Light-emitting element

Abstract

This invention provides a light-emitting element comprising: a light-emitting layer containing a guest, an n-type host, and a p-type host between a pair of electrodes, wherein the energy difference between the triplet excited state and the ground state of the n-type host (or p-type host) and the energy difference between the triplet excited state and the ground state of the guest is 0.15 eV or more. Alternatively, in this light-emitting element, the LUMO level of the n-type host is 0.1 eV or more higher than the LUMO level of the guest, or the HOMO level of the p-type host is 0.1 eV or more lower than the HOMO level of the guest.

Description

[Technical Field] This invention relates to a light-emitting element (hereinafter also referred to as an organic EL element) that utilizes the organic electroluminescence (EL) phenomenon. [Previous Technology] Organic EL elements are being actively researched and developed (see Patent Document 1, Non-Patent Document 1, and Non-Patent Document 2). The basic structure of an organic EL element consists of a layer containing a luminescent organic compound (hereinafter also referred to as a light-emitting layer) sandwiched between a pair of electrodes. Organic EL elements are attracting attention as next-generation flat panel display elements due to their characteristics such as thinness and lightweight design, high-speed response to input signals, and low-voltage DC drive capability. Furthermore, displays using this type of light-emitting element exhibit excellent contrast, clear image quality, and wide viewing angles. Moreover, since organic EL elements are planar light sources, it is desirable to apply them to light sources such as backlights for liquid crystal displays and illumination devices. The light-emitting mechanism of organic EL devices belongs to a carrier injection system. In other words, by applying a voltage between electrodes sandwiching the light-emitting layer, electrons and holes injected from the electrodes recombine, exciting the light-emitting material, and emitting light when the material returns to the ground state. There are two types of excited states: singlet excited states and triplet excited states. Furthermore, in light-emitting devices, the statistically significant ratio of singlet excited states to triplet excited states is considered to be 1:3. Additionally, in this specification, unless otherwise specified, a singlet excited state (or triplet excited state) refers to the singlet excited state (or triplet excited state) with the lowest energy level. The ground state of luminescent organic compounds is typically a singlet excited state. Therefore, luminescence from a singlet excited state is caused by electronic transitions between the same spin multiplicity and is thus called fluorescence. Conversely, luminescence from a triplet excited state is caused by electronic transitions between different spin multiplicity states and is thus called phosphorescence. In fluorescent compounds (hereinafter referred to as fluorescent compounds), phosphorescence is generally not observed at room temperature, and only fluorescence is observed. Therefore, based on the aforementioned ratio of singlet to triplet excited states (=1:3), the theoretical limit of the internal quantum efficiency (the ratio of generated photons to injected carriers) in a luminescent element using a fluorescent compound is considered to be 25%. On the other hand, if phosphorescent compounds are used (hereinafter referred to as phosphorescent compounds), the internal quantum efficiency can theoretically be increased by 100%. In other words, higher luminous efficiency can be obtained compared to fluorescent compounds. Based on the above reasons, in order to realize high-efficiency light-emitting elements, light-emitting elements using phosphorescent compounds have been actively developed in recent years. In particular, organometallic complexes with iridium or similar central metals have attracted attention as phosphorescent compounds due to their high phosphorescence quantum efficiency. For example, Patent Document 1 discloses organometallic complexes with iridium as central metals as phosphorescent materials. When the phosphorescent compound described above is used to form the luminescent layer of a light-emitting element, in order to suppress concentration quenching of the phosphorescent compound or quenching caused by triplet-triplet annihilation, the luminescent layer is usually formed by dispersing the phosphorescent compound in a matrix composed of another compound. Here, the compound used as the matrix is ​​referred to as the host, and the compound dispersed in the matrix, such as the phosphorescent compound, is referred to as the guest. There are several basic processes involved in the luminescence of phosphorescent compounds as guest light-emitting elements. These basic processes are explained below. (1) When electrons and holes recombine in a guest molecule and the guest molecule is in 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 guest molecule in the singlet excited state undergoes an intersystem crossing to a triplet excited state and emits phosphorescence. In other words, in the direct recombination process described above (1), as long as the intersystem crossing efficiency and phosphorescence quantum efficiency of the guest molecules are high, high luminescence efficiency can be obtained. (2) When electrons and holes recombine in the host molecule and the host molecule is in an excited state (energy transfer process). (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 is transferred from the host molecule to the guest molecule, causing the guest molecule to be in a triplet excited state. The guest molecule in the triplet excited state emits phosphorescence. In addition, it is theoretically possible for energy to be transferred to the energy level (S1 level) of the singlet excited state of