Light-emitting element and display device including light-emitting element

Abstract

To provide a light-emitting element with long lifetime.SOLUTION: A light-emitting element includes a pair of electrodes and a light-emitting layer between the pair of electrodes. The light-emitting layer includes a first host material, a second host material, and a first light-emitting material exhibiting thermal activation delay fluorescence. In the light-emitting layer, the concentration of the first host material is larger than that of the second host material, the band gap of the first host material is larger than that of the second host material, the singlet excited energy level of the first light-emitting material is lower than that of the first host material, and the triplet excited energy level of the second host material is lower than that of the first host material.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] One embodiment of the present invention relates to a light-emitting element and a display device having the light-emitting element. [Background technology]

[0002] In recent years, displays equipped with organic electroluminescent devices (OLEDs) have been widely used. In addition, organic electroluminescent devices exhibiting thermally activated delayed fluorescence and hyperfluorescence (registered trademark) have attracted attention due to their extremely high luminous efficiency, and vigorous research and development has been conducted (see, for example, Patent Documents 1 to 3). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent Publication No. 2021-048366 [Patent Document 2] JP 2020-013695 A [Patent Document 3] JP 2017-222820 A Summary of the Invention [Problem to be solved by the invention]

[0004] An object of one embodiment of the present invention is to provide a light-emitting element having a long lifetime, and an object of one embodiment of the present invention is to provide a display device having a long lifetime. [Means for solving the problem]

[0005] One embodiment of the present invention is a light-emitting device, which has a pair of electrodes and a light-emitting layer between the pair of electrodes, the light-emitting layer includes a first host material, a second host material, and a first light-emitting material exhibiting thermally activated delayed fluorescence, in which the concentration of the first host material is greater than the concentration of the second host material, the band gap of the first host material is greater than the band gap of the second host material, the singlet excitation energy level of the first light-emitting material is lower than the singlet excitation energy level of the first host material, and the triplet excitation energy level of the second host material is lower than the triplet excitation energy level of the first host material.

[0006] One embodiment of the present invention is a display device. The display device includes a first pixel having a first light-emitting element and a second pixel having a second light-emitting element, the first light-emitting element has a pair of electrodes and a first light-emitting layer between the pair of electrodes, the first light-emitting layer includes a first host material, a second host material, and a first light-emitting material exhibiting thermally activated delayed fluorescence, the second light-emitting element has a pair of electrodes and a second light-emitting layer between the pair of electrodes, the second light-emitting layer includes a second host material and a second light-emitting material having a singlet excitation energy level higher than the singlet excitation energy level of the first light-emitting material, and in the first light-emitting layer, the concentration of the first host material is higher than the concentration of the second host material, the band gap of the first host material is higher than the band gap of the second host material, and the triplet excitation energy level of the second host material is lower than the triplet excitation energy level of the first host material. [Brief description of the drawings]

[0007]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Figure 13

Figure 14

[0008] Hereinafter, each embodiment of the present invention will be described with reference to the drawings, etc. However, the present invention can be embodied in various forms without departing from the spirit of the present invention, and the present invention should not be interpreted as being limited to the description of the embodiments exemplified below.

[0009] In order to clarify the description, the drawings may show the width, thickness, shape, etc. of each part in a schematic manner compared to the actual embodiment, but these are merely examples and do not limit the interpretation of the present invention. In this specification and each drawing, elements having the same functions as those explained in the previous drawings may be given the same reference numerals, and duplicate explanations may be omitted.

[0010] In this specification and the claims, when describing an aspect in which a structure is placed on top of another structure, the term "on top" is used, unless otherwise specified, to include both a case in which another structure is placed directly on top of a structure so as to be in contact with the structure, and a case in which another structure is placed above a structure via yet another structure.

[0011] In this specification and claims, the expression "a structure is exposed from another structure" means an embodiment in which a part of a structure is not covered by another structure, and includes an embodiment in which the part not covered by the other structure is covered by yet another structure. The embodiment expressed by this expression also includes an embodiment in which a structure is not in contact with the other structure.

[0012] First Embodiment In this embodiment, a light emitting device 100 which is one embodiment of the present invention will be described. (Overall composition)

[0013] A schematic end view of a light-emitting element 100 is shown in Fig. 1. As shown in Fig. 1, the light-emitting element 100 includes a pair of electrodes (anode 102 and cathode 104), and includes an electroluminescent layer (hereinafter also referred to as an EL layer) 110 between the anode 102 and the cathode 104. The EL layer 110 includes at least a light-emitting layer 120.

[0014] As shown in FIG. 1, the EL layer 110 is composed of a light-emitting layer 120 and a plurality of functional layers including organic compounds. The EL layer 110 may have a hole injection layer 112, a hole transport layer 114, an electron blocking layer 116, a hole blocking layer 122, an electron transport layer 124, an electron injection layer 126, and the like. By creating a voltage difference between the anode 102 and the cathode 104, holes and electrons are injected from the anode 102 and the cathode 104, respectively, into the EL layer 110, and these carriers are recombined in the light-emitting layer 120. The light-emitting material present in the light-emitting layer 120 is excited by the recombination of holes and electrons, and the energy released when this excited state returns to the ground state can be extracted as light. Each component will be described below.

[0015] (Pair of electrodes) The pair of electrodes may include an anode 102 and a cathode 104. The anode 102 is an electrode that injects holes into the EL layer 110. When light is extracted through the anode 102 obtained from the EL layer 110, the anode 102 is configured to transmit visible light, and therefore the anode 102 is made of a conductive oxide that transmits visible light, such as indium-tin oxide (ITO) or indium-zinc (IZO). On the other hand, when light is extracted through the cathode 104, the anode 102 is configured to function as a reflective electrode that efficiently reflects light. In this case, the anode 102 is configured to include a metal with high reflectivity, such as silver or aluminum, or an alloy thereof. For example, the anode 102 may have a configuration in which a film containing a metal is sandwiched between films containing a conductive oxide.

