Light emitting element

Abstract

To provide a light emitting element with high external quantum efficiency, and a light emitting element with low drive voltage.SOLUTION: A light emitting element which has light-emitting layer between a pair of electrodes containing a phosphorescent compound and a substance that exhibits thermally activated delayed fluorescence, in which the peak of the fluorescence spectrum and / or the peak of the phosphorescence spectrum of the substance that exhibits heat activation delayed fluorescence overlaps with the absorption band on the lowest energy side of the absorption spectrum of the phosphorescent compound, and in which when voltage is applied between a pair of electrodes, the phosphorescent compound emits phosphorescence in the light-emitting layer, is provided.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] Organic electroluminescence (EL) phenomenon The present invention relates to a light-emitting device (hereinafter also referred to as an organic EL device) utilizing the above. [Background technology]

[0002] Research and development of organic EL elements is actively underway. The basic structure of an organic EL element is a pair of A light-emitting layer containing a light-emitting material is sandwiched between electrodes, making it possible to reduce thickness and weight, and to respond quickly to input signals. It is expected to be the next generation flat panel display due to its characteristics such as being able to respond to high-speed signals and being able to be driven at low DC voltages. In addition, displays using such light-emitting elements are attracting attention. It also has the characteristics of excellent contrast and image quality, as well as a wide viewing angle. Since the L element is a surface light source, it can be used as a light source for backlighting of LCD displays and lighting. It is also being considered for use.

[0003] The light-emitting mechanism of organic EL elements is a carrier injection type. In other words, the light-emitting layer is sandwiched between electrodes. By applying a voltage, electrons and holes injected from the electrode recombine and emit light. The light-emitting substance is excited and emits light when the excited state returns to the ground state. The types of states include the singlet excited state (S * ) and triplet excited states (T * ) is possible. The statistical generation rate in the light-emitting element is S * :T * = 1:3 There are.

[0004] The ground state of luminescent organic compounds is usually a singlet state. Therefore, the singlet excited state (S *) is called fluorescence because it is an electron transition between atoms of the same spin multiplicity. , triplet excited state (T * ) is an electron transition between different spin multiplicities, Here, a compound that emits fluorescence (hereinafter referred to as a fluorescent compound) emits light at room temperature. Therefore, phosphorescence is not observed and only fluorescence is observed. The internal quantum efficiency (the ratio of photons generated to injected carriers) of a light-emitting device The theoretical limit is S * :T * It is said to be 25% based on the ratio = 1:3.

[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 be increased to 1 In other words, it is possible to obtain a higher luminous efficiency than fluorescent compounds. For this reason, in order to realize a highly efficient light-emitting device, phosphorescent compounds are used. In recent years, the development of light-emitting devices using phosphorescent compounds has been actively pursued. Due to the high phosphorescence quantum yield of iridium, organometallic complexes with iridium as the central metal have attracted attention. For example, Patent Document 1 discloses that an organometallic complex having iridium as the central metal is a phosphorescent material. and is disclosed as such.

[0006] When the light-emitting layer of the light-emitting element is formed using the above-mentioned phosphorescent compound, the concentration quenching of the phosphorescent compound is To suppress quenching by light and triplet-triplet annihilation, the compound is placed in a matrix of other compounds. In most cases, the phosphorescent compound is dispersed in a matrix. The compound is the host material, and the compound dispersed in the matrix, such as a phosphorescent compound, is the guest material. It is called a fee. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] International Publication No. 00 / 70655 Brochure Summary of the Invention [Problem to be solved by the invention]

[0008] However, the light extraction efficiency of organic EL elements is generally said to be around 20% to 30%. Therefore, when considering the absorption of light by the reflective electrode and transparent electrode, it is recommended to use a phosphorescent compound. The limit of external quantum efficiency of light-emitting devices is thought to be about 25%.

[0009] As mentioned above, organic EL elements are being considered for use in displays and lighting. One of the issues to be addressed is reducing power consumption. Therefore, it is important to reduce the driving voltage of the organic EL element.

[0010] In view of the above, an object of one embodiment of the present invention is to provide a light-emitting element with high external quantum efficiency. Another object of one embodiment of the present invention is to provide a light-emitting element with low driving voltage. .

[0011] The invention disclosed below aims to solve at least one of the above problems. Let's say. [Means for solving the problem]

[0012] In one embodiment of the present invention, a phosphorescent compound and a thermally activated delayed fluorescence (TDF) compound are disposed between a pair of electrodes. lly activated delayed fluorescence (TADF) and a fluorescent spectrum peak of the material exhibiting thermally activated delayed fluorescence. The absorption band of the phosphorescent compound overlaps with the lowest energy absorption band of the absorption spectrum of the phosphorescent compound, forming a pair of electrons. By applying a voltage between the electrodes, the phosphorescent compound in the light-emitting layer emits phosphorescence. He is a child.

[0013] In this specification and the like, the fluorescence spectrum of a substance that exhibits thermally activated delayed fluorescence is Spectra (thermally activated delayed fluorescence spectra) are included.

[0014] Here, delayed fluorescence is an emission that has the same spectrum as normal fluorescence but has a significantly longer lifespan. Its lifetime is 10 -6 seconds or more, preferably 10 -3 More than a second.

[0015] In the light-emitting device, the peak energy of the fluorescence spectrum of the substance exhibiting thermally activated delayed fluorescence is The energy value and the peak energy of the absorption band on the lowest energy side of the absorption spectrum of the phosphorescent compound It is preferable that the difference between the energy value is within 0.3 eV.

[0016] Another embodiment of the present invention is a light-emitting device comprising: a phosphorescent compound and a material exhibiting thermally activated delayed fluorescence between a pair of electrodes; The phosphorescence spectrum peak of a material that exhibits thermally activated delayed fluorescence and the phosphorescence The absorption band at the lowest energy side of the absorption spectrum of the conductive compound overlaps with the absorption band at the lowest energy side of the A light-emitting element in which a phosphorescent compound in the light-emitting layer emits phosphorescence by applying pressure. .

[0017] In the light-emitting device, the peak energy of the phosphorescence spectrum of the substance exhibiting thermally activated delayed fluorescence is The energy value and the peak energy of the absorption band on the lowest energy side of the absorption spectrum of the phosphorescent compound It is preferable that the difference between the energy value is within 0.3 eV.

[0018] Another embodiment of the present invention is a light-emitting device comprising: a phosphorescent compound and a material exhibiting thermally activated delayed fluorescence between a pair of electrodes; The peak of the fluorescence spectrum and the phosphorescence of a material having an emitting layer containing a thermally activated delayed fluorescence The spectral peaks are on the lowest energy side of the absorption spectrum of the phosphorescent compound. The absorption band overlaps with that of the luminescent layer, and by applying a voltage between the pair of electrodes, phosphorescence occurs in the luminescent layer. The compound is a light-emitting element that emits phosphorescence.

[0019] In the light-emitting device, the peak energy of the fluorescence spectrum of the substance exhibiting thermally activated delayed fluorescence is The energy value of the peak of the phosphorescence spectrum and the energy value of the absorption spectrum of the phosphorescent compound are The difference between the energy value of the lowest energy absorption band peak is within 0.3 eV It is preferable that:

[0020] In the light-emitting element, the substance exhibiting thermally activated delayed fluorescence has a peak in the fluorescence spectrum The difference between the energy value of the phosphorescence spectrum and the peak energy value of the phosphorescence spectrum is within 0.3 eV. It is preferable that there is.

