Light-emitting element

Abstract

To provide a light-emitting element with high luminous efficiency or long life, which contains a smaller amount of a phosphorescent compound.SOLUTION: A light-emitting element includes, between a pair of electrodes, a light-emitting layer including a phosphorescent compound, a first organic compound, and a second organic compound. The first organic compound and the second organic compound are combination forming an excitation complex (exciplex). The light-emitting element causes energy transfer by using overlapping between a light emission spectrum of the excitation complex and an absorption spectrum of the phosphorescent compound; therefore, energy transfer efficiency is high even if concentration of the phosphorescent compound is low.SELECTED DRAWING: None

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 has been actively carried out (Patent Document 1, Patent Document 2 and Non-Patent Document 3). The basic structure of an organic EL element is a pair of electrodes between which a light-emitting organic compound is placed. This allows for a thin and lightweight structure and high sensitivity to input signals. Due to its characteristics such as fast response and low DC voltage drive, it is expected to be the next generation flat panel display. It is attracting attention as a display element.

[0003] Furthermore, displays using such light-emitting elements have excellent contrast and image quality, and have a wide viewing angle. In addition, since organic EL elements are surface light sources, they are much lighter than LCDs. Applications as a light source for backlighting and illumination for play are also being considered.

[0004] The light-emitting mechanism of organic EL elements is the carrier injection type. In other words, the light-emitting layer is sandwiched between electrodes. By applying a voltage, the electrons and holes injected from the electrodes recombine and The luminescent material becomes excited and emits light when the excited state returns to the ground state. The types of states include singlet excited states (S * ) and triplet excited states (T * ) is possible. The statistical generation ratio of the light-emitting element is S * :T * = 1:3 is.

[0005] Luminescent organic compounds usually have a singlet ground state. Therefore, luminescence from the singlet excited state (S * ) is an electronic transition between the same spin multiplicities and is called fluorescence. On the other hand, luminescence from the triplet excited state (T * ) is an electronic transition between different spin multiplicities and is called phosphorescence. Usually, compounds that emit fluorescence (hereinafter referred to as fluorescent compounds) exhibit only fluorescence and no phosphorescence is observed at room temperature. Therefore, the theoretical limit of the internal quantum efficiency (the ratio of photons generated to the injected carriers) in a light-emitting device using a fluorescent compound is set at 25% based on S :T being 1:3. S * :T * =1:3.

[0006] On the other hand, if a compound that emits phosphorescence (hereinafter referred to as a phosphorescent compound) is used, theoretically, the internal quantum efficiency can be increased up to 100%. That is, it becomes possible to obtain higher luminous efficiency compared to fluorescent compounds. For these reasons, in order to realize a highly efficient light-emitting device, the development of light-emitting devices using phosphorescent compounds has been actively carried out in recent years. In particular, as phosphorescent compounds, organometallic complexes having iridium or the like as a central metal have attracted attention due to their high phosphorescence quantum efficiency. For example, Patent Document 1 discloses an organometallic complex having iridium as a central metal as a phosphorescent material. In particular, as phosphorescent compounds, organometallic complexes having iridium or the like as a central metal have attracted attention due to their high phosphorescence quantum efficiency. For example, Patent Document 1 discloses an organometallic complex having iridium as a central metal as a phosphorescent material. compounds, due to their high phosphorescence quantum efficiency, organometallic complexes having iridium or the like as a central metal have attracted attention. For example, Patent Document 1 discloses an organometallic complex having iridium as a central metal as a phosphorescent material. When forming the light-emitting layer of a light-emitting device using the above-described phosphorescent compound, in order to suppress concentration quenching of the phosphorescent compound and quenching due to triplet-triplet annihilation, the phosphorescent compound is often formed so as to be dispersed in a matrix composed of other compounds. At this time, the compound serving as the matrix is often formed so that the phosphorescent compound is dispersed in a matrix composed of other compounds. At this time, the compound serving as the matrix

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

[0008] In addition, in Patent Document 2 or Non-Patent Document 1, the light-emitting layer is made of a material with good electron transport properties and a material with good hole transport properties. For example, Non-Patent Document 1 states that Tris-8-quinolinolatoaluminum complex (Alq3) with good transport properties and hole transport 4,4'-bis[N-(1-naphthyl)-N-phenylamino]biphenyl ( The two materials are NPB and methylquinacridone as a guest (dopant). A light-emitting device using mqa (abbreviation: mqa) has been proposed.

[0009] In the light-emitting layer with this structure, two types of hosts each take on different functions, and both electrons and holes are In other words, electrons are conducted through Alq3, and holes are conducted through NP B, both of which reach mqa, which can be excited. mqa is However, in Patent Document 2, a phosphorescent compound is used as a guest to emit triple fluorescence. It is disclosed that light emission can be obtained from the first excited state. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] International Publication No. 2000 / 070655 [Patent Document 2] U.S. Patent No. 7,572,522 [Non-patent literature]

[0011] [Non-Patent Document 1] Vi-En Choong et al. ”Organic light-emitting diodes with a bipolar transport layer”, Appl. Phys. Lett., 75, 172 (1999).

Summary of the Invention

Problems to be Solved by the Invention

[0012] Iridium is a rare element (abundance 1×10 -7 %), and the amount buried near the earth's surface is less than that of platinum (abundance 5×10 -7 %) and gold (abundance 5×10 -7 %). Therefore, there are concerns about its stable supply, and a reduction in its usage amount is demanded.

[0013] However, generally when a phosphorescent compound is a guest, if the concentration of the guest is excessively decreased, the light emission efficiency decreases. This can be explained as follows. There are roughly two processes for exciting guest molecules to an excited state. One is a direct recombination process in which electrons and holes are injected into the guest molecules to excite the guest molecules. The other is an energy transfer process in which the state of the excited host molecules moves to the guest molecules. To increase the ratio of the direct recombination process, it is necessary for the guest to be present at a high concentration. Therefore, when the concentration of the guest decreases, the recombination probability decreases and the light emission efficiency decreases.

[0014]

[0015] On the other hand, regarding the energy transfer process, two theories, the Förster mechanism and the Dexter mechanism, have been proposed.

[0016] ​​​​​​The Förster mechanism does not require direct contact between molecules for energy transfer, and assumes that energy transfer occurs through the resonance phenomenon of dipole vibrations between host molecules and guest molecules. Due to the resonance phenomenon of dipole vibrations, the host molecule transfers energy to the guest molecule , the host molecule returns to the ground state, and the guest molecule enters the excited state. The rate constant k of the Förster mechanism is shown in Equation (1). h * →g is shown in Equation (1).

[0017]

Equation

[0018] In Equation (1), ν represents the frequency, f’ h (ν) represents the normalized emission spectrum of the host molecule (fluorescence spectrum when discussing energy transfer from the singlet excited state, phosphorescence spectrum when discussing energy transfer from the triplet excited state), ε g (ν ) represents the molar extinction coefficient of the guest molecule, N represents Avogadro's number, n represents the refractive index of the medium, R represents the intermolecular distance between the host molecule and the guest molecule, τ represents the measured lifetime of the excited state (fluorescence lifetime or phosphorescence lifetime), c represents the speed of light, φ represents the emission quantum efficiency (fluorescence quantum efficiency when discussing energy transfer from the singlet excited state, phosphorescence quantum efficiency when discussing energy transfer from the triplet excited state), K is a coefficient (0 to 4) representing the orientation of the transition dipole moments between the host molecule and the guest molecule. In the case of random orientation, 2 K = 2 / 3. K 2 = 2 / 3.

[0019] The Dexter mechanism assumes that as the host molecule and the guest molecule approach the contact effective distance where orbital overlap occurs, energy transfer takes place through the exchange of electrons between the excited host molecule and the ground state guest molecule. The rate constant k of the Dexter mechanism is shown in Equation (2). The rate constant k of the Dexter mechanism is shown in Equation (2). h * →g is shown in Equation (2) as follows.

[0020]

Equation

[0021] In Equation (2), h is the Planck constant, K is a constant with the dimension of energy, ν represents the frequency, f’ (ν) represents the normalized emission spectrum of the host molecule h (fluorescence spectrum when discussing energy transfer from the singlet excited state, phosphorescence spectrum when discussing energy transfer from the triplet excited state), ε’ (ν) represents the normalized absorption spectrum of the guest molecule, L represents the effective molecular radius, and R represents the intermolecular distance between the host molecule and the guest molecule. g (ν) represents the normalized absorption spectrum of the guest molecule, L represents the effective molecular radius, and R represents the intermolecular distance between the host molecule and the guest molecule. molecule, L represents the effective molecular radius, and R represents the intermolecular distance between the host molecule and the guest molecule.

[0022] Generally, the distance over which energy transfer by the Dexter mechanism is possible is up to about 1 nm, and the distance over which energy transfer by the Förster mechanism is possible is up to about 10 nm. Therefore, as the concentration of the guest decreases and the distance between the guest molecule and the excited host molecule increases, the efficiency of energy transfer decreases significantly. For this reason, the concentration of the guest has been required to be 3% or more. Therefore, as the concentration of the guest decreases and the distance between the guest molecule and the excited host molecule increases, the efficiency of energy transfer decreases significantly. For this reason, the concentration of the guest has been required to be 3% or more. Therefore, as the concentration of the guest decreases and the distance between the guest molecule and the excited host molecule increases, the efficiency of energy transfer decreases significantly. For this reason, the concentration of the guest has been required to be 3% or more. Therefore, as the concentration of the guest decreases and the distance between the guest molecule and the excited host molecule increases, the efficiency of energy transfer decreases significantly. For this reason, the concentration of the guest has been required to be 3% or more. Therefore, as the concentration of the guest decreases and the distance between the guest molecule and the excited host molecule increases, the efficiency of energy transfer decreases significantly. For this reason, the concentration of the guest has been required to be 3% or more.

[0023] As described above, the elementary processes from carrier recombination to the formation of the excited state of the guest Considering this, in order to efficiently excite the guest to the excited state, it is necessary to increase the concentration of the guest. However, on the other hand, when the concentration of the guest is high, the emission efficiency decreases due to aggregation of the guest, so-called concentration quenching occurs, and even if the excited state of the guest can be efficiently generated, as a result, the emission efficiency of the device decreases. That is, when the concentration of the guest is low, the generation efficiency of the excited state of the guest deteriorates, and when the concentration of the guest is high, concentration quenching occurs, so there is a dilemma that the emission efficiency decreases in any case.

[0024] In view of such a situation, one aspect of the present invention aims to provide a light-emitting device using a novel light-emitting mechanism, which reduces the concentration of the guest. Another aspect of the present invention aims to provide a highly efficient light-emitting device. Another aspect of the present invention aims to provide a long-life light-emitting device. Another aspect of the present invention aims to provide a light-emitting device with less deterioration. Another aspect of the present invention aims to provide a highly reliable light-emitting device. One aspect of the present invention solves at least one of the above problems.

Means for Solving the Problems

[0025] One aspect of the present invention is a light-emitting device having a light-emitting layer containing a phosphorescent compound, a first organic compound, and a second organic compound between a pair of electrodes, wherein the first organic compound and the second organic compound form an exciplex, and the weight ratio of the phosphorescent compound to the sum of the weights of the first organic compound and the second organic compound is 0.1% or more and 2.5% or less, preferably 0.1% or more and 1.5% or less, more preferably 0.1% or more and 0.5% or less. It is a light-emitting device.

[0026] In addition, one aspect of the present invention includes a phosphorescent compound, a first organic compound, and a second organic compound and has a light-emitting layer between a pair of electrodes, wherein at least one first organic compound and at least one second organic compound form a combination that forms an exciplex, and the exciplex acts on the phosphorescent compound to cause the phosphorescent compound to emit phosphorescence, and the ratio of the weight of the phosphorescent compound to the sum of the weights of the first organic compound and the second organic compound is 0.1% or more and 2.5% or less, preferably 0 .1% or more and 1.5% or less, more preferably 0.1% or more and 0.5% or less, which is a light-emitting device. .

[0027] In addition, one aspect of the present invention includes a phosphorescent compound, a first organic compound, and a second organic compound and has a light-emitting layer between a pair of electrodes, wherein the first organic compound or the second organic compound is one in which an exciplex of the first organic compound and the second organic compound is formed from its singlet exciton, and the ratio of the weight of the phosphorescent compound to the sum of the weights of the first organic compound and the second organic compound is 0.1% or more and 2.5% or less, preferably 0.1% or more and 1.5% or less, more preferably 0 .1% or more and 0.5% or less, which is a light-emitting device.

