Light-emitting element and display device having the light-emitting element

Abstract

To provide a highly efficient blue light emitting element that can be driven at a low voltage, and a display device that consumes low power. The light-emitting element includes an anode, a cathode, and an electroluminescent layer between the anode and the cathode. The electroluminescent layer has a light-emitting layer containing a host material and a first fluorescent material. The host material has a band gap of 3.0 eV or more and 3.4 eV or less. The difference in the lowest unoccupied molecular orbital levels between the host material and the first fluorescent material is 1.0 eV or more. The difference in the highest occupied molecular orbital levels between the host material and the first fluorescent material is 0.3 eV or less.

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, display devices equipped with organic electroluminescent devices (OLEDs) have been widely used. In addition, organic electroluminescent devices that exhibit thermally activated delayed fluorescence or hyperfluorescence (registered trademark) have attracted attention due to their extremely high luminous efficiency, and vigorous research and development has been conducted on them (see, for example, Patent Documents 1 to 4). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent Publication No. 2021-048366 [Patent Document 2] Japanese Patent Application Publication No. 2020-013695 [Patent Document 3] Japanese Patent Application Publication No. 2019-062127 [Patent Document 4] Japanese Patent Application Publication No. 2017-092329 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 new structure and a display device having the light-emitting element. Alternatively, an object of one embodiment of the present invention is to provide a highly efficient blue light-emitting element that can be driven at a low voltage and a display device that consumes low power. [Means for solving the problem]

[0005] One embodiment of the present invention is a light-emitting device. The light-emitting device includes an anode, a cathode, and an electroluminescent layer between the anode and the cathode. The electroluminescent layer has an emitting layer containing a host material and a first fluorescent material. The first fluorescent material exhibits thermally activated delayed fluorescence. The host material has a band gap of 3.0 eV or more and 3.4 eV or less. The lowest unoccupied molecular orbital level of the host material is shallower than the lowest unoccupied molecular orbital level of the first fluorescent material, with the difference therebetween being 0.3 eV or less. The highest occupied molecular orbital level of the host material is shallower than the highest occupied molecular orbital level of the first fluorescent material that exhibits thermally activated delayed fluorescence, with the difference therebetween being 1.0 eV or more.

[0006] One embodiment of the present invention is a display device. The display device includes a red-emitting pixel, a green-emitting pixel, and a blue-emitting pixel, each having a red-emitting element, a green-emitting element, and a blue-emitting element. The blue-emitting element has an anode, a cathode, and an electroluminescent layer between the anode and the cathode. The electroluminescent layer has an emissive layer including a host material and a first fluorescent material. The first fluorescent material exhibits thermally activated delayed fluorescence. The band gap of the host material is 3.0 eV or more and 3.4 eV or less. The lowest unoccupied molecular orbital level of the host material is shallower than the lowest unoccupied molecular orbital level of the first fluorescent material, and the difference therebetween is 0.3 eV or less. The highest occupied molecular orbital level of the host material is shallower than the highest occupied molecular orbital level of the first fluorescent material, and the difference therebetween is 1.0 eV or more. [Brief explanation of the drawings]

[0007] [Figure 1] 1 is a schematic end view of a light-emitting device according to an embodiment of the present invention; [Figure 2] 1 is a schematic top view of a display device according to an embodiment of the present invention. [Figure 3] 1 is an equivalent circuit diagram of a pixel of a display device according to an embodiment of the present invention. [Figure 4] FIG. 2 is a schematic end view of a light-emitting element provided in a display device according to an embodiment of the present invention. [Figure 5] 1 is a schematic end view of a display device according to an embodiment of the present invention; [Figure 6] 1 shows voltage-current density curves of light-emitting elements of Examples and Comparative Examples. [Figure 7] 1 shows current density-normalized current efficiency curves (L / J) of light-emitting elements according to examples and comparative examples. DETAILED DESCRIPTION OF THE INVENTION

[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 should not be construed 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 schematically 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 assigned the same reference numerals, and duplicate explanations may be omitted.

[0010] In this specification and claims, when expressing an aspect of placing another structure on top of a certain 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 certain structure so as to be in contact with the certain structure, and a case in which another structure is placed above a certain structure via yet another structure.

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

[0012] Hereinafter, blue emission refers to emission having a maximum emission peak wavelength in the range of 400 nm to 500 nm, green emission refers to emission having a maximum emission peak wavelength in the range of 500 nm to 650 nm, and red emission refers to emission having a maximum emission peak wavelength in the range of 650 nm to 750 nm.

[0013] First Embodiment In this embodiment, an electroluminescent element (hereinafter, sometimes simply referred to as a light-emitting element) 100 according to one embodiment of the present invention will be described. FIG. 1 shows a schematic end view of the light-emitting element 100. The light-emitting element 100 is a light-emitting element that emits blue light. As shown in FIG. 1, the light-emitting element 100 includes an anode 102 and a cathode 104 facing each other, and an electroluminescent layer (hereinafter, also referred to as an EL layer) 110 between the anode 102 and the cathode 104. The EL layer 110 is composed of multiple functional layers containing organic compounds. The EL layer 110 includes an emissive layer 118 as a functional layer responsible for emitting light, and may include other functional layers such as a hole injection layer 112, a hole transport layer 114, an electron blocking layer 116, a hole blocking layer 120, an electron transport layer 122, and an electron injection layer 124. Preferably, the EL layer 110 includes an emissive layer 118 and an electron blocking layer 116 located between the emissive layer 118 and the anode 102 and in contact with the emissive layer 118. 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 recombine in the light-emitting layer 118. The light-emitting material capable of emitting blue light present in the light-emitting layer 118 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.

[0014] 1. Anode and cathode The anode 102 is an electrode that injects holes into the EL layer 110. When light obtained in the EL layer 110 is extracted through the anode 102, 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 contain a highly reflective metal 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.

