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

Abstract

A light-emitting element according to the present invention comprises an anode, a light-emitting layer, a first electron transport layer, a second electron transport layer, and a cathode. The light-emitting layer is positioned on the anode. The first electron transport layer is positioned on the light-emitting layer. The second electron transport layer is positioned on the first electron transport layer, is in contact with the first electron transport layer, and has a thickness greater than that of the first electron transport layer. The cathode is positioned on the second electron transport layer. The first electron transport layer and the second electron transport layer both contain an electron transport material and (8-quinolinolato) lithium. The concentration of (8-quinolinolato) lithium in the first electron transport layer is lower than the concentration of (8-quinolinolato) lithium in the second electron transport layer.

Description

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

[0001] One embodiment of the present invention relates to an electroluminescent element and a display device including the electroluminescent element.

[0002] In recent years, display devices including organic electroluminescent elements (OLEDs, hereinafter also simply referred to as light-emitting elements) have been widely used. In addition, organic electroluminescent elements exhibiting thermally activated delayed fluorescence and Hyperfluorescence (registered trademark) have attracted attention due to their extremely high luminous efficiency, and intensive research and development have been conducted (see, for example, Patent Documents 1 and 2).

[0003] Japanese Patent Application Laid-Open No. 2021-122042 Japanese Patent Application Laid-Open No. 2024-138021

[0004] One object of an embodiment of the present invention is to provide a light-emitting element having a novel structure and a display device including the same. Alternatively, one object of an embodiment of the present invention is to provide a display device that exhibits high efficiency and high reliability and can be manufactured at low cost. Alternatively, one object of an embodiment of the present invention is to provide a method for manufacturing the above-described display device.

[0005] One embodiment of the present invention is a light-emitting element. The light-emitting element includes an anode, a light-emitting layer, a first electron transport layer, a second electron transport layer, and a cathode. The light-emitting layer is located on the anode. The first electron transport layer is located on the light-emitting layer. The second electron transport layer is located on the first electron transport layer, is in contact with the first electron transport layer, and has a greater thickness than the first electron transport layer. The cathode is located on the second electron transport layer. Both the first electron transport layer and the second electron transport layer contain an electron transport material and (8-quinolinolato)lithium. The concentration of (8-quinolinolato)lithium in the first electron transport layer is lower than that in the second electron transport layer.

[0006] One embodiment of the present invention is a display device. This display device comprises a red light-emitting element, a green light-emitting element, and a blue light-emitting element. Each of the red light-emitting element, the green light-emitting element, and the blue light-emitting element has an anode, a light-emitting layer, a first electron transport layer, a second electron transport layer, and a cathode. The light-emitting layer is located on the anode. The first electron transport layer is located on the light-emitting layer. The second electron transport layer is located on the first electron transport layer, is in contact with the first electron transport layer, and has a greater thickness than the first electron transport layer. The cathode is located on the second electron transport layer and is shared by the red light-emitting element, the green light-emitting element, and the blue light-emitting element. Both the first electron transport layer and the second electron transport layer contain an electron transport material and (8-quinolinolato)lithium. The concentration of (8-quinolinolato)lithium in the first electron transport layer is lower than that in the second electron transport layer.

[0007] A schematic end view of a display element according to one embodiment of the present invention. A schematic end view of a display element according to one embodiment of the present invention. A schematic end view of a display element according to one embodiment of the present invention. A schematic top view of a display device according to one embodiment of the present invention. An equivalent circuit of a pixel provided in a display device according to one embodiment of the present invention. A schematic end view of a light-emitting element provided in a pixel of a display device according to one embodiment of the present invention. A schematic end view of a light-emitting element provided in a pixel of a display device according to one embodiment of the present invention. A schematic end view of a light-emitting element provided in a pixel of a display device according to one embodiment of the present invention. A schematic end view of a display device according to one embodiment of the present invention.

[0008] The embodiments of the present invention will be described below with reference to the drawings and other documents. However, the present invention can be implemented in various forms without departing from its essence, and is not to be construed as being limited to the embodiments described below.

[0009] While drawings may schematically represent the width, thickness, shape, etc., of each part compared to the actual embodiment in order to clarify the explanation, these are merely examples and do not limit the interpretation of the present invention. In this specification and in each figure, elements having the same function as those described in previously shown figures are denoted by the same reference numerals, and redundant explanations may be omitted.

[0010] In this specification and the claims, when describing a configuration in which one structure is placed on top of another structure, unless otherwise specified, the term "on top of" includes both cases: one in which the other structure is placed directly on top of the other structure so as to be in contact with it, and another in which the other structure is placed above the other structure via yet another structure.

[0011] In this specification and claims, the expression "a structure is exposed from another structure" means a portion of a structure that is not covered by another structure, and this portion that is not covered by another structure may also be covered by yet another structure. Furthermore, the expression also includes a portion of a structure that is not in contact with another structure.

[0012] In embodiments of the present invention, when a single film is formed across multiple light-emitting elements, this film functions as a functional layer in each of the multiple light-emitting elements. However, since the film is formed simultaneously in the same process, it has substantially the same layer structure, the same material, the same composition, and the same morphology across the multiple light-emitting elements. Therefore, the film is defined as existing in the same layer across the multiple light-emitting elements.

[0013] <First Embodiment> In this embodiment, the structure of a light-emitting element 100 according to one embodiment of the present invention will be described. A schematic end view of the light-emitting element 100 is shown in Figure 1. The light-emitting element 100 is provided on a substrate not shown in Figure 1 and has an anode 102, a cathode 104 on the anode 102, and an electroluminescent layer (hereinafter referred to as EL layer) 110 provided between the anode 102 and the cathode 104. The EL layer 110 is a laminate of a plurality of functional layers and includes at least an emissive layer 118 and an electron transport layer 122 on the emissive layer 118. The EL layer 110 may further include a hole injection layer 112 in contact with the anode 102, a hole transport layer 114 on the hole injection layer 112, an electron blocking layer 116 on the hole transport layer 114, a hole blocking layer 120 on the emissive layer 118, and an electron injection layer 124 between the electron transport layer 122 and the cathode 104. These configurations will be described in detail below.

[0014] (1) Anode and Cathode The anode 102 functions as an electrode that injects holes into the EL layer 110. When light obtained in the EL layer 110 is extracted via 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 via 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 made of a highly reflective metal or alloy thereof, such as silver or aluminum. For example, a configuration in which a film containing a metal is covered or sandwiched with a film containing a conductive oxide may be applied to the anode 102.

[0015] The cathode 104 is an electrode that injects electrons into the EL layer 110. When the light obtained from the EL layer 110 is extracted via the anode 102, the cathode 104 also functions as a reflective electrode, so the cathode 104 is configured to include the above-mentioned metal or alloy (for example, an alloy of silver and a metal such as magnesium with a low work function). Conversely, when the light obtained from the EL layer 110 is extracted via 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 that transmits visible light (for example, 5 nm to 20 nm) (for example, a film containing magnesium, an alloy of magnesium and silver, etc.) 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. As will be described later, in a display device in which a plurality of light-emitting elements 100 are provided on a substrate, unlike the anode 102, the cathode 104 is continuous and shared among the plurality of light-emitting elements 100 without being separated between them.

[0016] (2) Hole implantation layer The hole implantation layer 112 has the function of promoting hole implantation from the anode 102 to the EL layer 110. Compounds that are easily implanted with holes, i.e., easily oxidized (electron-donating), can be used in the hole implantation layer 112. In other words, compounds with shallow highest occupied molecular orbital (HOMO) levels can be used. For example, benzidine derivatives, aromatic amines such as triarylamines, carbazole derivatives, thiophene derivatives, phthalocyanine derivatives such as copper phthalocyanine can be used. Alternatively, polymer materials such as polythiophene, polyaniline, and their derivatives can be used, one example being poly(ethylenedioxythiophene) / poly(styrenesulfonic acid).