[0021] In the light-emitting element, the absorption spectrum of the phosphorescent compound on the lowest energy side is The band is triplet MLCT (Metal to Ligand Charge Trans). It is preferable that the absorption band derived from the fer transition is included.

[0022] In the light-emitting element, the phosphorescent compound is preferably an organometallic complex. Lithium complexes are particularly preferred.

[0023] In the light-emitting element, the absorption spectrum of the phosphorescent compound on the lowest energy side is The molar extinction coefficient of the absorbance is preferably 5000 / M·cm or more.

[0024] In the light-emitting element, the substance exhibiting thermally activated delayed fluorescence is a π-excess heteroaromatic ring and The heterocyclic compound preferably has a π-deficient heteroaromatic ring, and the heterocyclic compound preferably has a π-excess heteroaromatic ring. and a π-deficient heteroaromatic ring are particularly preferably heterocyclic compounds in which the π-deficient heteroaromatic ring and the π-deficient heteroaromatic ring are directly bonded to each other.

[0025] The light-emitting element of one embodiment of the present invention can be applied to light-emitting devices, electronic devices, and lighting devices. . [Effects of the Invention]

[0026] According to one embodiment of the present invention, a light-emitting element with high external quantum efficiency can be provided. In one embodiment, a light-emitting element with low driving voltage can be provided. [Brief explanation of the drawings]

[0027] [Figure 1] FIG. 1 illustrates a concept of one embodiment of the present invention. [Figure 2] 1A and 1B illustrate light-emitting elements of one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0028] The embodiments will be described in detail with reference to the drawings. However, the present invention is not limited to the following description. The present invention is not limited to the above embodiments, and various changes and modifications may be made in the form and details thereof without departing from the spirit and scope of the present invention. It will be readily understood by those skilled in the art that the present invention can be achieved by the following embodiments. It should not be construed as being limited to the contents of the description. The same reference numerals are used in different drawings to denote the same parts or parts having similar functions. A repeated explanation will be omitted.

[0029] (Embodiment 1) In this embodiment, a light-emitting element of one embodiment of the present invention will be described with reference to FIG.

[0030] The light-emitting element of one embodiment of the present invention includes a guest material that is a light-emitting substance and a host material in which the guest material is dispersed. The light-emitting layer contains a host material. The guest material is a phosphorescent compound. A substance that exhibits thermally activated delayed fluorescence is used as the light-emitting element of one embodiment of the present invention. Phosphorescence and fluorescence spectra of the material, and absorption spectra of the phosphorescent compounds A conceptual diagram is shown in Figure 1. In Figure 1, the vertical axis represents the absorption intensity and emission intensity, and the horizontal axis represents the energy. - indicates.

[0031] The light-emitting layer of the light-emitting element of one embodiment of the present invention has a host material content higher than the guest material content. By dispersing the guest material in the host material, the crystallization of the light-emitting layer can be improved. In addition, concentration quenching caused by a high concentration of the guest material can be suppressed, and the emission The light emitting efficiency of the optical element can be increased.

[0032] In this embodiment, the triplet excitation energy level (T1 level) of the host material is It is preferable that the T1 level of the host material is higher than the T1 level of the guest material. If the T1 level is lower than that of the host material, the triplet excitation energy of the guest material that contributes to the emission is This is because the material will be quenched, resulting in a decrease in luminous efficiency.

[0033] <Elemental process of luminescence> First, we will explain the general elementary process of light emission in a light-emitting device that uses a phosphorescent compound as a guest material. explain.

[0034] (1) When electrons and holes recombine in the guest molecule, the guest molecule enters 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 intersystem crossing to the triplet excited state, emitting phosphorescence.

[0035] In other words, in the direct recombination process (1) above, the intersystem crossing efficiency and phosphorescence intensity of the guest molecules If the molecular yield is high, high luminous efficiency can be obtained. The T1 level of the molecule is preferably higher than the T1 level of the guest molecule.

[0036] (2) When electrons and holes recombine in the host molecule, the host molecule enters an excited state. (energy transfer process). (2-1) When the excited state of the host molecule is a triplet excited state If the T1 level of the host molecule is higher than the T1 level of the guest molecule, the host molecule The excitation energy is transferred to the guest molecule, and the guest molecule enters a triplet excited state. The guest molecule emits phosphorescence. Although energy transfer to the S1 level of the guest molecule is formally possible, in most cases the energy transfer is to the S1 level of the guest molecule. is located on the higher energy side than the T1 level of the host molecule, and is the main energy transfer pathway. Since it is difficult to achieve this level, we will not go into detail here. (2-2) When the excited state of the host molecule is a singlet excited state If the S1 level of the host molecule is higher than the S1 and T1 levels of the guest molecule, The excitation energy is transferred from the molecule to the guest molecule, and the guest molecule enters a singlet excited state or a triplet excited state. The guest molecules in the triplet excited state emit phosphorescence. The guest molecule in this state undergoes intersystem crossing to a triplet excited state and emits phosphorescence.

[0037] In other words, in the energy transfer process (2) above, the triplet excitation energy of the host molecule It is important to transfer both the singlet and singlet excitation energy to the guest molecule efficiently. This becomes:

[0038] <Energy transfer process> Next, the intermolecular energy transfer process will be explained in detail.

[0039] The following two mechanisms have been proposed for the energy transfer between molecules. The molecule that provides the excitation energy is the host molecule, and the molecule that receives the excitation energy is the guest molecule. It is written as the to molecule.

[0040] <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 vibration between the molecule and the guest molecule. The host molecule transfers energy to the guest molecule through the vibrational resonance phenomenon, 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).

[0041]

number

[0042] In formula (1), ν represents the frequency, and f' h (ν) is the normalized Emission spectrum (fluorescence spectrum when discussing energy transfer from singlet excited states, When discussing energy transfer from triplet excited states, 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 measured excitation state. represents the lifetime of the state (fluorescence lifetime or phosphorescence lifetime), c represents the speed of light, and φ represents the luminescence quantum yield (single When discussing energy transfer from single excited states, the fluorescence quantum yield is used, and when discussing energy transfer from triplet excited states, the fluorescence quantum yield is used. When discussing energy transfer, κ represents the phosphorescence quantum yield, and 2 is the ratio of the host molecule and the guest molecule This is a coefficient (0 to 4) that represents the orientation of the transition dipole moment. In the case of random orientation, κ 2 =2 / 3.

[0043] Dexter mechanism (electron exchange interaction) The Dexter mechanism occurs when the host and guest molecules approach the effective contact distance where orbital overlap occurs. 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 equation (2).

[0044]

number

[0045] In equation (2), h is Planck's constant, and K is a constant with the dimension of energy. where ν represents the frequency and f' h (ν) is the normalized emission spectrum of the host molecule (When discussing energy transfer from the singlet excited state, the fluorescence spectrum, triplet excited state represents the phosphorescence spectrum 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.

[0046] Here, the energy transfer efficiency from the host molecule to the guest molecule Φ ET is expressed by equation (3). It is thought that r The emission process of the host molecule (energy transfer from the singlet excited state) The rate of fluorescence is used when discussing energy transfer from the triplet excited state, and the rate of phosphorescence is used when discussing energy transfer from the triplet excited state. represents the coefficient constant, k n represents the rate constant of the non-radiative process (thermal deactivation and intersystem crossing) of the host molecule. , τ represents the measured lifetime of the excited state of the host molecule.