[0028] In addition, one aspect of the present invention includes a phosphorescent compound, a first organic compound, and a second organic compound and has a light-emitting layer between a pair of electrodes, wherein the first organic compound and the second organic compound are those in which an exciplex of the first organic compound and the second organic compound is formed from the anion of the first organic compound and the cation of the second organic compound, and the ratio of the weight of the phosphorescent compound to the sum of the weights of the first organic compound and the second organic compound is 0.1% or more and 2.5% or less, preferably 0 .1% or more and 1.5% or less, more preferably 0.1% or more and 0.5% or less, which is a light-emitting device. 。

[0029] In the above light-emitting element, it is preferable that the excitation energy of the exciplex transfers to the phosphorescent compound, and the phosphorescent compound emits phosphorescence.

[0030] In the above light-emitting element, it is preferable that at least one of the first organic compound and the second organic compound is a fluorescent compound.

[0031] In the above light-emitting element, the phosphorescent compound is preferably an organometallic complex having iridium.

[0032] In the above light-emitting element, the molar absorption coefficient of the phosphorescent compound at the peak of the emission spectrum of the exciplex is preferably 5000 M -1 cm -1 or more.

[0033] In the above light-emitting element, it is preferable that the first organic compound has higher electron transporting property than hole transporting property, and the second organic compound has higher hole transporting property than electron transporting property.

[0034] The light-emitting element according to one aspect of the present invention can be applied to a light-emitting device, an electronic device, and a lighting device.

Advantages of the Invention

[0035] In the above energy transfer process, the energy transfer efficiency Φ from the host molecule to the guest molecule is considered to be represented by the mathematical formula (3). k ET represents the rate constant of the emission process of the host molecule (fluorescence when discussing the energy transfer from the singlet excited state of the host r molecule, phosphorescence when discussing the energy transfer from the triplet excited state of the host molecule), and k represents the rate constant of the non-emission process n ​​​represents the rate constant of the process (thermal deactivation and cross - terms), and τ represents the lifetime of the excited state of the measured host molecule. Lifetime.

[0036]

Equation

[0037] From Equation (3), to increase the energy transfer efficiency Φ ET it can be seen that the rate constant k of energy transfer h * →g should be much larger than the other competing rate constants k r +k n (=1 / τ). And to increase the rate constant k of that energy transfer h * →g it can be seen from Equation (1) and Equation (2) that in either the Förster mechanism or the Dexter mechanism, when discussing energy transfer from the singlet excited state of the host molecule (fluorescence spectrum when discussing energy transfer from the singlet excited state, phosphorescence spectrum when discussing energy transfer from the triplet excited state), it is better that the overlap between the emission spectrum of the host molecule (fluorescence spectrum or phosphorescence spectrum) and the absorption spectrum of the guest molecule is larger. That is, in either mechanism, it is better that the overlap between the emission spectrum of the host molecule (fluorescence spectrum when discussing energy transfer from the singlet excited state, phosphorescence spectrum when discussing energy transfer from the triplet excited state) and the absorption spectrum of the guest molecule is larger. And when discussing energy transfer from the singlet excited state of the host molecule (fluorescence spectrum), or from the triplet excited state (phosphorescence spectrum), it is better that the overlap between the emission spectrum of the host molecule and the absorption spectrum of the guest molecule is larger. When discussing energy transfer from the singlet excited state, it is the fluorescence spectrum; when discussing energy transfer from the triplet excited state, it is the phosphorescence spectrum. It can be seen that it is better that the overlap between the emission spectrum of the host molecule (fluorescence spectrum when discussing energy transfer from the singlet excited state, phosphorescence spectrum when discussing energy transfer from the triplet excited state) and the absorption spectrum of the guest molecule is larger. That is.

[0038] Also, it can be seen that the higher the absorbance of the guest molecule at the peak of the emission spectrum of the host molecule, the more likely the energy transfer process from the host molecule to the guest molecule will occur. That is, by setting the energy levels of the excited state of the host molecule and the excited state of the guest molecule to be approximately equal, and increasing the transition probability to the excited state of the guest molecule, energy transfer becomes easier.

[0039] That is, by setting the energy levels of the excited state of the host molecule and the excited state of the guest molecule to be approximately equal, and increasing the transition probability to the excited state of the guest molecule, energy transfer becomes easier. And by increasing the transition probability to the excited state of the guest molecule, energy transfer becomes easier. That is, energy transfer becomes easier.

[0040] However, under this condition, when the host is a single material as in the prior art, conversely, the process in which the energy of the excited guest molecules transfers to the ground state host molecules is likely to occur, and ultimately, the luminescence efficiency decreases.

[0041] Regarding this problem, it can be solved by using an exciplex. Here, the exciplex will be described in detail. An exciplex (exciplex) is formed by the interaction between different molecules in the excited state. It is generally known that an exciplex is likely to be formed between a material having a relatively deep lowest unoccupied molecular orbital (LUMO) level and a material having a shallow highest occupied molecular orbital (HOMO) level.

[0042] Here, the HOMO levels and LUMO levels of the first organic compound and the second organic compound applied in one aspect of the present invention are different. Specifically, the energy levels satisfy the relationship: the HOMO level of the first organic compound < the HOMO level of the second organic compound < the LUMO level of the first organic compound < the LUMO level of the second organic compound.

[0043] Note that even if this condition is satisfied, an exciplex is not necessarily formed. For example, as described in Non-Patent Document 1, an exciplex cannot be formed between Alq3 and NPB. It should be noted that when an exciplex cannot be formed, the following effects cannot be obtained.

[0044] When an exciplex is formed by the first organic compound and the second organic compound, the LUMO of the exciplex ​​​​​​​​​​​​The MO level is derived from the first organic compound, and the HOMO level is derived from the second organic compound. . Therefore, the energy difference of the exciplex is smaller than the energy differences of the first organic compound and the second organic compound. That is, the emission wavelength of the exciplex becomes longer than the emission wavelengths of each of the first organic compound and the second organic compound.

[0045] The formation process of the exciplex can be roughly divided into the following two processes. The first is due to the formation of an electroplex . In this specification, an electroplex refers to the direct formation of an exciplex from the first organic compound in the ground state and the second organic compound in the ground state. Specifically, when the anion of the first organic compound and the cation of the second organic compound are adjacent, the two form an exciplex.

[0046] From the above relationship, since the first organic compound is an electron-trapping compound and the second organic compound is a hole-trapping compound, an electroplex is directly formed from the anion of the first organic compound and the cation of the second organic compound .

[0047] The emission spectrum of the formed electroplex exists on the longer wavelength side compared to the emission wavelengths of each of the first organic compound and the second organic compound.

[0048] The overlap between the emission spectrum of the first organic compound (or the second organic compound) and the absorption spectrum of the phosphorescent compound is smaller than the overlap between the emission spectrum of the electroplex and the absorption spectrum of the phosphorescent compound. The light-emitting element of one aspect of the present invention is an electroplex By utilizing the overlap between the emission spectrum of the host and the absorption spectrum of the phosphorescent compound as the guest, energy transfer occurs, and the energy transfer efficiency is high. Therefore, in one aspect of the present invention it is possible to realize a light-emitting device with high external quantum efficiency.

[0049] As described above, generally, when electrons and holes recombine in the host, the excited host molecule transfers the excitation energy to the guest molecule, the guest molecule reaches the excited state, and emits light .

[0050] Here, before the excitation energy is transferred from the host molecule to the guest molecule, the host molecule itself emits light at an unwanted wavelength, or the excitation energy becomes thermal energy, resulting in a loss (deactivation) of part of the excitation energy. In particular, when the host molecule is in the singlet excited state, compared to the case where it is in the triplet excited state, the excitation lifetime is short, so the deactivation of singlet excitons occurs easily . The deactivation of excitons is one of the factors leading to a decrease in the lifetime of the light-emitting device.

[0051] On the other hand, in one aspect of the present invention, the first organic compound and the second organic compound form an electrocomplex from the state (cation or anion) having carriers, so that the formation of singlet excitons with a short excitation lifetime can be suppressed. That is, there may be a process of directly forming an exciplex without forming singlet excitons. As a result, the deactivation of the above-mentioned singlet excitons can also be suppressed. Therefore, a light-emitting device with a long lifetime can be realized.

[0052] The concept of suppressing the generation of the singlet excited state of the host in this way and performing energy transfer from the electrocomplex to the guest molecule to obtain a light-emitting device with high luminous efficiency is unprecedented . ​​​。

[0053] As another process, there is a process in which one of the first organic compound and the second organic compound as the host forms a singlet exciton and then interacts with the other in the ground state to form an exciplex. This is considered. Different from an electroplex, in this case, once a singlet exciton of the host is generated, since this is rapidly converted into an exciplex, the deactivation of the singlet exciton can still be suppressed. That the singlet exciton of the host is rapidly converted into an exciplex is also clear from the experimental results described later. Therefore, it is possible to suppress the host from deactivating. For this reason, in one aspect of the present invention, a light-emitting device having a long lifetime can be realized. 。

[0054] Also, in this case as well, the emission spectrum of the formed exciplex exists on the longer wavelength side compared to the emission wavelengths of each of the first organic compound and the second organic compound. 。 。

[0055] Also, in this case as well, the overlap between the emission spectrum of the formed exciplex and the absorption spectrum of the phosphorescent compound is larger than the overlap between the emission spectrum of the first organic compound (or the second organic compound) and the absorption spectrum of the phosphorescent compound. The light-emitting device according to one aspect of the present invention utilizes the overlap between the emission spectrum of the exciplex and the absorption spectrum of the phosphorescent compound to perform energy transfer, and thus the energy transfer efficiency is high. Therefore, in one aspect of the present invention, a light-emitting device having a high external quantum efficiency can be realized. 。

[0056] From the above relationship, the first organic compound is an electron-trapping compound, while the second 。 。 。 。 。

[0057] Note that from the above relationship, the first organic compound is an electron-trapping compound, while the second The organic compound is a hole-trapping compound. When the difference in the HOMO levels of these compounds and the difference in the LUMO levels are large (specifically, the difference is 0.3 eV or more), electrons preferentially enter the first organic compound, and holes preferentially enter the second organic compound. In this case, the process of forming an electrocomplex is considered to be prioritized over the process of forming an exciplex through singlet excitons. As described above.

[0058] By the way, the concept that the exciplex has an energy level only in the excited state holds. Therefore, the molecules constituting the exciplex are, in the ground state, two or more independent molecules, and these molecules do not form any complex in the ground state.

[0059] For example, if energy moves from an excited exciplex in the excited state to a guest molecule in the ground state and the guest molecule becomes excited, even if the guest molecule tries to excite other molecules, in one aspect of the present invention, the molecules to be excited are either the first organic compound or the second organic compound. And to excite the first organic compound or the second organic compound, since the energy of the excited guest molecule is insufficient, these cannot be excited. That is, the phenomenon that the excited state of the guest molecule undergoes reverse energy transfer to the exciplex and the guest molecule is deactivated before emitting light (i.e., the luminescence efficiency is impaired) is considered not to occur in principle.

[0060] This is also one of the reasons why the luminescence efficiency can be increased. As described above, in one aspect of the present invention, the energy transfer between molecules in the excited state and the ground state is as follows.

[0061] As described above, in one aspect of the present invention, the energy transfer between molecules in the excited state and the ground state It is preferable to make the energy differences substantially equal. That is, the excitation energy level of the exciplex is made substantially equal to the excitation energy level of the guest. On the other hand, the excitation energy levels of the first organic compound and the second organic compound constituting the exciplex are higher than the excitation energy level of the exciplex.

[0062] That is, even if the guest molecule in the excited state attempts to excite the first organic compound or the second organic compound, there is insufficient energy, so the probability of energy transfer from the guest molecule to other molecules becomes extremely low. For this reason, in one aspect of the present invention, the luminous efficiency is enhanced. Also, thermal deactivation can be suppressed, thus extending the lifespan of the light-emitting element, suppressing deterioration, and enhancing reliability.

[0063] Also, as described above, an exciplex may be formed from the singlet excitons of the first organic compound or the second organic compound. That is, a process in which an exciplex is formed by one of the first organic compound and the second organic compound forming a singlet exciton and then interacting with the other in the ground state is conceivable. And this process is considered to occur very rapidly.

[0064] Since singlet excitons have a short excitation lifetime (small τ), usually, before the excitation energy is transferred from the singlet exciton of the host to the guest, a part of the excitation energy is lost (emits light at a wavelength other than the desired wavelength or undergoes thermal deactivation) (Φ in Equation (3) tends to be small). ET

[0065] However, in one aspect of the present invention, since the singlet excitons of the host rapidly form an exciplex, such deactivation can be suppressed. Moreover, the excitation lifetime of the exciplex is relatively​​​​ Since it is long, it is considered that the energy transfer efficiency Φ ET will be increased. Therefore, it is considered that not only the efficiency of the host singlet excitons, which also affect the lifetime, but also the lifetime of the host singlet excitons can be suppressed, and a light-emitting device with a long

[0066] lifetime can be realized. In addition, it is preferable that the excitation energy of the exciplex is sufficiently transferred to the phosphorescent compound, and the light emission from the exciplex is not substantially observed from the device. Therefore, it

[0067] is preferable that the energy is transferred to the phosphorescent compound through the host exciplex, and the phosphorescent compound emits phosphorescence. In addition, from the concept of energy transfer described above, it is

[0068] effective when at least one of the first organic compound and the second organic compound is a fluorescent compound (that is, a compound in which luminescence or thermal deactivation easily occurs from the singlet excited state). Therefore, it is preferable that at least one of the first organic compound and the second organic compound is a fluorescent compound. In addition, when a phosphorescent compound is used for the organic

[0069] compound (the first organic compound and / or the second organic For example, in the method described in Non-Patent Document 1, an exciplex is formed. Therefore, the guest molecule can only be excited by a direct recombination process, and the emission efficiency is not sufficient. It's not too expensive.