[0016] The cathode 104 is an electrode that injects electrons into the EL layer 110. When light obtained in the EL layer 110 is extracted through the anode 102, the cathode 104 also functions as a reflective electrode, and therefore the cathode 104 is configured to include the above-mentioned metal or alloy (for example, an alloy of silver and a metal having a small work function, such as magnesium). Conversely, when light obtained in the EL layer 110 is extracted through the cathode 104, the cathode 104 is configured to include a conductive oxide that transmits visible light. Alternatively, a metal-containing film having a thickness (for example, 5 nm or more and 20 nm or less) that can transmit visible light may be used as the cathode 104. In the latter case, a conductive oxide film that transmits visible light may be provided on the metal-containing film.

[0017] (Hole injection layer) The hole injection layer 112 has a function of promoting hole injection from the anode 102 to the EL layer 110. For the hole injection layer 112, a compound that is easy for holes to be injected into, that is, a compound that is easy to be oxidized (electron-donating) can be used. In other words, a compound with a shallow highest occupied molecular orbital (HOMO) level can be used. For example, aromatic amines such as benzidine derivatives and triarylamines, carbazole derivatives, thiophene derivatives, phthalocyanine derivatives such as copper phthalocyanine, etc. can be used. Alternatively, polymer materials such as polythiophene, polyaniline, and their derivatives can be used, and examples include poly(ethylenedioxythiophene) / poly(styrenesulfonic acid). A mixture of an electron-donating compound and an electron acceptor such as the above-mentioned aromatic amines, carbazole derivatives, or aromatic hydrocarbons can also be used. Examples of the electron acceptor include transition metal oxides such as vanadium oxide and molybdenum oxide, nitrogen-containing heteroaromatic compounds, and aromatic compounds having a strong electron-withdrawing group such as a cyano group. In the hole injection layer 112, the mixing ratio of the electron-donating compound and the electron acceptor is preferably 20 to 50 vol% for the electron-donating compound and 1 to 1.5 vol% for the electron acceptor. The hole injection layer 112 may have a single-layer structure or may be composed of a plurality of layers containing different materials.

[0018] (Hole Transport Layer) The hole transport layer 114 is provided so as to be in contact with the hole injection layer 112. The hole transport layer 114 has a function of transporting the holes injected into the hole injection layer 112 to the light-emitting layer 120 side, and materials similar to or the same as those usable in the hole injection layer 112 can be used. For example, a material with a deeper HOMO level than the hole injection layer 112 but with a difference of about 0.5 eV or less can be used. Typically, aromatic amines such as benzidine derivatives can be used. The hole transport layer 114 may also have a single-layer structure or may be composed of a plurality of layers containing different materials.

[0019] (Electron Blocking Layer) The electron blocking layer 116 is provided so as to be in contact with the hole transport layer 114. The electron blocking layer 116 has a function of preventing electrons injected from the cathode 104 from passing through the light emitting layer 120 and being injected into the hole transport layer 114 without contributing to recombination in the light emitting layer 120, thereby confining the electrons within the light emitting layer 120, and also prevents the excitation energy obtained in the light emitting layer 120 from being transferred to the molecules in the hole transport layer 114. This makes it possible to prevent a decrease in luminous efficiency.

[0020] For the electron blocking layer 116, it is preferable to use a material having a hole transporting property higher than or equal to the electron transporting property, a lowest unoccupied molecular orbital (LUMO) level shallower than the molecules in the light emitting layer 120, and a large band gap. Specifically, the difference between the LUMO level of the molecules in the electron blocking layer 116 and that of the molecules in the light emitting layer 120 is preferably 0.2 eV, 0.3 eV, or 0.5 eV or more. Also, the difference between the band gap of the molecules in the electron blocking layer 116 and that of the molecules in the light emitting layer 120 is preferably 0.2 eV, 0.3 eV, or 0.5 eV or more. Specifically, aromatic amine derivatives, carbazole derivatives, 9,10-dihydroacridine derivatives, benzofuran derivatives, benzothiophene derivatives, and the like can be used for the electron blocking layer 116. The electron blocking layer 116 may also have a single layer structure, or may be composed of multiple layers containing different materials.

[0021] (Light Emitting Layer) The light-emitting layer 120 contains a first host material as a main component, a second host material, and a first light-emitting material that is responsible for emitting light. The correlation between the energy bands of the first host material, the second host material, and the first light-emitting material will be described with reference to FIG.

[0022] 2 is a schematic diagram showing the correlation between the energy bands of the first host material, the second host material, and the first light-emitting material in the light-emitting layer 120. The LUMO level (LUMO H1 ) is the LUMO level of the second host material (LUMO H2) higher than the HOMO level of the first host material (HOMO H1 ) is the HOMO level of the second host material (HOMO H2 ) is preferably lower than or equal to the energy difference between the LUMO level and the HOMO level of the first host material. H1 ) is the band gap (ΔE H2 ) is larger than the LUMO level of the first host material. The LUMO level of the first host material is, for example, -2.4 eV to -2.6 eV, and the HOMO level of the first host material is -5.9 eV to -6.1 eV. The LUMO level of the second host material is, for example, -2.8 eV to -3.0 eV, and the HOMO level of the second host material is -5.8 eV to -6.0 eV. Since the first host material and the second host material have such an energy band correlation, carriers injected from a pair of electrodes are injected into the first host material, and then quickly injected into the second host material.

[0023] The LUMO level of the first host material is 100% lower than the LUMO level of the first light-emitting material (LUMO EM1 ) The difference between the LUMO level of the first host material and the LUMO level of the first light-emitting material is, for example, 0.7 eV or more and 1.1 eV or less. The LUMO level of the first host material is higher than the LUMO level of the first light-emitting material, and the difference is large.

[0024] The HOMO level of the first host material is preferably lower than or equal to the HOMO level of the first light-emitting material. H1 is the band gap ΔE of the first light-emitting material EM1 Such correlation of energy levels between the first host material and the first light-emitting material makes the LUMO level of the first host material higher than the LUMO level of the first light-emitting material.

[0025] The LUMO level of the second host material is preferably higher than the LUMO level of the first light-emitting material, and the HOMO level of the second host material is preferably higher than the HOMO level of the first light-emitting material. H2 is the band gap ΔE of the first light-emitting material EM1 It is preferable that the band gap is larger than that of the light emitting material that emits blue light. For example, when the light emitting material in the light emitting layer emits green light, the second host material has a band gap larger than that of the light emitting material that emits blue light. Note that blue light emission is light emission whose maximum emission peak wavelength is in the range of 400 nm to 500 nm. The band gap of the second host material is preferably, for example, about 3 eV. Such a correlation between the energy levels of the second host material and the first light emitting material suppresses the emission energy of the first light emitting material from being absorbed or transferred to the second host material, and the emission energy of the first light emitting material can be obtained as the emission of the light emitting element 100.