[0047]

number

[0048] First, from equation (3), the energy transfer efficiency Φ ET To increase the The rate constant k h * →g , other competing rate constants k r +k n (=1 / τ) Then, the rate constant of the energy transfer, k h * →gTo increase From equations (1) and (2), it is possible to determine whether the mechanism is a Forster mechanism or a Dexter mechanism. In the structure, the emission spectrum of the host molecule (energy transfer from the singlet excited state) When discussing energy transfer from triplet excited states, we use the fluorescence spectrum. It can be seen that it is better for the absorption spectrum of the guest molecule to overlap as much as possible.

[0049] Here, the present inventors have found that the overlap between the emission spectrum of the host molecule and the absorption spectrum of the guest molecule When considering the absorption spectrum of the guest molecule, We believe that the absorption bands of

[0050] In this embodiment, a phosphorescent compound is used as the guest material. The absorption band that is thought to contribute most strongly to the emission is the singlet ground state The absorption wavelength corresponding to the direct transition to the triplet excited state and its vicinity are the longest wavelengths ( This is an absorption band that appears on the low energy side of the emission spectrum of the host material ( The lowest energy absorption spectrum of a phosphorescent compound is called the fluorescent spectrum or phosphorescence spectrum. It is considered preferable that the absorption band overlaps with that of the ghee side.

[0051] For example, in organometallic complexes, especially in luminescent iridium complexes, the lowest energy absorption In many cases, a broad absorption band appears around 2.0 to 2.5 eV (of course, lower (Sometimes it appears on the lower or higher energy side.) This absorption band is mainly It is due to the triplet MLCT transition. However, this absorption band also contains the triplet π-π * Transition and singlet MLC The absorption bands derived from the T transition are also included, and these overlap to form the lowest energy band in the absorption spectrum. In other words, the lowest singlet excited state is The difference between the lowest triplet excited state and the lowest triplet excited state is small, and the absorption bands derived from these overlap, resulting in the absorption spectrum It is thought that a broad absorption band is formed on the lowest energy side of the spectrum. Therefore, when using an organometallic complex (especially an iridium complex) as a guest material, The broad absorption band exists on the low energy side, and the emission spectrum of the host material is large. An overlapping state is preferred.

[0052] From the above discussion, in the energy transfer from the triplet excited state of the host material, The phosphorescence spectrum of the material overlaps greatly with the lowest energy absorption band of the guest material. In addition, in the energy transfer from the singlet excited state of the host material, The overlap between the fluorescence spectrum of the material and the lowest energy absorption band of the guest material becomes large. That's fine.

[0053] In other words, energy transfer from the triplet excited state and energy transfer from the singlet excited state From the above discussion, in order to efficiently achieve both of these, the phosphorescence spectrum and the fluorescence spectrum of the host material must be considered. The optical spectrum must be designed so that both the optical spectrum and the optical spectrum overlap with the lowest energy absorption band of the guest material. It must be.

[0054] However, in general, the S1 level and the T1 level are significantly different (S1 level > T1 level). The emission wavelength of light and the emission wavelength of phosphorescence are also significantly different (emission wavelength of fluorescence < emission wavelength of phosphorescence). For example, in light-emitting devices using phosphorescent compounds, 4,4 '-Di(N-carbazolyl)biphenyl (abbreviation: CBP) has a peak near 500 nm (3.1 e V), while the fluorescence spectrum is around 400 nm (2.5 eV), which is a gap of 100 nm (equivalent to an energy difference of 0.6e (The difference is more than 1000 V.) Considering this example, it is clear that the fluorescent spectrum of the host material is It is extremely difficult to design a host material that lies in a similar position to the vector.

[0055] Here, in the light-emitting element of one embodiment of the present invention, a substance exhibiting thermally activated delayed fluorescence is used as a host material. is used.

[0056] One aspect of the present invention is a light-emitting device including a phosphorescent compound and a substance exhibiting thermally activated delayed fluorescence between a pair of electrodes. The fluorescent spectrum peak of the material exhibiting thermally activated delayed fluorescence and the ... The absorption band at the lowest energy side of the absorption spectrum of the substance overlaps with the absorption band at the lowest energy side of the substance. By adding the phosphorescent compound, the light-emitting layer emits phosphorescence.

[0057] Another aspect of the present invention is a device comprising a phosphorescent compound and a thermally activated delayed fluorescence compound disposed between a pair of electrodes. a light-emitting layer containing a substance, and a peak in the phosphorescence spectrum of the substance that exhibits thermally activated delayed fluorescence and a phosphorescent The absorption band of the photoactive compound overlaps with the lowest energy absorption band of the photoactive compound, and A light-emitting device in which a phosphorescent compound in a light-emitting layer emits phosphorescence when a voltage is applied. do.

[0058] The substances that exhibit thermally activated delayed fluorescence have the lowest triplet excitation energy and the lowest singlet excitation energy. In other words, the difference in the emission spectrum from the singlet state of a material that exhibits thermally activated delayed fluorescence is small. The emission spectra from the triplet and triplet states are close to each other. The peak of the fluorescence spectrum or the peak of the phosphorescence spectrum of a substance that exhibits delayed fluorescence is determined by the phosphorescence When designed to overlap the lowest energy absorption band of a compound, thermal activation delay Both the fluorescence spectrum and the phosphorescence spectrum of the fluorescent substance are at the lowest energy of the phosphorescent compound. This overlaps with (or is very close to) the absorption band located on the higher energy side (see Figure 1). This means that the thermally activated delayed fluorescence of a substance can be detected from both the singlet and triplet states. This means that energy can be efficiently transferred to the phosphorescent compound.

[0059] The light-emitting element of one embodiment of the present invention has a peak or a peak in the fluorescence spectrum of a substance that exhibits thermally activated delayed fluorescence. The peak of the phosphorescence spectrum is at the lowest energy in the absorption spectrum of the phosphorescent compound. By overlapping the absorption bands on the other side, the singlet excited state and Energy transfer from both triplet excited states to the phosphorescent compound is smooth, Thus, in one embodiment of the present invention, a light-emitting element with high external quantum efficiency can be obtained. This can be achieved.

[0060] In addition, considering the energy transfer process described above, the excitation energy is transferred from the host molecule to the guest molecule. Before the transfer of the electrons, the host molecule itself releases its excitation energy as light or heat and is deactivated. In one embodiment of the present invention, the energy Since the transfer is smooth, the deactivation of the excitation energy can be suppressed. This makes it possible to realize a light emitting element with a long life.

[0061] Here, the peak energy of the fluorescence spectrum of the material that exhibits thermally activated delayed fluorescence is too high. When the wavelength is too short, the energy is transferred from the thermally activated delayed fluorescence material to the phosphorescent compound. This requires a larger voltage to activate the phosphorescent compound and make it emit light, resulting in extra energy. This is not desirable because it consumes energy.

[0062] From this viewpoint, in one embodiment of the present invention, the fluorescent spectrum of a substance exhibiting thermally activated delayed fluorescence is The lower the peak energy (longer the wavelength), the lower the light emission start voltage of the light emitting element. The light-emitting element of one embodiment of the present invention is preferably a light-emitting element having a fluorescence spectrum of a substance exhibiting thermally activated delayed fluorescence. Since the energy of the peak is low, high luminous efficiency (external quantum efficiency) can be achieved while reducing the driving voltage. ) can be obtained, thereby achieving high power efficiency.

