As the anode, it is preferable to use a metal, an alloy, a conductive compound, or a mixture thereof having a large work function (specifically, 4.0 eV or more). Specifically, for example, indium oxide . Also, the first carrier injection layer 105 and the second carrier injection layer 107 are either a hole injection layer or an electron injection layer, and the first carrier transport layer 106 and the second carrier transport layer 108 are either a hole transport layer or an electron transport layer .

[0100] As the anode, it is preferable to use a metal, an alloy, a conductive compound, or a mixture thereof having a large work function (specifically, 4.0 eV or more). Specifically, for example, indium oxide . Containing indium tin oxide (ITO), silicon or silicon oxide Indium tin oxide having, indium zinc oxide (Indium Zinc Oxide), indium oxide containing tungsten oxide and zinc oxide (IWZO) and the like. These conductive metal oxide films are usually formed by sputtering, but may also be produced by applying a sol-gel method or the like.

[0101] For example, an indium zinc oxide film can be formed by sputtering using a target containing 1 to 20 wt% of zinc oxide with respect to indium oxide. Also, an IWZO film can be formed by sputtering using a target containing 0.5 to 5 wt% of tungsten oxide and 0 .1 to 1 wt% of zinc oxide with respect to indium oxide. In addition, graphene, gold, platinum, nickel, tungsten, chromium, molybdenum, iron , cobalt, copper, palladium, or nitrides of metal materials (for example, titanium nitride) and the like can be mentioned.

[0102] However, when the layer formed in contact with the anode in the EL layer 110 is formed using a composite material obtained by mixing an organic compound described later with an electron acceptor, the material used for the anode can be various metals, alloys, electrically conductive compounds, and mixtures thereof regardless of the work function. For example, aluminum, silver, alloys containing aluminum (for example, Al-Si), etc. can also be used. The anode can be formed by, for example, sputtering

[0102] or vapor deposition (including vacuum vapor deposition) and the like.

[0103] ​​​​​The cathode is preferably formed using a metal, alloy, electroconductive compound having a low work function (preferably 3.8 eV or less), , and mixtures thereof. Specifically, elements belonging to Group 1 or Group 2 of the periodic table, that is, alkali metals such as lithium and cesium, and alkaline earth metals such as calcium and strontium, magnesium, and alloys containing these (e.g., Mg-Ag, Al-Li), europium, ytterbium, and other rare earth metals and alloys containing these, as well as aluminum, silver, etc. can be used. However, when a layer formed in contact with the cathode in the EL layer 110 uses a composite material formed by mixing an organic compound and an electron donor (donor) described later, regardless of the work function, various conductive materials such as Al, Ag, ITO, indium tin oxide containing silicon or silicon oxide, etc. can be used. When forming the cathode, a vacuum evaporation method or a sputtering method can be used. Also, when using a silver paste or the like, a coating method

[0104] or an inkjet method can be used. The hole injection layer is a layer containing a substance with high hole injection properties. As substances with high hole injection properties, metal oxides such as molybdenum oxide, titanium oxide, vanadium oxide, rhenium oxide, ruthenium oxide, chromium oxide, zirconium oxide, hafnium oxide, tantalum oxide, silver oxide, tungsten oxide, manganese oxide, etc. can be used. Also, phthalocyanine-based compounds such as phthalocyanine (abbreviation: H2Pc), copper(II) phthalocyanine (abbreviation: CuPc), etc. can be used.

[0105]

[0106] ​​​​​​​In addition, 4,4’,4’’-tris(N,N-diphenylamino ) triphenylamine (abbreviation: TDATA), 4,4’,4’’-tris[N-(3-meth ylphenyl)-N-phenylamino] triphenylamine (abbreviation: MTDATA), 4 ,4’-bis[N-(4-diphenylaminophenyl)-N-phenylamino] biphenyl (abbreviation: DPAB), 4,4’-bis(N-{4-[N’-(3-methylphenyl)- N’-phenylamino] phenyl}-N-phenylamino) biphenyl (abbreviation: DNTP D), 1,3,5-tris[N-(4-diphenylaminophenyl)-N-phenylamino benzene (abbreviation: DPA3B), 3-[N-(9-phenylcarbazol-3-yl )-N-phenylamino]-9-phenylcarbazole (abbreviation: PCzPCA1), 3, 6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9- phenylcarbazole (abbreviation: PCzPCA2), 3-[N-(1-naphthyl)-N-( 9-phenylcarbazol-3-yl) amino]-9-phenylcarbazole (abbreviation: P CzPCN1), etc., aromatic amine compounds, etc. can be used.

[0107] Furthermore, high molecular compounds (oligomers, dendrimers, polymers, etc.) can also be used. For example, poly(N-vinylcarbazole) (abbreviation: PVK), poly(4-vinyltri enylamine) (abbreviation: PVTPA), poly[N-(4-{N’-[4-(4-diphen ylamino) phenyl] phenyl-N’-phenylamino} phenyl) methacrylamide] (abbreviation: PTPDMA), poly[N,N’-bis(4-butylphenyl)-N,N’-bi Examples thereof include polymer compounds such as sphenylbenzidine (abbreviation: Poly-TPD). In addition, poly(3,4-ethylenedioxythiophene) / poly(styrenesulfonic acid) (PEDOT / PSS), polyaniline / poly(styrenesulfonic acid) (PAni / PSS ), and other polymer compounds to which an acid is added can be used.

[0108] Further, as the hole injection layer, a composite material formed by mixing an organic compound and an electron acceptor may be used. Since holes are generated in the organic compound by such an electron acceptor in such a composite material, it has excellent hole injection properties and hole transport properties. In this case, the organic compound is preferably a material excellent in transporting the generated holes (a substance having high hole transportability). Since holes are generated in the organic compound by such an electron acceptor in such a composite material, it has excellent hole injection properties and hole transport properties. In this case, the organic compound is preferably a material excellent in transporting the generated holes (a substance having high hole transportability). Since holes are generated in the organic compound by such an electron acceptor in such a composite material, it has excellent hole injection properties and hole transport properties. In this case, the organic compound is preferably a material excellent in transporting the generated holes (a substance having high hole transportability). Since holes are generated in the organic compound by such an electron acceptor in such a composite material, it has excellent hole injection properties and hole transport properties. In this case, the organic compound is preferably a material excellent in transporting the generated holes (a substance having high hole transportability).