[0070] In general, when exciplexes are used in the light-emitting layer of a light-emitting device, they have the advantage of being able to control the color of emitted light. However, the luminescence efficiency is usually significantly reduced. Optical elements have traditionally been considered unsuitable for achieving highly efficient light emitting elements.

[0071] However, as shown in one embodiment of the present invention, the energy transfer from the exciplex to a phosphorescent compound is The inventors have found that by using the material as a moving medium, the luminous efficiency can be increased to the limit. This is a technological concept that contradicts conventional stereotypes.

[0072] In one embodiment of the present invention, the excitation energy level of the exciplex and the energy of the excited state of the guest are The energy levels are set to be nearly equal, and the probability of transition to an excited state of the guest is increased. This increases the probability of the energy transfer process. % or more and 2.5% or less, preferably 0.1% or more and 1.5% or less, more preferably 0.1% or less Even at a concentration of 0.5% or less, sufficient energy transfer can be achieved. Since concentration quenching can be suppressed, the luminous efficiency can be maximized. [Brief description of the drawings]

[0073]

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Mode for Carrying Out the Invention

[0074] The embodiments will be described in detail with reference to the drawings. However, the present invention is not limited to the following description, and those skilled in the art can easily understand that the form and details can be variously changed without departing from the spirit and scope of the present invention. Therefore, the present invention should not be construed as being limited to the description of the embodiments shown below. In the configuration of the invention described below, the same reference numerals are commonly used for the same parts or parts having the same function in different drawings, and the repeated description thereof will be omitted. It will not be limited to the description of the embodiments shown below, and those skilled in the art can easily understand that the form and details can be variously changed without departing from the spirit and scope of the present invention. Therefore, the present invention should not be construed as being limited to the description of the embodiments shown below. In the configuration of the invention described below, the same reference numerals are commonly used for the same parts or parts having the same function in different drawings, and the repeated description thereof will be omitted. It will not be limited to the description of the embodiments shown below, and those skilled in the art can easily understand that the form and details can be variously changed without departing from the spirit and scope of the present invention. Therefore, the present invention should not be construed as being limited to the description of the embodiments shown below. In the configuration of the invention described below, the same reference numerals are commonly used for the same parts or parts having the same function in different drawings, and the repeated description thereof will be omitted. It will not be limited to the description of the embodiments shown below, and those skilled in the art can easily understand that the form and details can be variously changed without departing from the spirit and scope of the present invention. Therefore, the present invention should not be construed as being limited to the description of the embodiments shown below. In the configuration of the invention described below, the same reference numerals are commonly used for the same parts or parts having the same function in different drawings, and the repeated description thereof will be omitted. In the configuration of the invention described below, the same reference numerals are commonly used for the same parts or parts having the same function in different drawings, and the repeated description thereof will be omitted. The repeated description thereof will be omitted.

[0075] (Embodiment 1) In the present embodiment, a light-emitting element according to an aspect of the present invention will be described. The light-emitting element of the present embodiment has a guest that is a light-emitting substance and a host that disperses the guest in a light-emitting layer. Specifically, a phosphorescent compound is used as the guest, and a first organic compound and a second organic compound are used as the host. Further, the first organic compound and the second organic compound are a combination that forms an exciplex. In the present embodiment, a light-emitting element according to an aspect of the present invention will be described. The light-emitting element of the present embodiment has a guest that is a light-emitting substance and a host that disperses the guest in a light-emitting layer. Specifically, a phosphorescent compound is used as the guest, and a first organic compound and a second organic compound are used as the host. Further, the first organic compound and the second organic compound are a combination that forms an exciplex. Specifically, a phosphorescent compound is used as the guest, and a first organic compound and a second organic compound are used as the host. Further, the first organic compound and the second organic compound are a combination that forms an exciplex. Specifically, a phosphorescent compound is used as the guest, and a first organic compound and a second organic compound are used as the host. Further, the first organic compound and the second organic compound are a combination that forms an exciplex. Specifically, a phosphorescent compound is used as the guest, and a first organic compound and a second organic compound are used as the host. Further, the first organic compound and the second organic compound are a combination that forms an exciplex.

[0076] In addition, in the present embodiment, the triplet excitation energy levels of the organic compounds used as hosts are preferably higher than the T level of the guest. If the T level of the host is lower than the T level of the guest, the host quenches the triplet excitation energy of the guest that contributes to luminescence, leading to a decrease in luminescence efficiency. level (T * level) is preferably higher than the T * level of the guest. T * level of the host is lower than the T * level of the guest, the host quenches the triplet excitation energy of the guest that contributes to luminescence, resulting in a decrease in luminescence efficiency. - because the host quenches (quenches) the triplet excitation energy of the guest that contributes to luminescence, leading to a decrease in luminescence efficiency.

[0077] In addition, as described above, when the T * level of the guest is made substantially equal to the T * level of the exciplex, the T * level of the guest is lower than the T * level of the first organic compound (or the second organic compound). Therefore, in many cases, this problem can be overcome.

[0078] The first organic compound and the second organic compound that constitute the host form an exciplex. When the formed exciplex emits light, its emission wavelength is on the longer wavelength side compared to the respective emission wavelengths (fluorescence wavelengths) of the first organic compound and the second organic compound. In other words, by forming an exciplex, the fluorescence spectrum of the first organic compound and the fluorescence spectrum of the second organic compound can be converted into an emission spectrum located on the longer wavelength side. When the formed exciplex emits light, its emission wavelength is on the longer wavelength side compared to the respective emission wavelengths (fluorescence wavelengths) of the first organic compound and the second organic compound. In other words, by forming an exciplex, the fluorescence spectrum of the first organic compound and the fluorescence spectrum of the second organic compound can be converted into an emission spectrum located on the longer wavelength side. Therefore, even if the fluorescence spectrum of the first organic compound (or the second organic compound) is located on the shorter wavelength side compared to the absorption band located on the longest wavelength side of the phosphorescent compound and there is no overlap with the absorption band, by forming an exciplex, its fluorescence spectrum can be converted into a longer wavelength emission spectrum and the overlap with the absorption band can be increased. emission spectrum, and the overlap with the absorption band can be increased.

[0079] Therefore, even if the fluorescence spectrum of the first organic compound (or the second organic compound) is located on the shorter wavelength side compared to the absorption band located on the longest wavelength side of the phosphorescent compound and there is no overlap with the absorption band, by forming an exciplex, its fluorescence spectrum can be converted into a longer wavelength emission spectrum and the overlap with the absorption band can be increased. absorption band, and by forming an exciplex, its fluorescence spectrum can be converted into a longer wavelength emission spectrum and the overlap with the absorption band can be increased. emission spectrum and the overlap with the absorption band can be increased. emission spectrum and the overlap with the absorption band can be increased.

[0080] The light-emitting element of this embodiment utilizes the overlap between the emission spectrum of this exciplex and the absorption spectrum of the phosphorescent compound to transfer energy from the exciplex to the phosphorescent compound. Therefore, even when the concentration of the guest is 0.1% or more and 2.5% or less, preferably 0.1% or more and 1.5% or less, more preferably 0.1% or more and 0.5% or less, sufficient energy transfer efficiency can be obtained. .

[0081] In addition, in order to sufficiently overlap the emission spectrum of the exciplex and the absorption spectrum of the guest, the difference between the energy value of the peak of the emission spectrum and the energy value of the peak of the absorption band on the lowest energy side of the absorption spectrum is preferably within 0.3 eV. More preferably, it is within 0.2 eV, and most preferably within 0.1 eV.

[0082] The light-emitting element of this embodiment will be described with reference to FIG. 1.

[0083] FIG. 1(A) is a diagram showing a light-emitting element having an EL layer 102 between a first electrode 101 and a second electrode 103. The light-emitting element in FIG. 1(A) is sequentially stacked on the first electrode 101 with a hole injection layer 111, a hole transport layer 112, a light-emitting layer 113, an electron transport layer 114, an electron injection layer 115, and further composed of a second electrode 103 provided thereon. As the first electrode 101, it is preferable to use a metal, alloy

[0084] with a large work function (specifically, 4.0 eV or more), a conductive compound, and a mixture thereof. Specifically, for example, , indium tin oxide (ITO: Indium Tin Oxide), silicon, or , indium tin oxide containing silicon oxide, indium zinc oxide, tungsten oxide , etc. ​​​Examples include indium oxide containing tin and zinc oxide (IWZO).

[0085] These conductive metal oxide films are usually formed by sputtering, but sol-gel methods or the like can also be applied for fabrication. For example, an indium oxide-zinc oxide film can be formed by sputtering using a target with 1 to 20 wt% of zinc oxide added to indium oxide. Also, an IWZO film can be formed by sputtering using a target containing 0.5 to 5 wt% of tungsten oxide and 0.1 to 1 wt% of zinc oxide with respect to indium oxide.

[0086] In addition, graphene, gold, platinum, nickel, tungsten, chromium, molybdenum, iron, cobalt, copper, palladium, or nitrides of metal materials (e.g., titanium nitride) etc. can be mentioned.

[0087] However, when the layer formed in contact with the first electrode 101 in the EL layer 102 is formed using a composite material obtained by mixing an organic compound and an electron acceptor, substances used for the first electrode 101 can be various metals, alloys, electrically conductive compounds, and mixtures thereof regardless of the work function. For example, aluminum, silver, alloys containing aluminum (e.g., Al-Si) etc. can also be used.

[0088] The first electrode 101 can be formed, for example, by sputtering or vapor deposition (including vacuum vapor deposition) etc.

[0089] The second electrode 103 is a metal, alloy, or electrically ​​​​​It is preferable to form the conductive layer using a conductive compound or a mixture thereof. Specifically, An element in group 1 or 2 of the periodic table, i.e. an alkali such as lithium or cesium Alkaline earth metals such as calcium and strontium, magnesium, and alloys containing these (e.g., Mg-Ag, Al-Li), europium, ytterbium In addition to rare earth metals such as nickel and alloys containing these, aluminum and silver can also be used. Cut.

[0090] However, the layer of the EL layer 102 formed in contact with the second electrode 103 is an organic compound as described later. When a composite material made by mixing a compound and an electron donor is used, the work function is large. Indium oxide containing Al, Ag, ITO, silicon or silicon oxide, regardless of size A variety of conductive materials can be used, such as tin oxide.

[0091] In addition, when forming the second electrode 103, a vacuum deposition method or a sputtering method is used. In addition, when using silver paste, the coating method or the inkjet method can be used. It can be used.

[0092] The EL layer 102 includes at least a light-emitting layer 113. It is also possible to use either a low molecular weight compound or a high molecular weight compound. The material for forming the EL layer 102 is not limited to organic compounds. This also includes configurations that partially contain inorganic compounds.

[0093] The EL layer 102 includes a light-emitting layer 113 and a material having a high hole-injecting property as shown in FIG. the hole transport layer 112 containing a material with high hole transport properties; An electron transport layer 114 containing a highly transportable substance, an electron injection layer 115 containing a highly electron-injectable substance, etc. are laminated by appropriately combining them to form the same. The hole injection layer 111 is a layer containing a substance with high hole injection properties. As the substance with high hole injection properties,

[0094] metal oxides such as molybdenum oxide, titanium oxide, vanadium oxide, rhenium oxide, ruthenium oxide, chromium oxide, zirconium oxide, hafnium oxide, tantalum oxide, silver oxide, tungsten oxide, manganese oxide, etc. can be used. Further, phthalocyanine-based compounds such as phthalocyanine (abbreviation: H2Pc), copper(II) phthalocyanine (abbreviation: CuPc), etc. can be used.

[0095] 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 (abbreviation: DPAB), 4,4’-bis(N-{4-[N’-(3-methylphenyl)-N’-phenylamino]phenyl}-N-phenylamino)biphenyl (abbreviation: DNTPD), 1,3,5-tris[N-(4-diphenylaminophenyl)-N-phenylamino]benzene (abbreviation: DPA3B), 3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole (abbreviation: PCzPCA1), 3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole (abbreviation: PCzPCA2), etc., which are low-molecular organic compounds, can also be used.​​​​​​​​​​​​​Phenylcarbazole (abbreviation: PCzPCA2), 3-[N-(1-naphthyl)-N-( 9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole (abbreviation: P CzPCN1), and other aromatic amine compounds can be used.