[0063] From this viewpoint, the peak or The peak of the phosphorescence spectrum is on the lowest energy side of the absorption spectrum of the phosphorescent compound. Within the range where the absorption band overlaps with In this case, the relatively high energy This is because the driving voltage of the light emitting element can be reduced while maintaining the efficiency.

[0064] In particular, the peaks of the fluorescence spectrum and the phosphorescence spectrum of a substance that exhibits thermally activated delayed fluorescence Both of these bands overlap with the lowest energy absorption band in the absorption spectrum of the phosphorescent compound. By having this, a light-emitting device with particularly high energy transfer efficiency and particularly high external quantum efficiency can be realized. This is preferable because it can

[0065] Specifically, another aspect of the present invention is a device comprising a phosphorescent compound and a thermally activated delayed fluorescent compound disposed between a pair of electrodes. and a fluorescent spectrum peak of the material exhibiting thermally activated delayed fluorescence. and the peak of the phosphorescence spectrum are at the lowest energy of the absorption spectrum of the phosphorescent compound, respectively. By applying a voltage between the pair of electrodes, The phosphorescent compound is a light-emitting element that emits phosphorescence.

[0066] In addition, the emission spectrum of a substance exhibiting thermally activated delayed fluorescence and the absorption spectrum of a phosphorescent compound were To achieve sufficient overlap, the energy of the peak in the fluorescence spectrum of the thermally activated delayed fluorescence substance must be The energy value and the peak energy of the absorption band on the lowest energy side of the absorption spectrum of the phosphorescent compound The difference between the energy value is preferably within 0.3 eV, more preferably within 0.2 eV. The thermally activated delayed fluorescence is preferably within 0.1 eV, more preferably within 0.1 eV. The energy value of the peak of the phosphorescence spectrum and the lowest energy of the absorption spectrum of the phosphorescent compound It is preferable that the difference between the energy value of the absorption band peak on the low-energy side and the energy value of the absorption band peak on the high-energy side is within 0.3 eV. It is more preferably within 0.2 eV, and particularly preferably within 0.1 eV.

[0067] As described above, the thermally activated delayed fluorescence used as a host material in the light-emitting element of one embodiment of the present invention The lowest triplet excitation energy and the lowest singlet excitation energy of the material shown in this figure are close to each other. In the light-emitting element of one embodiment of the present invention, a peak in the fluorescence spectrum of a substance exhibiting thermally activated delayed fluorescence is The energy value of the peak of the phosphorescence spectrum of the thermally activated delayed fluorescence material is It is preferable that the difference from the Gee value is within 0.3 eV.

[0068] In the light-emitting device, the absorption band on the lowest energy side of the absorption spectrum of the phosphorescent compound is , preferably includes an absorption band due to triplet MLCT transition. is the lowest triplet excited state of the phosphorescent compound, which is the guest material. In other words, from the triplet MLCT excited state, there is no decay other than emission. It is believed that increasing the proportion of this excited state as much as possible leads to high luminous efficiency. For this reason, the thermal activation can be achieved by utilizing the absorption band derived from the triplet MLCT transition. Direct energy transfer from activated delayed fluorescence materials to triplet MLCT excited states It can be said that it is preferable that there are many electron transfer processes. is preferably an organometallic complex, particularly an iridium complex.

[0069] The excitation energy of the material exhibiting thermally activated delayed fluorescence is sufficient for the phosphorescent compound. The fluorescence emission from the singlet excited state (delayed fluorescence emission) is not observed. preferable.

[0070] Furthermore, the energy transfer from the singlet excited state of a material exhibiting thermally activated delayed fluorescence is The star mechanism is considered to be important. Considering this, from formula (1), the phosphorescent compound The molar absorption coefficient of the absorption band located on the lowest energy side is preferably 2000 / M cm or more. Preferably, it is 5000 / M·cm or more.

[0071] This embodiment mode can be combined with other embodiment modes as appropriate.

[0072] (Embodiment 2) In this embodiment, a light-emitting element of one embodiment of the present invention will be described with reference to FIGS.

[0073] The light-emitting element exemplified in this embodiment has a pair of electrodes (a first electrode and a second electrode) and and an EL layer provided between a pair of electrodes. The pair of electrodes is an anode and a cathode. The EL layer has at least a light-emitting layer, and the light-emitting layer contains a luminescent material, a gate electrode, and a gate electrode. The guest material is a phosphorescent material, and the host material disperses the guest material. As the host material, a substance exhibiting thermally activated delayed fluorescence is used.

[0074] The light-emitting element of one embodiment of the present invention may have a top emission structure, a bottom emission structure, or the like. Both bottom-emission and dual-emission structures can be applied. can be done.

[0075] Specific structural examples of the light-emitting element of one embodiment of the present invention will be described below.

[0076] The light-emitting element shown in FIG. 2A has an EL layer 20 between a first electrode 201 and a second electrode 205. In this embodiment, the first electrode 201 functions as an anode, and the second electrode 20 5 acts as the cathode.

[0077] A voltage higher than the threshold voltage of the light-emitting element is applied between the first electrode 201 and the second electrode 205. When this occurs, holes are injected into the EL layer 203 from the first electrode 201 side, and electrons are injected from the second electrode 205 side. The injected electrons and holes recombine in the EL layer 203, and The luminescent material contained in 3 emits light.

[0078] As described above, the EL layer 203 has at least a light-emitting layer. As the layer, a material with high hole injection properties, a material with high hole transport properties, a hole blocking material, an electron transport material, A material with high electron transporting properties, a material with high electron injecting properties, or a bipolar material (electron transporting and hole transporting properties) The layer may further include a layer containing a substance having high transport properties.

[0079] The EL layer 203 can be made of known materials, such as low molecular weight compounds and high molecular weight compounds. Any of these may be used, and may contain an inorganic compound.

[0080] A specific example of the configuration of the EL layer 203 is shown in FIG. 2(B). The layer 301 includes a hole injection layer, a hole transport layer, a light emitting layer, an electron transport layer, and an electron injection layer. The insulating layer 305 is laminated in this order from the first electrode 201 side.

[0081] The light-emitting element shown in FIG. 2C has an EL layer 20 between a first electrode 201 and a second electrode 205. 3, and further includes an intermediate layer 207 between the EL layer 203 and the second electrode 205.

[0082] A specific example of the configuration of the intermediate layer 207 is shown in FIG. The intermediate layer 207 has at least the electron linkage layer 308 as a layer other than the charge generation region 308. It may further include a layer 307 and an electron injection buffer layer 306 .

[0083] A voltage higher than the threshold voltage of the light-emitting element is applied between the first electrode 201 and the second electrode 205. When this occurs, holes and electrons are generated in the charge generation region 308, and the holes move to the second electrode 205. The electrons move to the electron relay layer 307. The electron relay layer 307 has high electron transport properties. The electrons generated in the charge generation region 308 are quickly transferred to the electron injection buffer layer 306. The electron injection buffer layer 306 reduces the barrier to injecting electrons into the EL layer 203, Therefore, the electrons generated in the charge generation region 308 are injected into the electron relay. layer 307 and the electron injection buffer layer 306, and are injected into the LUMO level of the EL layer 203. do.

[0084] The electron relay layer 307 is made of a material that constitutes the charge generation region 308 and an electron injection buffer. The materials that make up layer 306 react at the interface, causing interactions that impair each other's functions. It can be prevented.