[0026] The LUMO level of the first light-emitting material is preferably lower than the LUMO levels of the first host material and the second host material, and higher than or equal to the HOMO level of the first host material. EM1 is the band gap ΔE of the first host material H1 and the band gap ΔE of the second host material H2 The LUMO level of the first light-emitting material is, for example, −3.3 to −3.5 eV, and the HOMO level of the first light-emitting material is, for example, −5.8 to −6.1 eV. Since the first light-emitting material has such an energy level correlation and a small band gap, the light-emitting element 100 can emit light of the emission color of the first light-emitting material.

[0027] Next, the energy levels of the first host material, the second host material, and the first light-emitting material in the light-emitting layer 120 will be described with reference to FIG.

[0028] 3 is a schematic diagram showing the correlation between the energy levels of the first host material, the second host material, and the first light-emitting material. In the light-emitting layer 120, the singlet excitation energy (S1) level (S H1 ) is the S1 level S of the first light-emitting material EM1 higher than the triplet excitation energy (T1) level of the first host material (T H1 ) is the T1 level (T EM1 Such an energy level correlation between the first host material and the first light-emitting material allows the singlet excitation energy and triplet excitation energy of the singlet excited state and triplet excited state of the first host material formed by recombination of carriers in the first host material to be transferred to the S1 level and T1 level of the first light-emitting material, respectively.

[0029] The first light-emitting material is a material that exhibits thermally activated delayed fluorescence. The difference between the T1 level and the S1 level of the first light-emitting material is small, for example, 5 meV to 20 meV. Therefore, the triplet excited state of the first light-emitting material generated by energy transfer can undergo reverse intersystem crossing to a singlet excited state by extremely small thermal energy at room temperature or below. Furthermore, as described below, the concentration of the first light-emitting material in the light-emitting layer 120 is relatively high, so that singlet excited excitons can be formed by TTA of the first light-emitting material. As a result, radiation deactivation from the singlet excited state generated by TTA can be used for light emission, so that high luminous efficiency can be achieved. However, when TTA is used, only one molecule of singlet excited exciton can be generated from two triplet excitons. On the other hand, the first light-emitting material can undergo reverse intersystem crossing from the triplet excited state to the singlet excited state by small thermal energy as described above. Therefore, by utilizing the contribution of the reverse intersystem crossing, the contribution of TTA can be reduced, and singlet excitons can be efficiently formed from triplet excitons of the first light-emitting material, resulting in the light-emitting element 100 exhibiting high luminous efficiency.

[0030] The S1 level (SH2 ) is higher than the S1 level of the first light-emitting material (Em1) and lower than the S1 level of the first host material. The T1 level (T H2 ) is lower than the T1 level of the first light-emitting material and the T1 level of the first host material. Such an energy level correlation between the second host material, the first light-emitting material, and the first host material allows the singlet excitation energy and triplet excitation energy of the first host material to be efficiently transferred to the S1 level and T1 level of the first light-emitting material. Conventionally, the generation of singlet excitons in the first light-emitting material has been promoted by increasing the concentration of the first light-emitting material. However, the second host material can further promote the generation of the singlet excited state of the first light-emitting material by having the above-mentioned energy level correlation.

[0031] In addition, in the light-emitting layer 120, the concentration of the second host material is smaller than that of the first host material. In other words, the concentration of the first host material is larger than that of the second host material in the light-emitting layer 120. In the light-emitting layer 120, the concentration of the first host material is preferably 20 vol% or more and 80 vol% or less, and the concentration of the second host material is preferably 5 vol% or more and 10 vol% or less. The energy level of the second host material is correlated as described above, and the concentration of the second host material is small, so that the singlet excitation energy and triplet excitation energy of the first host material can be more efficiently transferred to the S1 level and T1 level of the first light-emitting material.

[0032] Furthermore, the concentration of the second host material is preferably lower than the concentration of the first light-emitting material in the light-emitting layer 120. The concentration of the first light-emitting material may be higher than the concentration of the first host in the light-emitting layer 120. The concentration of the first light-emitting material is preferably 20 vol% or more and 60 vol% or less in the light-emitting layer 120. By having the concentration of the second host material lower than the concentration of the first light-emitting material in the light-emitting layer 120, the TTA of the first light-emitting material can be promoted.

[0033] Various compounds can be used as the first host material depending on the emission wavelength of the light-emitting material. For example, in addition to zinc or aluminum-based metal complexes, oxadiazole derivatives, triazole derivatives, benzimidazole derivatives, quinoxaline derivatives, dibenzoquinoxaline derivatives, dibenzothiophene derivatives, dibenzofuran derivatives, pyrimidine derivatives, triazine derivatives, pyridine derivatives, bipyridine derivatives, phenanthroline derivatives, aromatic amine derivatives, carbazole derivatives, etc. can be used as the first host material.

[0034] As described above, the second host material is a material having a band gap smaller than the band gap of the first host material. -11 cm 2 / Vs or more 1×10 -6 cm 2 It is preferable to use a material having a resistivity of 1 / Vs or less. As the second host material, for example, a carbazole derivative, a stilbene derivative, a distilbenzene derivative, an anthracene derivative, a rubrene derivative, or the like can be used.

[0035] As described above, the first light-emitting material is a material that exhibits thermally activated delayed fluorescence (TADF) (thermally activated delayed fluorescent material). It is preferable to use a material that emits light between green and yellow as the first light-emitting material. Here, green light emission is light emission whose maximum emission peak wavelength is in the range of 500 nm to 570 nm, and yellow light emission is light emission whose maximum emission peak wavelength is in the range of 570 nm to 650 nm.

[0036] Examples of the thermally activated delayed fluorescence material include fullerenes and their derivatives, acridine derivatives such as proflavine, and eosin. Further, metal-containing porphyrins containing magnesium, zinc, cadmium, tin, platinum, indium, palladium, or the like are also included. Examples of the metal-containing porphyrins include protoporphyrin-tin fluoride complex, mesoporphyrin-tin fluoride complex, hematoporphyrin-tin fluoride complex, coproporphyrin tetramethyl ester-tin fluoride complex, octaethylporphyrin-tin fluoride complex, etioporphyrin-tin fluoride complex, octaethylporphyrin-platinum chloride complex, and the like.