[0017] Alternatively, a mixture of electron-donating compounds such as aromatic amines, carbazole derivatives, or aromatic hydrocarbons, and an electron acceptor may be used. Examples of electron acceptors include transition metal oxides such as vanadium oxide and molybdenum oxide, nitrogen-containing heteroaromatic compounds, and aromatic compounds having strong electron-withdrawing groups such as cyano groups. The composition of the electron-donating compound in the hole-implanted layer 112 may be set, for example, to 1% by volume or more and 5% by volume or to 1% by volume or more and 1.5% by volume or less.

[0018] The hole injection layer 112 may have a single-layer structure or may be composed of multiple layers containing different materials. When multiple light-emitting elements 100 are provided on a substrate, the hole injection layer 112 can also be provided so as to be shared by the multiple light-emitting elements 100. In this case, the hole injection layer 112 exists within the same layer among the light-emitting elements 100 and is continuous without being interrupted between them. In other words, the configuration (composition, structure, thickness) of the hole injection layer 112 is the same among the multiple light-emitting elements 100.

[0019] (3) Hole transport layer The hole transport layer 114 is provided on the hole injection layer 112 and in contact with the hole injection layer 112. The hole transport layer 114 has the function of transporting holes injected into the hole injection layer 112 to the light-emitting layer 118, and can be made of the same or similar material as that which can be used in the hole injection layer 112. For example, a material can be used that has a deeper HOMO level compared to the hole injection layer 112, but the difference is about 0.5 eV or less. Typically, aromatic amines such as benzidine derivatives can be used. The hole transport layer 114 may also have a single-layer structure, or it may be composed of multiple layers containing different materials. When multiple light-emitting elements 100 are provided on a substrate, the hole transport layer 114 can also be provided so as to be shared by the multiple light-emitting elements 100. In this case, the hole injection layer 112 exists in the same layer among the multiple light-emitting elements 100 and is continuous among the light-emitting elements 100 without being separated. In other words, the composition (structure, thickness) of the hole transport layer 114 is the same among the multiple light-emitting elements 100.

[0020] (4) Electron Blocking Layer The electron blocking layer 116 is provided on the hole transport layer 114 so as to be in contact with the hole transport layer 114. The electron blocking layer 116 confines electrons within the light-emitting layer 118 by preventing 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. Furthermore, the electron blocking layer 116 has the function of preventing the excitation energy obtained in the light-emitting layer 118 from moving to the molecules of the hole transport layer 114. Due to these characteristics, a decrease in luminescence efficiency can be prevented.

[0021] The electron-blocking material contained in the electron-blocking layer 116 preferably has higher or equal hole transport properties than electron transport properties, and preferably has a shallower lowest unoccupied molecular orbital (LUMO) level and a larger band gap than the molecules in the light-emitting layer 118. Specifically, the difference between the LUMO level of the electron-blocking material and that of the molecules contained in the light-emitting layer 118 (specifically, the host material described later) is preferably 0.2 eV, 0.3 eV, or 0.5 eV or more. Also, the difference between the band gap of the electron-blocking material and that of the molecules contained in the light-emitting layer 118 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, etc. can be used as electron-blocking materials. The electron-blocking layer 116 may also have a single-layer structure or be composed of multiple layers containing different materials.

[0022] When multiple light-emitting elements 100 are provided on a substrate, the electron block layer 116 can also be provided so as to be shared by the multiple light-emitting elements 100. In this case, the electron block layer 116 exists within the same layer among the multiple light-emitting elements 100, is continuous without being divided among the light-emitting elements 100, and has the same configuration (composition, structure, and thickness) among the light-emitting elements 100.

[0023] (5) The light-emitting layer light-emitting element 100 is configured to emit light in the three primary colors of red, green, or blue. In any case, the light-emitting layer 118 contains a host material as the main component and a light-emitting material responsible for light emission, and the color of emission is controlled by the light-emitting material. Here, red emission, green emission, and blue emission refer to emission where the maximum emission peak wavelength is located at 650 nm to 750 nm, 500 nm to 650 nm, and 400 nm to 500 nm, respectively.

[0024] A. Light-emitting layer of a blue-emitting light-emitting element The light-emitting layer 118 provided on the blue-emitting light-emitting element 100 includes a host material and a blue-emitting fluorescent material as the light-emitting material. The volume ratio of the host material to the light-emitting material (light-emitting material / host material) may be, for example, 0.01 or more and 0.20 or less. As the host material, for example, in addition to zinc and 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.

[0025] As the light-emitting material, a blue-emitting fluorescent material that does not exhibit thermally activated delayed fluorescence (TADF) is used. Specifically, the maximum emission peak wavelength is located in the range of 400 nm to 500 nm, and the fluorescence lifetime is 10 －１２ seconds (1ps) or more 10 －９ Fluorescent materials with fluorescence times of less than 1 second (1 ns) are used. Examples of fluorescent materials include anthracene derivatives, stilbene derivatives, and pyrene derivatives.

[0026] Unlike the light-emitting element 100 that emits blue light, the light-emitting elements 100 that emit red and green light have light-emitting layers 118 that use thermally activated delayed fluorescence materials (thermally activated delayed fluorescence materials) that emit red and green light, respectively. Furthermore, the concentration of the light-emitting material (i.e., thermally activated delayed fluorescence material) in the light-emitting layer 118 is relatively high, and the volume ratio of host material to light-emitting material (light-emitting material / host material) is set to, for example, 0.30 or more and 0.60 or less. In thermally activated delayed fluorescence materials, the difference between the triplet excitation energy level and the singlet excitation energy level is small, for example, 5 meV or more and 20 meV or less. Therefore, the triplet excited state of the light-emitting material produced by carrier (hole and electron) recombination can inter-system crossover to the singlet excited state with extremely small thermal energy at or below room temperature. As a result, the rate of non-radiative deactivation of the triplet excited state is relatively reduced, and radiative deactivation from the singlet excited state is promoted. Due to this mechanism, thermally activated delayed fluorescence materials exhibit luminescence with a significantly longer lifetime while having a spectrum similar to that of ordinary fluorescence. The fluorescence lifetime of thermally activated delayed fluorescence materials is 10 －９ 1 second (1 ns) or longer, preferably 10 －６ The time interval is 1 μs or longer. Since the probability of generating a triplet excited state through hole-electron recombination is approximately three times that of a singlet excited state, the efficiency of the light-emitting element 100 can be dramatically improved by using a thermally activated delayed fluorescence material.

[0027] Examples of thermally activated delayed fluorescence materials include fullerenes and their derivatives, acridine derivatives such as proflavin, and eosin. Also, metal-containing porphyrins containing magnesium, zinc, cadmium, tin, platinum, indium, or palladium are used. 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.

[0028] Furthermore, compounds in which electron donor components and electron acceptor components are linked may be used as thermally activated delayed fluorescence materials. Examples of electron donor components and electron acceptor components include π-electron-rich heteroaromatic rings and π-electron-deficient heteroaromatic rings, respectively. Examples of basic skeletons for π-electron-deficient heteroaromatic rings include pyridine skeletons, diazine skeletons, and triazine skeletons. Examples of basic skeletons for π-electron-rich heteroaromatic rings include acridine skeletons, phenoxazine skeletons, phenothiazine skeletons, furan skeletons, thiophene skeletons, and pyrrole skeletons. Examples of such compounds include 2-(biphenyl-4-yl)-4,6-bis(12-phenylindoro[2,3-a]carbazole-11-yl)-1,3,5-triazine, 9-(4,6-diphenyl-1,3,5-triazine-2-yl)-9'-phenyl-9H,9'H-3,3'-bicarbazole, 9-[4-(4,6-diphenyl-1,3,5-triazine-2-yl)phenyl]-9'-phenyl-9H,9'H-3,3'-bicarbazole, and 2-[4-(10H-phenoxazine-10-yl)phenyl]-4,6-diphenyl-1,3,5-triazine.