[0015] 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 is configured to contain the above-mentioned metal or alloy (e.g., an alloy of silver and a metal with a low 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 contain a conductive oxide that transmits visible light. Alternatively, a metal-containing film having a thickness that allows visible light to transmit (e.g., 5 nm to 20 nm) may be used as the cathode 104. In the latter case, a conductive oxide film that transmits visible light may be further provided on the metal-containing film.

[0016] 2. Hole injection layer The hole injection layer 112 functions to facilitate hole injection from the anode 102 to the EL layer 110. The hole injection layer 112 can be made of a compound that easily injects holes, i.e., is easily oxidized (electron-donating). In other words, a compound with a shallow highest occupied molecular orbital (HOMO) level can be used. Examples of such compounds include aromatic amines such as benzidine derivatives and triarylamines, carbazole derivatives, thiophene derivatives, phthalocyanine derivatives such as copper phthalocyanine, and aromatic hydrocarbons. Alternatively, polymeric materials such as polythiophene, polyaniline, and their derivatives can be used, such as poly(ethylenedioxythiophene) / poly(styrenesulfonic acid). A mixture of electron-donating compounds such as the aromatic amines, carbazole derivatives, and aromatic hydrocarbons and an electron acceptor can also be used. Examples of electron acceptors include transition metal oxides such as vanadium oxide and molybdenum oxide, nitrogen-containing heteroaromatic compounds, and aromatic compounds with strong electron-withdrawing groups such as cyano groups. The hole injection layer 112 can have a single layer structure or can be composed of multiple layers containing different materials.

[0017] 3. Hole transport layer The hole transport layer 114 is provided in contact with the hole injection layer 112. The hole transport layer 114 transports holes injected into the hole injection layer 112 toward the light-emitting layer 118, and may be made of the same or similar materials as those usable for the hole injection layer 112. For example, a material having a deeper HOMO level than that of the hole injection layer 112, but with a difference of approximately 0.5 eV or less, may be used. Typically, an aromatic amine such as a benzidine derivative may be used. The hole transport layer 114 may also have a single-layer structure or may be made of multiple layers containing different materials.

[0018] 4.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 prevents electrons injected from the cathode 104 from passing through the light-emitting layer 118 and being injected into the hole transport layer 114 without contributing to recombination within the light-emitting layer 118, thereby confining the electrons within the light-emitting layer 118, and also prevents the excitation energy obtained in the light-emitting layer 118 from being transferred to the molecules in the hole transport layer 114. This prevents a decrease in luminous efficiency.

[0019] The electron blocking layer 116 contains or is made of an electron blocking material. The electron blocking material preferably has hole-transporting properties that are higher than or equal to electron-transporting properties, a shallower lowest unoccupied molecular orbital (LUMO) level, and a larger band gap than the host material (described below) in the emitting layer 118. Preferably, the difference in LUMO levels between the electron blocking material and the host material is greater than 0 eV and equal to or less than 0.1 eV or 0.2 eV. By providing a relatively small LUMO level difference between the electron blocking layer 116 and the emitting layer 118, electron penetration through the emitting layer 118 is suppressed while an increase in driving voltage is suppressed. Energy transfer from the emitting layer 118 is also prevented, suppressing non-radiative recombination of carriers accumulated between these layers. The difference in HOMO levels between the electron blocking material and the host material can be determined arbitrarily, and the HOMO level of the electron blocking material may be shallower or deeper than the HOMO level of the host material. The difference between these may be, for example, 0.1 eV or more and 0.3 eV or less.

[0020] Specific examples of electron blocking materials include aromatic amine derivatives, carbazole derivatives, 9,10-dihydroacridine derivatives, benzofuran derivatives, and benzothiophene derivatives.

[0021] 5. Emitting layer The light-emitting layer 118 contains a host material as a main component, and also contains a blue-emitting light-emitting material (hereinafter, sometimes referred to as a first fluorescent material) that is responsible for emitting light. When the electron blocking layer 116 is provided, the light-emitting layer 118 is provided on the electron blocking layer 116 so as to be in contact with the electron blocking layer 116. When the electron blocking layer 116 is not used, the light-emitting layer 118 may be formed on the hole-transporting layer 114 so as to be in contact with the hole-transporting layer 114.

[0022] Various compounds can be used as the host material depending on the emission wavelength of the light-emitting material. Preferably, a host material widely used in green-emitting phosphorescent devices is used. Specifically, a material with a band gap of 3.0 eV to 3.4 eV is used. It is also preferable to use a material with a lowest triplet excited state (T1) level of 1.9 eV to 2.5 eV. Furthermore, the host material is selected so that its LUMO level and HOMO level are shallower than those of the light-emitting material. Specifically, the host material is selected so that the difference in LUMO level between the host material and the light-emitting material is 1 eV or more and the difference in HOMO level between the host material and the light-emitting material is 0.3 eV or less. There is no restriction on the maximum difference in LUMO level between the host material and the light-emitting material, but it is, for example, 1.5 eV or less. Similarly, there is no restriction on the minimum difference in HOMU level between the host material and the light-emitting material, as long as it is greater than 0 eV.

[0023] The host material preferably has a hole transporting property higher than its electron transporting property. That is, it is preferable to use a compound having a hole mobility higher than its electron mobility as the host material. For example, a compound having a hole mobility 10 times or more and 1000 times or less than the electron mobility can be used as the host material.

[0024] Specific examples of the host material that can be used include zinc and aluminum-based metal complexes, as well as 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, and carbazole derivatives.

[0025] A fluorescent material (thermally activated delayed fluorescent material) exhibiting thermally activated delayed fluorescence is used as the light-emitting material (first fluorescent material). A thermally activated delayed fluorescent material is a compound in which the T1 level and the lowest singlet excited state (S1) level are close to each other, with the difference being between 5 meV and 20 meV. Therefore, the lowest triplet excited state of the light-emitting material, which is generated by carrier recombination, can undergo reverse intersystem crossing to the lowest singlet excited state with extremely small thermal energy at room temperature or below. As a result, the rate of non-radiative deactivation of the lowest triplet excited state is relatively reduced, and radiative deactivation from the lowest singlet excited state is promoted. This can dramatically improve the efficiency of the light-emitting element 100. Because light emission occurs through this mechanism, the thermally activated delayed fluorescent material exhibits light emission with a spectrum similar to that of ordinary fluorescence, but with a significantly longer lifetime. The fluorescence lifetime of a thermally activated delayed fluorescent material is 10 -6 seconds (1 ns) or more, preferably 10 -3 seconds (1 μs) or more.