[0109] As the organic compound used in the composite material, various compounds such as aromatic amine compounds, carbazole derivatives, aromatic hydrocarbons, and polymer compounds (oligomers, dendrimers, polymers, etc.) can be used. Note that the organic compound used in the composite material is preferably an organic compound having high hole transportability. Specifically, it is preferably a substance having a hole mobility of 10 cm / Vs or more. However, as long as it is a substance having higher hole transportability than electrons, other substances may be used. Hereinafter, the organic compounds that can be used in the composite material will be specifically listed. As the organic compound used in the composite material, various compounds such as aromatic amine compounds, carbazole derivatives, aromatic hydrocarbons, and polymer compounds (oligomers, dendrimers, polymers, etc.) can be used. Note that the organic compound used in the composite material is preferably an organic compound having high hole transportability. Specifically, it is preferably a substance having a hole mobility of 10 -6 cm 2 / Vs or more. However, as long as it is a substance having higher hole transportability than electrons, other substances may be used. Hereinafter, the organic compounds that can be used in the composite material will be specifically listed. As the organic compound used in the composite material, various compounds such as aromatic amine compounds, carbazole derivatives, aromatic hydrocarbons, and polymer compounds (oligomers, dendrimers, polymers, etc.) can be used. Note that the organic compound used in the composite material is preferably an organic compound having high hole transportability. Specifically, it is preferably a substance having a hole mobility of 10 cm / Vs or more. However, as long as it is a substance having higher hole transportability than electrons, other substances may be used. Hereinafter, the organic compounds that can be used in the composite material will be specifically listed.

[0110] Examples of the organic compound that can be used in the composite material include, for example, TDATA, MTDATA , DPAB, DNTPD, DPA3B, PCzPCA1, PCzPCA2, PCzPCN 1, 4,4'-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB or α-NPD), N,N'-bis(3-methylphenyl)-N,N'-diphenyl- -[1,1'-biphenyl]-4,4'-diamine (abbreviation: TPD), 4-phenyl -4'-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: BPAFL P), etc. aromatic amine compounds, 4,4'-di(N-carbazolyl)biphenyl (abbreviation: CBP), 1,3,5-tris[4-(N-carbazolyl)phenyl]benzene (abbreviation: TCPB), 9-[4-(N-carbazolyl)]phenyl-10-phenylanthracene (abbreviation: CzPA), 9-phenyl-3-[4-(10-phenyl-9-anthryl)ph enyl]-9H-carbazole (abbreviation: PCzPA), 1,4-bis[4-(N-carb azolyl)phenyl]-2,3,5,6-tetraphenylbenzene and other carbazole derivatives can be used.

[0111] Also, 2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation: t- BuDNA), 2-tert-butyl-9,10-di(1-naphthyl)anthracene, 9 ,10-bis(3,5-diphenylphenyl)anthracene (abbreviation: DPPA), 2-t ert-butyl-9,10-bis(4-phenylphenyl)anthracene (abbreviation: t-B uDBA), 9,10-di(2-naphthyl)anthracene (abbreviation: DNA), 9,10- diphenylanthracene (abbreviation: DPAnth), 2-tert-butylanthracene( abbreviation: t-BuAnth), 9,10-bis(4-methyl-1-naphthyl)anthracene (abbreviation: DMNA), 9,10-bis[2-(1-naphthyl)phenyl]-2-tert -butylanthracene, 9,10-bis[2-(1-naphthyl)phenyl]anthracene , aromatic hydrocarbon compounds such as 2,3,6,7-tetramethyl-9,10-di(1-naphthyl)anthracene can be used.

[0112] Furthermore, 2,3,6,7-tetramethyl-9,10-di(2-naphthyl)anthracene, 9,9'-bianthryl, 10,10'-diphenyl-9,9'-bianthryl, 10, 10'-bis(2-phenylphenyl)-9,9'-bianthryl, 10,10'-bis [(2,3,4,5,6-pentaphenyl)phenyl]-9,9'-bianthryl, anth racene, tetracene, rubrene, perylene, 2,5,8,11-tetra(tert-but yl)perylene, pentacene, coronene, 4,4'-bis(2,2-diphenylvinyl) biphenyl (abbreviation: DPVBi), 9,10-bis[4-(2,2-diphenylvinyl) phenyl]anthracene (abbreviation: DPVPA) and other aromatic hydrocarbon compounds can be used.

[0113] In addition, as the electron acceptor, organic compounds such as 7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation: F4-TCNQ), chloranil, and transition metal oxides can be mentioned. Also, oxides of metals belonging to Groups 4 to 8 in the periodic table of elements can be mentioned. Specifically, vanadium oxide, niobium oxide, tantalum oxide, chromium oxide, molybdenum oxide, tungsten oxide, manganese oxide, and rhenium oxide are preferred because of their high electron accepting properties. Among them, molybdenum oxide is particularly preferred because it is stable in the atmosphere, has low hygroscopicity, and is easy to handle.

[0114] ​​ In addition, composite materials can be formed using the above-described polymer compounds such as PVK, PVTPA, PTPDMA, and Poly-TPD, and they may be used for the hole injection layer. The hole transport layer is a layer containing a substance with high hole transport properties. Examples of substances with high hole transport properties include

[0115] NPB, TPD, BPAFLP, 4,4'-bis[N-(9,9-dimethylfluoren- 2-yl)-N-phenylamino]biphenyl (abbreviation: DFLDPBi), 4,4'-bi s[N-(spiro-9,9'-bifluoren-2-yl)-N-phenylamino]biphe nyl (abbreviation: BSPB) and other aromatic amine compounds can be used. The substances described here mainly have a hole mobility of 10 cm -6 / Vs or more. However, as long as the substance has higher hole transport properties than electrons, other substances may also be used. Note that the layer containing a substance with high hole transport properties may be not only a single layer but also a laminate of two or more layers composed of the above substances. 2 / Vs or more. However, as long as the substance has higher hole transport properties than electrons, other substances may also be used. Note that the layer containing a substance with high hole transport properties may be not only a single layer but also a laminate of two or more layers composed of the above substances. / Vs or more. However, as long as the substance has higher hole transport properties than electrons, other substances may also be used. Note that the layer containing a substance with high hole transport properties may be not only a single layer but also a laminate of two or more layers composed of the above substances. / Vs or more. However, as long as the substance has higher hole transport properties than electrons, other substances may also be used. Note that the layer containing a substance with high hole transport properties may be not only a single layer but also a laminate of two or more layers composed of the above substances. / Vs or more. However, as long as the substance has higher hole transport properties than electrons, other substances may also be used. Note that the layer containing a substance with high hole transport properties may be not only a single layer but also a laminate of two or more layers composed of the above substances.