[0029] Both or one of the light-emitting layers 118 provided on the light-emitting element 100 that emits red and green light may further include, as a light-emitting material, a fluorescent material (hereinafter also referred to as a second fluorescent material) capable of receiving the excitation singlet energy of the thermally activated delayed fluorescent material and forming a singlet excited state, in addition to the thermally activated delayed fluorescent material. The second fluorescent material is selected such that the energy level of its singlet excited state is lower than that of the thermally activated delayed fluorescent material and its band gap is smaller than that of the thermally activated delayed fluorescent material. Since the second fluorescent material does not exhibit thermally activated delayed fluorescence, it exhibits a relatively short fluorescence lifetime (e.g., 1 ps or more and less than 1 ns). Specifically, examples of fluorescent materials include coumarin derivatives, pyran derivatives, quinacridone derivatives, tetracene derivatives, pyrene derivatives, anthracene derivatives, and pyran derivatives. Generally, the emission spectrum exhibited by thermally activated delayed fluorescent materials is broader and has lower color purity compared to that of fluorescent materials that do not exhibit thermally activated delayed fluorescence. In contrast, the fluorescent material described above provides an emission spectrum with a relatively narrow half-width, enabling emission with high color purity. Therefore, by further adding the second fluorescent material to the light-emitting layer 118, it is possible to provide a light-emitting element 100 that not only has high luminescence efficiency due to the thermally activated delayed fluorescence material but also excellent color purity. As a result, a display device with high color reproducibility can be provided.

[0030] (6) Hole-blocking layer The hole-blocking layer 120 confines the holes within the light-emitting layer 118 by preventing the 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. The hole-blocking layer 120 further has the function of preventing the excitation energy obtained in the light-emitting layer 118 from moving to molecules in the electron transport layer 122. Due to these properties, a decrease in luminescence efficiency can be prevented.

[0031] The hole-blocking material constituting the hole-blocking layer 120 is a material that has higher or equal electron transport properties than hole transport properties, and has a deeper HOMO level and a larger band gap than the molecules in the light-emitting layer 118 (more specifically, the host material). Furthermore, a material having a LUMO level shallower than the LUMO level of the electron transport layer 122 is used. Specifically, the difference between the HOMO level of the hole-blocking material and that of the host material is preferably 0.2 eV, 0.3 eV, or 0.5 eV or more. Also, the difference between the band gap of the hole-blocking material and that of the host material is preferably 0.2 eV, 0.3 eV, or 0.5 eV or more. Specifically, examples 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-quinolinolate)(4-hydroxybiphenylyl)aluminum. The hole block layer 120 may have a single-layer structure, or it may be composed of multiple layers containing different materials.

[0032] When multiple light-emitting elements 100 are formed on a substrate, the hole block layer 120 can also be provided so as to be shared by the multiple light-emitting elements 100. In this case, the hole block layer 120 exists within the same layer among the light-emitting elements 100, is continuous without being divided among the light-emitting elements 100, and has the same composition and thickness among the light-emitting elements 100.

[0033] (7) Electron transport layer The electron transport layer 122 has the function of transporting electrons injected from the cathode 104 via the electron injection layer 124 to the light-emitting layer 118. As shown in Figure 1, the electron transport layer 122 comprises two layers (a first electron transport layer 122-1 and a second electron transport layer 122-2). The first electron transport layer 122-1 is located on the light-emitting layer 118 side. The second electron transport layer 122-2 is located on the cathode 104 side and is in contact with the first electron transport layer 122-1. Both the first electron transport layer 122-1 and the second electron transport layer 122-2 contain lithium (8-quinolinolate) (hereinafter referred to as Liq), which is a lithium complex exhibiting electron transport properties, and a material other than Liq that exhibits electron transport properties (hereinafter, materials other than Liq that exhibit electron transport properties are simply referred to as electron transport materials). The first electron transport layer 122-1 and the second electron transport layer 122-2 may both be made of Liq and an electron transport material.

[0034] As electron transport materials, compounds that are easily reduced (electron-accepting) can be used. In other words, compounds with shallow LUMO levels can be used. Examples include metal complexes other than (8-quinolinolato)lithium that contain ligands having benzoquinolinol as a basic skeleton, such as tris(8-quinolinolato)aluminum and tris(4-methyl-8-quinolinolato)aluminum, and metal complexes that contain ligands having oxadiazole or thiazole as a basic skeleton. In addition to these metal complexes, compounds having electron-deficient heteroaromatic rings, such as oxadiazole derivatives, thiazole derivatives, triazole derivatives, and phenanthroline derivatives, can be used. However, the electron transport material is selected such that its LUMO level is deeper than that of Liq. The difference in LUMO levels between the electron transport material and Liq is preferably 0.5 eV to 0.8 eV. Furthermore, it is preferable that the electron transport material is configured such that its HOMO level is deeper than that of the hole-blocking material, and the difference between them is 0.3 eV to 0.4 eV. Similarly, it is preferable that the electron transport material is configured such that its LUMO level is deeper than that of the hole-blocking material, and the difference between them is 0.4 eV to 0.7 eV. In addition, it is preferable that the electron mobility of the electron transport material is about the same as that of the hole-blocking material. Specifically, it is preferable that the electron mobility of the electron transport material is 0.1 to 10 times that of the hole-blocking material.

[0035] The composition ratio (volume ratio) of electron transport material to Liq differs between the first electron transport layer 122-1 and the second electron transport layer 122-2. Specifically, the electron transport layer 122 is configured such that the volume ratio of electron transport material to Liq (Liq / electron transport material) in the first electron transport layer 122-1 is lower than that of the second electron transport layer 122-2. In other words, the first electron transport layer 122-1 contains Liq at a lower volume ratio than the second electron transport layer 122-2. For example, the composition of Liq in the first electron transport layer 122-1 is greater than 0 volume%, 50 volume%, or 20 volume%, while the composition of Liq in the second electron transport layer 122-2 is greater than 50 volume%, 60 volume%, 70 volume%, or 80 volume%, or 80 volume%, or 60 volume%, or 60 volume%, or 70 volume%, or 80 volume%, or 6 Furthermore, the thicknesses of the first electron transport layer 122-1 and the second electron transport layer 122-2 are different, with the former being smaller than the latter. For example, the thickness of the first electron transport layer 122-1 is between 3 nm and 5 nm, and the thickness of the second electron transport layer 122-2 is between 25 nm and 27 nm.

[0036] When multiple light-emitting elements 100 are provided on a substrate, the electron transport layer 122 is shared by the multiple light-emitting elements 100 and is continuous without being interrupted between them. In other words, the configuration (composition, structure, and thickness) of the electron transport layer 122 is the same among the multiple light-emitting elements 100.

[0037] (8) Electron injection layer The electron injection layer 124 can be 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 (specifically, Liq and the electron transport material mentioned above) and an electron donor such as lithium or magnesium can be used. Alternatively, an inorganic compound such as lithium fluoride or calcium fluoride may be used. The electron injection layer 124 can also be provided so as to be shared by multiple light-emitting elements 100, without being interrupted between the light-emitting elements 100. In other words, the configuration (composition, structure, thickness) of the electron injection layer 124 may be the same among the multiple light-emitting elements 100.