[0026] Examples of thermally activated delayed fluorescent materials include fullerenes and their derivatives, acridine derivatives such as proflavine, and eosin. Other examples include metal-containing porphyrins containing magnesium, zinc, cadmium, tin, platinum, indium, or palladium. Examples of metal-containing porphyrins include protoporphyrin-tin fluoride complexes, mesoporphyrin-tin fluoride complexes, hematoporphyrin-tin fluoride complexes, coproporphyrin tetramethyl ester-tin fluoride complexes, octaethylporphyrin-tin fluoride complexes, etioporphyrin-tin fluoride complexes, and octaethylporphyrin-platinum chloride complexes.

[0027] Furthermore, compounds in which an electron donor component and an electron acceptor component are linked can be used as thermally activated delayed fluorescent materials. Examples of the electron donor component and the electron acceptor component include π-electron-rich heteroaromatic rings and π-electron-deficient heteroaromatic rings, respectively. Examples of the basic skeleton of a π-electron-deficient heteroaromatic ring include a pyridine skeleton, a diazine skeleton, and a triazine skeleton. Examples of the basic skeleton of a π-electron-rich heteroaromatic ring include an acridine skeleton, a phenoxazine skeleton, a phenothiazine skeleton, a furan skeleton, a thiophene skeleton, and a pyrrole skeleton.Such compounds include 2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN), 2,4,5,6-tetra(3,6-dimethyl-9H-carbazol-9-yl)isophthalonitrile (m4CzIPN), 2,4,6-tri(4-diphenylaminophenyl)-1,3,5-tricyanobenzene (3DPA3CN), 2-[4-(10H-phenoxazin-10-yl)phenyl]-4,6-diphenyl- 1,3,5-triazine (PXZ-TRZ), 2-(biphenyl-4-yl)-4,6-bis(12-phenylindolo[2,3-a]carbazol-11-yl)-1,3,5-triazine (PIC-TRZ), 2-[4-(10H-phenoxazin-10-yl)phenyl]-4,6-diphenyl-1,3,5-triazine (PXZ-TRZ), 10-[4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl]-9,9-dimethicone 2,4,5,6-tetrakis(3,6-di-t-butyl-9H-carbazol-9-yl)isophthalonitrile (t4XaIPN), 2-{4-[3-(N-phenyl-9H-carbazol-3-yl)-9H-carbazol-9-yl]phenyl}-4,6-diphenyl-1,3,5-triazine (PCCzPTzn), 3-[4-(5-phenyl-5,10-dihydrophenazine-10- Examples include bis[4-(9,9-dimethyl-9,10-dihydroacridine)phenyl]sulfone (DMAC-DPS), 10-phenyl-10H,10'H-spiro[acridine-9,9'-anthracene]-10'-one (ACRSA), and the like.

[0028] In addition to the thermally activated delayed fluorescent material, the light-emitting layer 118 may further include a light-emitting material (hereinafter also referred to as a second fluorescent material) that can receive the lowest singlet excited energy of the thermally activated delayed fluorescent material to transition from the ground state to the lowest singlet excited state and emit blue light by radiative deactivation from the lowest singlet excited state. If the second fluorescent material is not a compound exhibiting the multiple resonance effect described below, it is selected so that its S1 level is lower than that of the thermally activated delayed fluorescent material, i.e., so that its band gap is smaller than that of the thermally activated delayed fluorescent material. Furthermore, the second fluorescent material is selected so that the difference between its T1 level and S1 level exceeds 50 meV. Therefore, the second fluorescent material does not exhibit thermally activated delayed fluorescence in the light-emitting device 100 and exhibits a relatively short fluorescence lifetime (e.g., 1 ps or more but less than 1 ns). Examples of second fluorescent materials exhibiting such characteristics include coumarin derivatives, quinocridone derivatives, pyrene derivatives, anthracene derivatives, and pyran derivatives, which exhibit a maximum emission peak wavelength in the range of 400 nm to 500 nm. Thermally activated delayed fluorescent materials generally exhibit broad emission spectra and low color purity. In contrast, the fluorescent materials described above exhibit emission spectra with relatively narrow half-widths, enabling emission of light with high color purity. Therefore, by further adding a second fluorescent material to the light-emitting layer 118, it is possible to provide a light-emitting device 100 that not only exhibits high luminous efficiency due to the thermally activated delayed fluorescent material but also has excellent color purity.

[0029] Furthermore, in the light-emitting layer 118, a compound exhibiting a multiple resonance effect may be used as the second fluorescent material. The multiple resonance effect is an effect in which the contribution of a resonance structure having an anion and a cation is increased by incorporating heteroatoms such as boron and nitrogen into a carbon-conjugated π-electron system. Therefore, in the light-emitting device 100, a compound containing heteroatoms such as boron and nitrogen incorporated into a carbon-conjugated π-electron system may be used as the second fluorescent material. This further promotes reverse intersystem crossing from the lowest triplet excited state to the lowest singlet excited state, resulting in improved luminous efficiency. Furthermore, compounds exhibiting a multiple resonance effect also produce an emission spectrum with a relatively narrow half-width, enabling narrowing of the emission spectrum and improving color purity.