[0085] As in the light-emitting element shown in FIGS. 2(E) and 2(F), In this case, an intermediate layer may be provided between the stacked EL layers. For example, the light-emitting element shown in FIG. An intermediate layer 207 is provided between the first EL layer 203a and the second EL layer 203b. The light emitting element has n EL layers (n is a natural number of 2 or more), and the mth EL layer 203(m) (m is a natural number of 1 or more and (n-1) or less), and the (m+1)th EL layer 203(m+1) There is an intermediate layer 207 between them.

[0086] The electric field in the intermediate layer 207 provided between the EL layer 203(m) and the EL layer 203(m+1) The behavior of electrons and holes will be explained. When a voltage higher than the threshold voltage of the molecule is applied, holes and electrons are generated in the intermediate layer 207, and the positive The holes move to the EL layer 203(m+1) provided on the second electrode 205 side, and the electrons move to the EL layer 203(m+1) provided on the first electrode 205 side. Moves to EL layer 203(m) provided on the electrode 201 side. Injected into EL layer 203(m+1) The holes are recombined with electrons injected from the second electrode 205 side, and are transported to the EL layer 203 ( The luminescent material contained in the EL layer 203(m+1) emits light. The electrons recombine with the holes injected from the first electrode 201 side and are contained in the EL layer 203(m). Therefore, the holes and electrons generated in the intermediate layer 207 are Light emission occurs in the different EL layers.

[0087] In addition, when the EL layers are provided in contact with each other, the same structure as the intermediate layer is formed between them. For example, a charge generating region can be provided on one side of the EL layer. When the light-emitting layer is formed on the surface of the organic electroluminescent layer, an EL layer can be further provided in contact with the surface of the organic electroluminescent layer.

[0088] In addition, by making the luminescent color of each EL layer different, the desired luminescent color can be obtained as a whole. For example, in a light-emitting element having two EL layers, the first By making the luminescent color of the first EL layer and the luminescent color of the second EL layer complementary to each other, It is also possible to obtain a light emitting element that emits white light as a whole. In other words, colors that are complementary to each other are obtained from luminescent materials. By mixing the emitted light, white light can be obtained. The same applies to the light emitting element.

[0089] 2(A) to 2(F) can be used in combination with each other. For example, FIG. 2(F) An intermediate layer 207 may be provided between the second electrode 205 and the EL layer 203(n).

[0090] Examples of materials that can be used for each layer are shown below. Each layer is not limited to a single layer. Two or more layers may be laminated.

[0091] <anode> The electrode functioning as an anode (first electrode 201 in this embodiment) is made of a conductive metal. The layer can be formed by using one or more of a material such as a conductive material, an alloy, or a conductive compound. It is preferable to use a material with a large electron transport potential (4.0 eV or more). For example, indium tin oxide Indium Tin Oxide (ITO), silicon or silicon oxide containing Containing indium tin oxide, indium zinc oxide, tungsten oxide and zinc oxide Indium oxide, graphene, gold, platinum, nickel, tungsten, chromium, molybdenum , iron, cobalt, copper, palladium, or nitrides of metallic materials (for example, titanium nitride). It can be obtained.

[0092] When the anode is in contact with the charge generating region, various conductive materials can be used without considering the magnitude of the work function. For example, aluminum, silver, and alloys containing aluminum can be used. You can be there.

[0093] <cathode> The electrode functioning as a cathode (the second electrode 205 in this embodiment) is made of a conductive metal. The material can be formed by using one or more of a conductive material, an alloy, a conductive compound, etc. It is preferable to use a material with a small electron density (3.8 eV or less). For example, Elements belonging to Group 1 or Group 2 (e.g., alkali metals such as lithium and cesium, calcium , alkaline earth metals such as strontium, magnesium, etc.), alloys containing these elements (e.g. For example, rare earth metals such as Mg-Ag, Al-Li), europium, ytterbium, etc. For example, alloys containing rare earth metals, aluminum, silver, etc. can be used.

[0094] When the cathode is in contact with the charge generating region, various conductive materials can be used without considering the magnitude of the work function. For example, ITO, indium containing silicon or silicon oxide, Tin oxide and the like can also be used.

[0095] The light-emitting element has a conductive film on either the anode or cathode that transmits visible light, and the other that reflects visible light. Alternatively, both the anode and cathode may be conductive films that transmit visible light. A certain configuration may be used.

[0096] The conductive film that transmits visible light is, for example, indium oxide, ITO, indium zinc oxide, It can be formed using zinc oxide, zinc oxide doped with gallium, etc. Platinum, nickel, tungsten, chromium, molybdenum, iron, cobalt, copper, palladium, Alternatively, metal materials such as titanium, or nitrides of these metal materials (for example, titanium nitride), etc. The graphene or the like can be used by forming it thin enough to have light-transmitting properties. You can use it.

[0097] The conductive film that reflects visible light is made of, for example, aluminum, gold, platinum, silver, nickel, or tungsten. Metallic materials such as zinc, chromium, molybdenum, iron, cobalt, copper, or palladium, aluminum Aluminum and titanium alloys, aluminum and nickel alloys, aluminum and neodymium alloys Alloys containing aluminum, such as gold (aluminum alloys), or silver, such as silver-copper alloys The electrode can be formed using an alloy. An alloy of silver and copper is preferable because of its high heat resistance. Even if lanthanum, neodymium, germanium, etc. are added to the above metal materials or alloys, good.

[0098] The electrodes may be formed by vacuum deposition or sputtering. When paste or the like is used, a coating method or an ink jet method may be used.

[0099] <Hole injection layer 301> The hole injection layer 301 is a layer containing a substance with high hole injection properties.

[0100] Examples of materials with high hole injection properties include molybdenum oxide, titanium oxide, and vanadium oxide. oxide, rhenium oxide, ruthenium oxide, chromium oxide, zirconium oxide, huff Gold oxide, tantalum oxide, silver oxide, tungsten oxide, manganese oxide, etc. Metal oxides and the like can be used.

[0101] In addition, phthalocyanine (abbreviated as HPc), copper(II) phthalocyanine (abbreviated as CuPc Phthalocyanine compounds such as

[0102] In addition, the low molecular weight organic compound 4,4',4''-tris(N,N-diphenylamino) ) triphenylamine (abbreviation: TDATA), 4,4',4''-tris[N-(3-methylphenyl) (N-phenylamino)triphenylamine (abbreviation: MTDATA), 4 ,4'-Bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl DPAB, 4,4'-bis(N-{4-[N'-(3-methylphenyl)- N'-phenylamino]phenyl}-N-phenylamino)biphenyl (abbreviated as DNTP D), 1,3,5-tris[N-(4-diphenylaminophenyl)-N-phenylamino] 3-[N-(9-phenylcarbazol-3-yl)benzene (abbreviation: DPA3B) )-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 Aromatic amine compounds such as CzPCN1) can be used.

[0103] In addition, poly(N-vinylcarbazole) (abbreviation: PVK), poly(4-vinyltriphenyl) PVTPA), poly[N-(4-{N'-[4-(4-diphenylamine] N'-phenylamino)phenyl]phenyl-N'-phenylamino}phenyl)methacrylamide Name: PTPDMA), poly[N,N'-bis(4-butylphenyl)-N,N'-bis( phenyl)benzidine] (abbreviation: Poly-TPD), and other polymer compounds, such as poly(3,4- ethylenedioxythiophene) / poly(styrenesulfonic acid) (PEDOT / PSS), Acid-added polymers such as polyaniline / poly(styrene sulfonic acid) (PAni / PSS) A child compound can be used.