[0038] (9) Other functional layers The light-emitting element 100 may also have a buffer layer in addition to the various functional layers described above. For example, as shown in Figure 2, a buffer layer (first buffer layer) 126 may be provided between the electron block layer 116 and the light-emitting layer 118, in contact with them. The first buffer layer 126 contains the host material contained in the light-emitting layer 118. In other words, the material contained in the first buffer layer 126 is the same as the host material contained in the light-emitting layer 118. Preferably, the first buffer layer 126 consists of the host material and substantially does not contain other components. The thickness of the first buffer layer 126 is relatively small, for example, 2.0 nm to 10 nm or 2.0 nm to 8.5 nm. By using the first buffer layer 126, an excellent carrier balance can be established in the light-emitting element 100, especially in a red-emitting light-emitting element 100 containing a thermally activated delayed fluorescence material as the light-emitting material, thereby obtaining high luminous efficiency and a low driving voltage. Furthermore, by providing the first buffer layer 126, an excellent carrier balance can be maintained even when many functional layers (for example, the hole block layer 120, the electron block layer 116, the electron transport layer 122, etc.) are formed to be shared among all the light-emitting elements 100. As a result, the number of deposition masks required to manufacture the display device described later is reduced, and as a result, it becomes possible to provide a display device at a lower cost.

[0039] Alternatively, as shown in Figure 3, the light-emitting element 100 may have a buffer layer (second buffer layer) 128 in contact with the hole-blocking layer 120 and the light-emitting layer 118. The thickness of the second buffer layer 128 is also relatively small, preferably 2 nm to 8.5 nm, or 2 nm to 5 nm. The second buffer layer 128 is configured such that the difference in LUMO levels between the second buffer layer 128 and the hole-blocking layer 120 is 0.1 eV to 0.3 eV, and the difference in LUMO levels between the second buffer layer 128 and the light-emitting layer 118 is 0.1 eV to 0.3 eV. Furthermore, the second buffer layer 128 is configured such that the difference in HOMO levels between the second buffer layer 128 and the hole block layer 120 is 0.1 eV or more and 0.3 eV or less, and the difference in HOMO levels between the second buffer layer 128 and the light-emitting layer 118 is 0.1 eV or more and 0.3 eV or less. For example, a material that satisfies the above-described relationship between HOMO levels and LUMO levels may be selected from the host material or hole block material described above.

[0040] Alternatively, the second buffer layer 128 may consist substantially of the host material contained in the light-emitting layer 118 and may not contain substantially any other components. In this case, since the second buffer layer 128 does not contain a material with a smaller band gap than the second buffer layer 128, the hole and electron injection characteristics are reduced compared to the light-emitting layer 118. As a result, the second buffer layer 128 can act as a resistive component between the hole-blocking layer 120 and the light-emitting layer 118.

[0041] Preferably, the second buffer layer 128 is selectively provided on the blue light-emitting element 100. By providing the second buffer layer 128, an appropriate energy barrier is formed between the hole block layer 120 and the light-emitting layer 118, improving the carrier balance of the light-emitting element 100. As a result, the driving voltage of the light-emitting element 100, especially the blue light-emitting element 100, can be reduced and the element lifespan can be improved.

[0042] Although not shown in the figure, similarly to the hole transport layer 114, the electron transport layer 122 may be configured such that the thickness increases in the order of the light-emitting elements 100-3, 100-2, and 100-1 that emit blue, green, and red light, respectively. Therefore, for example, the light-emitting element 100-3 that emits blue light may be provided with the electron transport layer 122 of a single-layer structure, the light-emitting element 100-2 that emits green light may be provided with the electron transport layer 122 of a two-layer structure, and the light-emitting element 100-1 that emits red light may be provided with the electron transport layer 122 of a three-layer structure. By adopting such a configuration, an appropriate resonator structure can be formed in each of the light-emitting elements 100.

[0043] As described above, the electron transport layer 122 of the light-emitting element 100 includes two electron transport layers (a first electron transport layer 122-1 and a second electron transport layer 122-2). In an electron transport layer 122 having such a two-layer structure, the LUMO level of Liq is shallower than the LUMO level of the electron transport material, so the LUMO level of the first electron transport layer 122-1 is deeper than the LUMO level of the second electron transport layer 122-2. As a result, the electron transport barrier between the electron transport layer 122 and the hole block layer 120 increases, which increases the number of electrons accumulated at the interface between the hole block layer 120 and the electron transport layer 122. Furthermore, by making the thickness of the second electron transport layer 122-2 greater than the thickness of the first electron transport layer 122-1, the diffusion of accumulated electrons can be prevented. Based on this principle, it becomes possible to precisely control the balance of carriers injected into the light-emitting layer 118. As a result, as described in the examples, the light-emitting element 100, particularly the blue-emitting light-emitting element 100, can have its lifespan significantly improved along with a reduction in the driving voltage without a decrease in luminous efficiency. Furthermore, in the green-emitting light-emitting element 100 containing a thermally activated delayed fluorescence material, the driving voltage can be reduced without affecting various characteristics such as luminous efficiency and lifespan. Moreover, in the red-emitting light-emitting element 100 containing a thermally activated delayed fluorescence material, the provision of the electron transport layer 122 with the above structure does not significantly affect the carrier balance, and therefore does not significantly affect its characteristics. Accordingly, by configuring a display device using red, green, and blue-emitting light-emitting elements 100 that share the electron transport layer 122 having the above structure, it is possible to provide a full-color display device that is low in power consumption and highly reliable.

[0044] <Second Embodiment> In this embodiment, a display device 200 equipped with the light-emitting element 100 described in the first embodiment will be described. Configurations that are similar to or identical to those described in the first embodiment may be omitted from the description.

[0045] 1. FIG. 4 is a schematic top view of an overall structure of a display device 200. As shown in FIG. 4, the display device 200 includes a substrate 202 and a counter substrate not shown in FIG. 4, and various patterned insulating films, semiconductor films, and conductor films are stacked between these substrates. By appropriately stacking these films, a plurality of pixels 210, drive circuits for driving the pixels 210 (a scanning line drive circuit 204 and a signal line drive circuit 206), a plurality of terminals 208 and the like are formed on the substrate 202. The substrate 202 and the counter substrate are fixed by an adhesive such as a sealing material (not shown), whereby the pixels 210, the scanning line drive circuit 204, and the signal line drive circuit 206 are sealed and protected. The terminals 208 are electrically connected to an external circuit (not shown) via a connector such as a flexible printed circuit (FPC) substrate. Various signals and power supplies for displaying images are supplied from the external circuit to the scanning line drive circuit 204 and the signal line drive circuit 206 via the terminals 208. Note that one or both of the scanning line drive circuit 204 and the signal line drive circuit 206 do not need to be formed directly on the substrate 202, and a drive circuit formed on a substrate different from the substrate 202 (such as a semiconductor substrate) may be provided on the substrate 202 or a connector as the scanning line drive circuit 204 and / or the signal line drive circuit 206.

[0046] A pixel circuit is formed in each pixel 210, and a light emitting element that emits light of three primary colors, that is, a light emitting element 100 that emits red, green, or blue light is further disposed. Based on various signals supplied from an external circuit, signals for operating the pixel circuits are generated by the scanning line drive circuit 204 and the signal line drive circuit 206, and supplied to each pixel 210. Accordingly, the light emitting element 100 connected to the pixel circuit emits light, and each pixel 210 functions as a minimum unit that provides color information. As a result, full-color display can be achieved.

[0047] 2. Pixel Structure FIG. 5 shows an equivalent circuit of the pixel 210. In this figure, equivalent circuit diagrams of three continuously arranged pixels 210 are shown. A plurality of scanning lines 220 and a plurality of video signal lines 222 extend from the scanning line drive circuit 204 and the signal line drive circuit 206, respectively. The 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.

[0048] There are no restrictions on the configuration of the pixel circuit 230; as shown in Figure 5, it is sufficient to have at least two transistors (switching transistor 232, driving transistor 234) and a retaining capacitance element 248. Therefore, although not shown, the pixel circuit 230 may further include one or more transistors and one or more capacitance elements. As shown in Figure 5, the gate of the switching transistor 232 is electrically connected to the scan 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 retaining capacitance element 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 held by 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 retaining capacitance element 248 and the pixel electrode corresponding to the anode 102 of the light-emitting element 100 described in the first embodiment. The cathode 104 of the light-emitting element 100 is electrically connected to the common line 226 and receives a constant potential. With this configuration, when the drive transistor 234 is ON, the current supplied from the current supply line 224 flows through the light-emitting element 100, and light can be emitted from each pixel 210.