[0116] In addition, for the hole transport layer, carbazole derivatives such as CBP, CzPA, and PCzPA, or anthracene derivatives such as t-BuDNA, DNA, and DPAnth may be used.

[0117] In addition, polymer compounds such as PVK, PVTPA, PTPDMA, and Poly-TPD can also be used for the hole transport layer. In addition, polymer compounds such as PVK, PVTPA, PTPDMA, and Poly-TPD can also be used for the hole transport layer.

[0118] The light-emitting layer 102 is a layer containing a light-emitting substance. The light-emitting layer 102 of the present embodiment has a phosphorescent compound as a guest and an n-type host and a p-type host as hosts. The n-type host ( has a phosphorescent compound as a guest and an n-type host and a p-type host as hosts. The n-type host ( Alternatively, two or more types of p-type hosts can be used.

[0119] As the phosphorescent compound, an organometallic complex is preferable, and an iridium complex is particularly preferable. Considering the energy transfer by the above-described Förster mechanism, the molar extinction coefficient of the absorption band located on the longest wavelength side of the phosphorescent compound is preferably 2000 M ·cm -1 ·cm -1 or more, and particularly preferably 5 000 M -1 ·cm -1 or more.

[0120] Examples of the compound having such a large molar extinction coefficient include [Ir(mppr-M e)2(dpm)] and [Ir(dppm)2(acac)]. In particular, When a material such as [Ir(dppm)2(acac)] having a molar extinction coefficient of 5000 M -1 ·cm - 1 or more is used, a light-emitting element having an external quantum efficiency of about 30% can be obtained.

[0121] Examples of the n-type host include, in addition to the above-described 2mDBTPDBq-II, 2-[4-( 3,6-diphenyl-9H-carbazol-9-yl)phenyl]dibenzof[h]quinoxaline (abbreviation: 2CzPDBq-III), 7-[3-(dibenzothiophen-4- yl)phenyl]dibenzof[h]quinoxaline (abbreviation: 7mDBTPDBq-II) , and 6-[3-(dibenzothiophen-4-yl)phenyl]dibenzof[h]quinoxaline (abbreviation: 6mDBTPDBq-II), and any one of the compounds that easily accept electrons can be used.

[0122] In addition to the above-mentioned PCBNBB, as the p-type host, compounds that easily accept holes, such as 4,4'-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB or α-NPD), and 4-phenyl-4'-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBA1BP), may be used. However, it is not limited to these. For example, any combination of an n-type host and a p-type host that satisfies the energy level relationship shown in Embodiment 1 or Embodiment 2 may be used.

[0123] The electron transport layer is a layer containing a substance with high electron transport properties. Examples of substances with high electron transport properties include metal complexes such as Alq3, tris(4-methyl-8-quinolinolato)aluminum (abbreviation: Almq3), bis(10-hydroxybenzo[h]quinolinato)beryllium (abbreviation: BeBq2), BAlq, Zn(BOX)2, and bis[2-(2-hydroxyphenyl)benzothiazolato]zinc (abbreviation: Zn(BTZ)2).

[0124] In addition, 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation: PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene (abbreviation: OXD-7), 3-(4-tert-butylphenyl)-4-phenyl-5-(4-biphenylyl)-1,2,4-triazole (abbreviation: TAZ), 3-(4-tert-butylphenyl)-4-(4-ethylphenyl)-5-(4-biphenylyl)-1,2,4-triazole (abbreviation: p-EtTAZ), bathophenanthroline (abbreviation: BPhen), bathocuproine Abbreviation: BCP), 4,4'-bis(5-methylbenzoxazol-2-yl)stilbene Heteroaromatic compounds such as and

[0125] can also be used.

[0125] In addition, polymer compounds such as poly(2,5-pyridine-diyl) (abbreviation: PPy), poly[(9,9-dihexylfluorene-2,7-diyl)-co-(pyridine-3,5-diyl)] (abbreviation: PF -Py), and poly[(9,9-dioctylfluorene-2,7-diyl)-co-(2,2 '-bipyridine-6,6'-diyl)] (abbreviation: PF-BPy) can also be used. The substances described here mainly have an electron mobility of 10 cm -6 / Vs or more. 2 As long as the substance has higher electron transportability than holes, substances other than the above can also be used as the electron transport layer.

[0126] In addition, the electron transport layer may be not only a single layer but also a structure in which two or more layers composed of the above substances are stacked.

[0127] The electron injection layer is a layer containing a substance with high electron injection properties. For the electron injection layer, alkali metals, alkaline earth metals, or their compounds such as lithium, cesium, calcium, lithium fluoride, cesium fluoride, calcium fluoride, lithium oxide, etc. can be used. In addition, rare earth metal compounds such as erbium fluoride can be used. Also, the substances constituting the above-described electron transport layer can be used.

[0128] Alternatively, a composite material formed by mixing an organic compound and an electron donor can be used for the electron injection layer. In such a composite material, electrons are generated in the organic compound by the electron donor.​​​​​​ Therefore, it is excellent in electron injection property and electron transport property. In this case, as the organic compound, it is preferably a material excellent in transporting the generated electrons. Specifically, for example, the substances (such as metal complexes and heteroaromatic compounds) constituting the above-described electron transport layer can be used. As the electron donor, any substance that exhibits electron-donating properties to the organic compound may be used. Specifically, alkali metals, alkaline earth metals, and rare earth metals are preferable, and examples include lithium, cesium, magnesium, calcium, erbium, ytterbium, etc. Also, alkali metal oxides and alkaline earth metal oxides are preferable, and examples include lithium oxide, calcium oxide, barium oxide, etc. Further, a Lewis base such as magnesium oxide can also be used. Also, organic compounds such as tetrathiafulvalene (abbreviation: TTF) can also be used.

[0129] Moreover, the above-described hole injection layer, hole transport layer, light-emitting layer 102, electron transport layer, and electron injection layer can each be formed by a method such as a vapor deposition method (including a vacuum vapor deposition method), an inkjet method, or a coating method.