the guest molecule, but in many cases the S1 level of the guest molecule is higher than the T1 level of the host molecule. Therefore, the energy transfer to the energy level (S1 level) of the singlet excited state of the guest molecule is not likely to become the main energy transfer process, so it is omitted here. (2-2) When the excited state of the host molecule is a singlet excited state, and the energy level (S1 level) of the singlet excited state of the host molecule is higher than the S1 and T1 energy levels of the guest molecule, the excitation energy is transferred from the host molecule to the guest molecule, and the guest molecule is thus in a singlet or triplet excited state. The guest molecule in the triplet excited state emits phosphorescence. In addition, the guest molecule in the singlet excited state undergoes intersystem crossing to the triplet excited state and emits phosphorescence. In other words, in the energy transfer process described in (2) above, it is important to efficiently transfer both the triple excitation energy and the single excitation energy of the host molecule to the guest molecule. Given the energy transfer process described above, if the host molecule itself releases the excitation energy in the form of light or heat and becomes inactive before the excitation energy is transferred from the host molecule to the guest molecule, the luminescence efficiency will decrease. <Energy Transfer Process> The following section details the energy transfer process between molecules. First, as a mechanism for energy transfer between molecules, the following two mechanisms are proposed. Here, the molecule that imparts excitation energy is denoted as the host molecule, and the molecule that receives excitation energy is denoted as the guest molecule. The Förster Mechanism (Dipole-Dipole Interactions) The Foster mechanism does not require direct intermolecular contact for energy transfer. Energy transfer occurs through the resonance of dipole oscillations between the host and guest molecules. Through this resonance, the host molecule supplies energy to the guest molecule, thus the host molecule is in its ground state and the guest molecule is in its excited state. Equation (1) shows the rate constant kh * → g in the Foster mechanism. In formula (1), ν represents the frequency, f'h(ν) represents the normalized emission spectrum of the host molecule (fluorescence spectrum from energy transfer of the singlet excited state, phosphorescence spectrum from energy transfer of the triplet excited state), εg(ν) represents the molar absorptivity of the guest molecule, N represents the Avogadro number, n represents the refractive index of the medium, R represents the intermolecular distance between the host and guest molecules, τ represents the lifetime of the measured excited state (fluorescence lifetime or phosphorescence lifetime), c represents the speed of light, Φ represents the luminescence quantum efficiency (fluorescence quantum efficiency from energy transfer of the singlet excited state, phosphorescence quantum efficiency from energy transfer of the triplet excited state), and K2 represents the alignment coefficient (0 to 4) of the transition dipole moments of the host and guest molecules. Furthermore, in random alignment, K2 = 2 / 3. The Dexter Mechanism (Electron Exchange Interaction) In the Dexter mechanism (also known as Dexter electron transfer), the host molecule and the guest molecule approach each other to an effective contact distance where their orbits overlap, and energy transfer occurs by exchanging electrons between the excited-state host molecule and the ground-state guest molecule. Equation 2 shows the rate constant kh * → g in the Dexter mechanism. In formula (2), h represents Planck's constant, K represents a constant with energy dimension, ν represents frequency, f'h(ν) represents the normalized emission spectrum of the host molecule (fluorescence spectrum from energy transfer from singlet excited state, phosphorescence spectrum from energy transfer from triplet excited state), ε'g(ν) represents the normalized absorption spectrum of the guest molecule, L represents effective molecular radius, and R represents the intermolecular distance between the host molecule and the guest molecule. Here, the energy transfer efficiency ΦET from the host molecule to the guest molecule can be represented by formula (3). kr represents the rate constant of the luminescence process of the host molecule (fluorescence from the energy transfer of the singlet excited state of the host molecule and phosphorescence from the energy transfer of the triplet excited state of the host molecule), kn represents the rate constant of the non-luminescence process of the host molecule (thermal deactivation or intersystem crossing), and τ represents the lifetime of the excited state of the host molecule detected. First, as can be seen from formula (3), in order to improve the energy transfer efficiency ΦET, the energy transfer rate constant kh * →g should be much larger than the other competing rate constants kr + kn (=1 / τ). Moreover, in order to increase the energy transfer rate constant kh * →g, as can be seen from formulas (1) and (2), in the Foster mechanism and the Dexter mechanism, the greater the overlap between the emission spectrum of the host molecule (fluorescence spectrum from energy transfer from singlet excited state, phosphorescence spectrum from energy transfer from triplet excited state) and the absorption spectrum of the guest molecule (which usually represents the energy difference between triplet excited state and ground state since it is phosphorescence), the better. For example, by selecting materials in a manner that overlaps the energy difference between the triplet excited state and the ground state of the host molecule with the energy difference between the triplet excited state and the ground state of the guest molecule, energy transfer from the