[0030] Compounds that exhibit multiple resonance effects include, for example, 5,9-diphenyl-5,9-diaza-13b-boranaphtho[3,2,1-de]anthracene (DABNA1), 9-([1,1'-diphenyl]-3-yl)-N,N,5,11-tetraphenyl-5,9-dihydro-5,9-diaza-13b-boranaphtho[3,2,1-de]anthracene-3-amine (D ABNA2), 2,12-di(tert-butyl)-5,9-di(4-tert-butylphenyl)-N,N-diphenyl-5H,9H-[1,4]benzazaborino[2,3,4-kl]phenazaborin-7-amine (DPhA-tBu4DABNA), 2,12-di(tert-butyl)-N,N,5,9-tetra(4-tert-butylphenyl)-5H, 9H-[1,4]benzazaborino[2,3,4-kl]phenazaborin-7-amine (tBuDPhA-tBu4DABNA), 2,12-di(tert-butyl)-5,9-di(4-tert-butylphenyl)-7-methyl-5H,9H-[1,4]benzazaborino[2,3,4-kl]phenazaborine (Me-tBu4DABNA), N7,N7,N1 Examples include nitrogen- and boron-containing fused heteroaromatic compounds such as 3,N13,5,9,11,15-octaphenyl-5H,9H,11H,15H-[1,4]benzazaborino[2,3,4-kl][1,4]benzazaborino[4',3',2':4,5][1,4]benzazaborino[3,2-b]phenazaborine-7,13-diamine (ν-DABNA).

[0031] Among the above-mentioned compounds, a thermally activated delayed fluorescent material having a maximum emission peak wavelength of 400 nm or more and 500 nm or less is used as the light-emitting material in the light-emitting device 100.

[0032] 6. Hole Block Layer The hole blocking layer 120 prevents holes injected from the anode 102 from passing through the light-emitting layer 118 and being injected into the electron transport layer 122 without contributing to recombination, thereby confining the holes within the light-emitting layer 118, and also prevents the excitation energy obtained in the light-emitting layer 118 from being transferred to molecules in the electron transport layer 122. This prevents a decrease in luminous efficiency.

[0033] The hole blocking layer 120 preferably uses a material that has electron transport properties that are higher or equal to its hole transport properties, a deeper HOMO level than the host material in the light-emitting layer 118, and a larger band gap. Specifically, the difference between the HOMO level of the hole blocking material in the hole blocking layer 120 and that of the host material in the light-emitting layer 118 is preferably 0.2 eV, 0.3 eV, or 0.5 eV or more. The difference in band gap between the hole blocking material and the host material is preferably 0.2 eV, 0.3 eV, or 0.5 eV or more. Specific examples of hole blocking materials include phenanthroline derivatives, oxadiazole derivatives, triazole derivatives, and metal complexes with relatively large band gaps (e.g., 2.8 eV or more), such as bis(2-methyl-8-quinolinolato)(4-hydroxybiphenylyl)aluminum. The hole blocking layer 120 may have a single-layer structure or may be composed of multiple layers containing different materials.

[0034] 7.Electron transport layer The electron transport layer 122 transports electrons injected from the cathode 104 through the electron injection layer 124 to the light-emitting layer 118. The electron transport layer 122 can be made of an easily reduced (electron-accepting) compound. In other words, it can be made of a compound with a shallow LUMO level. Examples of such a compound include metal complexes containing a ligand with a benzoquinolinol skeleton, such as tris(8-quinolinolato)aluminum and tris(4-methyl-8-quinolinolato)aluminum, and metal complexes containing a ligand with an oxadiazole or thiazole skeleton. In addition to these metal complexes, compounds with electron-deficient heteroaromatic rings, such as oxadiazole derivatives, thiazole derivatives, triazole derivatives, and phenanthroline derivatives, can also be used. The electron transport layer 122 can also have a single-layer structure or can be made of multiple layers containing different materials.

[0035] 8.Electron injection layer The electron injection layer 124 can be made of a compound that promotes electron injection from the cathode 104 to the electron transport layer 122. For example, a mixture of a compound that can be used in the electron transport layer 122 and an electron donor such as lithium or magnesium can be used. Alternatively, an inorganic compound such as lithium fluoride or calcium fluoride can be used.

[0036] A light-emitting device containing a blue light-emitting material that exhibits thermally activated delayed fluorescence can emit light with extremely high efficiency compared to a light-emitting device that uses a conventional blue fluorescent material (i.e., a fluorescent material in which the rate of inverse intersystem crossover from the T1 level to the S1 level is negligibly small and which does not exhibit thermally activated delayed fluorescence). However, the driving voltage is high, and therefore, when a blue light-emitting device containing a thermally activated delayed fluorescent material is used in a display device, the power consumption of the display device increases.

[0037] However, as demonstrated in the examples, by selecting a host material and an emitting material so as to satisfy the above-described relationships between the host material and the emitting material, specifically, the band gap, the difference in LUMO level, and the difference in HOMO level, the driving voltage can be reduced. Therefore, by applying embodiments of the present invention, it is possible to provide a blue light-emitting device that is highly efficient and has a reduced driving voltage. Therefore, embodiments of the present invention enable the manufacture of a highly efficient display device with low power consumption.

[0038] Second Embodiment In this embodiment, a display device including the light emitting device 100 described in the first embodiment will be described. Description of configurations that are the same as or similar to those described in the first embodiment may be omitted.

[0039] 1.Overall structure FIG. 2 is a schematic top view of a display device 200 according to an embodiment of the present invention. As shown in FIG. 2, the display device 200 includes a substrate 202 on which various patterned insulating films, semiconductor films, and conductive films are laminated. By appropriately combining these films, a plurality of pixels 210 and drive circuits for driving the pixels 210 (scanning line drive circuit 204, signal line drive circuit 206), etc., are formed on the substrate 202. A counter substrate (not shown in FIG. 2) is provided on the pixels 210, scanning line drive circuit 204, and signal line drive circuit 206. The substrate 202 and the counter substrate are fixed together with a sealant, thereby sealing and protecting the pixels 210, scanning line drive circuit 204, and signal line drive circuit 206. A plurality of terminals 208 formed of conductive films 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 driving circuit 204 and the signal line driving circuit 206 via terminals 208. Note that either or both of the scanning line driving circuit 204 and the signal line driving circuit 206 do not need to be formed directly on the substrate 202; a driving 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. These configurations will be described in detail below.