[0037] Furthermore, a compound in which an electron donor component and an electron acceptor component are linked may be used. Examples of the electron donor component and the electron acceptor component include a π-electron excess type heteroaromatic ring and a π-electron deficient type heteroaromatic ring, respectively. Examples of the basic skeleton of the π-electron deficient type heteroaromatic ring include a pyridine skeleton, a diazine skeleton, and a triazine skeleton. Examples of the basic skeleton of the π-electron excess type heteroaromatic ring include an acridine skeleton, a phenoxazine skeleton, a phenothiazine skeleton, a furan skeleton, a thiophene skeleton, and a pyrrole skeleton. Examples of such compounds include 2-(biphenyl-4-yl)-4,6-bis(12-phenylindolo[2,3-a]carbazol-11-yl)-1,3,5-triazine, 9-(4,6-diphenyl-1,3,5-triazin-2-yl)-9'-phenyl-9H,9'H-3,3'-bicarbazole, 9-[4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl]-9'-phenyl-9H,9'H-3,3'-bicarbazole, 2-[4-(10H-phenoxazin-10-yl)phenyl]-4,6-diphenyl-1,3,5-triazine, and the like.

[0038] The light-emitting layer 120 may further include, in addition to the first light-emitting material that is a thermally activated delayed fluorescence material, a fluorescent material (hereinafter also referred to as the second light-emitting material) that can receive the singlet excitation energy of the first light-emitting material to form a singlet excited state. Referring to FIG. 4, the energy correlation between the second light-emitting material and other materials in the light-emitting layer 120 will be described.

[0039] FIG. 4 is a schematic diagram showing the energy levels of the first host material, the second host material, the first light-emitting material, and the second light-emitting material in the light-emitting layer. As shown in FIG. 4, the second light-emitting material (Em2) preferably has an S1 level (S EM2 ) lower than the S1 level of the first light-emitting material. The second light-emitting material preferably has a T1 level (T EM2 ) lower than the T1 level of the first light-emitting material. The second light-emitting material preferably has a T1 level higher than the T1 level of the second host material. The second light-emitting material preferably has a bandgap smaller than or equivalent to the bandgap of the first light-emitting material. Since the energy levels of the second light-emitting material and the first light-emitting material have such a correlation, the singlet excitation energy of the first light-emitting material can move to the S1 level of the second light-emitting material, and the light-emitting element 100 can exhibit light emission of the emission color of the second light-emitting material.

[0040] The second light-emitting material is preferably included in the light-emitting layer 120 at a concentration lower than the concentration of the first light-emitting material. For example, the concentration of the second light-emitting material is 0.1 vol% or more and 1.0 vol% or less in the light-emitting layer 120.

[0041] The second light-emitting material is preferably a fluorescent material. The second light-emitting material is preferably a material that emits light between green, yellow, and red. The second light-emitting material may be, for example, a fluorescent material such as a coumarin derivative, a pyran derivative, a quinocridone derivative, a tetracene derivative, a pyrene derivative, an anthracene derivative, or a pyran derivative. In general, the emission spectrum of a thermally activated delayed fluorescent material is broad and has low color purity. In contrast, the above-mentioned fluorescent material provides an emission spectrum with a relatively narrow half-width, so that light emission with high color purity is possible. Therefore, by further adding a second light-emitting material to the light-emitting layer 120, it is possible to provide a light-emitting device 100 that has not only high luminous efficiency due to the thermally activated delayed fluorescent material, but also excellent color purity.

[0042] (Hole Block Layer) The hole blocking layer 122 has a function of preventing holes injected from the anode 102 from passing through the light-emitting layer 120 and being injected into the electron transport layer 124 without contributing to recombination, thereby confining the holes within the light-emitting layer 120, and also prevents the excitation energy obtained in the light-emitting layer 120 from being transferred to molecules in the electron transport layer 124. This makes it possible to prevent a decrease in luminous efficiency.

[0043] For the hole blocking layer 122, it is preferable to use a material having a higher or equal electron transporting property than a hole transporting property, a deeper HOMO level than the molecules in the light emitting layer 120, and a larger band gap. Specifically, the difference between the HOMO level of the molecules in the hole blocking layer 122 and that of the molecules in the light emitting layer 120 is preferably 0.2 eV, 0.3 eV, or 0.5 eV or more. Also, the difference between the band gap of the molecules in the hole blocking layer 122 and that of the molecules in the light emitting layer 120 is preferably 0.2 eV, 0.3 eV, or 0.5 eV or more. Specifically, metal complexes having a relatively large band gap (for example, 2.8 eV or more), such as phenanthroline derivatives, oxadiazole derivatives, triazole derivatives, and bis(2-methyl-8-quinolinolato)(4-hydroxy-biphenylyl)aluminum, may be used. The hole blocking layer 122 may also have a single layer structure, or may be composed of multiple layers containing different materials.

[0044] (electron transport layer) The electron transport layer 124 has a function of transporting electrons injected from the cathode 104 through the electron injection layer 126 to the light emitting layer 120. The electron transport layer 124 may be made of a compound that is easily reduced (electron accepting). In other words, a compound with a shallow LUMO level may be used. For example, metal complexes containing a ligand having benzoquinolinol as a basic skeleton, such as tris(8-quinolinolato)aluminum and tris(4-methyl-8-quinolinolato)aluminum, and metal complexes containing a ligand having oxadiazole or thiazole as a basic skeleton may be used. In addition to these metal complexes, compounds having an electron-deficient heteroaromatic ring, such as oxadiazole derivatives, thiazole derivatives, triazole derivatives, and phenanthroline derivatives, may be used. The electron transport layer 124 may also have a single layer structure, or may be made of multiple layers containing different materials.

[0045] (electron injection layer) The electron injection layer 126 may be made of a compound that promotes electron injection from the cathode 104 to the electron transport layer 124. For example, a mixture of a compound that can be used in the electron transport layer 124 and an electron donor such as lithium or magnesium may be used. Alternatively, an inorganic compound such as lithium fluoride or calcium fluoride may be used.