[0096] Furthermore, a high molecular compound (oligomer, dendrimer, polymer, etc.) can also be used. . For example, poly(N-vinylcarbazole) (abbreviation: PVK), poly(4-vinyltri enylamine) (abbreviation: PVTPA), poly[N-(4-{N’-[4-(4-diphenyl amino)phenyl]phenyl-N’-phenylamino}phenyl)methacrylamide] (abbreviation: PTPDMA), poly[N,N’-bis(4-butylphenyl)-N,N’-bi s(phenyl)benzidine] (abbreviation: Poly-TPD), and other high molecular compounds can be mentioned. In addition, poly(3,4-ethylenedioxythiophene) / poly(styrenesulfonic acid) (PEDOT / PSS), polyaniline / poly(styrenesulfonic acid) (PAni / PS S), and other high molecular compounds added with an acid can be used.

[0097] Also, as the hole injection layer 111, a composite material formed by mixing an organic compound and an electron acceptor may be used. Since such a composite material generates holes in the organic compound by the electron acceptor, it has excellent hole injection properties and hole transport properties. In this case, the organic compound is preferably a material excellent in transporting the generated holes (a substance with high hole transportability). The organic compounds used in the composite material include aromatic amine compounds, carbazole derivatives, aromatic compounds, etc.

[0098] As the organic compound used in the composite material, aromatic amine compounds, carbazole derivatives, aromatic ​Various compounds such as aromatic hydrocarbons and polymer compounds (oligomers, dendrimers, polymers, etc.) As the organic compound used for the composite material, a compound having a high hole transporting property can be used. It is preferable that the organic compound is a compound having a low molecular weight. -6 cm 2 Hole transfer above / Vs However, it is preferable that the material has a higher hole transporting property than the electron transporting property. In the following, organic compounds that can be used in the composite material are described. The compounds are specifically listed below.

[0099] Examples of organic compounds that can be used in composite materials include TDATA and MTDATA. , DPAB, DNTPD, DPA3B, PCzPCA1, PCzPCA2, PCzPCN 1,4,4'-Bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB or α-NPD), N,N'-bis(3-methylphenyl)-N,N'-diphenyl Nyl-[1,1'-biphenyl]-4,4'-diamine (abbreviation: TPD), 4-phenyl -4'-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: BPAFL P) and other aromatic amine compounds, such as 4,4'-di(N-carbazolyl)biphenyl (abbreviation: CBP), 1,3,5-tris[4-(N-carbazolyl)phenyl]benzene (abbreviation: TCPB), 9-[4-(N-carbazolyl)]phenyl-10-phenylanthracene (abbreviation: CzPA), 9-phenyl-3-[4-(10-phenyl-9-anthryl)phenyl] phenyl]-9H-carbazole (abbreviation: PCzPA), 1,4-bis[4-(N-carbazolyl) Carbazole derivatives such as [2,3,5,6-tetraphenylbenzene]-2,3,5,6-zolylphenyl can be used.

[0100] Also, aromatic hydrocarbon compounds such as 2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation: t- BuDNA), 2-tert-butyl-9,10-di(1-naphthyl)anthracene, 9 ,10-bis(3,5-diphenylphenyl)anthracene (abbreviation: DPPA), 2-t ert-butyl-9,10-bis(4-phenylphenyl)anthracene (abbreviation: t-B uDBA), 9,10-di(2-naphthyl)anthracene (abbreviation: DNA), 9,10- diphenylanthracene (abbreviation: DPAnth), 2-tert-butylanthracene ( abbreviation: t-BuAnth), 9,10-bis(4-methyl-1-naphthyl)anthracene (abbreviation: DMNA), 9,10-bis[2-(1-naphthyl)phenyl]-2-tert -butylanthracene, 9,10-bis[2-(1-naphthyl)phenyl]anthracene , 2,3,6,7-tetramethyl-9,10-di(1-naphthyl)anthracene, etc. can be used. Furthermore, aromatic hydrocarbon compounds such as 2,3,6,7-tetramethyl-9,10-di(2-naphthyl)anthracene,

[0101] 9,9'-bianthryl, 10,10'-diphenyl-9,9'-bianthryl, 10, 10'-bis(2-phenylphenyl)-9,9'-bianthryl, 10,10'-bis [(2,3,4,5,6-pentaphenyl)phenyl]-9,9'-bianthryl, anth racene, tetracene, rubrene, perylene, 2,5,8,11-tetra(tert-b utyl)perylene, pentacene, coronene, 4,4'-bis(2,2-diphenylvinyl) biphenyl (abbreviation: DPVBi), 9,10-bis[4-(2,2-diphenylvinyl) biphenyl can be used. It is possible to use aromatic hydrocarbon compounds such as phenyl anthracene (abbreviation: DPVPA). It is possible.

[0102] In addition, examples of the electron acceptor include organic compounds such as 7,7,8,8 - tetracyano - 2,3,5,6 - tetrafluorobenzoquinodimethane (abbreviation: F4 - TCNQ) and chloranil, and transition metal oxides. Examples thereof also include oxides of metals belonging to Groups 4 to 8 in the periodic table of elements. Specifically, vanadium oxide, niobium oxide, tantalum oxide, chromium oxide, molybdenum oxide, tungsten oxide, manganese oxide, and rhenium oxide are preferable because they have high electron - accepting properties. Among them, molybdenum oxide is particularly preferable because it is stable in the atmosphere, has low hygroscopicity, and is easy to handle. Examples thereof also include oxides of metals belonging to Groups 4 to 8 in the periodic table of elements. Specifically, vanadium oxide, niobium oxide, tantalum oxide, chromium oxide, molybdenum oxide, tungsten oxide, manganese oxide, and rhenium oxide are preferable because they have high electron - accepting properties. Among them, molybdenum oxide is particularly preferable because it is stable in the atmosphere, has low hygroscopicity, and is easy to handle. Examples thereof also include oxides of metals belonging to Groups 4 to 8 in the periodic table of elements. Specifically, vanadium oxide, niobium oxide, tantalum oxide, chromium oxide, molybdenum oxide, tungsten oxide, manganese oxide, and rhenium oxide are preferable because they have high electron - accepting properties. Among them, molybdenum oxide is particularly preferable because it is stable in the atmosphere, has low hygroscopicity, and is easy to handle. Examples thereof also include oxides of metals belonging to Groups 4 to 8 in the periodic table of elements. Specifically, vanadium oxide, niobium oxide, tantalum oxide, chromium oxide, molybdenum oxide, tungsten oxide, manganese oxide, and rhenium oxide are preferable because they have high electron - accepting properties. Among them, molybdenum oxide is particularly preferable because it is stable in the atmosphere, has low hygroscopicity, and is easy to handle. Examples thereof also include oxides of metals belonging to Groups 4 to 8 in the periodic table of elements. Specifically, vanadium oxide, niobium oxide, tantalum oxide, chromium oxide, molybdenum oxide, tungsten oxide, manganese oxide, and rhenium oxide are preferable because they have high electron - accepting properties. Among them, molybdenum oxide is particularly preferable because it is stable in the atmosphere, has low hygroscopicity, and is easy to handle. Examples thereof also include oxides of metals belonging to Groups 4 to 8 in the periodic table of elements. Specifically, vanadium oxide, niobium oxide, tantalum oxide, chromium oxide, molybdenum oxide, tungsten oxide, manganese oxide, and rhenium oxide are preferable because they have high electron - accepting properties. Among them, molybdenum oxide is particularly preferable because it is stable in the atmosphere, has low hygroscopicity, and is easy to handle.

[0103] In addition, a composite material may be formed using the above - mentioned polymer compounds such as PVK, PVTPA, PTPDMA, and Poly - TPD and the above - mentioned electron acceptor, and used for the hole - injection layer 111. In addition, a composite material may be formed using the above - mentioned polymer compounds such as PVK, PVTPA, PTPDMA, and Poly - TPD and the above - mentioned electron acceptor, and used for the hole - injection layer 111.

[0104] The hole - transport layer 112 is a layer containing a substance with high hole - transporting properties. As the substance with high hole - transporting properties, aromatic amine compounds such as NPB, TPD, BPAFLP, 4,4’ - bis[N - (9,9 - dimethylfluoren - 2 - yl) - N - phenylamino]biphenyl (abbreviation: DFLDPBi), and 4,4’ - bis[N - (spiro - 9,9’ - bifluoren - 2 - yl) - N - phenylamino]biphenyl (abbreviation: BSPB) can be used. The substances described here mainly have a hole mobility of 10 cm / Vs or more. However, as long as the substance has higher hole - transporting properties than electrons, other substances may be used. Note that Examples thereof also include oxides of metals belonging to Groups 4 to 8 in the periodic table of elements. Specifically, vanadium oxide, niobium oxide, tantalum oxide, chromium oxide, molybdenum oxide, tungsten oxide, manganese oxide, and rhenium oxide are preferable because they have high electron - accepting properties. Among them, molybdenum oxide is particularly preferable because it is stable in the atmosphere, has low hygroscopicity, and is easy to handle. Examples thereof also include oxides of metals belonging to Groups 4 to 8 in the periodic table of elements. Specifically, vanadium oxide, niobium oxide, tantalum oxide, chromium oxide, molybdenum oxide, tungsten oxide, manganese oxide, and rhenium oxide are preferable because they have high electron - accepting properties. Among them, molybdenum oxide is particularly preferable because it is stable in the atmosphere, has low hygroscopicity, and is easy to handle. Examples thereof also include oxides of metals belonging to Groups 4 to 8 in the periodic table of elements. Specifically, vanadium oxide, niobium oxide, tantalum oxide, chromium oxide, molybdenum oxide, tungsten oxide, manganese oxide, and rhenium oxide are preferable because they have high electron - accepting properties. Among them, molybdenum oxide is particularly preferable because it is stable in the atmosphere, has low hygroscopicity, and is easy to handle. -6 cm 2 / Vs or more. However, as long as the substance has higher hole - transporting properties than electrons, other substances may be used. Note that Examples thereof also include oxides of metals belonging to Groups 4 to 8 in the periodic table of elements. Specifically, vanadium oxide, niobium oxide, tantalum oxide, chromium oxide, molybdenum oxide, tungsten oxide, manganese oxide, and rhenium oxide are preferable because they have high electron - accepting properties. Among them, molybdenum oxide is particularly preferable because it is stable in the atmosphere, has low hygroscopicity, and is easy to handle. The layer containing a substance with high hole transport properties may be not only a single layer but also a layer formed by laminating two or more layers of the above substance. It may also be laminated one on top of the other.

[0105] In addition, for the hole transport layer 112, carbazole derivatives such as CBP, CzPA, and PCzPA or anthracene derivatives such as t-BuDNA, DNA, and DPAnth may be used. It is okay.

[0106] In addition, for the hole transport layer 112, any polymer compound such as PVK, PVTPA, PTPDMA, and Poly-TPD can be used.

[0107] The light-emitting layer 113 is a layer containing a light-emitting substance. The light-emitting layer 113 of this embodiment has a phosphorescent compound as a guest and a first organic compound and a second organic compound as hosts. Two or more hosts can be used. With respect to the total weight of the hosts (when the hosts are the first organic compound and the second organic compound, the sum of their weights), the weight ratio of the guest is 0.1 % or more and 2.5% or less, preferably 0.1% or more and 1.5% or less, more preferably 0.1% or more and 0.5% or less.

[0108] As the phosphorescent compound, an organometallic complex is preferable, and an iridium complex is particularly preferable. Considering the energy transfer by the above Förster mechanism, the molar extinction coefficient of the absorption band located on the longest wavelength side of the phosphorescent compound is preferably 2000 M ·cm or more, and particularly preferably 5 -1 ·cm -1 or more. 000 M -1 ·cm -1 or more.

[0109] Examples of compounds having such a large molar extinction coefficient include, for example, bis(3,5-dimethyl ​(μ-2-Phenylpyrazinato)(dipivaloylmethanato)iridium(III) (abbreviation: [Ir(mppr-Me)2(dpm)]), (acetylacetonato)bis(4,6-di phenylpyrimidinato)iridium(III) (abbreviation: [Ir(dppm)2(acac )]), (acetylacetonato)bis[4-(2-norbornyl)-6-phenylpyrim idinato]iridium(III) (abbreviation: [Ir(nbppm)2(acac)]), (2 ,6-Dimethyl-3,5-heptanedionato-κ 2 O,O’)bis[4-methyl-2-( 3-methyl-4-pyrimidinyl-κN3)phenyl-κC]iridium(III) (abbreviation : [Ir(5mdppm)2(dibm)]), (acetylacetonato)bis(2-phen yldibenzo[f,h]quinoxalinato)iridium(III) (abbreviation: [Ir(dbq -P)2(acac)]) and the like can be mentioned. In particular, like [Ir(dppm)2(acac) , when a material with a molar extinction coefficient reaching 5000 M -1 ·cm -1 or more is used, a light-emitting element with an external quantum efficiency reaching about 30% can be obtained.