host to the guest can occur more efficiently. However, the same energy transfer occurs from the triplet excited state of the guest molecule to the ground state of the host molecule. This means that by selecting materials in a manner where the energy difference between the triplet excited state and the ground state of the host molecule is equal to or close to the energy difference between the triplet excited state and the ground state of the guest molecule, energy transfer from the triplet excited state of the guest molecule to the triplet excited state of the host molecule is readily achieved. Unfortunately, this results in a decrease in luminescence efficiency. For example, as described in Non-Patent Document 1, in order to solve such a problem, a method is proposed as follows: the energy difference between the triplet excited state and the ground state of the host molecule is greater than the energy difference between the triplet excited state and the ground state of the guest molecule. In Non-Patent Literature 1, the transfer from the triplet excited state of the guest molecule to the triplet excited state of the host molecule is prevented by making the energy difference between the triplet excited state and the ground state of the host molecule 0.3 eV greater than the energy difference between the triplet excited state and the ground state of the guest molecule (now revised to 0.15 eV). That is, by making the energy difference between the triplet excited state and the ground state of the host molecule 0.15 eV or more greater than the energy difference between the triplet excited state and the ground state of the guest molecule, the transfer from the triplet excited state of the guest molecule to the triplet excited state of the host molecule can be effectively prevented. [Patent Document 1] International Patent Publication No. 2000 / 070655 [Non-patent document 1] Shizuo Tokito et al., "Confinement of triplet energy on phosphorescent molecules for highly-efficient organic blue-light-emitting devices", Appl. Phys. Lett., 83, 569 (2003). [Non-Patent Literature 2] Vi-En Choong et al., “Organic light-emitting diodes with a bipolar transport layer”, Appl. Phys. Lett., 75, 172 (1999). However, the difference in energy difference between the host molecules and the guest molecules means that the aforementioned Foster or Dexter mechanisms are not easily generated, resulting in a decrease in luminous efficiency. One specific embodiment of the present invention provides a light-emitting element based on a novel principle that resolves this contradiction. Furthermore, as mentioned above, although various excitation processes exist, the excitation process with the lowest deactivation rate is the direct recombination process. Therefore, it is preferable to increase the ratio of direct recombination processes to improve luminous efficiency or external quantum efficiency. A specific embodiment of the present invention aims to provide a method for efficiently performing a direct recombination process. Another specific embodiment of the present invention aims to provide a light-emitting element with high external quantum efficiency. [Summary of the Invention] One specific embodiment of the present invention is a light-emitting element comprising: a light-emitting layer containing a phosphorescent compound (guest), a first organic compound, and a second organic compound between a pair of electrodes, wherein the energy difference between the triplet excited state and the ground state of the first organic compound and the energy difference between the triplet excited state and the ground state of the second organic compound are both 0.15 eV or more greater than the energy difference between the triplet excited state and the ground state of the guest. In the above structure, the combination of the first organic compound and the second organic compound can also form an exciplex. Alternatively, the first organic compound may have superior electron transport properties compared to its hole transport properties, and the second organic compound may have superior hole transport properties compared to its electron transport properties. When these properties are present, the first organic compound and the second organic compound are referred to as the n-type host and the p-type host, respectively. In another specific embodiment of the present invention, a light-emitting element is provided, comprising: a light-emitting layer containing a guest, an n-type host, and a p-type host between a pair of electrodes, wherein the LUMO (Lowest Unoccupied Molecular Orbital) energy level of the n-type host is 0.1 eV or more higher than the LUMO energy level of the guest. Furthermore, when the LUMO level of the guest is too low compared to the LUMO level of the n-type host, the conductivity is not optimal. Therefore, it is preferable that the difference between the LUMO level En of the n-type host and the LUMO level Ea of the guest (En-Ea) is 0.1 eV to 0.5 eV. In another specific embodiment of the present invention, a light-emitting element is provided, comprising: a light-emitting layer containing a guest, an n-type host, and a p-type host between a pair of electrodes, wherein the HOMO (Highest Occupied Molecular Orbital) energy level of the p-type host is 0.1 eV or more lower than the HOMO energy level of the guest. Furthermore, when the HOMO level of the guest is too high compared to the HOMO level of the p-type host, it is not ideal in terms of conductivity. Therefore, it is preferable that the difference between the HOMO level Ep of the p-type host and the HOMO level Eb of the guest (Ep-Eb) is -0.5 eV to -0.1 eV. In the above-described light-emitting element, the guest is preferably an organometallic complex. In the above-described light-emitting element, at least one of the n-type and p-type host may also be a fluorescent compound. The light-emitting element of one specific embodiment of the present invention