[0046] The anode 102 is formed by chemical vapor deposition (CVD), evaporation, sputtering, or the like. Each layer in the EL layer 110 is formed by inkjet printing, spin coating, printing, or evaporation. In the light-emitting layer 120, three types of materials, a first host material, a second host material, and a first light-emitting material, or four types of materials including a second light-emitting material may be simultaneously evaporated. In this case, the light-emitting layer 120 may be formed by a four-component co-evaporation method, or a three-component co-evaporation method using a (premixed) material in which the first host material and the second host material are mixed. The cathode 104 is formed by chemical vapor deposition (CVD), evaporation, sputtering, or co-evaporation.

[0047] Fig. 5 is a schematic diagram showing the correlation of energy bands of a light-emitting element according to one embodiment of the present invention. Fig. 5 shows the HOMO level and the LUMO level of the functional layer and the light-emitting layer in the EL layer 110. In Fig. 5, 111 indicates the Fermi level of the anode 102, and 104 indicates the Fermi level of the cathode.

[0048] 5, the LUMO levels decrease in the order of the hole injection layer 112, the hole transport layer 114, the electron blocking layer 116, and the light-emitting layer 120. The LUMO level of the light-emitting layer 120 is lower than the LUMO level of the hole blocking layer 122. The LUMO levels decrease in the order of the hole blocking layer 122, the electron transport layer 124, and the electron injection layer 126. The HOMO levels decrease in the order of the hole injection layer 112, the hole transport layer 114, the electron blocking layer 116, the light-emitting layer 120, the hole blocking layer 122, the electron transport layer 124, and the electron injection layer 126.

[0049] As described above, in the light-emitting element 100, the light-emitting layer has a first host material, a second host material, and a first light-emitting material exhibiting thermally activated delayed fluorescence. The band gap of the first host material is larger than that of the second host material, and the triplet excitation energy level of the second host material is lower than that of the first host material, so that high light-emitting efficiency can be obtained without increasing the concentration of the first light-emitting material, and the light-emitting lifetime is significantly improved. Therefore, a display device with high reliability can be provided.

[0050] <Second embodiment> In this embodiment, a display device 200 including the light emitting device 100 described in the first embodiment will be described. Descriptions of configurations that are the same as or similar to those described in the first embodiment may be omitted.

[0051] (Overall structure) A schematic top view of the display device 200 is shown in FIG. 6. As shown in FIG. 6, the display device 200 includes a substrate 202, on which various insulating films, semiconductor films, and conductive films are patterned and laminated. By appropriately laminating these films, a plurality of pixels 210 and driving circuits (scanning line driving circuit 204, signal line driving circuit 206) for driving the pixels 210 are formed on the substrate 202. An opposing substrate (not shown in FIG. 2) is provided on the pixels 210, the scanning line driving circuit 204, and the signal line driving circuit 206. The substrate 202 and the opposing substrate are fixed by an adhesive such as a sealant, thereby sealing and protecting the pixels 210, the scanning line driving circuit 204, and the signal line driving circuit 206. A plurality of terminals 208 formed of a conductive film are provided on the substrate 202, and the terminals 208 are electrically connected to an external circuit (not shown) via a connector such as a flexible printed circuit (FPC) board. Various signals and power sources for displaying images are supplied from an external circuit to the scanning line driver circuit 204 and the signal line driver circuit 206 via terminals 208. Note that either or both of the scanning line driver circuit 204 and the signal line driver circuit 206 do not need to be formed directly on the substrate 202, and a driver circuit formed on a substrate (such as a semiconductor substrate) different from the substrate 202 may be provided on the substrate 202 or a connector.

[0052] Each pixel 210 is formed with a pixel circuit, and further includes any one of the three primary color light-emitting elements (i.e., red light-emitting element, green light-emitting element, and blue light-emitting element). Based on various signals supplied from an external circuit, a signal for operating the pixel circuit is generated by the scanning line driving circuit 204 and the signal line driving circuit 206. This causes the light-emitting element connected to the pixel circuit to emit light, and each pixel 210 functions as a minimum unit for providing color information. As a result, full color display is possible. Here, the red light-emitting element, the green light-emitting element, and the blue light-emitting element refer to elements that have emission peak wavelengths in the ranges of 650 nm to 750 nm, 500 nm to 570 nm, and 400 nm to 500 nm, respectively.

[0053] There is no restriction on the arrangement of the pixels 210. For example, as shown in FIG. 7, a stripe arrangement may be adopted in which a red light-emitting pixel 210-1, a green light-emitting pixel 210-2, and a blue light-emitting pixel 210-3 that respectively provide red, green, and blue colors are arranged in order in the row direction, and pixels 210 that provide the same emission color are arranged in the same column. Alternatively, although not shown, various arrangements such as a mosaic arrangement in which a red light-emitting pixel 210-1, a green light-emitting pixel 210-2, and a blue light-emitting pixel 210-3 are arranged in order in both the row direction and the column direction, a delta arrangement, a pentile arrangement, etc. may be adopted. Alternatively, as shown in FIG. 8, a plurality of pixels 210 may be arranged such that one or more red light-emitting pixels 210-1 and one or more green light-emitting pixels 210-2 are sandwiched between adjacent blue light-emitting pixels 210-3. In this case, by arranging the multiple pixels 210 so that the area of ​​the blue light-emitting pixel 210-3, in which the blue light-emitting element with the lowest light-emitting efficiency is arranged, is larger than the area of ​​the other pixels 210, the burden on the blue light-emitting element can be reduced and the reliability of the display device 200 can be improved.

[0054] The light-emitting element 100 described in the first embodiment is disposed in at least one of the red light-emitting pixel 210-1, the green light-emitting pixel 210-2, and the blue light-emitting pixel 210-3. For example, as shown in FIG. 9, light-emitting elements having a structure different from that of the light-emitting element 100 can be disposed in the red light-emitting pixel 210-1 and the blue light-emitting pixel 210-3, and the light-emitting element 100 can be disposed in the green light-emitting pixel 210-2. In this case, the second host material used in the light-emitting layer 120 of the green light-emitting pixel 210-2 can be used as the host material of the light-emitting layer 120 of the blue light-emitting pixel 210-3. Furthermore, the light-emitting material of the light-emitting layer 120 of the blue light-emitting pixel 210-3 can be a light-emitting material having a singlet excitation energy level higher than the singlet excitation energy level of the first light-emitting material of the light-emitting layer 120 of the green light-emitting pixel 210-2. In other words, the emission color of the light-emitting material of the light-emitting layer 120 of the blue light-emitting pixel can be blue. The red light-emitting pixel 210-1 and the blue light-emitting pixel 210-3 may have a light-emitting element in the light-emitting layer 120 that includes a fluorescent material (that does not exhibit activated delayed fluorescence) or a phosphorescent material.