[0130]

[0131]

[0131] Also, as shown in FIG. 1(C), a plurality of EL layers 110a and 110b may be laminated between the anode and the cathode. In this case, each of the EL layers 110a and 110b has at least a light-emitting layer. It is preferable to provide a charge generation layer 111 between the laminated first EL layer 110a and the second EL layer 110b. The charge generation layer 111 can be formed of the above-described composite material. Also, the charge generation layer 111 may have a laminated structure of a layer made of a composite material and a layer made of another material.

[0132] In this case, the layer made of the other material includes a material having high electron donating property and a material having high electron transporting property. A layer made of a transparent conductive film or a layer made of a transparent conductive film can be used. The device is less susceptible to problems such as energy transfer and quenching, and the range of materials that can be selected is wider. It is easy to obtain a light emitting element having both high light emitting efficiency and a long life. It is also easy to obtain phosphorescence in the EL layer and fluorescence in the other. It can be used in combination with the structure.

[0133] In addition, by making the emission color of each EL layer different, the light-emitting device as a whole can have a desired color. For example, the color of the light emitted from the first EL layer 110a and the color of the light emitted from the second EL layer 110b can be obtained. By making the emission color of 110b complementary, the light emitting element as a whole emits white light. It is also possible to obtain a light-emitting device having three or more EL layers. The same is true.

[0134] Alternatively, as shown in FIG. 1D, a hole injection layer 20 is provided between the anode 201 and the cathode 209. 2, hole transport layer 203, light emitting layer 204, electron transport layer 205, electron injection buffer layer 206 EL layer 210 having a composite material layer 208 in contact with an electron relay layer 207 and a cathode 209. may be formed.

[0135] By providing a composite material layer 208 in contact with the cathode 209, it is possible to form a cathode by, in particular, using a sputtering method. This is preferable because it can reduce damage to the EL layer 210 when forming the electrodes. The composite material layer 208 is formed by adding an acceptor substance to the organic compound having a high hole transporting property. Composite materials containing the same may also be used.

[0136] Furthermore, by providing the electron injection buffer layer 206, the injection barrier between the composite material layer 208 and the electron transport layer 2 05 can be relaxed, so that the electrons generated in the composite material layer 208 can be easily injected into the electron transport layer 205.

[0137] The electron injection buffer layer 206 can be made of substances with high electron injection properties such as alkali metals, alkaline earth metals, rare earth metals, and their compounds (including alkali metal compounds (such as oxides like lithium oxide, halides, carbonates such as lithium carbonate and cesium carbonate), alkaline earth metal compounds (including oxides, halides, carbonates), or compounds of rare earth metals (including oxides, halides, carbonates)).

[0138] When the electron injection buffer layer 206 is formed by including a substance with high electron transport properties and a donor substance, it is preferable to add the donor substance at a ratio of 0.001 or more and 0.1 or less by mass ratio with respect to the substance with high electron transport properties. As the substance with high electron transport properties, the same materials as those of the electron transport layer 205 described above can be used for formation.

[0139] In addition, as the donor substance, in addition to alkali metals, alkaline earth metals, rare earth metals, and their compounds (including alkali metal compounds (such as oxides like lithium oxide, halides, carbonates such as lithium carbonate and cesium carbonate), alkaline earth metal compounds (including oxides, halides, carbonates), or compounds of rare earth metals (including oxides, halides, carbonates)), tetrathianaphthacene (abbreviation: TTN), nickelocene, decamethylnickel Organic compounds such as selenium can also be used.

[0140] Furthermore, it is preferable to form an electron relay layer 207 between the electron injection buffer layer 206 and the composite material layer 208. The electron relay layer 207 does not necessarily have to be provided, but by providing an electron relay layer 207 with high electron transportability, electrons can be quickly sent to the electron injection buffer layer 206.

[0141] The structure in which the electron relay layer 207 is sandwiched between the composite material layer 208 and the electron injection buffer layer 206 is a structure in which the acceptor substance contained in the composite material layer 208 and the donor substance contained in the electron injection buffer layer 206 are less likely to interact with each other and are less likely to inhibit each other's functions. Therefore, an increase in the driving voltage can be prevented.

[0142] The electron relay layer 207 contains a substance with high electron transportability, and the LUMO level of the substance with high electron transportability is formed to be between the LUMO level of the acceptor substance contained in the composite material layer 208 and the LUMO level of the substance with high electron transportability contained in the electron transport layer 205.

[0143] When the electron relay layer 207 contains a donor substance, the donor level of the donor substance is also set to be between the LUMO level of the acceptor substance in the composite material layer 208 and the LUMO level of the substance with high electron transportability contained in the electron transport layer 205. As specific numerical values of the energy level, the LUMO level of the substance with high electron transportability contained in the electron relay layer 207 is preferably -5.0 eV or higher, more preferably -5.0 eV or higher and -3.0 eV or lower.

[0144] ​​​​​​​​​​As a material with high electron transport properties contained in the electron relay layer 207, a phthalocyanine-based material or it is preferable to use a metal complex having a metal-oxygen bond and an aromatic ligand.

[0145] As the phthalocyanine-based material contained in the electron relay layer 207, specifically, CuPc, S nPc (Phthalocyanine tin(II) complex), ZnPc (Phthalocyanine zinc complex), CoPc (Cobal t(II)phthalocyanine, β-form), FePc (Phthal ocyanine Iron) and PhO-VOPc (Vanadyl 2,9,16, 23-tetraphenoxy-29H,31H-phthalocyanine) is preferably used.

[0146] As the metal complex having a metal-oxygen bond and an aromatic ligand contained in the electron relay layer 207, it is preferable to use a metal complex having a metal-oxygen double bond. Since the metal-oxygen double bond has acceptor properties (the property of easily accepting electrons), the transfer (donation and acceptance) of electrons becomes easier. In addition, a metal complex having a metal-oxygen double bond is considered to be stable. Therefore, by using a metal complex having a metal-oxygen double bond, the light-emitting element can be driven more stably at a low voltage.

[0147] As the metal complex having a metal-oxygen bond and an aromatic ligand, a phthalocyanine-based material is preferable . Specifically, VOPc (Vanadyl phthalocyanine), SnO Pc (Phthalocyanine tin(IV) oxide complex) and either TiOPc (Phthalocyanine titanium oxide co mplex) is preferable because the metal-oxygen double bond in its molecular structure easily acts on other molecules and has high acceptor properties.