[0104] The hole injection layer 301 may also be used as a charge generation region. When the charge generation region is the charge generation region, various conductive materials can be used for the anode without considering the work function. The materials constituting the charge generating region will be described later.

[0105] <Hole transport layer 302> The hole transport layer 302 is a layer containing a substance with a high hole transport property.

[0106] The material having a high hole transporting property may be a material having a higher hole transporting property than an electron transporting property, and in particular , 10 -6 cm 2 It is preferable that the material has a hole mobility of 1 / Vs or more.

[0107] For example, 4,4'-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation N,N'-bis(3-methylphenyl)-N,N'-diphenyl ether (NPB or α-NPD) Phenyl-[1,1'-biphenyl]-4,4'-diamine (abbreviation: TPD), 4-phenyl 4'-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: BPAF LP), 4,4'-bis[N-(9,9-dimethylfluoren-2-yl)-N-phenyl 4,4'-bis[N-(spiro-9, 9'-Bifluoren-2-yl)-N-phenylamino]biphenyl (abbreviation: BSPB) Aromatic amine compounds such as the above can be used.

[0108] In addition, 4,4'-di(N-carbazolyl)biphenyl (abbreviation: CBP), 9-[4-(1 0-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: CzPA), 9-phenyl-3-[4-(10-phenyl-9-anthryl)phenyl]-9H-chlor Carbazole derivatives such as PCzPA (abbreviation: PCzPA) can be used.

[0109] In addition, 2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation: t- BuDNA), 9,10-di(2-naphthyl)anthracene (abbreviation: DNA), 9,10 - Use of aromatic hydrocarbon compounds such as diphenylanthracene (abbreviation: DPAnth) This can be done.

[0110] In addition, polymer compounds such as PVK, PVTPA, PTPDMA, and Poly-TPD are used. It is possible.

[0111] <Light-emitting layer 303> The light-emitting layer 303 is made of a guest material, which is a light-emitting substance, and a host material in which the guest material is dispersed. The guest material is a phosphorescent compound that emits phosphorescence. As the fluorescent substance, a substance exhibiting thermally activated delayed fluorescence is used.

[0112] The phosphorescent compound serving as the guest material is preferably an organometallic complex, and particularly preferably an iridium complex. In addition, considering the energy transfer by the Förster mechanism described above, phosphorescence The molar absorption coefficient of the lowest energy absorption band of the compound is 2000 / M cm or more. A molar extinction coefficient of 5000 / M·cm or higher is preferable, and 5000 / M·cm or higher is even more preferable. Examples of compounds having the formula include aryldiazines (pyridazines and pyrimidins having an aryl group attached thereto). Phosphorescent organometallic iridium complexes having a ligand of iridium fluoride, ... The carbon of the phenyl group of the phenylpyrimidine derivative or phenylpyrazine derivative is iridium Phosphorescent orthometalated iridium complexes linked to bis(3,5 -Dimethyl-2-phenylpyrazinato)(dipivaloylmethanato)iridium(III) (abbreviation: [Ir(mppr-Me)2(dpm)]), (acetylacetonato)bis(4 ,6-diphenylpyrimidinato)iridium(III) (synonym: bis[2-(6-phenyl [2,4-pentanedionato-κ](4-pyrimidinyl-κN3)phenyl-κC](2,4-pentanedionato-κ 2 O ,O') Iridium(III) (abbreviation: [Ir(dppm)2(acac)]), bis (2,3,5-triphenylpyrazinato)(dipivaloylmethanato)iridium(III) ) (abbreviation: [Ir(tppr)2(dpm)]), (acetylacetonato)bis(6-methyl bis[2-(6-methyl-4-phenylpyrimidinato)iridium(III) [2,4-pentanedionato-κ](4-pyrimidinyl-κN3)phenyl-κC](2,4-pentanedionato-κ 2 O ,O') Iridium(III) (abbreviation: [Ir(mppm)2(acac)]), (A Cetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium Bis[2-(6-tert-butyl-4-pyrimidinyl-κN3] Phenyl-κC](2,4-pentanedionato-κ 2 O,O')iridium(III) (abbreviation: [Ir(tBuppm)2(acac)])

[0113] As the host material, a known substance exhibiting thermally activated delayed fluorescence can be used. Examples of substances that exhibit delayed fluorescence include fullerene and its derivatives, and amines such as proflavine. Examples include cridine derivatives and eosin.

[0114] In addition, substances that exhibit thermally activated delayed fluorescence include magnesium (Mg), zinc (Zn), and Cadmium (Cd), tin (Sn), platinum (Pt), indium (In), or palladium Examples of such metal-containing porphyrins include metal-containing porphyrins containing palladium (Pd) and the like. For example, protoporphyrin-tin fluoride complex (abbreviation: S nF2(Proto IX)), mesoporphyrin-tin fluoride complex (abbreviated as SnF2( Meso IX), hematoporphyrin-tin fluoride complex (abbreviated as SnF2(Hema to IX), coproporphyrin tetramethyl ester-tin fluoride complex (abbreviation: S nF2(Copro III-4Me)), octaethylporphyrin-tin fluoride complex (abbreviation: SnF2(OEP)), etioporphyrin-tin fluoride complex (abbreviation: SnF2 (Etio I)), octaethylporphyrin-platinum chloride complex (abbreviated as PtCl2(O EP)) and others.

[0115] [ka]

[0116] [ka]

[0117] Furthermore, as a substance that exhibits thermally activated delayed fluorescence, 2-(biphenyl)-2-(phenyl)-1-(2- ... -4-yl)-4,6-bis(12-phenylindolo[2,3-a]carbazole-1 π-excess heteroaromatic rings such as (1-yl)-1,3,5-triazine (abbreviation: PIC-TRZ) and heterocyclic compounds having a π-deficient heteroaromatic ring. , and electron-transporting and hole-transporting properties due to the presence of π-rich and π-deficient heteroaromatic rings. In addition, a compound in which a π-rich heteroaromatic ring and a π-deficient heteroaromatic ring are directly bonded is also preferable. The quality is that the donor property of the π-rich heteroaromatic ring and the acceptor property of the π-deficient heteroaromatic ring are both strong. This is particularly preferable because the energy difference between S1 and T1 becomes small.

[0118] [ka]

[0119] In addition, by providing multiple light-emitting layers and making each layer emit a different color, the entire light-emitting element can be As a result, light of a desired color can be obtained. For example, in a light-emitting device having two light-emitting layers, In this case, the luminescent color of the first luminescent layer and the luminescent color of the second luminescent layer are made to have a complementary color relationship. In this way, it is possible to obtain a light emitting element that emits white light as a whole. The same applies to the case of a light emitting element having three or more of these.

[0120] <Electron transport layer 304> The electron transporting layer 304 is a layer containing a substance with a high electron transporting property.

[0121] As a substance with a high electron transporting property, an organic compound that transports electrons more than holes may be used. , especially, 10 -6 cm 2 It is preferable that the material has an electron mobility of 1 / Vs or more.