[0055] FIG. 10 is a schematic diagram of an end surface along the dashed line AB in FIG. 7. In FIG. 10, the end surfaces of a red light-emitting pixel 210-1, a green light-emitting pixel 210-2, and a blue light-emitting pixel 210-3 that are successively arranged are shown. The configuration of the pixel circuit formed in each pixel 210 can be determined arbitrarily, and a known configuration may be applied. In the example shown in FIG. 10, one transistor 220 and a capacitance element (auxiliary capacitance element) 240 connected to the transistor 220 are shown as part of the elements that constitute the pixel circuit. Furthermore, a light-emitting element connected to the pixel circuit is provided in each pixel 210. In the example shown in FIG. 10, the combination shown in FIG. 9 is applied. That is, a light-emitting element having a structure different from the light-emitting element 100 is arranged in the red light-emitting pixel 210-1 and the blue light-emitting pixel 210-3, and the light-emitting element 100 is arranged in the green light-emitting pixel 210-2. Here, an example is shown in which light from the light-emitting layer 120 in all light-emitting elements is extracted through the opposing substrate 250, but the display device 200 may also be configured so that light from the light-emitting layer 120 is extracted through the substrate 202.

[0056] (Substrate and opposing substrate) The substrate 202 and the counter substrate 250 are provided to provide physical strength to the display device 200 and to protect the pixels 210, the scanning line driving circuit 204, and the signal line driving circuit 206. The substrate 202 and the counter substrate 250 may be substrates containing inorganic materials such as crystalline semiconductor substrates, glass substrates, and quartz substrates, or may contain polymers such as polyimide, polyamide, and polycarbonate. The substrate 202 and the counter substrate 250 may be flexible or may not be flexible. In the former case, the substrate 202 and / or the counter substrate 250 may be flexible enough to be elastically deformed, or may be highly flexible enough to be plastically deformed. When light emitted from the light emitting element is taken out through the counter substrate 250, at least the counter substrate 250 is configured to transmit visible light. Conversely, when light emitted from the light emitting element is taken out through the substrate 202, at least the substrate 202 is configured to transmit visible light.

[0057] (Pixel circuit) As described above, a known configuration can be applied to the pixel circuit, so detailed description will be omitted. In the example shown in FIG. 10, a transistor 220 functioning as a driving transistor is provided on a substrate 202. The transistor 220 may be provided directly on the substrate 202, or may be formed on the substrate 202 via an undercoat 212 that prevents diffusion of impurities contained in the substrate 202. The transistor 220 shown in FIG. 10 is composed of a semiconductor film 222, a gate insulating film 224 on the semiconductor film 222, a gate electrode 226 on the gate insulating film 224, an interlayer insulating film 228 on the gate electrode 226, and a pair of terminals 230 and 232 provided on the interlayer insulating film 228 and electrically connected to the semiconductor film 222. The transistor 220 shown here is a top-gate transistor, but there is no restriction on the structure of the transistor 220, and a bottom-gate transistor or a transistor having gate electrodes above and below the semiconductor film may be used as the transistor 220.

[0058] A planarization film 236 is provided on the transistor 220 to absorb unevenness caused by elements such as the transistor 220 included in the pixel circuit and provide a flat surface. A capacitance electrode 242, a capacitance insulating film 244 on the capacitance electrode 242, and a pixel electrode 246 can be arranged on the planarization film 236, and these can form a capacitance element 240. Here, the pixel electrode 246 functions as the anode 102 of the light-emitting element 100. An opening for exposing the terminal 232 is provided in the planarization film 236, and the pixel electrode 246 is electrically connected to the terminal 232 through this opening, either directly or via a connection electrode 234 covering this opening. A partition wall 238, which is an insulating film, is provided to cover the end of the pixel electrode 246, and the EL layer 110 is arranged to cover the pixel electrode 246 and the partition wall 238. This electrically insulates adjacent light-emitting elements 100, and prevents the EL layer 110 from being cut by the end of the pixel electrode 246. In the example shown in FIG. 10, the pixel electrode 246 is shared by the light emitting element 100 and the capacitor element 240 .

[0059] (Light emitting element) As described above, light-emitting elements different from the light-emitting element 100 are disposed in the red light-emitting pixel 210-1 and the blue light-emitting pixel 210-3, and the light-emitting element 100 is disposed in the green light-emitting pixel 210-2. For this reason, all or a part of the functional layers other than the light-emitting layer 120 can be provided so as to be shared by all the pixels 210 and to be continuous across all the pixels 210. For example, as shown in FIG. 10, the electron blocking layer 116 can be formed so as to be shared by all the pixels 210 and to be continuous across all the pixels. As shown in FIG. 10, in addition to the electron blocking layer 116, the hole injection layer 112, the hole transport layer 114, the hole blocking layer 122, the electron transport layer 124, the electron injection layer 126, and the cathode 104 may also be provided so as to be shared by all the pixels 210 and to be continuous across all the pixels.

[0060] (Other configurations) As an optional configuration, one or more cap layers 130 may be provided on the cathode 104 to resonate the light extracted from the cathode 104 to improve color purity and brightness in the front direction. Furthermore, a protective film 132 may be provided on the light emitting element to prevent impurities such as water and oxygen from entering the EL layer 110. The protective film 132 may be formed, for example, of a film containing silicon nitride, a film containing a polymer such as an acrylic resin or an epoxy resin, or a laminate of these. EXAMPLES

[0061] Example 1 In this example, the fabrication of the light-emitting element 100 described in the first embodiment and evaluation of its characteristics are shown. Specifically, a light-emitting element 1 according to an embodiment of the present invention and a comparative light-emitting element 1 as a comparative example were fabricated. The light-emitting element 1 is one of the light-emitting elements 100 according to the first embodiment, and the comparative light-emitting element 1 are all identical to each other, except for the configuration in which the light-emitting layer 120 does not contain the second host material. The configurations of the light-emitting element 1 and the comparative light-emitting element 1 are shown in Table 1. The configuration of the light-emitting layer 120 of each light-emitting element is shown in Table 2.