[0049] Figure 6 shows schematic end views of the red-emitting light-emitting element 100-1, the green-emitting light-emitting element 100-2, and the blue-emitting light-emitting element 100-3, which are provided on pixels 210-1, 210-2, and 210-2, respectively, which emit red, green, and blue light. As shown in Figure 6, all functional layers of these light-emitting elements may have the same structure, except for the material contained in the light-emitting layer 118 and the thickness of the light-emitting layer 118. In the example shown here, in all light-emitting elements 100, the EL layer 110 is composed of a hole injection layer 112, a hole transport layer 114, an electron blocking layer 116, a light-emitting layer 118, a hole blocking layer 120, an electron transport layer 122, and an electron injection layer 124, and the electron transport layer 122 includes a first electron transport layer 122-1 and a second electron transport layer 122-2, as described in the first embodiment. The functional layers constituting the EL layer 110 are usually formed using a vapor deposition method. Therefore, by adopting the above configuration, it becomes possible to simultaneously form functional layers other than the light-emitting layer 118 across all pixels 210, thereby significantly reducing the number of metal masks used. This contributes to reducing the manufacturing cost of the display device 200.

[0050] Alternatively, as shown in Figure 7, the light-emitting element 100 may be formed such that the thickness of the hole transport layer 114 differs depending on the emission color of the light-emitting element 100. Specifically, the hole transport layer 114 may be configured such that its thickness increases as the emission wavelength of the light-emitting element 100 increases. In this case, first, a metal mask that exposes all the light-emitting elements 100 is used to simultaneously form the first hole transport layer 114-1 of all the light-emitting elements 100. Then, a metal mask that exposes the pixels 210 that emit red and green light is used to form the second hole transport layer 114-2 on the first hole transport layer 114-1 of the light-emitting elements 100-1 and 100-2. Furthermore, a metal mask that exposes the pixels 210 that emit red light is used to form the third hole transport layer 114-3 on the second hole transport layer 114-2 of the light-emitting element 100-1. The first hole transport layer 114-1, the second hole transport layer 114-2, and the third hole transport layer 114-3 may have the same composition or different compositions, but by using the same composition, the display device 200 can be manufactured more efficiently.

[0051] Light obtained from the light-emitting layer 118 travels isotropically and is extracted from the anode 102 and / or cathode 104, but in the process, it repeatedly reflects between the anode 102 and cathode 104. As a result, the anode 102 and cathode 104 form a resonator structure. Therefore, by adjusting the thickness of the hole transport layer 114 and appropriately adjusting the distance between the anode 102 and the light-emitting layer 118, the obtained light can be amplified by resonance, and the brightness in the front direction of the display device 200 can be increased. Since the distance required for resonance (optical distance) increases with increasing wavelength, by forming the hole transport layer 114 thicker in the order of light-emitting elements 100-3, 100-2, and 100-1 that emit blue, green, and red light, an appropriate resonator structure can be formed in each pixel 210.

[0052] Alternatively, as shown in Figure 8, the display device 200 may employ a light-emitting element 100 provided with the first buffer layer 126 and / or the second buffer layer 128 described in the first embodiment. Preferably, the first buffer layer 126 is provided on the red-emitting light-emitting element 100-1, and the second buffer layer 128 is provided on the blue-emitting light-emitting element 100-3. By adopting such a configuration, the carrier balance is optimized in each light-emitting element 100, making it possible to place highly efficient and reliable light-emitting elements 100 in each pixel 210. As a result, a highly reliable display device with reduced power consumption can be provided. Note that the first buffer layer 126 and / or the second buffer layer 128 do not need to be provided on the green-emitting light-emitting element 100-2. In this case, in the light-emitting element 100-2, the light-emitting layer 118 is in direct contact with the hole-blocking layer 120 and / or the electron-blocking layer 116.

[0053] Figure 9 shows a schematic end view of the display device 200. Figure 9 shows an example in which three consecutively arranged pixels 210-1, 210, and 210-3 are each provided with light-emitting elements 100-1, 100-2, and 100-3, as shown in Figure 8. As can be seen from Figure 9, the light-emitting elements 100 are arranged between the substrate 202 and the opposing substrate 212. In Figure 9, for ease of viewing, the hole injection layer 112, the hole transport layer 114, and the hole block layer 120 are shown together as a single layer. Although not shown, the thickness of the hole transport layer 114 may differ between the light-emitting elements 100.

[0054] The substrate 202 and the opposing substrate 212 are provided to give the display device 200 physical strength and to protect the multiple pixels 210, the scan line driving circuit 204, and the signal line driving circuit 206. The substrate 202 and the opposing substrate 212 may be inorganic material-containing substrates such as crystalline semiconductor substrates, glass substrates, or quartz substrates, or they may contain polymers such as polyimide, polyamide, or polycarbonate. The substrate 202 and the opposing substrate 212 may or may not be flexible. In the former case, the substrate 202 and / or the opposing substrate 212 may have flexibility to the extent that they can be elastically deformed, or they may have high flexibility that allows for plastic deformation. When light emitted from the light-emitting element 100 is extracted to the outside through the opposing substrate 212, at least the opposing substrate 212 is configured to transmit visible light. Conversely, when light emitted from the light-emitting element is extracted to the outside through the substrate 202, at least the substrate 202 is configured to transmit visible light.

[0055] As mentioned above, a known configuration can be applied as the pixel circuit, so a detailed explanation will be omitted. In the example shown in Figure 9, the drive transistor 234 is provided on the substrate 202. The drive transistor 234 may be provided directly on the substrate 202, or it may be formed on the substrate 202 via an undercoat 214 that prevents the diffusion of impurities contained in the substrate 202. The drive transistor 234 shown in Figure 9 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, 246 provided on the interlayer insulating film 242 and electrically connected to the semiconductor film 236. The drive transistor 234 shown here is a top-gate type transistor, but there are no restrictions on the structure of the drive transistor 234, and a bottom-gate type transistor or a transistor having gate electrodes on the top and bottom of the semiconductor film may also be used as the drive transistor 234.

[0056] A planarization film 256 is provided on the drive transistor 234 to absorb irregularities caused by elements such as the drive transistor 234 included in the pixel circuit and provide a flat surface. A capacitive electrode 250, a capacitive insulating film 252 on the capacitive electrode 250, and an anode 102 on the capacitive insulating film 252 can be arranged on the planarization film 256, and these constitute a retaining capacitance element 248. In this structure, the anode 102 is shared by the light-emitting element 100 and the retaining capacitance element 248. An opening is provided in the planarization film 256 that exposes the terminal 246, and the anode 102 is electrically connected to the terminal 246 at this opening, either directly or via a connecting electrode 254 that covers the opening. The anode 102 is also called a pixel electrode. A partition wall 258, which is an insulating film, is provided to cover the end of the anode 102, and the EL layer 110 and cathode 104 are arranged to cover the anode 102 and the partition wall 258. This electrically insulates adjacent light-emitting elements 100 and prevents the EL layer 110 from being cut by the edge of the anode 102.

[0057] In the example shown in Figure 9, the light-emitting element 100-1, which is located in the pixel 210-1 that emits red light, has a first buffer layer 126 between the electron blocking layer 116 and the light-emitting layer 118, and the light-emitting element 100-3, which is located in the pixel 210-3 that emits blue light, has a second buffer layer 128 between the hole blocking layer 120 and the light-emitting layer 118. Also, as described in the first embodiment, the electron transport layer 122 is formed to have a first electron transport layer 122-1 and a second electron transport layer 122-2.