can be applied to light-emitting devices, electronic devices, and lighting devices. In one specific embodiment of the invention, the light-emitting layer comprises n-type host molecules, p-type host molecules, and guest molecules. Of course, the molecules do not need to be arranged in a regular pattern; they can also be in a nearly random state. In particular, 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 choose a combination of materials that are not easily crystallized. Alternatively, as shown in FIG1A, a specific embodiment of the present invention can also be a light-emitting element, wherein a first electrode 103, a light-emitting layer 102 having the above-described structure, and a second electrode 104 are disposed overlappingly on a substrate 101. Here, the first electrode 103 is one of an anode and a cathode, and the second electrode 104 is the other of an anode and a cathode. Additionally, as shown in FIG1B, one specific embodiment of the present invention can also be a light-emitting element, wherein, in addition to the first electrode 103, the light-emitting layer 102, and the second electrode 104, a first carrier injection layer 105, a first carrier transport layer 106, a second carrier injection layer 107, and a second carrier transport layer 108 are also disposed in an overlapping manner. Here, the first carrier is one of electrons and holes, and the second carrier is the other of electrons and holes. Furthermore, 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. In one specific embodiment 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) molecule 0.15 eV or more higher than the energy difference between the triplet excited state and the ground state of the guest molecule, the transition from the triplet excited state of the guest molecule to the triplet excited state of the host (n-type host and p-type host) molecule can be effectively prevented, thereby providing a light-emitting element with high external quantum efficiency. On the other hand, regarding energy transfer processes using the Foster or Dexter mechanism, energy can be transferred from the exciton complex generated by the n-type and p-type host molecules to the guest molecule. The exciton complex splits into the n-type and p-type host molecules during the energy transfer phase. The energy difference between the triplet excited state and the ground state of the n-type (or p-type) host molecule is 0.15 eV or more higher than the energy difference between the triplet excited state and the ground state of the guest molecule, thus preventing energy transfer from the triplet excited state of the guest molecule to the triplet excited state of the n-type (or p-type) host molecule. Furthermore, in one specific embodiment of the invention, for example, since the LUMO energy level of the n-type host molecule is 0.1 eV or more higher than that of the guest molecule, electrons from the n-type host molecule preferentially enter the LUMO energy level of the guest molecule. As a result, the guest molecule becomes an anion, attracting holes and causing holes and electrons to recombine within the guest molecule. Furthermore, in one specific embodiment of the present invention, for example, since the HOMO energy level of the p-type host molecule is 0.1 eV or more lower than that of the guest molecule, the hole in the p-type host molecule preferentially enters the HOMO energy level of the guest molecule. As a result, the guest molecule becomes a cation, attracts electrons, and causes the hole and electron to recombine within the guest molecule. In this way, by applying a specific embodiment of the present invention, carriers can be efficiently injected into guest molecules, increasing the rate of direct recombination processes. In particular, in one embodiment of the present invention, since an n-type host and a p-type host are mixed in the luminescent layer, electrons tend to pass through the n-type host molecule, and holes tend to pass through the p-type host molecule. As a result, electrons are injected from the n-type host molecule into the LUMO level of the guest molecule, while holes are injected from the p-type host molecule into the HOMO level of the guest molecule. [Simplified Explanation of the Diagram] In the diagram: Figures 1A to 1D are diagrams illustrating various specific embodiments of the present invention; Figures 2A to 2C are diagrams illustrating the principle of a specific embodiment of the present invention; Figures 3A to 3C are diagrams illustrating the principle of a specific embodiment of the present invention; Figure 4 is a graph showing the brightness-current density characteristics of the light-emitting element in Example 1; Figure 5 is a graph showing the brightness-voltage characteristics of the light-emitting element in Embodiment 1; Figure 6 is a graph showing the current efficiency-brightness characteristics of the light-emitting element in Embodiment 1; Figure 7 is a graph showing the external quantum efficiency-brightness characteristics of the light-emitting element of Example 1; Figure 8 is a graph showing the emission spectrum of the light-emitting element of Example 1; Figure 9 is a graph showing the results of a reliability test of the light-emitting element in Example 1; Figure 10 is a graph showing the brightness-current density characteristics of the light-emitting element in Embodiment 2; Figure 11 is a graph showing the brightness-voltage characteristics of the light-emitting element in Embodiment 2; Figure 12 is a graph showing the current efficiency-brightness characteristics of the light-emitting element in Embodiment 2; Figure 13 is a graph showing the external quantum efficiency-brightness characteristics of the light-emitting element of Example 2; Figure 14 is a graph showing the emission spectrum of the light-emitting element of Example 2; Figure 15 is a graph showing the results of a reliability test of the light-emitting element in Example 2.