[0122] For example, tris(8-quinolinolato)aluminum(III) (abbreviation: Alq), tris (4-methyl-8-quinolinolato)aluminum (abbreviation: Almq3), bis(10-hydroxybenzoate) hydroxybenzo[h]quinolinato)beryllium (abbreviation: BeBq2), bis(2-methyl -8-quinolinolato)(4-phenylphenolato)aluminum(III) (abbreviation: BA lq), bis[2-(2-hydroxyphenyl)benzoxazolato]zinc (abbreviation: Zn (BOX)2), bis[2-(2-hydroxyphenyl)benzothiazolato]zinc (abbreviation Metal complexes such as Zn(BTZ)2 can be used.

[0123] Also, 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4 -Oxadiazole (abbreviation: PBD), 1,3-bis[5-(p-tert-butylphenyl) Nyl)-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)stilbe Heteroaromatic compounds such as benzophenone (abbreviation: BzOs) can be used.

[0124] In addition, poly(2,5-pyridinediyl) (abbreviation: PPy), poly[(9,9-dihexyl fluorene-2,7-diyl)-co-(pyridine-3,5-diyl)] (abbreviation: PF- Py), poly[(9,9-dioctylfluorene-2,7-diyl)-co-(2,2' -bipyridine-6,6'-diyl)] (abbreviation: PF-BPy) It is possible.

[0125] <Electron injection layer 305> The electron injection layer 305 is a layer containing a substance with high electron injection properties.

[0126] Materials with high electron injection properties include, for example, lithium, cesium, calcium, and lithium oxide. Lithium carbonate, cesium carbonate, lithium fluoride, cesium fluoride, calcium fluoride alkali metals, alkaline earth metals, rare earth metals such as erbium fluoride, etc., or Compounds (oxides, carbonates, halides, etc.) of the above can be used.

[0127] The electron injection layer 305 contains the above-described substance with high electron transport properties and a donor substance. For example, by adding magnesium (Mg) to Alq, the electron injection layer 3 When a substance with high electron transporting properties and a donor substance are contained, The mass ratio of the donor substance to the high-energy substance is preferably 0.001 or more and 0.1 or less. I wish.

[0128] Donor substances include lithium, cesium, magnesium, calcium, and erbium. , ytterbium, lithium oxide, calcium oxide, barium oxide, magnesium oxide Alkali metals, alkaline earth metals, rare earth metals, or their compounds (acids) such as aluminum compounds), Lewis bases, tetrathiafulvalene (abbreviation: TTF), tetrathianafluorene, Using organic compounds such as nickelocene (abbreviated as TTN), nickelocene, and decamethylnickelocene It is possible.

[0129] <Charge generation region> The charge generation region constituting the hole injection layer and the charge generation region 308 are made of a material having a high hole transporting property. This is a region containing an acceptor substance (electron acceptor). The acceptor substance is a hole transporting substance. It is preferable that the weight ratio of the highly resistant substance to the resistant substance is 0.1 or more and 4.0 or less. It's nice.

[0130] In addition, the charge generation region contains a substance with high hole transporting properties and a substance with acceptor properties in the same film. In addition to the case where a layer containing a substance with high hole transporting properties and a layer containing a substance with high hole accepting properties are stacked, However, when the charge generating region is provided on the cathode side, a layer having a high hole transporting property may be used. In the case of a laminated structure in which the layer containing the substance is in contact with the cathode and the charge generation region is provided on the anode side In this case, the layer containing the acceptor material is in contact with the anode.

[0131] As a substance with a high hole transporting property, an organic compound that transports holes more efficiently than electrons may be used. , especially, 10 -6 cm 2 It is preferable that the organic compound has a hole mobility of 1 / Vs or more. stomach.

[0132] Specifically, aromatic amine compounds such as NPB and BPAFLP, CBP, CzPA, and PCz Carbazole derivatives such as PA, t-BuDNA, DNA, aromatic hydrocarbons such as DPAnth The hole transport layer 302 may be made of a polymer compound such as PVK or PVTPA. As a material capable of transporting holes, the materials having high hole transport properties exemplified above can be used.

[0133] Acceptor substances include 7,7,8,8-tetracyano-2,3,5,6-tetramethyl ... Fluoroquinodimethane (abbreviation: F4-TCNQ), organic compounds such as chloranil, transition metals oxides, and oxides of metals belonging to groups 4 to 8 in the periodic table. Specifically, vanadium oxide, niobium oxide, tantalum oxide, chromium oxide, molyb oxide Tungsten oxide, manganese oxide, and rhenium oxide are preferred due to their high electron-accepting properties. In particular, molybdenum oxide is preferred because it is stable in the air, has low hygroscopicity, and is easy to handle. It's nice.

[0134] <Electron injection buffer layer 306> The electron injection buffer layer 306 is a layer containing a material with high electron injection properties. The charge generating region 308 is electrically conductive and the charge generating region 308 is electrically conductive. The above-mentioned materials can be used as the material with high electron injection properties. The electron transport layer 306 may contain the above-described substance with high electron transport properties and a donor substance.

[0135] <Electronic Relay Layer 307> In the electron relay layer 307, the acceptor material is extracted in the charge generation region 308. Accepts electrons quickly.

[0136] The electron-relay layer 307 contains a substance with a high electron-transporting property. Phthalocyanine-based materials or metal complexes with metal-oxygen bonds and aromatic ligands are used. It is preferable that:

[0137] Specific examples of the phthalocyanine-based material include CuPc and SnPc (Phthalocyanine anine tin(II) complex), ZnPc(Phthalocyani ne zinc complex), CoPc(Cobalt(II)phthaloc yanine, β-form), FePc (Phthalocyanine Iron ), PhO-VOPc(Vanadyl 2,9,16,23-tetraphenox y-29H,31H-phthalocyanine) and the like.

[0138] The metal complex having a metal-oxygen bond and an aromatic ligand includes a metal complex having a metal-oxygen double bond. It is preferable to use a metal complex in which the metal-oxygen double bond has acceptor properties. This makes it easier for electrons to move (donate).

[0139] In addition, as the metal complex having the metal-oxygen bond and the aromatic ligand, a phthalocyanine-based material In particular, VOPc (vanadyl phthalocyanine), S nOPc(Phthalocyanine tin(IV) oxide comple x), TiOPc(Phthalocyanine titanium oxide c Complex) is a compound whose molecular structure makes the metal-oxygen double bond more likely to interact with other molecules. , is preferred because of its high acceptor properties.

[0140] The phthalocyanine-based material is preferably one having a phenoxy group, specifically P Phthalocyanine derivatives having a phenoxy group, such as hO-VOPc, are preferred. The phthalocyanine derivative having an oxy group is soluble in a solvent, and therefore can be used to form a light-emitting element. It has the advantage that it is easy to handle and the maintenance of the equipment used for film formation is easy. do.