[0058] As an optional configuration, a cap layer 262 may be provided on the cathode 104 to resonate the light extracted from the cathode 104 and improve color purity and brightness in the front direction. The cap layer 262 can be made of a material included in any functional layer that constitutes the EL layer 110. Alternatively, the cap layer 262 may be formed to include a polymer such as acrylic resin, epoxy resin, silicone resin, polytetrafluoroethylene, or fluorine-containing resin such as polyvinylidene fluoride. Alternatively, the cap layer 262 may include an inorganic compound, such as a metallic fluoride such as lithium fluoride, magnesium fluoride, or calcium fluoride. The cap layer 262 may have the same thickness in all light-emitting elements 100, or the cap layer 262 may be configured such that at least one of the cap layers 262 of light-emitting elements 100-1, 100-2, and 100-3 has a different thickness from that of the other one.

[0059] As an optional configuration, a protective film 270 may be provided on the light-emitting element 100 (or on the cap layer 262 if a cap layer 262 is provided) to prevent impurities such as water and oxygen from entering the EL layer 110. The protective film 270 can be formed from, for example, a film containing a silicon-containing inorganic compound such as silicon nitride, or a layer containing a polymer such as acrylic resin or epoxy resin. For example, as shown in Figure 9, the protective film 270 may be composed of a first layer 272 and a second layer 276 containing silicon nitride, and a polymer-containing layer 274 provided between them.

[0060] Typically, each functional layer constituting the EL layer 110 is formed using a vapor deposition method. Therefore, a metal mask is used to selectively arrange the functional layers in a predetermined area; however, increasing the number of metal masks directly leads to increased manufacturing costs for the display device. In the display device 200, however, all or part of the functional layers other than the light-emitting layer 118, the first buffer layer 126, and the second buffer layer 128 can be shared by all pixels 210 and provided simultaneously in a continuous manner across all pixels 210. Therefore, for example, all or part of the hole injection layer 112, hole block layer 120, electron block layer 116, electron transport layer 122, and electron injection layer 124 can be formed on all pixels 210 using the same metal mask, eliminating the need for separate coating using multiple metal masks. If the hole transport layer 114 has the same thickness across all light-emitting elements 100, the hole transport layer 114 can also be formed on all pixels 210 using the same metal mask. Furthermore, the first buffer layer 126 and the light-emitting layer 118 of the red-emitting light-emitting element 100-1 can also be formed from the same metal mask, and the light-emitting layer 118 and the second buffer layer 128 of the blue-emitting light-emitting element 100-3 can also be formed from the same metal mask. Therefore, by applying embodiments of the present invention, it is possible to prevent an increase in manufacturing costs due to an increase in the number of metal masks, and to provide a display device at a lower cost.

[0061] However, achieving an appropriate carrier balance in the light-emitting element 100, particularly in the red-emitting element 100-1 and the green-emitting element 100-2 which contain a thermally activated delayed fluorescence material, is relatively difficult, and the carrier balance is greatly influenced by the structure of the electron blocking layer 116 and hole blocking layer 120 located near the light-emitting layer 118. For this reason, if a common structure for the electron blocking layer 116 and hole blocking layer 120 is adopted for all light-emitting elements 100, the carrier balance of some of the light-emitting elements 100 will be disrupted, which is likely to lead to a decrease in the efficiency of the light-emitting elements 100, an increase in the driving voltage, and a decrease in the lifespan of the elements.

[0062] However, as described above, in the display device 200 according to one embodiment of the present invention, the electron transport layer 122 having a two-layer structure can be shared by all light-emitting elements 100 and formed to be continuous across all light-emitting elements 100. Furthermore, as shown in the examples, even if a thermally activated delayed fluorescence material is used in the light-emitting layer 118 of light-emitting elements 100-1 and 100-2 that emit red and green light respectively, the two-layer structure of the electron transport layer 122 described in the first embodiment allows for a reduction in the driving voltage of light-emitting element 100-2 without disrupting the carrier balance of light-emitting elements 100-1 and 100-2, and also allows for a reduction in the driving voltage of light-emitting element 100-3 and an improvement in element lifespan. Therefore, by applying embodiments of the present invention, a display device with low power consumption and high reliability can be manufactured at low cost.

[0063] 3. Method for Manufacturing the Display Device The display device 200 can be manufactured by sequentially laminating functional layers on a substrate 202 having pixel circuits and anodes 102, which has been manufactured by known methods. Specifically, a hole injection layer 112 is formed on the anodes 102 provided on the substrate 202 by spin coating, dip coating, inkjet, or vapor deposition. Thereafter, a hole transport layer 114 and an electron block layer 116 are sequentially formed by vapor deposition. When forming the hole transport layer 114 such that the thickness differs between the light-emitting elements 100-1, 100-2, and 100-3, as described above, a first hole transport layer 114-1 is formed using a metal mask that exposes all pixels 210, a second hole transport layer 114-2 is formed using a metal mask that exposes the pixels 210-1 and 210-2 that provide green and red, and a third hole transport layer 114-3 is formed using a metal mask that exposes the pixel 210-1 that provides red.

[0064] After this, the electron block layer 116 is formed by vapor deposition. Since color separation is not required at this stage, a metal mask that exposes all pixels 210 can be used, and the electron block layer 116 can be formed simultaneously on all pixels 210.

[0065] Subsequently, the light-emitting layers 118 of the light-emitting elements 100-1, 100-2, and 100-3 are formed using a vapor deposition method. At this stage, the light-emitting layers 118 are painted separately using a metal mask that exposes pixels 210-1, 210-2, or 210-3. When forming the first buffer layer 126, the first buffer layer 126 is formed on pixel 210-1 using a metal mask that exposes pixel 210-1, and then the light-emitting layer 118 is formed using the same metal mask. Similarly, when forming the second buffer layer 128, the light-emitting layer 118 is formed on pixel 210-3 using a metal mask that exposes pixel 210-3, and then the second buffer layer 128 is formed using the same metal mask. Therefore, when forming the first buffer layer 126 and light-emitting layer 118 of pixel 210-1, it is unnecessary to replace or realign the metal mask. Similarly, when forming the light-emitting layer 118 and the second buffer layer 128 of pixels 210-3, there is no need to replace or realign the metal mask.

[0066] Next, the hole block layer 120, electron transport layer 122, and electron injection layer 124 are formed by vapor deposition. Since no color separation is required at this stage, a metal mask that exposes all pixels 210 can be used to simultaneously form the hole block layer 120, electron transport layer 122, and electron injection layer 124 at all pixels 210. The first electron transport layer 122-1 and the second electron transport layer 122-2 that constitute the electron transport layer 122 are formed by co-depositing an electron transport material and Liq.

[0067] The cathode 104 is formed by vapor deposition or sputtering. For example, a cathode 104 containing a silver-magnesium alloy can be formed by vapor deposition, while a cathode 104 containing ITO or IZO can be formed by sputtering.

[0068] Thus, in the manufacturing method of the display device 200 according to one embodiment of the present invention, many functional layers, including an electron blocking layer 116, a hole blocking layer 120, and an electron transport layer 122, can be formed simultaneously in all pixels 210, thus reducing the number of metal masks. Furthermore, in a display device 200 in which light-emitting elements 100 containing thermally activated delayed fluorescence material and light-emitting elements 100 containing fluorescent material that does not exhibit thermally activated delayed fluorescence are mixed, even if an electron blocking layer 116 and a hole blocking layer 120 with the same structure are formed to be shared by all light-emitting elements 100, by using the two-layer electron transport layer 122 described in the first embodiment, it is possible to achieve a good carrier balance in all pixels 210, and in particular, it is possible to reduce the driving voltage of the green and blue light-emitting elements 100-2 and 100-3 while improving the element life of the blue light-emitting element 100-3.