[0062] [Table 1]

[0063] [Table 2]

[0064] The current density-voltage characteristics were measured for the light-emitting element 1 and the comparative light-emitting element 1. FIG 11 shows the measurement results of the current density-voltage characteristics.

[0065] 11, the light-emitting element 1 showed current density-voltage characteristics similar to those of the comparative light-emitting element 1, and showed high current efficiency similar to that of the comparative light-emitting element 1. The light-emitting element 1 and the comparative light-emitting element 1 exhibited green emission.

[0066] Next, the light-emitting element 1 and the comparative light-emitting element 1 were measured at a temperature of 30° C. and 50 mA / cm 2A constant current drive test was conducted at 1000 s. The results of the constant current drive test are shown in Figure 12 and Table 3.

[0067] [Table 3]

[0068] For light-emitting element 1, the time until the initial emission intensity decreases by 5% (LT 95 ) is the LT of comparative light-emitting element 1 95 It was shown that the light-emitting element according to the embodiment of the present invention can obtain good reliability while maintaining high current efficiency by including a second host material in the light-emitting layer.

[0069] Example 2 In this example, the fabrication of the light-emitting element 100 described in the first embodiment and evaluation of its characteristics are shown. Specifically, a light-emitting element 2 according to an embodiment of the present invention and a comparative light-emitting element 2 as a comparative example were fabricated. The light-emitting element 2 is one of the light-emitting elements 100 according to the first embodiment, and is identical to the light-emitting element 1 except for the configuration of the light-emitting layer 120 (see Table 1). The comparative light-emitting element 2 differs from the light-emitting element 2 in that the light-emitting layer does not contain a second host material. The configuration of the light-emitting layer 120 of each light-emitting element is shown in Table 4.

[0070] [Table 4]

[0071] The current density-voltage characteristics were measured for the light-emitting element 2 and the comparative light-emitting element 2. FIG 13 shows the measurement results of the current density-voltage characteristics.

[0072] 13, the light-emitting element 2 showed current density-voltage characteristics similar to those of the comparative light-emitting element 2 and showed high current efficiency similar to that of the comparative light-emitting element 2. The light-emitting element 2 and the comparative light-emitting element 2 exhibited green emission. The light-emitting element 2 exhibited emission at a wavelength longer than the maximum emission peak wavelength in the emission spectrum of the light-emitting element 1.

[0073] Next, the light-emitting element 2 and the comparative light-emitting element 2 were measured at a temperature of 30° C. and 50 mA / cm 2 A constant current drive test was conducted at 1000 s. The results of the constant current drive test are shown in Figure 14 and Table 5.

[0074] [Table 5]

[0075] For light-emitting element 2, the time until the initial emission intensity decreases by 5% (LT 95 ) is the LT of comparative light-emitting element 2 95 It was shown that the light-emitting efficiency was increased by about four times. This suggests that the light-emitting element according to the embodiment of the present invention can achieve good reliability while maintaining high current efficiency by including the second host material in the light-emitting layer.

[0076] Example 3 In this example, the fabrication of the light-emitting element 100 described in the first embodiment and evaluation of its characteristics are shown. Specifically, light-emitting elements 3 to 4 according to the embodiment of the present invention are all identical to light-emitting element 2 except for the configuration in which the concentration of the first light-emitting material and the concentration of the first host material in the light-emitting layer of light-emitting element 2 are different (see Table 1). Comparative light-emitting elements 3 and 4, which serve as comparative examples, are identical to light-emitting elements 3 and 4, respectively, except that the light-emitting layer does not contain a second host material. The configuration of the light-emitting layer 120 of each light-emitting element is shown in Table 6.

[0077] [Table 6]

[0078] The measured temperature of the light-emitting element 2 to the light-emitting element 4 and the comparative light-emitting element 2 to the comparative light-emitting element 4 was 30° C., and the measured value was 50 mA / cm 2 A constant current drive test was conducted at 1000 MHz. Table 7 shows the results of the constant current drive test.

[0079] [Table 7]

[0080] In Comparative Light-Emitting Element 2 to Comparative Light-Emitting Element 4, the reliability was improved by increasing the concentration of the first light-emitting material exhibiting TADF. This is known as a means for extending the life of a light-emitting element using a light-emitting material exhibiting TADF, and it is considered that in Comparative Light-Emitting Element 2 to Comparative Light-Emitting Element 4, TTA was promoted by increasing the concentration of the light-emitting material. As in Comparative Light-Emitting Element 2 to Comparative Light-Emitting Element 4, the reliability was also improved by increasing the concentration of the first light-emitting material. This suggests that the effect of improving reliability associated with an increase in the concentration of TADF is not impaired in Light-Emitting Element 2 to Light-Emitting Element 4 even when the light-emitting element contains a second host material.

[0081] Example 4 In this example, the fabrication of the light-emitting element 100 described in the first embodiment and evaluation of its characteristics are shown. Specifically, the light-emitting elements 5 to 9 according to the embodiment of the present invention differ mainly in the electron mobility of the second host material in the light-emitting layer of the light-emitting element 1 and the concentration of the second host material. The light-emitting elements 5 to 9 according to the embodiment of the present invention are all the same as the light-emitting element 1 except for the configuration of the light-emitting layer (see Table 1). The configuration of the light-emitting layer 120 of each light-emitting element is shown in Table 8.

[0082] [Table 8]

[0083] The measurement temperature of the light-emitting elements 1, 5 to 9, and the comparative light-emitting element 1 was 30° C., and the measured value was 50 mA / cm 2 A constant current drive test was conducted at 1000 MHz. Table 9 shows the results of the constant current drive test.

[0084] [Table 9]

[0085] Light-emitting elements 1, 5 to 9 LT 95It was shown that the reliability depends on the concentration and electron mobility of the second host material in the light-emitting layer. The results of the light-emitting elements 1 and 5 to 9 show that good reliability can be obtained when the concentration of the second host material in the light-emitting layer is at least 5 vol% to 10 vol%. The results of the light-emitting elements 1 and 5 to 9 show that good reliability can be obtained when the electron mobility of the second host material is at least 1×10 -11 cm 2 / Vs or more 1×10 -6 cm 2 It was shown that good reliability can be obtained at or below / Vs.