[0141] Other materials with high electron transport properties include 3,4,9,10-perylenetetate. Tetracarboxylic dianhydride (PTCDA), 3,4,9,10-perylenetetracarboxylic acid Xylic bisbenzimidazole (abbreviation: PTCBI), N,N'-dioctyl-3, 4,9,10-Perylenetetracarboxylic diimide (abbreviation: PTCDI-CH), N, N'-Dihexyl-3,4,9,10-perylenetetracarboxylic acid diimide (abbreviation: He x PTC) and pyrazino[2,3-f][1,10]phenanthro ... Phosphorus-2,3-dicarbonitrile (PPDN), 2,3,6,7,10,11-hexafluorophosphate Hexacyano-1,4,5,8,9,12-hexaazatriphenylene (abbreviation: HAT(C N)6), 2,3-diphenylpyrido[2,3-b]pyrazine (abbreviation: 2PYPR), 2 ,3-Bis(4-fluorophenyl)pyrido[2,3-b]pyrazine (abbreviation: F2PYP Nitrogen-containing condensed aromatic compounds such as benzophenone-1, benzophenone-2, benzophenone-3, benzophenone-4, benzophenone-5, benzophenone-6, benzophenone-7, benzophenone-8, benzophenone-9, benzophenone-10, benzophenone-11, benzophenone-12, benzophenone-13, benzophenone-14, benzophenone-15, benzophenone Therefore, it is preferable as a material to be used for forming the electron relay layer 307.

[0142] In addition, 7,7,8,8-tetracyanoquinodimethane (abbreviation: TCNQ), 1,4,5,8 -Naphthalenetetracarboxylic dianhydride (NTCDA), perfluoropentacene , copper hexadecafluorophthalocyanine (abbreviation: F 16 CuPc), N,N'-bis(2 ,2,3,3,4,4,5,5,6,6,7,7,8,8,8-Pentadecafluorooctyl NTCDI-C 8F), 3',4'-dibutyl-5,5''-bis(dicyanomethylene)-5,5''- Dihydro-2,2':5',2''-terthiophene (abbreviation: DCMT), methanofulva phenylenes (e.g., [6,6]-phenyl C 61 butyric acid methyl ester) can be used. do.

[0143] The electron relay layer 307 may further contain the donor material described above. By including a donor substance in the 07, electrons can be easily transferred, making the light-emitting element more It becomes possible to drive it at a low voltage.

[0144] The LUMO levels of the material with high electron transport properties and the donor material are included in the charge generation region 308. The LUMO level of the acceptor material to be used and the electron transporting material contained in the electron transport layer 304 are The LUMO level of the material (or the electron relay layer 307 or the electron injection buffer layer 306) The LUMO level is set to be between -5.0 and the LUMO level of the adjacent EL layer 203. It is preferable that the electron relay layer 307 has a donor property. When a material is contained, the accession number contained in the charge generation region 308 is used as a material with high electron transport properties. A material with a LUMO level higher than the acceptor level of the acceptor material can be used. do.

[0145] The layers constituting the EL layer 203 and the intermediate layer 207 are formed by evaporation (vacuum deposition). It can be formed by methods such as deposition, transfer, printing, inkjet, and coating. can.

[0146] The light-emitting element described in this embodiment mode can be used to produce a passive matrix light-emitting device or a transistor. An active matrix light-emitting device was fabricated in which the driving of light-emitting elements was controlled by a transistor. In addition, the light-emitting device can be applied to electronic devices, lighting devices, etc. .

[0147] This embodiment mode can be freely combined with other embodiment modes. [Explanation of symbols]

[0148] 201 First electrode 203 EL layer 203a First EL layer 203b Second EL layer 205 Second electrode 207 Middle Class 301 Hole injection layer 302 Hole transport layer 303 Light-emitting layer 304 Electron transport layer 305 Electron injection layer 306 Electron injection buffer layer 307 Electronic Relay Layer 308 Charge generation region

Claims

1. A first electrode and The second electrode and The device comprises a light-emitting layer between the first electrode and the second electrode, The light-emitting layer comprises a phosphorescent compound and a substance that exhibits thermally activated delayed fluorescence. The substance contains a π-deficient heteroaromatic ring, The fluorescence spectrum of the substance overlaps with the lowest energy absorption band of the absorption spectrum of the phosphorescent compound. A light-emitting device wherein the energy value of the peak in the fluorescence spectrum of the substance is lower than the energy value of the peak in the lowest energy absorption band of the absorption spectrum of the phosphorescent compound.

2. A first electrode and The second electrode and The device comprises a light-emitting layer between the first electrode and the second electrode, The light-emitting layer comprises a phosphorescent compound and a substance that exhibits thermally activated delayed fluorescence. The substance contains a π-deficient heteroaromatic ring, A light-emitting device wherein the energy value of the peak in the fluorescence spectrum of the substance is lower than the energy value of the peak in the lowest energy absorption band of the absorption spectrum of the phosphorescent compound.

3. A first electrode and The second electrode and The device comprises a light-emitting layer between the first electrode and the second electrode, The light-emitting layer comprises a phosphorescent compound and a substance containing a π-deficient heteroaromatic ring. The aforementioned substance has a difference of 0.3 eV or less between its lowest triplet excitation energy and its lowest singlet excitation energy. The fluorescence spectrum of the substance overlaps with the lowest energy absorption band of the absorption spectrum of the phosphorescent compound. A light-emitting device wherein the energy value of the peak in the fluorescence spectrum of the substance is lower than the energy value of the peak in the lowest energy absorption band of the absorption spectrum of the phosphorescent compound.

4. A first electrode and The second electrode and The device comprises a light-emitting layer between the first electrode and the second electrode, The light-emitting layer comprises a phosphorescent compound and a substance containing a π-deficient heteroaromatic ring. The aforementioned substance has a difference of 0.3 eV or less between its lowest triplet excitation energy and its lowest singlet excitation energy. A light-emitting device wherein the energy value of the peak in the fluorescence spectrum of the substance is lower than the energy value of the peak in the lowest energy absorption band of the absorption spectrum of the phosphorescent compound.

5. In claim 3 or claim 4, The aforementioned material is a light-emitting device that exhibits thermally activated delayed fluorescence.

6. In any one of Claims 1 to 5, A light-emitting device in which the difference between the energy value of the peak of the fluorescence spectrum of the substance and the energy value of the peak of the lowest energy absorption band of the absorption spectrum of the phosphorescent compound is 0.3 eV or less.

7. In any one of Claims 1 to 6, The light-emitting device includes an absorption band based on a triplet MLCT transition, with the lowest energy absorption band being the one described above.

8. In any one of Claims 1 to 7, The phosphorescent compound is an iridium complex, which is used in the light-emitting device.

9. In any one of Claims 1 to 8, A light-emitting device having a molar extinction coefficient of 5000 / M·cm or more in the lowest energy absorption band.

10. In any one of Claims 1 to 9, A light-emitting device in which the substance is a heterocyclic compound comprising a π-excess heteroaromatic ring and a π-deficient heteroaromatic ring.

11. In claim 10, The substance is a heterocyclic compound in which the π-excess heteroaromatic ring and the π-deficient heteroaromatic ring are directly bonded, and this is a light-emitting device.

12. In any one of Claims 1 to 11, A light-emitting device in which the phosphorescent compound functions as a guest material and the substance functions as a host material in the light-emitting layer.

13. In any one of Claims 1 to 12, The phosphorescence spectrum of the substance overlaps with the lowest energy absorption band of the absorption spectrum of the phosphorescent compound. A light-emitting device wherein the energy value of the peak in the phosphorescence spectrum of the substance is lower than the energy value of the peak in the lowest energy absorption band of the absorption spectrum of the phosphorescent compound.

14. In any one of Claims 1 to 13, The first electrode and the light-emitting layer have a hole transport layer, The hole transport layer comprises a carbazole derivative in the light-emitting device.

15. In any one of Claims 1 to 14, A light-emitting device wherein the triplet excitation energy level of the material is higher than the triplet excitation energy level of the phosphorescent compound.

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