[0069] 1. Fabrication and Characterization of Red-Emitting Light-Emitting Devices Red-emitting light-emitting devices (Examples 1 to 3 and Comparative Example 1) with different electron transport layer configurations were fabricated. In all light-emitting devices, ITO was used as the anode and a silver-magnesium co-evaporated film (thickness 13 nm) was used as the cathode. The size of the light-emitting region was 2.0 mm × 2.0 mm. The light-emitting layer contained a host material, a red-emitting thermally activated delayed fluorescence material, and a red-emitting fluorescent material that does not exhibit thermally activated delayed fluorescence, and lithium fluoride was used as the electron injection layer. The structure of the functional layers other than the electron transport layer was the same for all light-emitting devices. The thickness of the functional layers constituting the EL layer is shown in Table 1. As shown in Table 1, the electron transport layer of the light-emitting device of Comparative Example 1 has a single-layer structure and contains 40 volume% and 60 volume% of the electron transport material and Liq, respectively. In contrast, the electron transport layers of the light-emitting devices of Examples 1 to 3 have a two-layer structure. In the light-emitting elements of Examples 1 to 3, the composition of the second electron transport layer on the electron injection layer side is the same as that of the electron transport layer in Comparative Example 1, but the composition of the first electron transport layer on the light-emitting layer side is different from each other. Specifically, the Liq composition of the first electron transport layer in Examples 1 to 3 was 80% by volume, 40% by volume, and 20% by volume, respectively.

[0070]

[0071] The characteristics of these light-emitting elements are summarized in Table 2. As shown in Table 2, it can be seen that all light-emitting elements exhibit very high current efficiency and external quantum efficiency, due to the thermally activated delayed fluorescence material. Furthermore, no significant differences were observed in characteristics such as driving voltage, current efficiency, external quantum efficiency, and emission color. These results suggest that in red-emitting light-emitting elements that have a thermally activated delayed fluorescence material as the light-emitting material in the light-emitting layer, a good carrier balance is maintained without being significantly affected by the configuration of the electron transport layer. Although the element lifetime of the light-emitting elements in Examples 1 to 3 is lower than that of Comparative Example 1, it can still be seen from Table 2 that a very long element lifetime can be achieved.

[0072]

[0073] 2. Fabrication and Characterization of Green Emitting Light-Emitting Devices Green emitting light-emitting devices (Examples 4 to 6 and Comparative Example 2) with different electron transport layer configurations were fabricated. In all light-emitting devices, ITO was used as the anode and a silver-magnesium co-evaporated film (thickness 13 nm) was used as the cathode. The size of the light-emitting region was 2.0 mm × 2.0 mm. The light-emitting layer contained a host material, a green thermally activated delayed fluorescence material, and a green fluorescent material that does not exhibit thermally activated delayed fluorescence, and lithium fluoride was used as the electron injection layer. The configuration of the functional layers other than the electron transport layer was the same for all light-emitting devices. The thickness of the functional layers constituting the EL layer is shown in Table 3. As shown in Table 3, the electron transport layer of the light-emitting device of Comparative Example 2 has a single-layer structure and contains 40 volume% and 60 volume% of the electron transport material and Liq, respectively. In contrast, the electron transport layers of the light-emitting devices of Examples 4 to 6 have a two-layer structure. In the light-emitting elements of Examples 4 to 6, the composition of the second electron transport layer on the electron injection layer side is the same as that of the electron transport layer in Comparative Example 2, but the first electron transport layers on the light-emitting layer side are different. Specifically, the Liq compositions of the first electron transport layers in Examples 4 to 6 were 80% by volume, 40% by volume, and 20% by volume, respectively.

[0074]

[0075] The characteristics of these light-emitting elements are summarized in Table 4. As shown in Table 4, it can be seen that all light-emitting elements exhibit very high current efficiency and external quantum efficiency due to the thermally activated delayed fluorescence material. Furthermore, although no significant differences were observed in characteristics such as external quantum efficiency, emission color, and element lifetime, the driving voltage decreased in the order of Examples 4, 5, and 6, and the current efficiency increased in the same order. The current efficiency of the light-emitting elements in Examples 5 and 6, with Liq compositions of 40 vol% and 20 vol% respectively, was higher than that of Comparative Example 2. These results suggest that even in green-emitting light-emitting elements having a thermally activated delayed fluorescence material as the light-emitting material in the light-emitting layer, a good carrier balance can be maintained without being significantly affected by the configuration of the electron transport layer. In particular, it was confirmed that by reducing the Liq composition in the first electron transport layer (for example, setting it to 50 vol% or less), the current efficiency increased and the driving voltage and power consumption could be significantly reduced.

[0076]

[0077] 3. Fabrication and Characterization of Blue-Emitting Light-Emitting Devices Blue-emitting light-emitting devices (Examples 7 to 9 and Comparative Example 3) with different electron transport layer configurations were fabricated. In all light-emitting devices, ITO was used as the anode and a silver-magnesium co-evaporated film (thickness 13 nm) was used as the cathode. The size of the light-emitting region was 2.0 mm × 2.0 mm. The light-emitting layer contained a host material and a blue-emitting fluorescent material (i.e., a fluorescent material that does not exhibit thermally activated delayed fluorescence), and lithium fluoride was used as the electron injection layer. The configuration of the functional layers other than the electron transport layer was the same for all light-emitting devices. The thickness of the functional layers constituting the EL layer is shown in Table 5. As shown in Table 5, the electron transport layer of the light-emitting device of Comparative Example 3 has a single-layer structure and contains 40 volume% electron transport material and 60 volume% Liq, respectively. In contrast, the electron transport layers of the light-emitting devices of Examples 7 to 9 have a two-layer structure. In the light-emitting elements of Examples 7 to 9, the composition of the second electron transport layer on the electron injection layer side is the same as that of the electron transport layer in Comparative Example 3, but the composition of the first electron transport layer on the light-emitting layer side is different from each other. Specifically, the Liq composition of the first electron transport layer in Examples 7 to 9 was 80% by volume, 40% by volume, and 20% by volume, respectively.

[0078]

[0079] The characteristics of these light-emitting elements are summarized in Table 6. As can be seen from Table 6, although no significant differences were observed in characteristics such as current efficiency, external quantum efficiency, and emission color, it was found that the drive voltage tended to decrease as the Liq composition decreased (Examples 4, 5, and 6). Notably, the element lifetime improved significantly in the order of Examples 4, 5, and 6. These results indicate that in a blue light-emitting element having a fluorescent material that does not exhibit thermally activated delayed fluorescence as the light-emitting material in the light-emitting layer, it is possible to improve current efficiency and significantly improve element lifetime by adopting the two-layer electron transport layer structure described in the first embodiment. In particular, it was confirmed that the drive voltage decreased and the element lifetime could be significantly improved by reducing the Liq composition in the first electron transport layer (for example, setting it to 50 volume% or less).

[0080]

[0081] The above results show that the two-layer electron transport layer has almost no practical effect on the characteristics of the red light-emitting element having a thermally activated delayed fluorescence material, while reducing the driving voltage of the green light-emitting element having a thermally activated delayed fluorescence material and improving the driving voltage and element life of the blue light-emitting element having a fluorescent material that does not exhibit thermally activated delayed fluorescence. In other words, the two-layer electron transport layer can improve the characteristics of the blue light-emitting element while maintaining a good carrier balance in the red and green light-emitting elements having a thermally activated delayed fluorescence material. This tendency is particularly pronounced when the Liq composition of the first electron transport layer 122-1 is low (for example, 50 vol% or less, 40 vol% or less, or 20 vol% or less). Therefore, by using the light-emitting element according to the embodiment of the present invention, it is possible to provide a full-color display device with low power consumption and high reliability.