[0110] Examples of the first organic compound and the second organic compound include 2-[3-(dibenzothiophen hen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation: 2mDBTPDB q-II), 2-[4-(3,6-diphenyl-9H-carbazol-9-yl)phenyl yl]dibenzo[f,h]quinoxaline (abbreviation: 2CzPDBq-III), 7-[3-( dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation: 7 mDBTPDBq-II), and 6-[3-(dibenzothiophen-4-yl)phenyl Any one of the compounds that easily receive electrons, such as l]dibenzo[f,h]quinoxaline (abbreviation: 6mDBTPDBq-II), and 4,4'-bis[N-(1-naphthyl)- N-phenylamino]biphenyl (abbreviation: NPB or α-NPD), 4-phenyl-4 '-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PC BA1BP), and any one of the compounds that easily receive holes, such as 4,4'-di(1-naphthyl)-4''-(9-phenyl-9H- carbazol-3-yl)triphenylamine (abbreviation: PCBNBB), are preferably combined. However, it is not limited to these, and any combination that can form an exciplex may be used.

[0111] The main physical property values of 2mDBTPDBq-II, PCBNBB, and [Ir(dppm)2(acac)], [Ir(mppr-Me)2(dpm) which are suitable as phosphorescent compounds when using these are as shown in Table 1.

[0112]

Table 1

[0113] In the light-emitting layer 113 in which 2mDBTPDBq-II and PCBNBB are mixed, the LUMO level is -2.78 eV, and the HOMO level is -5.46 eV. This is the same as the LUMO level and HOMO level of the exciplex of 2mDBTPDB q-II and PCBNBB. And the LUMO level and HOM O level of the phosphorescent compound [Ir(mppr-Me)2(dpm)] are also at the same level.

[0114] On the one hand, since both the LUMO level and the HOMO level of [Ir(dppm)2(acac)] are lower than this , it can be seen that [Ir(dppm)2(acac)] is likely to trap electrons. Therefore, when [Ir(dppm)2(acac)] is used as a phosphorescent compound, the probability of the direct recombination process is suggested to be higher than when [Ir(mppr-Me)2(dpm)] is used.

[0115] Also, the energy levels (T levels) of the triplet excited states of both [Ir(mppr-Me)2(dpm)] and [Ir(dppm)2(acac)] are more than 0.1 electron volts lower than those of 2mDBTPDBq-II and PCBN * BB. Therefore, after [Ir(mppr-Me)2(dpm)] or [Ir(dppm)2(acac)] becomes a triplet excited state, the probability that its state transfers energy to the triplet excited states of 2mDBTPDBq-II and PCBN * BB is small. Especially for [Ir(dppm)2(acac)], it is as low as 0.18 electron volts, suggesting that [Ir(dppm)2(acac)] has higher luminescence efficiency.

[0116] The electron transport layer 114 is a layer containing a substance with high electron transport properties. Examples of substances with high electron transport properties include metal complexes such as Alq3, tris(4-methyl-8-quinolinolato)aluminum (abbreviation: Alm q3), bis(10-hydroxybenzo[h]quinolinato)beryllium (abbreviation: BeBq 2), BAlq, Zn(BOX)2, and bis[2-(2-hydroxyphenyl)benzothia zolato]zinc (abbreviation: Zn(BTZ)2). Also, 2-(4 -biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole -bis[5-(p-tert-butylphenyl)-1,3 ,4-oxadiazol-2-yl]benzene (abbreviation: OXD-7), 3-(4-ter t-butylphenyl)-4-phenyl-5-(4-biphenylyl)-1,2,4-tri azole (abbreviation: TAZ), 3-(4-tert-butylphenyl)-4-(4-ethylf enyl)-5-(4-biphenylyl)-1,2,4-triazole (abbreviation: p-EtTA Z), bathophenanthroline (abbreviation: BPhen), bathocuproin (abbreviation: BCP) , 4,4'-bis(5-methylbenzoxazol-2-yl)stilbene (abbreviation: Bz Os) and other heteroaromatic compounds can also be used. In addition, poly(2,5-pyridine- diyl) (abbreviation: PPy), poly[(9,9-dihexylfluorene-2,7-diyl) -co-(pyridine-3,5-diyl)] (abbreviation: PF-Py), poly[(9,9-dio ctylfluorene-2,7-diyl)-co-(2,2'-bipyridine-6,6'-diyl yl)] (abbreviation: PF-BPy) and other polymer compounds can also be used. The substances described here are mainly substances having an electron mobility of 10 -6 cm 2 / Vs or more. Note that, as long as the substance has higher electron transportability than hole , substances other than the above can also be used as the electron transport layer .

[0117] In addition, the electron transport layer may be not only a single layer but also a structure in which two or more layers made of the above substances are laminated .

[0118] The electron injection layer 115 is a layer containing a substance with high electron injection properties. The electron injection layer 115 contains lithium , cesium, calcium, lithium fluoride, cesium fluoride, calcium fluoride, Alkali metals, alkaline earth metals, or compounds thereof such as lithium oxide can be used. Further, rare earth metal compounds such as erbium fluoride can be used. Also, the materials constituting the above-described electron transport layer 114 can be used. Alternatively, a composite material formed by mixing an organic compound and an electron donor (donor) may be used for the electron injection layer 115. Since such a composite material generates electrons in the organic compound by the electron donor, it is excellent in electron injection property and electron transport property. In this case, the organic compound is preferably a material excellent in transporting the generated electrons. Specifically, for example, the materials (such as metal complexes and heteroaromatic compounds) constituting the above-described electron transport layer 114 can be used.

[0119] As the electron donor, any material that exhibits electron donating property to the organic compound may be used. Specifically, alkali metals, alkaline earth metals, and rare earth metals are preferable, and examples include lithium, cesium, magnesium, calcium, erbium, ytterbium, etc. Also, alkali metal oxides and alkaline earth metal oxides are preferable, and examples include lithium oxide, calcium oxide, barium oxide, etc. Further, Lewis bases such as magnesium oxide can be used. Also, organic compounds such as tetrathiafulvalene (abbreviation: TTF) can be used. Moreover, the above-described hole injection layer 111, hole transport layer 112, light emitting layer 113, electron transport layer 114, and electron injection layer 115 can be formed by vapor deposition method (including vacuum vapor deposition method), inkjet method, coating method, respectively.

[0120]

[0121]

[0122] It can be formed by a method such as a cloth method.

[0123] As shown in FIG. 1B, the EL layer is formed by disposing a plurality of layers between the first electrode 101 and the second electrode 103. In this case, the first EL layer 104 and the second EL layer 105 are stacked. It is preferable to provide a charge generating layer 106 between the above. The charge generating layer 106 is the above-mentioned composite. The material may be formed.

[0124] The charge generating layer 106 may also have a laminated structure of a layer made of a composite material and a layer made of another material. In this case, the layer made of the other material may be a layer made of an electron donating material and a material having high electron transport properties. A layer containing a conductive material, a layer made of a transparent conductive film, or the like can be used.

[0125] A light-emitting element having such a configuration is less susceptible to problems such as energy transfer and quenching. By expanding the range of materials to be selected, it will be possible to develop light-emitting devices that have both high luminous efficiency and a long lifespan. It is also easy to obtain phosphorescence in one EL layer and fluorescence in the other. This structure can be used in combination with the EL layer structure described above.

[0126] In addition, by making the emission color of each EL layer different, the light-emitting device as a whole can have a desired color. For example, in a light-emitting element having two EL layers, the first By making the emission color of the first EL layer and the emission color of the second EL layer complementary to each other, It is also possible to obtain a light-emitting device that emits white light as a whole. The same applies to the case of a light emitting element having a

[0127] As shown in FIG. 1C, the EL layer 102 is disposed between the first electrode 101 and the second electrode 103. It may further include a hole injection layer 111, a hole transport layer 112, a light-emitting layer 113, an electron transport layer 114, an electron injection buffer layer 116, an electron relay layer 117, and a composite material layer 1 18 in contact with the second electrode 103.

[0128] By providing the composite material layer 118 in contact with the second electrode 103, particularly when forming the second electrode 103 using a sputtering method, the damage to the EL layer 102 can be reduced, which is preferable. The composite material layer 118 can use a composite material containing an acceptor substance in the above-mentioned organic compound with high hole transportability. Furthermore, by providing the electron injection buffer layer 116, the injection barrier between the composite material layer 118 and the electron transport layer 1 14 can be alleviated, so that the electrons generated in the composite material layer 118 can be easily injected into the electron transport layer 1

[0129] 14. For the electron injection buffer layer 116, it is possible to use substances with high electron injection properties such as alkali metals, alkaline earth metals, rare earth metals, and their compounds (including alkali metal compounds (oxides such as lithium oxide, halides, carbonates such as lithium carbonate and cesium carbonate), alkaline earth metal compounds (including oxides, halides, carbonates), or compounds of rare earth metals (including oxides, halides, carbonates)).

[0130] When the electron injection buffer layer 116 is formed by including a substance with high electron transportability and a donor substance, it is preferable to add the donor substance at a ratio of 0.001 or more and 0.1 or less by mass ratio with respect to the substance with high electron transportability.

[0131]

[0132] ​​​​​​​In addition, as the donor substance, alkali metals, alkaline earth metals, rare earth metals, and these compounds (including alkali metal compounds (such as oxides like lithium oxide, halides, carbonates like lithium carbonate and cesium carbonate), alkaline earth metal compounds (including oxides, halides, carbonates), or compounds of rare earth metals (including oxides, halides, carbonates ))), in addition to this, organic compounds such as tetrathianaphthacene (abbreviation: TTN), nickelocene, decamethylnickelocene, etc. can also be used.

[0133] In addition, as the substance with high electron transport property, it can be formed using the same material as the material of the electron transport layer 114 described above.

[0134] Furthermore, it is preferable to form an electron relay layer 117 between the electron injection buffer layer 116 and the composite material layer 118. The electron relay layer 117 is not necessarily provided, but by providing an electron relay layer 117 with high electron transport property, it becomes possible to quickly send electrons to the electron injection buffer layer 116.

[0135] The structure in which the electron relay layer 117 is sandwiched between the composite material layer 118 and the electron injection buffer layer 116 is a structure in which the acceptor substance contained in the composite material layer 118 and the donor substance contained in the electron injection buffer layer 116 are less likely to interact with each other and are less likely to inhibit each other's functions. Therefore,

[0136] the increase in driving voltage can be prevented. The electron relay layer 117 contains a substance with high electron transport property, and the LUMO It is formed so as to be between the LUMO level of the highly electron-transporting substance contained in layer 114.

[0137] Also, when the electron relay layer 117 contains a donor substance, the donor level of the donor substance is also set to be between the LUMO level of the acceptor substance in the composite material layer 118 and the LUMO level of the highly electron-transporting substance contained in the electron transport layer 114. Specifically, as the numerical values of the energy levels, the LUMO level of the highly electron-transporting substance contained in the electron relay layer 117 is preferably -5.0 eV or more, more preferably -5.0 eV or more and -3.0 eV or less. As the highly electron-transporting substance contained in the electron relay layer 117, it is preferable to use a phthalocyanine-based material or a metal complex having a metal-oxygen bond and an aromatic ligand. Specific examples of the phthalocyanine-based material contained in the electron relay layer 117 include CuPc, SnPc (Phthalocyanine tin(II) complex), ZnPc (Phthalocyanine zinc complex), CoPc (Cobalt(II) phthalocyanine, β-form), FePc (Phthalocyanine Iron), and PhO-VOPc (Vanadyl 2,9,16,23-tetraphenoxy-29H,31H-phthalocyanine), and it is preferable to use any of them. As the metal complex having a metal-oxygen bond and an aromatic ligand contained in the electron relay layer 117, it is preferable to use a metal complex having a metal-oxygen double bond.

[0138]

[0139]

[0140] ​​​​​​​​​​​Since the combination has an acceptor property (the property of easily accepting electrons), the movement (transfer) of electrons becomes easier. In addition, a metal complex having a metal-oxygen double bond is considered to be stable. Therefore, by using a metal complex having a metal-oxygen double bond, it becomes possible to drive the light-emitting element more stably at a low voltage.

[0141] As the metal complex having a metal-oxygen bond and an aromatic ligand, a phthalocyanine-based material is preferable. Specifically, VOPc (Vanadyl phthalocyanine), SnO Pc (Phthalocyanine tin(IV) oxide complex) and TiOPc (Phthalocyanine titanium oxide co mplex) are each preferable because the metal-oxygen double bond in the molecular structure easily acts on other molecules and has a high acceptor property.