[0040] 2. Substrate and opposing substrate The substrate 202 and the counter substrate 212 are provided to provide physical strength to the display device 200 and to protect the multiple pixels 210, the scanning line driving circuit 204, and the signal line driving circuit 206. The substrate 202 and the counter substrate 212 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 212 may each be flexible or inflexible. In the former case, the substrate 202 and / or the counter substrate 212 may be flexible enough to allow elastic deformation, or may be highly flexible enough to allow plastic deformation. When light emitted from the light-emitting elements is extracted to the outside through the counter substrate, at least the counter substrate 212 is configured to transmit visible light. Conversely, when light emitted from the light-emitting elements is extracted to the outside through the substrate 202, at least the substrate 202 is configured to transmit visible light.

[0041] 3. Pixels The plurality of pixels 210 are configured to provide the three primary colors. Specifically, the plurality of pixels 210 are configured by a plurality of red-emitting pixels, a plurality of green-emitting pixels, and a plurality of blue-emitting pixels. A pixel circuit is formed in each pixel 210, and any one of a red light-emitting element, a green light-emitting element, and a blue light-emitting element is disposed therein. The light-emitting element 100 described in the first embodiment is used as the blue light-emitting element. A scanning line driving circuit 204 and a signal line driving circuit 206 generate signals for operating the pixel circuits based on various signals supplied from external circuits. This causes the light-emitting elements connected to the pixel circuits to emit light, and each pixel 210 functions as the smallest unit for providing color information. As a result, a full-color display is possible.

[0042] (1) Pixel circuit 3 shows an equivalent circuit diagram of a pixel 210. This diagram shows an equivalent circuit diagram of three pixels 210 arranged in succession. A plurality of scanning lines 220 and a plurality of video signal lines 222 extend from the scanning line driving circuit 204 and the signal line driving circuit 206, respectively. A pixel circuit 230 provided in each pixel 210 is electrically connected to a corresponding one of the plurality of scanning lines 220 and a corresponding one of the plurality of video signal lines 222.

[0043] The pixel circuit 230 may have any configuration, provided that it includes at least two transistors (a switching transistor 232 and a driving transistor 234) and a storage capacitor 248, as shown in FIG. 3 . Therefore, although not shown, the pixel circuit 230 may further include one or more transistors and one or more capacitors. As shown in FIG. 3 , the gate of the switching transistor 232 is electrically connected to the scanning line 220, and one terminal is electrically connected to the video signal line 222. The other terminal of the switching transistor 232 is connected to one electrode (capacitor electrode) of the storage capacitor 248 and the gate of the driving transistor 234. This allows various signals, such as video signals input via the video signal line 222, to be input to and stored in the driving transistor 234. One terminal of the driving transistor 234 is electrically connected to the current supply line 224, and the other terminal is electrically connected to the other electrode of the storage capacitor 248 and a pixel electrode constituting the light-emitting element 260. The other electrode of the light-emitting element 260 is electrically connected to the common line 226 and is supplied with a constant potential. With this configuration, when the drive transistor 234 is in an on state, a current supplied from the current supply line 224 flows through the light emitting element 260, and light can be emitted from each pixel 210. In the pixel 210 that emits blue light, the light emitting element 260 uses the light emitting element 100 described in the first embodiment.

[0044] (2) Light-emitting element 4 shows a schematic end view of a light-emitting element provided in the pixel 210. As described above, the red light-emitting pixel 210-1, the green light-emitting pixel 210-2, and the blue light-emitting pixel 210-3 are provided with a red light-emitting element 260-1, a green light-emitting element 260-2, and the light-emitting element 100 described in the first embodiment, respectively. There are no restrictions on the configuration of the red light-emitting element 260-1 and the green light-emitting element 260-2. Similar to the light-emitting element 100, the red light-emitting element 260-1 and the green light-emitting element 260-2 each have an EL layer 110 between the anode 102 and the cathode 104, and the EL layer 110 is composed of multiple functional layers. Specifically, the EL layer 110 of the red light-emitting element 260-1 and the green light-emitting element 260-2 each includes an emitting layer 118, a hole injection layer 112, a hole transport layer 114, an electron blocking layer 116, a hole blocking layer 120, an electron transport layer 122, an electron injection layer 124, and the like. The EL layer 110 of the red light emitting element 260-1 and the green light emitting element 260-2 may each include all or some of the above-mentioned functional layers. The materials used in these functional layers are the same as those described in the first embodiment. However, since the emission wavelengths are different, the red light emitting element 260-1 and the green light emitting element 260-2 are appropriately adjusted in terms of the light emitting material, the structure of the light emitting layer 118 containing the light emitting material, the configuration of the functional layer adjacent to the light emitting layer 118 (for example, the electron blocking layer 116 and the hole blocking layer), and the structure (thickness) of the hole transport layer 114 and the electron transport layer 122.

[0045] For example, the thickness of the hole transport layer 114 may be set to be larger in the red light-emitting element 260-1 and the green light-emitting element 260-2, which emit light of longer wavelengths, compared to the light-emitting element 100. Specifically, the pixel 210 may be configured so that the thickness of the hole transport layer 114 increases in the order of the light-emitting element 100, the green light-emitting element 260-2, and the red light-emitting element 260-1. For example, the red light-emitting element 260-1 and the green light-emitting element 260-2 may be provided with multiple hole transport layers 114 (in the example shown in FIG. 4, a first hole transport layer 114-1 and a second hole transport layer 114-2). By providing the first hole transport layer 114-1 in common to all pixels 210 and making the second hole transport layer 114-2 in the red light-emitting element 260-1 thicker than that in the green light-emitting element 260-2, the number of metal masks used to form the hole transport layer 114 can be reduced, and a resonant structure suitable for the emission wavelength can be formed between the anode 102 and the light-emitting layer 118 for each pixel 210.