[0082] The embodiments described above as embodiments of the present invention can be combined and implemented as appropriate, insofar as they do not contradict each other. Furthermore, any additions, deletions, or design changes to components, or additions, omissions, or changes to processes based on these embodiments, made by those skilled in the art, are also included within the scope of the present invention, as long as they retain the essence of the present invention.

[0083] Any effects or benefits other than those brought about by the embodiments described above, if they are clear from the description herein or easily predictable to a person skilled in the art, are naturally considered to be brought about by the present invention.

[0084] 100: Light-emitting element, 100-1: Light-emitting element, 100-2: Light-emitting element, 100-3: 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, 114-3: Third hole transport layer, 116: Electron block layer, 118: Light-emitting layer, 120: Hole block layer, 122: Electron transport layer, 122-1: First electron transport layer, 122-2: Second electron transport layer, 124: Electron injection layer, 126: First buffer layer, 128: Second buffer layer, 200: Display device, 202: Substrate, 204: Scan line drive circuit, 206: Signal line drive circuit, 208: Terminal 210: Multiple pixels, 210: Pixel, 210-1: Pixel, 210-2: Pixel, 210-3: Pixel, 212: Opposing substrate, 214: Undercoat, 220: Scan 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: Holding capacitance element, 250: Capacitive electrode, 252: Capacitive insulating film, 254: Connecting electrode, 256: Planarization film, 258: Partition, 262: Cap layer, 270: Protective film, 272: First layer, 274: Layer, 276: Second layer

Claims

1. A light-emitting element comprising an anode, a light-emitting layer on the anode, a first electron transport layer on the light-emitting layer, a second electron transport layer located on the first electron transport layer, in contact with the first electron transport layer, and having a greater thickness than the first electron transport layer, and a cathode on the second electron transport layer, wherein both the first electron transport layer and the second electron transport layer contain an electron transport material and (8-quinolinolato)lithium, and the concentration of (8-quinolinolato)lithium in the first electron transport layer is lower than that in the second electron transport layer.

2. The light-emitting element according to claim 1, wherein the light-emitting layer includes a light-emitting material that exhibits fluorescence with a maximum emission peak wavelength of 400 nm or more and 500 nm or less, and a fluorescence lifetime of 1 ps or more and less than 1 ns.

3. The light-emitting element according to claim 1, wherein the light-emitting layer includes a light-emitting material that exhibits thermally activated delayed fluorescence with a maximum emission peak wavelength of 650 nm to 750 nm or 500 nm to 650 nm.

4. The light-emitting element according to claim 3, wherein the light-emitting layer further comprises a fluorescent material having a fluorescence lifetime of 1 ps or more and less than 1 ns.

5. The light-emitting element according to claim 1, wherein the concentration of (8-quinolinolato)lithium in the first electron transport layer is greater than 0 volume% and 50 volume% or less, and the concentration of (8-quinolinolato)lithium in the second electron transport layer is greater than 50 volume% and 80 volume% or less.

6. The light-emitting element according to claim 1, wherein the thickness of the first electron transport layer is 3 nm or more and 5 nm or less, and the thickness of the second electron transport layer is 25 nm or more and 27 nm or less.

7. The light-emitting element according to claim 1, wherein the electron transport material is selected from triazine derivatives, hexaphenylbenzene derivatives, benzimidazole derivatives, azine derivatives, aluminum complexes, zinc complexes, and lithium complexes other than (8-quinolinolato)lithium.

8. The light-emitting element according to claim 1, further comprising a hole-blocking layer located between the light-emitting layer and the first electron-transporting layer, in contact with the light-emitting layer and the first electron-transporting layer, and comprising a hole-blocking material, wherein the difference in the maximum occupied molecular orbital levels between the electron-transporting material and the hole-blocking material is 0.3 eV or more and 0.4 eV or less.

9. The light-emitting element according to claim 1, further comprising a hole-blocking layer located between the light-emitting layer and the first electron-transporting layer, in contact with the light-emitting layer and the first electron-transporting layer, wherein the difference in the minimum unoccupied molecular orbital levels between the hole-blocking material and the electron-transporting material is 0.4 eV or more and 0.7 eV or less.

10. A display device comprising a red light-emitting element, a green light-emitting element, and a blue light-emitting element, each of the red light-emitting element, the green light-emitting element, and the blue light-emitting element having an anode, a light-emitting layer on the anode, a first electron transport layer on the light-emitting layer, a second electron transport layer located on the first electron transport layer, in contact with the first electron transport layer, and having a greater thickness than the first electron transport layer, and a cathode located on the second electron transport layer and shared by the red light-emitting element, the green light-emitting element, and the blue light-emitting element, wherein both the first electron transport layer and the second electron transport layer contain an electron transport material and (8-quinolinolato)lithium, and the concentration of (8-quinolinolato)lithium in the first electron transport layer is lower than that in the second electron transport layer.

11. The display device according to claim 10, wherein the light-emitting layer of the blue light-emitting element includes a light-emitting material that exhibits fluorescence with a maximum emission peak wavelength of 400 nm or more and 500 nm or less, and a fluorescence lifetime of 1 ps or more and less than 1 ns.

12. The display device according to claim 10, wherein the light-emitting layers of the red light-emitting element and the green light-emitting element each contain a light-emitting material exhibiting thermally activated delayed fluorescence with maximum peak wavelengths of 650 nm to 750 nm and 500 nm to 650 nm, respectively.

13. The display device according to claim 12, wherein the light-emitting layer of the red light-emitting element further comprises a fluorescent material having a maximum emission peak wavelength of 650 nm or more and 750 nm or less, and a fluorescence lifetime of 1 ps or more and less than 1 ns.

14. The display device according to claim 12, wherein the light-emitting layer of the green light-emitting element further comprises a fluorescent material having a maximum emission peak wavelength of 500 nm or more and 650 nm or less, and a fluorescence lifetime of 1 ps or more and less than 1 ns.

15. The display device according to claim 10, wherein the first electron transport layer and the second electron transport layer are continuous across the red light-emitting element, the green light-emitting element, and the blue light-emitting element.

16. The display device according to claim 10, wherein in each of the red light-emitting element, the green light-emitting element, and the blue light-emitting element, the concentration of (8-quinolinolato)lithium in the first electron transport layer is greater than 0 volume% and 50 volume% or less, and the concentration of (8-quinolinolato)lithium in the second electron transport layer is greater than 50 volume% and 80 volume% or less.

17. The display device according to claim 10, wherein in each of the red light-emitting element, the green light-emitting element, and the blue light-emitting element, the thickness of the first electron transport layer is 3 nm or more and 5 nm or less, and the thickness of the second electron transport layer is 25 nm or more and 27 nm or less.

18. The display device according to claim 10, wherein in each of the red light-emitting element, the green light-emitting element, and the blue light-emitting element, the electron transport material is selected from triazine derivatives, hexaphenylbenzene derivatives, benzimidazole derivatives, azine derivatives, aluminum complexes, zinc complexes, and lithium complexes excluding (8-quinolinolato)lithium.

19. The display device according to claim 10, wherein each of the red light-emitting element, the green light-emitting element, and the blue light-emitting element is located between the light-emitting layer and the first electron transport layer, is in contact with the light-emitting layer and the first electron transport layer, and further comprises a hole-blocking layer containing a hole-blocking material, wherein the difference in the maximum occupied molecular orbital levels between the electron transporting material and the hole-blocking material is 0.3 eV or more and 0.4 eV or less.

20. The display device according to claim 10, wherein each of the red light-emitting element, the green light-emitting element, and the blue light-emitting element is located between the light-emitting layer and the first electron transport layer, is in contact with the light-emitting layer and the first electron transport layer, and further comprises a hole-blocking layer containing a hole-blocking material, wherein the difference in the minimum unoccupied molecular orbital level between the electron transporting material and the hole-blocking material is 0.4 eV or more and 0.7 eV or less.

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