[0086] The reliability of the light-emitting elements 1 and 5 to 9 is improved as the concentration of the second host material in the light-emitting layer is reduced in the range of 5 vol % to 10 vol %. Furthermore, when the electron mobility of the second host material is less than 1×10 -11 cm 2 / Vs or more 1×10 -6 cm 2 In the range of 1 / Vs or less, the lower the electron mobility of the second host material, the more improved it was.

[0087] The above-described embodiments of the present invention may be combined as appropriate as long as they are not mutually inconsistent. In addition, a display device according to any of the embodiments may be combined as appropriate by a person skilled in the art to add or remove components or modify the design, or to add or omit steps or modify conditions, and the combination is included in the scope of the present invention as long as it includes the gist of the present invention.

[0088] Even if there are other effects and advantages different from those brought about by the aspects of each of the above-mentioned embodiments, those that are clear from the description in this specification or that can be easily predicted by a person skilled in the art are naturally understood to be brought about by the present invention. [Explanation of symbols]

[0089] 100: light-emitting element, 102: anode, 104: cathode, 110: electroluminescent layer, 112: hole injection layer, 114: comparative light-emitting element, 114: hole transport layer, 116: electron blocking layer, 118: buffer layer, 120: light-emitting layer, 122: hole blocking layer, 124: electron transport layer, 126: electron injection layer, 130: cap layer, 132: protective film, 200: display device, 202: substrate, 204: scanning line driving circuit, 206: signal line driving circuit, 208: terminal, 210: pixel, 210-1: red light emitting pixel, 210-2: green light emitting pixel, 210-3: blue light emitting pixel, 212: undercoat, 220: transistor, 222: semiconductor film, 224: gate insulating film, 226: gate electrode, 228: interlayer insulating film, 230: terminal, 232: terminal, 234: connection electrode, 236: planarizing film, 238: partition wall, 240: capacitance element, 242: capacitance electrode, 244: capacitance insulating film, 246: pixel electrode, 250: opposing substrate

Claims

1. A pair of electrodes and a light-emitting layer between the pair of electrodes, The light-emitting layer is A first host material, a second host material, and A first luminescent material exhibiting thermally activated delayed fluorescence, In the light-emitting layer, a concentration of the first host material is greater than a concentration of the second host material; the band gap of the first host material is greater than the band gap of the second host material; a singlet excitation energy level of the first light-emitting material is lower than a singlet excitation energy level of the first host material; A light-emitting device, wherein the triplet excitation energy level of the second host material is lower than the triplet excitation energy level of the first host material.

2. 10. The light-emitting device of claim 1, wherein the light-emitting layer further comprises a second light-emitting material having a singlet excitation energy level lower than a singlet excitation energy level of the first light-emitting material.

3. 2 . The light-emitting device according to claim 1 , wherein in the light-emitting layer, the concentration of the first host material is 20 vol % or more and 80 vol % or less, and the concentration of the second host material is 5 vol % or more and 10 vol % or less.

4. The light-emitting element according to claim 1 , wherein the concentration of the first light-emitting material in the light-emitting layer is 20 vol % or more and 60 vol % or less.

5. The light-emitting device according to claim 1 , wherein the LUMO level of the second host material is lower than the LUMO level of the first host material.

6. The light-emitting element of claim 1 , wherein a triplet excited energy level of the first light-emitting material is lower than a triplet excited energy level of the first host material.

7. The light-emitting element of claim 1 , wherein the triplet excited energy level of the second host material is lower than the triplet excited energy level of the first light-emitting material.

8. The light-emitting device of claim 2 , wherein the emission color of the second light-emitting material is between green and red.

9. The electron mobility of the second host material is 1×10 －１１ cm ２ / Vs or more 1×10 －６ cm ２ The light-emitting device according to claim 1 , wherein the n-th voltage Vs is equal to or lower than 1 Vs.

10. The light-emitting device of claim 1 , wherein the emission color of the first light-emitting material is between green and yellow.

11. a first pixel having a first light emitting element; and a second pixel having a second light emitting element; the first light-emitting element has a pair of electrodes and a first light-emitting layer between the pair of electrodes, The first light-emitting layer is A first host material, a second host material, and A first luminescent material exhibiting thermally activated delayed fluorescence, the second light-emitting element has the pair of electrodes and a second light-emitting layer between the pair of electrodes, The second light-emitting layer is the second host material, and a second emissive material having a singlet excitation energy level higher than the singlet excitation energy level of the first emissive material; In the first light-emitting layer, a concentration of the first host material is greater than a concentration of the second host material; the band gap of the first host material is greater than the band gap of the second host material; A display device, wherein the triplet excitation energy level of the second host material is lower than the triplet excitation energy level of the first host material.

12. 12. The display of claim 11, wherein the first light-emitting layer further comprises a third light-emitting material having a singlet excitation energy level lower than a singlet excitation energy level of the first light-emitting material.

13. 12. The display device according to claim 11, wherein in the first light-emitting layer, the concentration of the first host material is 20 vol% or more and 80 vol% or less, and the concentration of the second host material is 5 vol% or more and 10 vol% or less.

14. The display device according to claim 11 , wherein the concentration of the first light-emitting material in the first light-emitting layer is 20 vol % or more and 60 vol % or less.

15. The display device of claim 11 , wherein the LUMO level of the second host material is lower than the LUMO level of the first host material.

16. The display device of claim 11 , wherein the triplet excited energy level of the first light-emitting material is lower than the triplet excited energy level of the first host material.

17. The display device of claim 11 , wherein the triplet excited energy level of the second host material is lower than the triplet excited energy level of the first light-emitting material.

18. The electron mobility of the second host material is 1×10 －１１ cm ２ / Vs or more 1×10 －６ cm ２ The display device according to claim 11, wherein the Vref is equal to or lower than Vs.

19. The display device of claim 11 , wherein the emission color of the first luminescent material is between green and yellow.

20. The display device according to claim 11 , wherein the emission color of the second light-emitting material is blue.

Images