[0046] A phosphorescent material is used as the light-emitting material in at least one of the red light-emitting element 260-1 and the green light-emitting element 260-2. By using a phosphorescent material, light emission from a triplet excited state formed in the light-emitting layer 118 can be utilized, thereby achieving highly efficient light emission. In this case, the light-emitting layer 118 is configured so that the band gap of the host material is larger than that of the light-emitting material and the T1 level of the host material is shallower than that of the light-emitting material.

[0047] Examples of the host material that can be used include zinc and aluminum-based metal complexes, as well as 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, and carbazole derivatives.

[0048] As the light-emitting material, phosphorescent materials such as iridium orthometal complexes, platinum porphyrin complexes, and rare earth complexes can be used. For example, examples of light-emitting materials that can emit red light include iridium complexes having a pyrimidine skeleton, such as (diisobutyrylmethanato)bis[4,6-bis(3-methylphenyl)pyrimidinato]iridium(III) and bis[4,6-bis(3-methylphenyl)pyrimidinato](dipivaloylmethanato)iridium(III); iridium complexes having a pyrazine skeleton, such as (acetylacetonato)bis(2,3,5-triphenylpyrazinato)iridium(III) and bis(2,3,5-triphenylpyrazinato)(dipivaloylmethanato)iridium(III); and iridium complexes having a pyridine skeleton, such as tris(1-phenylisoquinolinato-N,C2′)iridium(III) and bis(1-phenylisoquinolinato-N,C2′)iridium(III) acetylacetonate. Alternatively, platinum complexes such as 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrinplatinum(II) and rare earth metal complexes such as tris(1,3-diphenyl-1,3-propanedionato)(monophenanthroline)europium(III) and tris[1-(2-thenoyl)-3,3,3-trifluoroacetonato](monophenanthroline)europium(III) may be used.

[0049] Examples of light-emitting materials capable of emitting green light include iridium complexes having a pyrimidine skeleton, such as tris(4-methyl-6-phenylpyrimidinato)iridium(III) and tris(4-t-butyl-6-phenylpyrimidinato)iridium(III), iridium complexes having a pyrazine skeleton, such as (acetylacetonato)bis(3,5-dimethyl-2-phenylpyrazinato)iridium(III) and (acetylacetonato)bis(5-isopropyl-3-methyl-2-phenylpyrazinato)iridium(III), iridium complexes having a pyridine skeleton, such as tris(2-phenylpyridinato-N,C2')iridium(III) and bis(2-phenylpyridinato-N,C2')iridium(III) acetylacetonate, and rare earth metal complexes, such as tris(acetylacetonato)(monophenanthroline)terbium(III).

[0050] Alternatively, a thermally activated delayed fluorescent material may be used in both or at least one of the red light-emitting element 260-1 and the green light-emitting element 260-2. In this case, the light-emitting layer 118 is configured so that the band gap of the host material is larger than that of the light-emitting material and the T1 level of the host material is higher than the T1 level of the light-emitting material. As the thermally activated delayed fluorescent material, among the thermally activated delayed fluorescent materials described in the first embodiment, red-emitting and green-emitting light-emitting materials may be used in the red light-emitting element 260-1 and the green light-emitting element 260-2. The use of a thermally activated delayed fluorescent material promotes reverse intersystem crossing from the T1 level to the S1 level of the thermally activated delayed fluorescent material formed in the light-emitting layer 118, allowing light emission from the S1 level to be utilized, thereby achieving highly efficient light emission.

[0051] Similarly to the light-emitting element 100, the light-emitting layers 118 of the red light-emitting element 260-1 and the green light-emitting element 260-2 may further include a fluorescent material (second fluorescent material) that does not exhibit thermally activated delayed fluorescence. Examples of suitable second fluorescent materials for the red light-emitting element 260-1 include pyran derivatives, tetracene derivatives, and anthraquinone derivatives. Examples of suitable second fluorescent materials for the green light-emitting element 260-2 include coumarin derivatives, quinocridone derivatives, anthracene derivatives, and pyrene derivatives.

[0052] The electron blocking layer 116, the electron transport layer 122, the electron injection layer 124, and the cathode 104 may be provided so as to be shared by all the pixels 210, or may be provided independently in one pixel 210 selected from the pixels 210-1, 210-2, and 210-3 and shared by the other two pixels 210. By providing functional layers so as to be shared by the pixels 210 that emit different colors of light, it is possible to reduce the number of metal masks, and the display device 200 can be provided at lower cost.

[0053] Although not shown, one or more cap layers may be provided on the cathode 104 in each pixel 210 so as to be in contact with the cathode 104. By providing the cap layer, light emitted from the light-emitting layer 118 can be repeatedly reflected between the upper and lower surfaces of the cap layer and resonate. This increases the luminance in the front direction of the display device 200 and improves color purity. The cap layer is made of a material selected to transmit at least a portion of visible light. For example, the material contained in the functional layer or a polymer such as an acrylic resin, an epoxy resin, or a silicone resin may be used. Alternatively, a fluorine-containing polymer material may be used. Alternatively, instead of an organic compound, an inorganic compound such as a metal fluoride, such as lithium fluoride, magnesium fluoride, or calcium fluoride, may be used. The cap layer may be provided so as to have the same structure in all pixels 210. Alternatively, a first cap layer may be provided in selected pixels 210, and a second cap layer may be provided so as to be shared by all pixels 210. This allows the cap layer to have a resonance structure more appropriate for the emission wavelength.