[0148] Note that as the phthalocyanine-based material described above, those having a phenoxy group are preferable. Specifically, phthalocyanine derivatives having a phenoxy group, such as PhO-VOPc, are preferable. Phthalocyanine derivatives having a phenoxy group are soluble in a solvent. Therefore, they have the advantage of being easy to handle when forming a light-emitting element. Also, since they are soluble in a solvent, they have the advantage that maintenance of the apparatus used for film formation becomes easy.

[0149] The electron relay layer 207 may further contain a donor substance. As the donor substance, alkali metals, alkaline earth metals, rare earth metals, and their compounds (alkali metal compounds such as oxides (such as lithium oxide), halides, carbonates (such as lithium carbonate and cesium carbonate) containing), alkaline earth metal compounds (including oxides, halides, carbonates), or rare earth metal compounds (including oxides, halides, carbonates)), in addition to organic compounds such as tetrathianaphthacene , nickelocene, and decamethylnickelocene can be used. By including these donor substances in the electron relay layer 207, the movement of electrons becomes easy and it becomes possible to drive the light-emitting element at a lower voltage.

[0150] When including a donor substance in the electron relay layer 207, as substances having high electron transport properties, in addition to the materials described above, from the acceptor level of the acceptor substance contained in the composite material layer 208 ​A substance having a high LUMO level can be used. As specific energy levels, , it is preferable to use a substance having a LUMO level of -5.0 eV or more, preferably in the range of -5.0 eV or more and -3.0 eV or less. Examples of such substances include perylene derivatives and nitrogen-containing condensed aromatic compounds. Since the nitrogen-containing condensed aromatic compound is stable, it is a preferable material as a material used to form the electron relay layer 207.

[0151] Specific examples of perylene derivatives include 3,4,9,10-perylenetetracarboxylic dianhydride (abbreviation: PTCDA), 3,4,9,10-perylenetetracarboxylic bisbenzimidazole (abbreviation: PTCBI), N,N'-dioctyl-3,4,9,10-perylenetetracarboxylic diimide (abbreviation: PTCDI-C8H), N,N'-dihexyl- 3,4,9,10-perylenetetracarboxylic diimide (abbreviation: Hex PTC), etc.

[0152] In addition, specific examples of nitrogen-containing condensed aromatic compounds include pyrazino[2,3-f][1,10] phenanthroline-2,3-dicarbonitrile (abbreviation: PPDN), 2,3,6,7,1 0,11-hexacyano-1,4,5,8,9,12-hexaazatriphenylene (abbreviation : HAT(CN)6), 2,3-diphenylpyrido[2,3-b]pyrazine (abbreviation: 2P YPR), 2,3-bis(4-fluorophenyl)pyrido[2,3-b]pyrazine (abbreviation : F2PYPR), etc.

[0153] In addition, 7,7,8,8,-tetracyanoquinodimethane (abbreviation: TCNQ), 1,4 ,5,8-Naphthalenetetracarboxylic dianhydride (abbreviation: NTCDA), perfluoro pentacene, copper hexadecafluorophthalocyanine (abbreviation: F 16 CuPc), N,N’ -bis(2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentadecafluoro octyl)-1,4,5,8-naphthalenetetracarboxylic diimide (abbreviation: NT CDI-C8F), 3’,4’-dibutyl-5,5’’-bis(dicyanomethylene)-5 ,5’’-dihydro-2,2’:5’,2’’-terthiophene) (abbreviation: DCMT), methanofullerene (e.g., [6,6]-phenyl C 61 methyl butyrate), etc. can be used. It is possible to use them.

[0154] When the electron relay layer 207 contains a donor substance, the electron relay layer 207 may be formed by a method such as co-evaporation of a substance with high electron transport property and a donor substance.

[0155] The hole injection layer 202, the hole transport layer 203, the light emitting layer 204, and the electron transport layer 205 may be formed using the above-described materials respectively. Thus, the EL layer 210 of the present embodiment can be fabricated. By the above, the EL layer 210 of the present embodiment can be fabricated. It is possible to fabricate it.

[0156] In the above-described light emitting element, a current flows due to the potential difference generated between the anode and the cathode, and light is emitted by the recombination of holes and electrons in the EL layer. Then, this light is taken out to the outside through either one or both of the anode and the cathode. Therefore, either one or both of the anode and the cathode become electrodes having translucency to visible light. And, this light emission is taken out to the outside through either one or both of the anode and the cathode. Therefore, either one or both of the anode and the cathode become electrodes having translucency to visible light. Accordingly, either one or both of the anode and the cathode become electrodes having translucency to visible light. One or both of the anode and the cathode become electrodes having translucency to visible light.

[0157] Note that the configuration of the layer provided between the anode and the cathode is not limited to the above. The light emitting region A part away from the anode and the cathode so as to prevent quenching caused by the proximity of the metal Any other configuration may be used as long as it has a light-emitting region where holes and electrons recombine at a position away from the anode and the cathode.

[0158] That is, the layer stacking structure is not particularly limited, and layers made of a substance with high electron transportability, a substance with high hole transportability, a substance with high electron injectability, a substance with high hole injectability, a bipolar substance (a substance with high transportability of both electrons and holes), or a hole blocking material can be freely combined with the light-emitting layer to form a structure. A substance with high hole transportability, a substance with high electron injectability, a substance with high hole injectability, a bipolar substance (a substance with high transportability of both electrons and holes), or a hole blocking material can be freely combined with the light-emitting layer to form a structure. That is, the layer stacking structure is not particularly limited, and layers made of a substance with high electron transportability, a substance with high hole transportability, a substance with high electron injectability, a substance with high hole injectability, a bipolar substance (a substance with high transportability of both electrons and holes), or a hole blocking material can be freely combined with the light-emitting layer to form a structure. formed.

[0159] Using the light-emitting element shown in this embodiment, a passive matrix type light-emitting device or an active matrix type light-emitting device in which the driving of the light-emitting element is controlled by a transistor can be manufactured. In addition, the light-emitting device can be applied to an electronic device, a lighting device, or the like.

Example

Example

[0160] In this example, a light-emitting element according to one aspect of the present invention will be described. The chemical formulas of the materials used in this example are shown below. The chemical formulas of the materials used in this example are shown below.

[0161]

Chemical formula

[0162] The manufacturing methods of the light-emitting element 1 and the comparative light-emitting element 2 of this example are shown below.