Implementation Method

Claims

1. A light-emitting element, comprising: anode; cathode; The light-emitting layer between the anode and the cathode; The first layer between the anode and the light-emitting layer includes: a phosphorescent compound; a first organic compound having electron transport properties; and a second organic compound having hole transport properties. The first layer includes a third organic compound and a fourth organic compound, wherein the third organic compound is an aromatic amine compound or a carbazole derivative, wherein the first organic compound forms an excimer complex with the second organic compound, wherein the fourth organic compound contains a fluorine group, wherein the emission spectrum of the excimer complex overlaps with the absorption band of the longest wavelength side of the phosphorescent compound, and wherein the highest occupied molecular orbital energy level of the second organic compound is 0.1 eV or more lower than the highest occupied molecular orbital energy level of the phosphorescent compound.

2. A light-emitting element, comprising: anode; cathode; The light-emitting layer between the anode and the cathode; The first layer between the anode and the light-emitting layer includes: a phosphorescent compound; a first organic compound having electron transport properties; and a second organic compound having hole transport properties. The first layer includes a third organic compound and a fourth organic compound, wherein the third organic compound is an aromatic amine compound or a carbazole derivative, wherein the first organic compound forms an excimer complex with the second organic compound, wherein the fourth organic compound contains a fluorine group, wherein the emission spectrum of the excimer complex overlaps with the absorption band of the longest wavelength side of the phosphorescent compound, wherein the energy difference between the triplet excited state and the ground state of the first organic compound is 0.15 eV or more higher than the energy difference between the triplet excited state and the ground state of the phosphorescent compound, and wherein the energy difference between the triplet excited state and the ground state of the second organic compound is 0.15 eV or more higher than the energy difference between the triplet excited state and the ground state of the phosphorescent compound.

3. A light-emitting element, comprising: anode; cathode; The light-emitting layer between the anode and the cathode; and a hole injection layer between the anode and the light-emitting layer and in contact with the anode, wherein the light-emitting layer comprises: a phosphorescent compound; a first organic compound having electron transport properties; and a second organic compound having hole transport properties, wherein the hole injection layer comprises a third organic compound and a fourth organic compound, wherein the third organic compound is an aromatic amine compound or a carbazole derivative, wherein the first organic compound forms an excimer complex with the second organic compound, wherein the fourth organic compound contains a fluorine group, wherein the emission spectrum of the excimer complex overlaps with the absorption band of the longest wavelength side of the phosphorescent compound, and wherein the highest occupied molecular orbital energy level of the second organic compound is 0.1 eV or more lower than the highest occupied molecular orbital energy level of the phosphorescent compound.

4. The light-emitting element of claim 1 or 2, further comprising a second layer between the cathode and the light-emitting layer, wherein the second layer comprises a metal complex.

5. The light-emitting element of claim 3, further comprising an electron transport layer between the cathode and the light-emitting layer, wherein the electron transport layer comprises a metal complex.

6. The light-emitting element of any one of claims 1 to 3, wherein the phosphorescent compound is an iridium complex.

7. The light-emitting element of any one of claims 1 to 3, wherein the molar absorptivity of the absorption band on the longest wavelength side of the phosphorescent compound is 2000 M⁻¹·cm⁻¹ or higher.

8. A light-emitting device comprising a light-emitting element as claimed in any one of claims 1 to 3.

9. A light-emitting element as claimed in any of claims 1 to 3, wherein the fourth organic compound is an electron acceptor.

10. The light-emitting element of any one of claims 1 to 3, wherein the fourth organic compound is 7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinone dimethylane.

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