[0054] (3) Pixel edge structure FIG. 5 is a schematic diagram of an end surface of a display device 200. Three consecutively arranged pixels 210-1, 210-2, and 210-3 are shown. In the example shown in FIG. 5, a pixel circuit 230 including a driving transistor 234 is provided on a substrate 202 via an undercoat 214 of any configuration. The driving transistor 234 shown in FIG. 5 is composed of a semiconductor film 236, a gate insulating film 238 on the semiconductor film 236, a gate electrode 240 on the gate insulating film 238, an interlayer insulating film 242 on the gate electrode 240, and a pair of terminals 244 and 246 provided on the interlayer insulating film 242 and electrically connected to the semiconductor film 236. While the driving transistor 234 shown here is a top-gate transistor, there are no restrictions on the structure of the driving transistor 234. A bottom-gate transistor or a transistor having gate electrodes above and below the semiconductor film may also be used as the driving transistor 234.

[0055] A planarization film 256 is provided on the driving transistor 234 to absorb unevenness caused by the pixel circuit 230 and provide a flat surface. A capacitance electrode 250, a capacitance insulating film 252 on the capacitance electrode 250, and a pixel electrode 262 can be disposed on the planarization film 256, and these constitute a storage capacitance element 248. Here, the pixel electrode 262 functions as the anode 102 for the light-emitting element 100, the red light-emitting element 260-1, and the green light-emitting element 260-2. Therefore, each pixel electrode 262 is shared by the light-emitting element 100, the red light-emitting element 260-1, or the green light-emitting element 260-2 and the storage capacitance element 248. An opening exposing the terminal 246 is provided in the planarization film 256, and the pixel electrode 262 is electrically connected to the terminal 246 through this opening either directly or via a connection electrode 254 covering this opening. A partition wall 258, which is an insulating film, is provided to cover the end of the pixel electrode 262, and the EL layer 110 is disposed to cover the pixel electrode 262 and the partition wall 258. This electrically insulates adjacent light-emitting elements 100 and prevents the EL layer 110 from being cut by the end of the pixel electrode 262.

[0056] In FIG. 5 , for ease of viewing, some functional layers, excluding the light-emitting layer 118, are shown as single layers. However, as described above, a red light-emitting element 260-1, a green light-emitting element 260-2, and a light-emitting element 100 are disposed in the red light-emitting pixel 210-1, the green light-emitting pixel 210-2, and the blue light-emitting pixel 210-3, respectively. The cathode 104 is provided so as to be shared by all pixels 210. As an optional configuration, a sealing film 270 may be formed directly on the cathode 104 or via a cap layer (not shown). There are no limitations on the configuration of the sealing film 270; for example, the sealing film 270 may be formed using a film containing a silicon-containing inorganic compound such as silicon nitride or a film containing a resin such as an epoxy resin or an acrylic resin. In the example shown in FIG. 5 , a sealing film 270 is used in which a resin-containing film 274 is sandwiched between films 272 and 276 containing a silicon-containing inorganic compound. An opposing substrate 212 is provided on the cathode 104 via a sealing film 270 and / or a cap layer, and the substrate 202 and the opposing substrate 212 are fixed to each other using a sealing material (not shown), thereby protecting the pixels 210.

[0057] As described in the first embodiment, the blue-emitting light-emitting device 100 exhibits extremely high efficiency because it contains a thermally activated delayed fluorescent material. Furthermore, the host material and the light-emitting material contained in the light-emitting layer 118 of the light-emitting device 100 are configured to satisfy the relationships described in the first embodiment with respect to the LUMO level, the HOMO level, and the band gap. Therefore, as shown in the examples, the light-emitting device 100 can be driven at a lower voltage than a conventional blue light-emitting device containing a thermally activated delayed fluorescent material in the light-emitting layer, and the light-emitting efficiency is improved. Therefore, by applying the embodiments of the present invention, it is possible to provide a full-color electroluminescent display device with low power consumption. [Example]

[0058] Light-emitting devices of Examples and Comparative Examples were fabricated and their characteristics were evaluated. Table 1 shows the functional layers constituting these light-emitting devices and their thicknesses. In each light-emitting device, an ITO thin film (50 nm thick) was used as the anode, and a co-evaporated film of silver and magnesium (160 nm thick) was used as the cathode. The size of the light-emitting region was 2 mm × 2 mm. In each light-emitting device, the light-emitting layer contained a host material, a blue-emitting thermally activated delayed fluorescent material, and a blue-emitting fluorescent material (i.e., a second fluorescent material that does not exhibit thermally activated delayed fluorescence). However, while the host material of the light-emitting device of Comparative Example is a host material commonly used in blue fluorescent light-emitting devices, the host material of the light-emitting device of Examples is a host material commonly used in green phosphorescent light-emitting devices. Therefore, in the light-emitting devices of Examples, the relationships described in the first embodiment hold between the host material and the thermally activated delayed fluorescent material in terms of the band gap, LUMO level, and HOMO level. In contrast, in the light-emitting device of the comparative example, the difference in LUMO level between the host material and the thermally activated delayed fluorescent material was 1.0 eV, and the difference in HOMO level between them was 0.3 eV.

[0059] The hole injection layer, hole transport layer, hole blocking layer, electron transport layer, and electron injection layer were made of the same materials and had the same thicknesses in the light-emitting devices of Examples and Comparative Examples. In all light-emitting devices, the electron blocking layer was formed so that the difference in LUMO level between the host material of the light-emitting layer and the material contained in the electron blocking layer was 0.1 eV to 0.2 eV to prevent the accumulation of a large amount of carriers.

[0060] [Table 1]

[0061] The voltage-current density curve of the fabricated light-emitting element is shown in Figure 6. From Figure 6, it can be seen that the driving voltage of the light-emitting element of the example is significantly reduced compared to the light-emitting element of the comparative example. For example, when a current of 1 mA / cm 2 When a current is passed through the light emitting device at a current density of 1.0 V, the difference in driving voltage reaches approximately 1.2 V. As can be seen from the current density-normalized luminous efficiency curve shown in FIG. 7, the light emitting device of the example has a higher luminous efficiency than that of the comparative example. In particular, the increase in luminous efficiency at low current densities is remarkable, which suggests that the light emitting device of the example suppresses non-radiative recombination of carriers accumulated at the interface of the functional layer (mainly the interface between the light emitting layer and the electron blocking layer).