[0163] (Light-emitting element 1) First, indium tin oxide (ITSO) containing silicon oxide was formed on a glass substrate by sputtering to form a first electrode that functions as an anode. The film thickness was 110 nm, and the electrode area was 2 mm × 2 mm. First, indium tin oxide (ITSO) containing silicon oxide was formed on a glass substrate by sputtering to form a first electrode that functions as an anode. The film thickness was 110 nm, and the electrode area was 2 mm × 2 mm. First, indium tin oxide (ITSO) containing silicon oxide was formed on a glass substrate by sputtering to form a first electrode that functions as an anode. The film thickness was 110 nm, and the electrode area was 2 mm × 2 mm.

[0164] Next, as a pretreatment for forming a light-emitting element on a substrate, the substrate surface was washed with water and baked at 200 °C for 1 hour, and then UV ozone treatment was performed for 370 seconds.

[0165] Thereafter, the substrate was introduced into a heating chamber in a vacuum deposition apparatus whose interior was evacuated to about 10 -4 Pa, and vacuum baking was performed at 170°C for 30 minutes. Then, the substrate was allowed to cool for about 30 minutes. Next, the substrate was introduced into the deposition chamber in the vacuum deposition apparatus, and the substrate on which the first electrode was formed was fixed to the substrate holder provided in the vacuum deposition apparatus so that the surface on which the first electrode was formed faced downward

[0166] Then, after reducing the pressure to about 10 Pa, a hole injection layer was formed by co-depositing BPAFLP and molybdenum(VI) oxide on the first electrode. The film thickness was 40 nm and the ratio of BPAFLP to molybdenum(VI) oxide was adjusted to a weight ratio of 4:2 (= BPAFLP: molybdenum(VI) oxide). -4 Pa. Thereafter, BPAFLP was formed into a film with a thickness of 20 nm on the hole injection layer to form a hole transport layer

[0167] Next, 2mDBTPDBq-II, PCBNBB, and [Ir(mppr-Me)2( dpm)] were co-deposited to form a light-emitting layer on the hole transport layer. Here, the weight ratio of 2mDBTPDBq

[0168] -II, PCBNBB, and [Ir(mppr-Me)2(dpm)] was adjusted to 0.8 :0.2:0.05 (= 2mDBTPDBq-II: PCBNBB: [Ir(mppr- Me)2(dpm)]). Also, the film thickness of the light-emitting layer was 40 nm. Me)2(dpm)]).

[0169] ​​​Next, 2mDBTPDBq-II was formed into a film with a thickness of 10 nm on the light-emitting layer, and the first electron transport layer was formed.

[0170] Next, BPhen was formed into a film with a thickness of 20 nm on the first electron transport layer, and the second electron transport layer was formed.

[0171] Furthermore, lithium fluoride (LiF) was deposited with a film thickness of 1 nm on the second electron transport layer to form an electron injection layer.

[0172] Finally, as the second electrode functioning as a cathode, aluminum was deposited to a film thickness of 200 nm to fabricate the light-emitting device 1 of this example.

[0173] (Comparative light-emitting device 2) The light-emitting layer of the comparative light-emitting device 2 was formed by co-depositing 2mDBTPDBq-II and [Ir(mppr-Me)2 (dpm)]. Here, the weight ratio of 2mDBTPDBq-II and [I r(mppr-Me)2(dpm)] was adjusted to 1:0.05 (=2mDBTPDBq -II:[Ir(mppr-Me)2(dpm)]). Also, the film thickness of the light-emitting layer was set to 40 nm. Except for the light-emitting layer, it was fabricated in the same manner as the light-emitting device 1.

[0174] In the deposition process described above, all depositions used the resistance heating method.

[0175] The device structures of the light-emitting device 1 and the comparative light-emitting device 2 obtained as described above are shown in Table 1. In this example, 2mDBTPDBq-II is the n-type host, PCBNBB is the p-type host, and [Ir (mppr-Me)2(dpm)] is the guest. That is, in the light-emitting device 1, both the n-type host and the p-type host are in the light-emitting layer, whereas in the comparative light-emitting device 2, the p-type host emits light. It does not exist in the optical layer.

[0176]

Table 1

[0177] After performing the operation of encapsulating these light-emitting elements in a glove box under a nitrogen atmosphere so that the light-emitting elements are not exposed to the atmosphere, the operating characteristics of the light-emitting elements were measured. Note that the measurement was performed at room temperature (atmosphere maintained at 25 °C).

[0178] The current density-luminance characteristics of Light-emitting element 1 and Comparative light-emitting element 2 are shown in FIG. 4. In FIG. 4, the horizontal axis represents the current density (mA / cm 2 ), and the vertical axis represents the luminance (cd / m 2 ). Also, the voltage-luminance characteristics are shown in FIG. 5. In FIG. 5, the horizontal axis represents the voltage (V), and the vertical axis represents the luminance (cd / m 2 ). Also, the luminance-current efficiency characteristics are shown in FIG. 6. In FIG. 6, the horizontal axis represents the luminance (cd / m 2 ), and the vertical axis represents the current efficiency (cd / A). Also, the luminance-external quantum efficiency characteristics are shown in FIG. 7. In FIG. 7, the horizontal axis represents the luminance (cd / m 2 ), and the vertical axis represents the external quantum efficiency (%).

[0179]

[0180] 2 Near 1000 cd / m, the voltage (V), current density (mA / cm 2 ), CIE chromaticity coordinates (x, y), current efficiency (cd / A) ), power efficiency (lm / W), and external quantum efficiency (%) of Light-emitting element 1 and Comparative light-emitting element 2 are shown in Table 2.

[0180]

Table 2

[0181] In addition, the emission spectra when a current of 0.1 mA was applied to the light-emitting element 1 and the comparative light-emitting element 2 were , which is shown in FIG. 8. In FIG. 8, the horizontal axis represents wavelength (nm) and the vertical axis represents emission intensity (arbitrary unit). Also, as shown in Table 2, 1200cd / m 2 CIE chromaticity coordinates of light-emitting element 1 at luminance of is (x,y)=(0.56,0.44), which is 960 cd / m 2 Comparison of brightness at The CIE chromaticity coordinates of element 2 were (x,y)=(0.55,0.44). The light-emitting element 1 and the comparative light-emitting element 2 are derived from [Ir(mppr-Me)2(dpm)]. It was found that orange light emission was obtained.