[0142] In addition, as the above-mentioned phthalocyanine-based material, those having a phenoxy group are preferable. Specifically, phthalocyanine derivatives having a phenoxy group such as PhO-VOPc are preferable. Phthalocyanine derivatives having a phenoxy group are soluble in a solvent. Therefore, they have the advantage of being easy to handle in forming the light-emitting element. In addition, since they are soluble in a solvent, they have the advantage of facilitating the maintenance of the apparatus used for film formation.

[0143] The electron relay layer 117 may further contain a donor substance. As the donor substance, an alkali metal, an alkaline earth metal, a rare earth metal, and their compounds (alkali metal compounds (oxides such as lithium oxide, halides, carbonates such as lithium carbonate and cesium carbonate In addition to alkaline earth metal compounds (including oxides, halides, carbonates), or rare earth metal compounds (including oxides, halides, carbonates)), organic compounds such as tetrathianaphthacene (abbreviation: TTN), nickelocene, decamethylnickelocene, etc. can be used. By including these donor substances in the electron relay layer 117, the movement of electrons becomes easier, and the light-emitting device can be driven at a lower voltage. When a donor substance is included in the electron relay layer 117, as a substance with high electron transport properties, in addition to the materials described above, a substance having a LUMO level higher than the acceptor level of the acceptor substance contained in the composite material layer 118 can be used.

[0144] Specifically, as the energy level, it is preferable to use a substance having a LUMO level in the range of -5.0 eV or higher, preferably -5.0 eV or higher and -3 .0 eV or lower. Examples of such substances include perylene derivatives and nitrogen-containing condensed aromatic compounds. Note that since nitrogen-containing condensed aromatic compounds are stable, they are preferable materials as the materials used to form the electron relay layer 117.

[0145] Specific examples of perylene derivatives include 3,4,9,10-perylenetetracarboxylic dianhydride (abbreviation: PTCDA), 3,4,9,10-perylenetetracarboxylic bisbenzimidazole (abbreviation: PTCBI), N,N'-dioctyl-3,4,9,10-perylenetetracarboxylic diimide (abbreviation: PTCDI-C8H), N,N'-dihexyl-3,4,9,10-perylenetetracarboxylic diimide (abbreviation: Hex PTC), etc.

[0146] (abbreviation: PTCBI), N,N'-dioctyl-3,4,9,10-perylenetetracarboxylic diimide (abbreviation: PTCDI-C8H), N,N'-dihexyl-3,4,9,10-perylenetetracarboxylic diimide (abbreviation: PTCDI-C8H), N,N'-dihexyl-3,4,9,10-perylenetetracarboxylic diimide (abbreviation: Hex PTC), etc. can be mentioned.

[0147] In addition, specific examples of the nitrogen-containing condensed aromatic compound include pyrazino[2,3-f][1,10] phenanthroline-2,3-dicarbonitrile (abbreviation: PPDN), 2,3,6,7,1 0,11-hexacyano-1,4,5,8,9,12-hexaazatriphenylene (abbreviation : HAT(CN)6), 2,3-diphenylpyrido[2,3-b]pyrazine (abbreviation: 2P YPR), 2,3-bis(4-fluorophenyl)pyrido[2,3-b]pyrazine (abbreviation : F2PYPR), etc. can be mentioned.

[0148] In addition, 7,7,8,8-tetracyanoquinodimethane (abbreviation: TCNQ), 1,4, 5,8-naphthalenetetracarboxylic dianhydride (abbreviation: NTCDA), perfluoropent tacene, copper hexadecafluorophthalocyanine (abbreviation: F 16 CuPc), N,N'-bis (2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentadecafluoro octyl)-1,4,5,8-naphthalenetetracarboxylic diimide (abbreviation: NTCD I-C8F), 3',4'-dibutyl-5,5''-bis(dicyanomethylene)-5,5 ''-dihydro-2,2':5',2''-terthiophene)(abbreviation: DCMT), metha fullerene (for example, [6,6]-phenyl C 61 methyl butyrate), etc. can be used. That is, when the electron relay layer 117 contains a donor substance, the electron relay layer 117 may be formed by a method such as co-evaporation of a substance with high electron transport property and a donor substance.

[0149] When the electron relay layer 117 contains a donor substance, the electron relay layer 117 may be formed by a method such as co-evaporation of a substance with high electron transport property and a donor substance. That is, when the electron relay layer 117 contains a donor substance, the electron relay layer 117 may be formed by a method such as co-evaporation of a substance with high electron transport property and a donor substance.

[0150] The hole injection layer 111, the hole transport layer 112, the light emitting layer 113, and the electron transport layer 114 may be formed using the above-described materials respectively.

[0151] As described above, the EL layer 102 of the present embodiment can be manufactured.

[0152] In the above-described light emitting element, current flows due to the potential difference generated between the first electrode 101 and the second electrode 103, and light is emitted by recombination of holes and electrons in the EL layer 102. Then, this light is taken out to the outside through either one or both of the first electrode 101 and the second electrode 103. Therefore, either one or both of the first electrode 101 and the second electrode 103 become electrodes having translucency with respect to visible light.

[0153] Note that the configuration of the layer provided between the first electrode 101 and the second electrode 103 is not limited to the above. In order to prevent quenching caused by the proximity of the light emitting region and the metal, a light emitting region where holes and electrons recombine at a site away from the first electrode 101 and the second electrode 103 may be provided, and other configurations may be used as long as they have such a structure.

[0154] That is, the stacking structure of the layers is not particularly limited, and layers made of a material with high electron transport property, a material with high hole transport property, a material with high electron injection property, a material with high hole injection property, a bipolar material (a material with high transport properties of both electrons and holes), or a hole blocking material, etc. can be freely combined with the light emitting layer to form a structure.

[0155] Using the light emitting element shown in the present embodiment, a passive matrix type light emitting device or an active matrix type light emitting device in which the driving of the light emitting element is controlled by a transistor can be manufactured. It is possible. Further, the light-emitting device can be applied to an electronic device, a lighting device, or the like.

[0156] As described above, a light-emitting element according to one embodiment of the present invention can be manufactured. This embodiment can be appropriately combined with other embodiments.

Example

[0157] In this example, an example of a combination of a first organic compound, a second organic compound, and a phosphorescent compound that can be applied to a light-emitting element according to one embodiment of the present invention will be described with reference to FIG. 2. .

[0158] The phosphorescent compound used in this example is bis(3,5-dimethyl-2-phenylpyrazinato) (dipivaloylmethanato)iridium(III) (abbreviation: [Ir(mppr-Me)2( dpm)]). The first organic compound used in this example is 2-[3-(diben zothiophen-4-yl)phenyl]dibenz[f,h]quinoxaline (abbreviation: 2mDB TPDBq-II). The second organic compound used in this example is 4,4'-di (1-naphthyl)-4''-(9-phenyl-9H-carbazol-3-yl)triphe nylamine (abbreviation: PCBNBB). The chemical formulas of the above materials are shown below.

[0159]

Chemical formula

[0160] <Absorption spectrum> FIGS. 2(A) and 2(B) show the ultraviolet-visible absorption spectrum (absorption spectrum A) of a dichloromethane solution of [Ir(mppr-Me)2(dpm )], which is a phosphorescent compound. Absorption ​For the measurement of the spectrum, a UV-visible spectrophotometer (V550 type manufactured by JASCO Corporation) was used. A dichloromethane solution (0.093 mmol / L) was placed in a quartz cell and measured at room temperature. Performed.

[0161] <Emission Spectrum> Also, in FIGS. 2(A) and 2(B), the emission spectrum of the thin film of 2mDBTPDBq-II, which is the first organic compound (Emission Spectrum 1), the emission spectrum of the thin film of PCBNBB, which is the second organic compound (Emission Spectrum 2), and the emission spectrum of the thin film of the mixed material of 2mDBTPDBq-II and PCBNBB (Emission Spectrum 3) are shown. In FIG. 2(A), the horizontal axis represents the wavelength (nm), and the vertical axis represents the molar absorption coefficient ε (M ·cm ) and the emission intensity (arbitrary unit). In FIG. 2(B), the horizontal axis represents the energy (eV) -1 ·cm -1 and the vertical axis represents the molar absorption coefficient ε (M -1 ·cm -1 ) and the emission intensity (arbitrary unit). .

[0162] From the absorption spectrum A in FIG. 2(A), it can be seen that [Ir(mppr-Me)2(dpm)] has a broad absorption band near 50 0 nm. This absorption band is considered to be the absorption band that strongly contributes to the emission.

[0163] Emission Spectrum 3 has a peak on the longer wavelength (lower energy) side than Emission Spectra 1 and 2. And the peak of Emission Spectrum 3 is compared with the peaks of Emission Spectra 1 and 2, present at a position closer to the absorption band. From FIGS. 2(A) and 2(B), the emission spectrum that has the largest overlap with the absorption band that strongly contributes to the emission of the absorption spectrum A is Emission Spectrum 3. ​​Specifically, the peak of absorption spectrum A and the peak of emission spectrum 3 The peak difference was 0.10 eV.

[0164] The emission spectrum of the mixture of 2mDBTPDBq-II and PCBNBB is It was found that the spectrum had a peak on the longer wavelength (lower energy) side. Therefore, by mixing 2mDBTPDBq-II and PCBNBB, an exciplex was formed. It was suggested that this is the case.

[0165] The emission spectrum of the mixed material is similar to the absorption spectrum of [Ir(mppr-Me)2(dpm)]. It was found that there is a large overlap with the absorption band that is thought to strongly contribute to the luminescence at 1000 .mu.m. Therefore, the mixed material of 2mDBTPDBq-II and PCBNBB was used as the host, [Ir(mpp The light-emitting device using the guest Zn(r-Me)2(dpm) has the emission spectrum of the mixed material. Energy transfer is achieved by utilizing the overlap with the absorption spectrum of the phosphorescent compound. This suggests that the energy transfer efficiency is high. Therefore, a light-emitting device with high external quantum efficiency can be obtained. It was suggested that this could be done. EXAMPLES

[0166] In this example, a first organic compound that can be used for a light-emitting element of one embodiment of the present invention, An example of a combination of a second organic compound and a phosphorescent compound will be described with reference to FIG. 3. .

[0167] The phosphorescent compound used in this example is (acetylacetonato)bis(4,6-diphenylpyridine). iridium(III) (abbreviation: [Ir(dppm)2(acac)]) It is. Also, the first organic compound used in this example is 2mDBTPDBq-II. Also, the second organic compound used in this example is PCBNBB. The chemical formulas of the materials used in this example are shown below. Note that the chemical formulas of the materials shown in Example 1 are omitted.

[0168] [Chemical formula]

[0169] [Absorption spectrum] Figures 3(A) and 3(B) show the ultraviolet-visible absorption spectra (Absorption Spectrum B) of a dichloromethane solution of the phosphorescent compound [Ir(dppm)2(acac)] . For the measurement of the absorption spectrum, a UV-visible spectrophotometer (V550 type manufactured by JASCO Corporation) was used. A dichloromethane solution (0.093 mmol / L) was placed in a quartz cell and measured at room temperature. .

[0170] [Emission spectrum] Also, Figures 3(A) and 3(B) similarly show the emission spectra (Emission Spectrum 4) of a thin film of the first organic compound 2mDBTPDBq-II, the emission spectra (Emission Spectrum 5) of a thin film of the second organic compound PCBNBB, and the emission spectra (Emission Spectrum 6) of a thin film of a mixed material of 2mDBTPDBq-II and PCBNBB. In Figure 3(A), the horizontal axis represents the wavelength (nm), and the vertical axis represents the molar absorption coefficient ε (M ·cm ) and the emission intensity (arbitrary unit). In Figure 3(B), the horizontal axis represents the energy (eV) and the vertical axis represents the molar absorption coefficient ε (M -1 ·cm -1 ) and the emission intensity (arbitrary unit). -1 ·cm -1 ) and the emission intensity (arbitrary unit). .

[0171] From the absorption spectrum B in Fig. 3(A), it can be seen that [Ir(dppm)2(acac)] has a broad absorption band around 520 n m. This absorption band is considered to be the absorption band that strongly contributes to luminescence.

[0172] The emission spectrum 6 has a peak on the longer wavelength (lower energy) side than the emission spectra 4 and 5. And the peak of the emission spectrum 6 is located closer to the absorption band compared to the peaks of the emission spectra 4 and 5. From Figs. 3(A) and 3(B), it was found that the emission spectrum with the largest overlap with the absorption band that strongly contributes to the luminescence of the absorption spectrum B is the emission spectrum 6. Specifically, the difference between the peak of the absorption spectrum B and the peak of the emission spectrum 6 was 0.02 eV.