[0062] The above results show that by applying an embodiment of the present invention, it is possible to provide a light-emitting element with low power consumption and improved luminous efficiency, and a low-power consumption display device including this light-emitting element.

[0063] The above-described embodiments of the present invention can be combined as appropriate as long as they are not mutually inconsistent. Furthermore, even if a person skilled in the art appropriately adds or deletes components or modifies designs, or adds or omits processes or modifies conditions based on the embodiments, such combinations are included within the scope of the present invention as long as they include the gist of the present invention.

[0064] 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]

[0065] 100: light-emitting element, 102: anode, 104: cathode, 110: electroluminescent layer, 112: hole injection layer, 114: hole transport layer, 114-1: first hole transport layer, 114-2: second hole transport layer, 116: electron blocking layer, 118: light-emitting layer, 120: hole blocking layer, 122: electron transport layer, 124: electron injection layer, 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: opposing substrate, 214: undercoat, 220: scanning line, 222: video Signal line, 224: current supply line, 226: common line, 230: pixel circuit, 232: switching transistor, 234: drive transistor, 236: semiconductor film, 238: gate insulating film, 240: gate electrode, 242: interlayer insulating film, 244: terminal, 246: terminal, 248: storage capacitor element, 250: capacitor electrode, 252: capacitor insulating film, 254: connection electrode, 256: planarizing film, 258: partition wall, 260: light-emitting element, 260-1: red light-emitting element, 260-2: green light-emitting element, 262: pixel electrode, 270: sealing film, 272: film containing silicon-containing inorganic compound, 274: film containing resin, 276: film containing silicon-containing inorganic compound

Claims

1. an anode, a cathode, and an electroluminescent layer between the anode and the cathode; the electroluminescent layer has a light-emitting layer including a host material and a first fluorescent material; the first fluorescent material exhibits thermally activated delayed fluorescence; the band gap of the host material is 3.0 eV or more and 3.4 eV or less; the lowest unoccupied molecular orbital level of the host material is shallower than the lowest unoccupied molecular orbital level of the first fluorescent material, and the difference therebetween is 0.3 eV or less; a highest occupied molecular orbital level of the host material is shallower than a highest occupied molecular orbital level of the first fluorescent material, and the difference therebetween is 1.0 eV or more;

2. The light-emitting device according to claim 1 , wherein the host material is configured to have a hole-transporting property higher than an electron-transporting property.

3. 2. The light-emitting element according to claim 1, wherein the energy difference between the lowest singlet excited state and the lowest triplet excited state of the first fluorescent material is 5 meV or more and 20 meV or less.

4. The light-emitting device of claim 1 , wherein the light-emitting layer further comprises a second fluorescent material.

5. the electroluminescent layer further includes an electron blocking layer located between the light-emitting layer and the anode and in contact with the light-emitting layer; 2. The light-emitting device according to claim 1, wherein the lowest unoccupied molecular orbital level of the electron blocking layer is shallower than the lowest unoccupied molecular orbital level of the host material, and the difference therebetween is 0.2 eV or less.

6. The light-emitting device according to claim 4 , wherein the second fluorescent material exhibits a multiple resonance effect.

7. The light-emitting device of claim 6 , wherein the second fluorescent material comprises boron and nitrogen.

8. a red light-emitting pixel, a green light-emitting pixel, and a blue light-emitting pixel, each having a red light-emitting element, a green light-emitting element, and a blue light-emitting element, the blue light-emitting element has an anode, a cathode, and an electroluminescent layer between the anode and the cathode; the electroluminescent layer has a light-emitting layer including a host material and a first fluorescent material; the first fluorescent material exhibits thermally activated delayed fluorescence; the band gap of the host material is 3.0 eV or more and 3.4 eV or less; the lowest unoccupied molecular orbital level of the host material is shallower than the lowest unoccupied molecular orbital level of the first fluorescent material, and the difference therebetween is 0.3 eV or less; a highest occupied molecular orbital level of the host material is shallower than a highest occupied molecular orbital level of the first fluorescent material, and the difference therebetween is 1.0 eV or more;

9. The display device according to claim 8 , wherein the host material is configured so that its hole transporting property is higher than its electron transporting property.

10. 9. The display device according to claim 8, wherein the energy difference between the lowest singlet excited state and the lowest triplet excited state of the first fluorescent material is 5 meV or more and 20 meV or less.

11. The display device of claim 8 , wherein the light-emitting layer further comprises a second fluorescent material.

12. the electroluminescent layer further includes an electron blocking layer located between the light-emitting layer and the anode and in contact with the light-emitting layer; 9. The display device according to claim 8, wherein the lowest unoccupied molecular orbital level of the electron blocking layer is shallower than the lowest unoccupied molecular orbital level of the host material, and the difference therebetween is 0.2 eV or less.

13. The display device of claim 11 , wherein the second fluorescent material exhibits a multiple resonance effect.

14. 14. The display device of claim 13, wherein the second fluorescent material comprises boron and nitrogen.

15. the red light-emitting element has an anode, a cathode, and an electroluminescent layer between the anode and the cathode; The display device of claim 8 , wherein the electroluminescent layer of the red light-emitting element comprises a phosphorescent material.

16. the red light-emitting element has an anode, a cathode, and an electroluminescent layer between the anode and the cathode; The display device according to claim 8 , wherein the electroluminescent layer of the red light-emitting element contains a fluorescent material that exhibits thermally activated delayed fluorescence.

17. the green light-emitting element has an anode, a cathode, and an electroluminescent layer between the anode and the cathode; The display device of claim 8 , wherein the electroluminescent layer of the green light-emitting element comprises a phosphorescent material.

18. the green light-emitting element has an anode, a cathode, and an electroluminescent layer between the anode and the cathode; The display device according to claim 8 , wherein the electroluminescent layer of the green light-emitting element contains a fluorescent material that exhibits thermally activated delayed fluorescence.

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