[0182] As can be seen from Table 2 and FIGS. 4 to 7, the light-emitting element 1 has a higher current than the comparative light-emitting element 2. The efficiency, power efficiency, and external quantum efficiency all showed high values. When the light is taken out, total reflection occurs between the substrate and the atmosphere, resulting in a decrease in the internal quantum efficiency. It is said that only 25% to 30% of the light can be extracted to the outside.

[0183] Considering this, the internal quantum efficiency of comparative light-emitting element 2 is at best just under 60%. It is estimated that the internal quantum efficiency of the light-emitting device 1 is increased to about 80%. From the above results, it is possible to realize an element with high external quantum efficiency by applying one embodiment of the present invention. It has been shown that this is possible.

[0184] Next, a reliability test was performed on the light-emitting element 1 and the comparative light-emitting element 2. The results of the reliability test are shown in FIG. In FIG. 9, the vertical axis indicates normalized luminance (%) when the initial luminance is 100%. The horizontal axis indicates the operation time (h) of the element. The reliability test was performed with an initial luminance of 5000 cd / m2 to set, and the light-emitting element 1 was driven under the condition of a constant current density.

[0185] For the comparative light-emitting element 2, the luminance after 120 hours was 58% of the initial luminance. Also, for the light-emitting element 1, the luminance after 630 hours was 65% of the initial luminance. From these results, it was found that the light-emitting element 1 is an element with a longer lifespan compared to the comparative light-emitting element 2. From the above results, it was shown that by applying one aspect of the present invention, a highly reliable element can be realized.

Example

[0186] In this example, a light-emitting element according to one aspect of the present invention will be described. The chemical formulas of the materials used in this example are shown below. The chemical formulas of the materials used in the previous example are omitted. The chemical formulas of the materials used in this example are shown below. The chemical formulas of the materials used in the previous example are omitted.

[0187]

Chemical formula

[0188] The manufacturing method of the light-emitting element 3 of this example is shown below.

[0189] (Light-emitting element 3) First, ITSO was deposited on a glass substrate by sputtering to form the first electrode that functions as an anode. The film thickness was set to 110 nm, and the electrode area was set to 2 mm × 2 mm. The film thickness was 110 nm, and the electrode area was 2 mm × 2 mm.

[0190] Next, as a pretreatment for forming the light-emitting element on the substrate, the substrate surface was washed with water, baked at 200 °C for 1 hour, and then UV ozone treatment was performed for 370 seconds.

[0191] Thereafter, the substrate was introduced into a vacuum evaporation apparatus whose internal pressure was reduced to about 10 -4 Pa, and vacuum evaporation ​In the heating chamber inside the device, after performing vacuum firing at 170 °C for 30 minutes, the substrate was allowed to cool for about 30 minutes.

[0192] Next, the substrate with the first electrode formed thereon was fixed to a substrate holder provided inside a vacuum deposition apparatus such that the surface on which the first electrode was formed faced downward, and the pressure was reduced to about 10 Pa. -4 Then, a hole injection layer was formed by co-depositing BPAFLP and molybdenum(VI) oxide on the first electrode. The film thickness was 40 nm, and the ratio of BPAFLP to molybdenum(VI) oxide was adjusted to a weight ratio of 4:2 (= BPAFLP: molybdenum(VI) oxide).

[0193] Next, BPAFLP was deposited on the hole injection layer to a film thickness of 20 nm to form a hole transport layer.

[0194] Furthermore, 2mDBTPDBq-II, PCBNBB, and [Ir(dppm)2(acac)] were co-deposited to form a light-emitting layer on the hole transport layer. Here, the weight ratio of 2mDBTPDBq-II, PCBNBB, and [Ir(dppm)2(acac)] was adjusted to 0.8:0.2:0.05 (= 2mDBTPDBq-II: PCBNBB: [Ir(dppm)2(acac)]). Also, the film thickness of the light-emitting layer was 40 nm.

[0195] Next, 2mDBTPDBq-II was deposited on the light-emitting layer to a film thickness of 10 nm to form a first electron transport layer.

[0196] Next, BPhen was deposited on the first electron transport layer to a film thickness of 20 nm to form a second electron transport layer.

[0197] ​​​​​​​​​​​​Furthermore, LiF was deposited on the second electron transport layer with a film thickness of 1 nm to form an electron injection layer. .

[0198] Finally, as the second electrode functioning as the cathode, aluminum was deposited to have a film thickness of 200 nm, thereby fabricating the light-emitting device 3 of this example.

[0199] Note that in the above-described deposition process, all depositions were performed using the resistance heating method.

[0200] The device structure of the light-emitting device 3 obtained as described above is shown in Table 3.

[0201]

Table 3

[0202] After performing the operation of sealing the light-emitting device 3 in a glove box under a nitrogen atmosphere so that the light-emitting device is not exposed to the atmosphere, the operating characteristics of the light-emitting device were measured. Note that the measurement was performed at room temperature (atmosphere maintained at 25°C).

[0203] The current density-luminance characteristics of the light-emitting device 3 are shown in FIG. 10. In FIG. 10, the horizontal axis represents the current density (m A / cm 2 ), and the vertical axis represents the luminance (cd / m 2 ). Also, the voltage-luminance characteristics are shown in FIG. 11 . In FIG. 11, the horizontal axis represents the voltage (V), and the vertical axis represents the luminance (cd / m 2 ). Also, the luminance -current efficiency characteristics are shown in FIG. 12. In FIG. 12, the horizontal axis represents the luminance (cd / m 2 ), and the vertical axis represents the current efficiency (cd / A). Also, the luminance-external quantum efficiency characteristics are shown in FIG. 13. In FIG. 13 , the horizontal axis represents the luminance (cd / m 2 ), and the vertical axis represents the external quantum efficiency (%).

[0204] Also, the voltage (V), current density (mA 2 / cm ), CIE chromaticity coordinates (x, y), current efficiency (cd / A), power efficiency (lm / W 2 ), and external quantum efficiency (%) of the light-emitting element 3 at a luminance of 1100 cd / m ) are shown in Table 4.

[0205]

Table 4

[0206] Also, the emission spectrum of the light-emitting element 3 when a current of 0.1 mA is passed through it is shown in Fig. 14. In Fig. 14, the horizontal axis represents the wavelength (nm), and the vertical axis represents the emission intensity (arbitrary unit). Also, as shown in Table 4, the CIE chromaticity coordinates of the light-emitting element 3 at a luminance of 1100 cd / m are (x, y) = 2 (0.54, 0.46). From this result, it was found that the light-emitting element 3 obtained orange emission derived from [Ir(dppm)2 (acac)].