[0173] The emission spectra of the mixed materials of 2mDBTPDBq-II and PCBNBB were found to have peaks on the longer wavelength (lower energy) side than the emission spectra of the individual substances. From this, it was suggested that an exciplex is formed by mixing 2mDBTPDBq-II and PCBNBB.

[0174] The peak of the emission spectrum of the mixed material was found to have a large overlap with the absorption band that is considered to strongly contribute to luminescence in the absorption spectrum of [Ir(dppm)2(acac)]. Also, the molar absorption coefficient is 5000 M -1 ·cm -1 or more. Therefore, for a light-emitting device using the mixed material of 2mDBTP DBq-II and PCBNBB as the host and [Ir(dppm)2(acac)] as the guest, the emission spectrum of the mixed material and the absorption spectrum of the phosphorescent compound ​​​​​​​​To utilize the overlap with toluene for energy transfer, high energy transfer efficiency is suggested. Furthermore, it is expected that energy transfer can occur sufficiently even at low guest concentrations.

Example

[0175] In this example, in a light-emitting device using an organometallic complex, which is a phosphorescent compound, (acetylacetonato)bis[4-( 2-norbornyl)-6-phenylpyrimidinato)]iridium(III) (abbreviation: [I r(nbppm)2(acac)]) as a guest, the characteristics were examined by increasing and decreasing the concentration of the guest. The structure of [Ir(nbppm)2(acac)] is shown below

[0176]

Chemical formula

[0177] Also, the first organic compound used in this example is 2mDBTPDBq-II. Also, the second organic compound used in this example is 4-phenyl-4'-(9-phenyl-9H-ca rbazol-3-yl)triphenylamine (abbreviation: PCBA1BP). Note that the combination of 2 mDBTPDBq-II and PCBA1BP forms an exciplex, similar to the combination of 2mDBTPDB q-II and PCBNBB in Example 1, and the position of its emission spectrum is also generally the same as that in Example 1.

[0178] In this example, light-emitting devices 1, 2, and 3 with different guest concentrations were fabricated and their characteristics were compared. The structures of light-emitting devices 1, 2, and 3 are shown in Figure 1(A). The structural formulas of the organic compounds used in this example are shown below. However, those already shown Excluding those that are

[0179]

Chemical formula

[0180] First, as the first electrode 101 that functions as an anode, a glass substrate with indium tin oxide (ITSO) containing silicon formed with a film thickness of 110 nm was prepared. The ITSO surface was covered with a polyimide film around it so that the surface was exposed with an angle of 2 m m, and the electrode area was set to 2 mm × 2 m m. As a pretreatment for forming a light-emitting element on this substrate, the substrate surface was washed with water , baked at 200 °C for 1 hour, and then UV ozone treatment was performed for 370 seconds. After that, in a heating chamber in a vacuum deposition apparatus whose interior was evacuated to about 10

[0181] Pa, vacuum baking was performed at 170 -4 °C for 30 minutes, and then the substrate was allowed to cool for about 30 minutes. Next, the substrate was fixed to a holder provided in the deposition chamber in the vacuum deposition apparatus so that the surface on which ITSO was formed faced downward. After that, the substrate was fixed to a holder provided in the deposition chamber in the vacuum deposition apparatus so that the surface on which ITSO was formed faced downward. After that, the substrate was fixed to a holder provided in the deposition chamber in the vacuum deposition apparatus so that the surface on which ITSO was formed faced downward. fixed.

[0182] After evacuating the deposition chamber to 10 -4 Pa, 1,3,5-tri(dibenzothiophen-4-yl )-benzene (abbreviation: DBT3P-II) and molybdenum(VI) oxide were co-evaporated so that the ratio of DBT3P- II: molybdenum(VI) oxide was 4:2 (weight ratio), thereby forming a hole injection layer 111. The film thickness was 40 nm. Note that co-evaporation is a deposition method in which a plurality of different substances are simultaneously evaporated from different evaporation sources. respectively different evaporation sources at the same time.

[0183] Subsequently, 4-phenyl-4'-(9-phenylfluoren-9-yl)triphenylamine The hole transport layer 112 was formed by vapor-depositing 20 nm of BPAFLP (abbreviation: BPAFLP).

[0184] Furthermore, on the hole transport layer 112, 2mDBTPDBq-II, PCBA1BP, and Ir(nbppm)2(acac)] were co-vapor-deposited at 40 nm so that 2mDBTPDBq-II:PCBA1BP: Ir(nbppm)2(acac)] = 0.8:0.2:x (weight ratio) to form the light-emitting layer 113. Here, in the light-emitting device 1, x = 0. 01, in the light-emitting device 2, x = 0.025, and in the light-emitting device 3, x = 0.05.

[0185] Next, the electron transport layer 114 was formed by vapor-depositing 10 nm of 2mDBTPDBq-II and then 20 nm of bathophenanthroline (abbreviation: BP hen). That is, in this embodiment, the electron transport layer 114 consists of a first electron transport layer of 2mDBTPDBq-II with a thickness of 10 nm and a second electron transport layer of BPhen with a thickness of 20 nm.

[0186] Furthermore, the electron injection layer 115 was formed by vapor-depositing lithium fluoride to a thickness of 1 nm on the electron transport layer 114. Thus, the EL layer 102 was formed. In the above vapor-deposition process

[0187] Finally, aluminum was deposited to a thickness of 200 nm as the second electrode 103 that functions as the cathode,

[0188]

Table 2

[0189] The light-emitting elements (light-emitting element 1 to light-emitting element 3) obtained as described above were sealed in a glove box under a nitrogen atmosphere so that the light-emitting elements were not exposed to the air (heat treatment at 80 °C for 1 hour during sealing). After that, the operating characteristics of these light-emitting elements were measured. Note that the measurement was performed at room temperature (an atmosphere maintained at 25 °C).

[0190] The current density-luminance characteristics of light-emitting element 1 to light-emitting element 3 are shown in FIG. 4. In FIG. 4, the vertical axis represents luminance (cd / m 2 ) and the horizontal axis represents current density (mA / cm )). Also, the voltage-luminance characteristics of light-emitting element 1 to light-emitting element 3 are shown in FIG. 5. In FIG. 5, the vertical axis represents luminance (cd / m 2 ) and the horizontal axis represents voltage (V). The voltage-current characteristics of light-emitting element 1 to light-emitting element 3 are shown in FIG. 6. In FIG. 6, the vertical axis represents current (mA) and the horizontal axis represents voltage (V). The chromaticity-luminance characteristics of light-emitting element 1 to light-emitting element 3 are shown in FIG. 7. In FIG. 7, the vertical axis represents chromaticity and the horizontal axis represents luminance (cd / m 2 )). 2 )).

[0191] The current efficiency-luminance characteristics of light-emitting element 1 to light-emitting element 3 are shown in FIG. 8. In FIG. 8, the vertical axis represents current efficiency (cd / A) and the horizontal axis represents luminance (cd / m )). The power efficiency-luminance characteristics of light-emitting element 1 to light-emitting element 3 are shown in FIG. 9. In FIG. 9, the vertical axis represents power efficiency (lm / W) and the horizontal axis represents luminance (cd 2 )). The external quantum efficiency-luminance characteristics of light-emitting element 1 to light-emitting element 3 are shown in FIG. 10. In FIG. 10, the vertical axis represents external quantum efficiency (%) and the horizontal axis represents luminance (cd / m )). The emission spectra of light-emitting element 1 to light-emitting element 3 are shown in FIG. 11. In FIG. 11, the vertical axis represents emission intensity (arbitrary unit). 2 )). 2 )). The emission spectra of light-emitting element 1 to light-emitting element 3 are shown in FIG. 11. In FIG. 11, the vertical axis represents emission intensity (arbitrary unit). , the horizontal axis indicates the wavelength (nm).

[0192] In FIGS. 4 to 11, LE1 represents light-emitting element 1, LE2 represents light-emitting element 2, and LE3 represents light-emitting element 3.

[0193] From FIGS. 4 to 11, it can be seen that light-emitting element 1 with a low guest concentration has a higher luminous efficiency than light-emitting elements 2 and 3. Table 3 shows the main characteristics of light-emitting elements 1 to 3. In Table 3, LE1 represents light-emitting element 1, LE2 represents light-emitting element 2, and LE3 represents light-emitting element 3. As can be seen from Table 3, even in light-emitting element 1 with a weight ratio of guest to the total weight of the host of 1%, characteristics superior to those of light-emitting elements 2 and 3 with a higher weight ratio of guest can be obtained.

[0194]

Table 3

Example

[0195] In this example, in a light-emitting element using an organometallic complex, which is a phosphorescent compound, (2,6-dimethyl-3,5-heptanedionato-κ O,O’)bis[4-methyl-2-(3-methyl-4-pyrimidinyl- 2 κN3)phenyl-κC]iridium(III) (abbreviation: [Ir(5mdppm)2(d ibm)]) as a guest, the characteristics were examined by increasing or decreasing the concentration of the guest. The structure of [Ir(5mdppm)2(dibm)] is shown below.

[0196]

Chemical formula

[0197] ​​​​​In addition, the first organic compound used in this example is 2mDBTPDBq-II. Also, the second organic compound used in this example is 4,4'-bis[N-(1-naphthyl)-N- phenylamino]biphenyl (abbreviation: NPB). The structural formula of NPB is shown below. Note that the combination of 2mDBTPDBq-II and NPB forms an exciplex, similar to Example 1, and the position of its emission spectrum is also generally the same as that of Example 1.

[0198]

Chemical formula

[0199] In this example, light-emitting devices 4, 5, and 6 with different guest concentrations were fabricated and their characteristics were compared. The structures of light-emitting devices 4, 5, and 6 are shown in Figure 1(A).

[0200] First, a glass substrate with indium tin oxide (ITSO) containing silicon formed thereon with a film thickness of 110 nm was prepared as the first electrode 101 that functions as an anode. The surface of the ITSO was covered with a polyimide film around it so that the surface was exposed with an angle of 2m m, and the electrode area was 2 mm × 2m m. As a pretreatment for forming a light-emitting device on this substrate, the substrate surface was washed with water and baked at 200 °C for 1 hour, and then UV ozone treatment was performed for 370 seconds. After that, in a heating chamber in a vacuum evaporation apparatus whose internal pressure was reduced to about 10

[0201] Pa, vacuum baking was performed at 170 -4 °C for 30 minutes, and then the substrate was allowed to cool for about 30 minutes. Next, the substrate was fixed to a holder provided in the evaporation chamber in the vacuum evaporation apparatus so that the surface on which the ITSO was formed faced downward and cooled for about 30 minutes. Then, the substrate was fixed to a holder provided in the evaporation chamber of the vacuum evaporation apparatus so that the surface on which the ITSO was formed faced downward. fixed.

[0202] The evaporation chamber was evacuated to 10 -4 Pa, and then DBT3P-II and molybdenum(VI) oxide were co-evaporated such that DBT3P-II:molybdenum(VI) oxide = 4:2 (weight ratio), thereby forming the hole injection layer 111. The film thickness was 40 nm.

[0203] Subsequently, BPAFLP was evaporated at 20 nm to form the hole transport layer 112.

[0204] Furthermore, on the hole transport layer 112, 2mDBTPDBq-II, NPB, and [Ir(5md ppm)2(dibm)] were co-evaporated at 40 nm such that 2mDBTPDBq-II:NPB:[Ir(5mdpp m)2(dibm)] = 0.8:0.2:x (weight ratio), thereby forming the light-emitting layer 113. Here, for the light-emitting device 4, x = 0.01; for the light-emitting device 5, x = 0.025; and for the light-emitting device 6, x = 0.05.

[0205] Next, 2mDBTPDBq-II was evaporated at 10 nm, and then BPhen was evaporated at 20 nm to form the electron transport layer 114. Furthermore, on the electron transport layer 114, lithium fluoride was evaporated to a thickness of 1 nm to form the electron injection layer 115. Thus, the EL layer 10 2 was formed. In the above-described evaporation process, all evaporation was performed using the resistance heating method.

[0206] Finally, aluminum was deposited at 200 nm as the second electrode 103 functioning as the cathode to complete the light-emitting device. The stacked structure of the light-emitting device is shown in Table 4. In the table, x is 0.01 for the light-emitting device 4, 0.025 for the light-emitting device 5, and 0.05 for the light-emitting device 6.

[0207]

Table 4

[0208] The light-emitting elements (light-emitting elements 4 to 6) obtained in the above manner were placed in a globe in a nitrogen atmosphere. The process of sealing the light-emitting element in the box so that it is not exposed to the atmosphere (the temperature is kept at 80°C during sealing). After performing a heat treatment for 1 hour at 400 K for 1 hour, the operating characteristics of these light-emitting devices were measured. The measurements were carried out at room temperature (atmosphere maintained at 25°C).