[0207] As can be seen from Table 4 and Figs. 10 to 13, the light-emitting element 3 showed high values for current efficiency, power efficiency, and external quantum efficiency, respectively. In particular, the external quantum efficiency at a luminance of 1100 cd / m 2 showed an extremely high value of 28%. When converted to the internal quantum efficiency, this is 90% or more. From the above results, it was shown that an element with a high external quantum efficiency can be realized by applying one aspect of the present invention.

[0208] Next, a reliability test of the light-emitting element 3 was conducted. The results of the reliability test are shown in Fig. 15. In Fig. 15 , the vertical axis represents the normalized luminance (%) with the initial luminance set to 100%, and the horizontal axis represents the driving time (h) of the element.

[0209] The reliability test was conducted by setting the initial luminance to 5000 cd / m 2 and driving the light-emitting element 3 under the condition of constant current density. Regarding the luminance after 320 hours, the light-emitting element 3 maintained 92% of the initial luminance. From the above results, it was shown that by applying one aspect of the present invention, a highly reliable element can be realized.

Example

[0210] The T1 level of an organic material can be determined by optical measurement of a thin film or solution of the organic material, but can also be obtained by molecular orbital calculation. For example, molecular orbital calculation can be used to estimate the T1 level of an unknown material. In this example, the T1 levels of Ir(dppm) 2acac used as a guest, Ir(mppr-Me)2dpm, 2mDBT used as an N-type host, PDBqII, and PCBNBB used as a P-type host were calculated respectively.

[0211] The calculation method is as follows. First, the most stable structures of the singlet ground state (S0) and the triplet excited state (T1) of each molecule were calculated using the density functional theory (DFT). Furthermore, vibrational analysis was performed on the most stable structures of S0 and T1, and the zero-point corrected energy was obtained. The T1 level was calculated from the difference in the zero-point corrected energies of S0 and T1.

[0212] In the calculations of the N-type host molecule and the P-type host molecule, as the basis functions for all atoms, 6-3 11G (basis functions of a triple split valence basis system using three contracted functions for each valence orbital) was used. By using the above basis functions, for example, for an H atom If so, orbits from 1s to 3s are considered. Also, for C atoms, orbits from 1s to 4s and 2p to 4p are considered. Furthermore, for improving the calculation accuracy, as a polarized basis system, a p function is added to H atoms and a d function is added to atoms other than H atoms. For the generalized function, B3LYP is used to define the weights of the parameters related to the

[0213] exchange and correlation energies. In the calculation of guest molecules, LanL2DZ is used for the basis function of Ir atoms. For the basis functions of atoms other than Ir atoms, 6-311G is used. Furthermore, for improving the calculation accuracy, as a polarized basis system, a p function is added to H atoms and

[0214] a d function is added to atoms other than H atoms. For the generalized function, B3PW91 is used to define the weights of the parameters related to the exchange and correlation energies.

[0215] The T1 level obtained by the calculation is 2.13 electron volts for Ir(dppm)2acac, 2.13 electron volts for Ir(mppr-Me)2dpm, 2. 42 electron volts for 2mDBTPDBqII, and 2.31 electron volts for PCBNBB. These values were close to those obtained by optical measurement.

[0216] From the above results, the T1 levels of 2mDBTPDBqII used as the N-type host and PCBNBB used as the P-type host are 0.15 eV or more higher than the T1 levels of Ir(dppm )2acac and Ir(mppr-Me)2dpm used as guests. It has been found. Therefore, the transition from the triplet excited state of the guest molecule to the triplet excited state of the N-type host molecule or the P-type host molecule can be sufficiently prevented, suggesting that a light-emitting device with high external quantum efficiency can be obtained.

[0217] The T1 level obtained by such optical measurement and the T1 level obtained by molecular orbital calculation are very close. Therefore, even without synthesizing a new organic compound, by performing molecular orbital calculation of the organic compound, its T1 level can be evaluated, and it can be determined whether the organic compound is useful for enhancing the luminescence efficiency.

Explanation of symbols

[0218] 101 Substrate 102 Light-emitting layer 103 First electrode 104 Second electrode 105 First carrier injection layer 106 First carrier transport layer 107 Second carrier injection layer 108 Second carrier transport layer 110 EL layer 110a EL layer 110b EL layer 111 Charge generation layer 201 Anode 202 Hole injection layer 203 Hole transport layer 204 Light-emitting layer 205 Electron transport layer 206 Electron injection buffer layer 207 Electron relay layer 208 Composite material layer 209 Cathode 210 EL layer​​

Claims

1. An anode, a cathode, and a hole injection layer and a light emitting layer between the anode and the cathode, the light-emitting layer includes a phosphorescent compound, a first organic compound having an electron transport property, and a second organic compound having a hole transport property; The hole injection layer comprises two materials: one of the two materials is an aromatic amine compound or a carbazole derivative; the first organic compound and the second organic compound are a combination that forms an exciplex, an emission spectrum of the exciplex overlaps with an absorption band located on the longest wavelength side of the phosphorescent compound; A light-emitting element, wherein the HOMO level of the second organic compound is lower than the HOMO level of the phosphorescent compound by 0.1 eV or more.

2. An anode, a cathode, and a hole injection layer and a light emitting layer between the anode and the cathode, the light-emitting layer includes a phosphorescent compound, a first organic compound having an electron transport property, and a second organic compound having a hole transport property; The hole injection layer comprises two materials: one of the two materials is an aromatic amine compound or a carbazole derivative; the other of the two materials is a fluorine-containing organic compound; the first organic compound and the second organic compound are a combination that forms an exciplex, an emission spectrum of the exciplex overlaps with an absorption band located on the longest wavelength side of the phosphorescent compound; A light-emitting element, wherein the HOMO level of the second organic compound is lower than the HOMO level of the phosphorescent compound by 0.1 eV or more.

3. In claim 1 or 2, The molar absorption coefficient of the absorption band located on the longest wavelength side of the phosphorescent compound is 2000 M －１ ・cm －１ The light-emitting element is as described above.

4. In any one of claims 1 to 3, The phosphorescent compound is an organometallic complex.

5. A light emitting device comprising the light emitting element according to claim 1 .

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