[0209] FIG. 12 shows current density-luminance characteristics of the light-emitting elements 4 to 6. In FIG. 12, the vertical axis indicates luminance (cd / m 2 ), the horizontal axis is the current density (mA / cm 2 In addition, light-emitting element 4 to light-emitting element The voltage-luminance characteristics of the transistor 6 are shown in FIG. 13. In FIG. 13, the vertical axis indicates the luminance (cd / m 2 ), the horizontal axis is the current FIG. 14 shows the voltage-current characteristics of the light-emitting elements 4 to 6. In the graph, the vertical axis indicates current (mA) and the horizontal axis indicates voltage (V). The chromaticity characteristics are shown in Fig. 15. In Fig. 15, the vertical axis is chromaticity and the horizontal axis is luminance (cd / m 2 ) is shown.

[0210] FIG. 16 shows current efficiency vs. luminance characteristics of the light-emitting elements 4 to 6. In FIG. 16, the vertical axis indicates current Efficiency (cd / A), horizontal axis is brightness (cd / m 2 The power of the light-emitting elements 4 to 6 is shown. The power efficiency-luminance characteristics are shown in Fig. 17. In Fig. 17, the vertical axis represents power efficiency (lm / W) and the horizontal axis represents luminance. Degree (cd / m 2 18 shows the external quantum efficiency vs. luminance characteristics of the light-emitting elements 4 to 6. In Fig. 18, the vertical axis shows the external quantum efficiency (%) and the horizontal axis shows the luminance (cd / m 2 ) is shown. The emission spectra of the elements 4 to 6 and the light-emitting element 6 are shown in FIG. 19. In FIG. 19, the vertical axis represents the emission intensity (in arbitrary units), and the horizontal axis represents the wavelength (nm). Note that in FIGS. 12 to 19, LE4 represents the light-emitting element 4, LE5 represents the light-emitting element 5, and LE

[0211] 6 represents the light-emitting element 6. From FIGS. 12 to 19, it can be seen that the light-emitting element 4 with a low guest concentration has a higher luminous efficiency than the light-emitting elements 5 and 6. Table 5 shows the main characteristics of the light-emitting elements 4 to 6. Note

[0212] that in Table 5, LE4 represents the light-emitting element 4, LE5 represents the light-emitting element 5, and LE6 represents the light-emitting element 6 . As can be seen from Table 5, even the light-emitting element 4 with a guest weight ratio of 1% with respect to the total weight of the host has better characteristics than the light-emitting elements 5 and 6 with a higher guest weight ratio .

[0213]

Table 5

Example

[0214] In this example, in a light-emitting element using an organometallic complex, (acetylacetonato)bis(2-phenyl dibenzo[f,h]quinoxalinato)iridium(III) (abbreviation: [Ir(db q-P)2(acac)]) as a guest, the characteristics were examined by increasing and decreasing the concentration of the guest. The structure of [Ir(dbq-P)2(acac)] is shown below.

[0215]

Chemical formula

[0216] In addition, the first organic compound used in this example is 2mDBTPDBq-II. Also, the second organic compound used in this example is NPB. Note that the combination of 2mDBTPDBq-II and NPB forms an exciplex, similar to Example 1 (the combination of 2mDBTPDBq-II and PCBNBB), and the position of its emission spectrum is also generally equivalent to that of Example 1.

[0217] In this example, light-emitting devices 7, 8, and 9 with different guest concentrations were fabricated and their characteristics were compared. The structures of light-emitting devices 7, 8, and 9 are shown in Fig. 1(A).

[0218] First, a glass substrate with indium tin oxide (ITSO) containing silicon formed thereon with a film thickness of 110 nm as the first electrode 101 functioning as an anode was prepared. The surface of the ITSO was covered with a polyimide film around it with a surface exposure angle of 2 m m, and the electrode area was 2 mm × 2 m. m. As a pretreatment for forming a light-emitting device on this substrate, the substrate surface was washed with water and baked at 200 °C for 1 hour, and then UV ozone treatment was performed for 370 seconds.

[0219] Thereafter, in the heating chamber of a vacuum deposition apparatus whose internal pressure was reduced to about 10 -4 Pa, vacuum baking was performed at 170 °C for 30 minutes, and then the substrate was allowed to cool for about 30 minutes. Next, the substrate was fixed to a holder provided in the deposition chamber of the vacuum deposition apparatus so that the surface on which the ITSO was formed faced downward.

[0220] After reducing the pressure in the deposition chamber to 10 -4 Pa, DBT3P-II and molybdenum(VI) oxide were ​​​​​​DBT3P-II:molybdenum oxide = 4:2 (weight ratio) Thus, a hole injection layer 111 was formed. The film thickness was set to 40 nm.

[0221] Subsequently, BPAFLP was evaporated to a thickness of 20 nm to form a hole transport layer 112 .

[0222] Further, on the hole transport layer 112, 2mDBTPDBq-II, NPB, and [Ir(dbq -P)2(acac)], 2mDBTPDBq-II:NPB:[Ir(dbq-P) 2(acac)] = 0.8:0.2:x (weight ratio) The light-emitting layer 113 was formed by the above procedure. Here, x=0.01 in the light-emitting element 7 and x=0.01 in the light-emitting element 8. For the light-emitting element 1, x was set to 0.025, and for the light-emitting element 9, x was set to 0.05.

[0223] Next, 10 nm of 2mDBTPDBq-II was evaporated, followed by 20 nm of BPhen. The electron transport layer 114 was formed by the above-mentioned method. Further, lithium fluoride was deposited on the electron transport layer 114 by 1000 nm. The electron injection layer 115 was formed by vapor deposition so that the thickness of the EL layer 10 was 1 nm. 2 was formed. In the above-mentioned deposition processes, the deposition was all performed using a resistance heating method.

[0224] Finally, an aluminum film is formed to a thickness of 200 nm as the second electrode 103 that functions as a cathode. The light-emitting element was completed. The layer structure of the light-emitting element is shown in Table 6. In the table, x represents the number of layers of the light-emitting element. For light emitting element 7, it is 0.01, for light emitting element 8, it is 0.025, and for light emitting element 9, it is 0.05.

[0225] [Table 6]

[0226] The light-emitting elements (light-emitting elements 7 to 9) obtained in the above manner were placed in a globe in a nitrogen atmosphere. The process of sealing the light-emitting element in the box so that it is not exposed to the atmosphere (the temperature is kept at 80°C during sealing). After performing a heat treatment for 1 hour at 400 K for 1 hour, the operating characteristics of these light-emitting devices were measured. The measurements were carried out at room temperature (atmosphere maintained at 25°C).

[0227] FIG. 20 shows current density-luminance characteristics of the light-emitting elements 7 to 9. In FIG. 20, the vertical axis indicates luminance (cd / m 2 ), the horizontal axis is the current density (mA / cm 2 In addition, light-emitting elements 7 to The voltage-luminance characteristics of the transistor 9 are shown in FIG. 21. In FIG. 21, the vertical axis indicates the luminance (cd / m 2 ), the horizontal axis is the current FIG. 22 shows the voltage-current characteristics of the light-emitting elements 7 to 9. In the graph, the vertical axis indicates current (mA) and the horizontal axis indicates voltage (V). The chromaticity characteristics are shown in Fig. 23. In Fig. 23, the vertical axis is chromaticity and the horizontal axis is luminance (cd / m 2 ) is shown.

[0228] FIG. 24 shows current efficiency vs. luminance characteristics of the light-emitting elements 7 to 9. In FIG. 24, the vertical axis indicates current Efficiency (cd / A), horizontal axis is brightness (cd / m 2 The power of the light-emitting elements 7 to 9 is shown. The power efficiency-luminance characteristics are shown in Fig. 25. In Fig. 25, the vertical axis represents power efficiency (lm / W) and the horizontal axis represents luminance. Degree (cd / m 2 26 shows the external quantum efficiency vs. luminance characteristics of the light-emitting elements 7 to 9. In Fig. 26, the vertical axis shows external quantum efficiency (%) and the horizontal axis shows luminance (cd / m 2 ) is shown. FIG. 27 shows the external energy efficiency vs. luminance characteristics of the light-emitting elements 7 to 9. In FIG. is the external energy efficiency (%), and the horizontal axis is the brightness (cd / m 2) is shown. The emission spectra of light-emitting elements 7 to 9 are shown in FIG. 28. In FIG. 28, the vertical axis represents the emission intensity (in arbitrary units), and the horizontal axis represents the wavelength (nm).

[0229] In FIGS. 20 to 28, LE7 represents light-emitting element 7, LE8 represents light-emitting element 8, and LE 9 represents light-emitting element 9.

[0230] From FIGS. 20 to 28, it can be seen that the light-emitting element 7 with a low guest concentration has a higher emission efficiency than the light-emitting elements 8 and 9. Table 7 shows the main characteristics of the light-emitting elements 7 to 9. Note that in Table 7, LE7 represents light-emitting element 7, LE8 represents light-emitting element 8, and LE9 represents light-emitting element 9 . As can be seen from Table 7, even the light-emitting element 7 with a weight ratio of the guest to the total weight of the host of 1% has characteristics superior to those of the light-emitting elements 8 and 9 with a higher weight ratio of the guest .

[0231]

Table 7

Explanation of Signs

[0232] 101 First electrode 102 EL layer 103 Second electrode 104 First EL layer 105 Second EL layer 106 Charge generation layer 111 Hole injection layer 112 Hole transport layer 113 Emission layer 114 Electron transport layer 115 Electron injection layer 116 Electron injection buffer layer 117 Electron relay layer 118 Composite material layer​​

Claims

1. Between a pair of electrodes, having a hole injection layer and a light-emitting layer, The hole injection layer contains an acceptor, The light-emitting layer contains a phosphorescent compound, a first organic compound, and a second organic compound which is an aromatic amine compound, The first organic compound and the second organic compound are a combination that forms an exciplex, The peak of the emission spectrum of the exciplex is at a position close to the absorption band located on the longest wavelength side of the phosphorescent compound, compared to the peak of the emission spectrum of the first organic compound and the peak of the emission spectrum of the second organic compound, A light-emitting device in which the triplet excitation energy level of the first organic compound and the triplet excitation energy level of the second organic compound are higher than the triplet excitation energy level of the phosphorescent compound.

2. Between a pair of electrodes, having a hole injection layer and a light-emitting layer, The hole injection layer contains an acceptor and a third organic compound, The light-emitting layer contains a phosphorescent compound, a first organic compound, and a second organic compound which is an aromatic amine compound, The first organic compound and the second organic compound are a combination that forms an exciplex, The peak of the emission spectrum of the exciplex is at a position close to the absorption band located on the longest wavelength side of the phosphorescent compound, compared to the peak of the emission spectrum of the first organic compound and the peak of the emission spectrum of the second organic compound, A light-emitting device in which the triplet excitation energy level of the first organic compound and the triplet excitation energy level of the second organic compound are higher than the triplet excitation energy level of the phosphorescent compound.

3. Between a pair of electrodes, having a hole injection layer and a light-emitting layer, The hole injection layer contains an acceptor, The light-emitting layer contains a phosphorescent compound, a first organic compound, and a second organic compound which is an aromatic amine compound, The first organic compound and the second organic compound are a combination that forms an exciplex, The peak of the emission spectrum of the exciplex is at a position close to the peak of the absorption band located on the longest wavelength side of the phosphorescent compound, compared to the peak of the emission spectrum of the first organic compound and the peak of the emission spectrum of the second organic compound, A light-emitting device in which the triplet excitation energy level of the first organic compound and the triplet excitation energy level of the second organic compound are higher than the triplet excitation energy level of the phosphorescent compound.

4. Between a pair of electrodes, there are a hole injection layer and a light emitting layer, The hole injection layer contains an acceptor and a third organic compound, The light emitting layer contains a phosphorescent compound, a first organic compound, and a second organic compound which is an aromatic amine compound, The first organic compound and the second organic compound are a combination that forms an exciplex, The peak of the emission spectrum of the exciplex is closer to the peak of the absorption band located on the longest wavelength side of the phosphorescent compound than the peak of the emission spectrum of the first organic compound and the peak of the emission spectrum of the second organic compound, A light emitting device in which the triplet excitation energy level of the first organic compound and the triplet excitation energy level of the second organic compound are higher than the triplet excitation energy level of the phosphorescent compound.

5. In Claim 2 or Claim 4, The third organic compound is at least one of an aromatic amine compound, a carbazole derivative, and an aromatic hydrocarbon compound, a light emitting device.

6. In any one of Claims 1 to 5, A light emitting device having a hole transport layer containing an aromatic amine compound having a fluorenyl group between the hole injection layer and the light emitting layer.

7. In any one of Claims 1 to 6, A light emitting device in which the ratio of the weight of the phosphorescent compound to the sum of the weights of the first organic compound and the second organic compound is 0.1% or more and 2.5% or less.

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