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

Abstract

To provide a light-emitting element exhibiting high efficiency, and a display device including this light-emitting element.SOLUTION: A light-emitting element includes an anode, a cathode, and an electroluminescence layer between the anode and the cathode. The electroluminescence layer includes an electron block layer, a buffer layer on the electron block layer, and a light-emitting layer existing on the buffer layer and including a host material and a first light-emitting material exhibiting thermal-active delay fluorescence. The buffer layer includes the host material. The light-emitting layer may further include a second light-emitting material. The energy level of the second light-emitting material in a singlet excited state is lower than the energy level of the first light-emitting material in the singlet excited state.SELECTED DRAWING: Figure 1

Description

[Technical field]

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

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

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

[0004] An object of one embodiment of the present invention is to provide a light-emitting element having a 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 reliable light-emitting element exhibiting high efficiency and a display device having the light-emitting element at low cost. [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 includes an electron blocking layer, a buffer layer on the electron blocking layer, and an emitting layer located on the buffer layer and including a host material and a first emitting material exhibiting thermally activated delayed fluorescence. The buffer layer includes the host material.

[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 of the red-emitting pixel, the green-emitting pixel, and the blue-emitting pixel has a pixel electrode, an electroluminescent layer on the pixel electrode, and a cathode on the electroluminescent layer, and the electroluminescent layer includes an electron blocking layer and an emitting layer on the electron blocking layer. The electroluminescent layer of the red-emitting pixel includes a first host material and a first red-emitting material exhibiting thermally activated delayed fluorescence. The red-emitting pixel further includes a first buffer layer including the first host material between the electron blocking layer and the emitting layer. The electron blocking layer is shared by the red-emitting pixel, the green-emitting pixel, and the blue-emitting pixel, and is continuous across the red-emitting pixel, the green-emitting pixel, and the blue-emitting pixel. [Brief description of the drawings]

[0007] [Figure 1] FIG. 1 is a schematic end view of a light-emitting device according to an embodiment of the present invention. [Diagram 2] FIG. 1 is a schematic top view of a display device according to an embodiment of the present invention. [Figure 3A] FIG. 1 is a schematic top view of a display device according to an embodiment of the present invention. [Figure 3B] FIG. 1 is a schematic top view 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 pixel of a display device according to an embodiment of the present invention. [Diagram 5] FIG. 1 is a schematic end view of a display device according to an embodiment of the present invention. [Figure 6] FIG. 2 is a schematic end view of a light-emitting element provided in a pixel of a display device according to an embodiment of the present invention. [Figure 7]FIG. 2 is a schematic end view of a light-emitting element provided in a pixel of a display device according to an embodiment of the present invention. [Figure 8] FIG. 2 is a schematic end view of a light-emitting element provided in a pixel of a display device according to an embodiment of the present invention. [Figure 9A] 4 shows voltage-current density curves of the light-emitting elements fabricated in the examples. [Figure 9B] 1 is a plot showing the relationship between the thickness of the buffer layer and the driving voltage of the light-emitting device fabricated in the examples. [Figure 10A] 1 is a plot showing the relationship between the thickness of the buffer layer and the current efficiency of the light-emitting device fabricated in the examples. [Figure 10B] 4 shows current density-normalized external quantum efficiency curves of the light-emitting elements fabricated in the examples. [Figure 11A] 4 shows voltage-capacity curves of light-emitting elements fabricated in Examples. [Figure 11B] 4 shows voltage-capacity curves of light-emitting elements fabricated in Examples. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

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

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

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

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

[0012] First Embodiment In this embodiment, 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.

[0013] 1.Overall structure A schematic end view of a light-emitting element 100 is shown in FIG. 1. As shown in FIG. 1, the light-emitting element 100 includes 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 a plurality of functional layers including an organic compound. The EL layer 110 includes an emitting layer 120 that emits light, as well as an electron blocking layer 116 and a carrier balance adjustment layer (hereinafter also referred to as a buffer layer) 118 that are disposed between the emitting layer 120 and the anode 102. The EL layer 110 may further include a hole injection layer 112, a hole transport layer 114, a hole blocking layer 122, an electron transport layer 124, an electron injection layer 126, and the like. By creating a voltage difference between the anode 102 and the cathode 104, holes and electrons are injected from the anode 102 and the cathode 104, respectively, into the EL layer 110, and these carriers recombine in the light-emitting layer 120. The light-emitting material present in the light-emitting layer 120 is excited by the recombination of holes and electrons, and the energy released when this excited state returns to the ground state can be extracted as light. Each component will be described below.

[0014] 2. 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 metal with high reflectivity, such as silver or aluminum, or an alloy thereof. For example, the anode 102 may have a configuration in which a film containing a metal is sandwiched between films containing a conductive oxide.

[0015] The cathode 104 is an electrode that injects electrons into the EL layer 110. When the light obtained in the EL layer 110 is extracted through the anode 102, the cathode 104 also functions as a reflective electrode, and therefore the cathode 104 is configured to contain the above-mentioned metal or alloy (for example, an alloy of silver and a metal having a small work function, such as magnesium). Conversely, when the 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 (for example, 5 nm or more and 20 nm or less) that can transmit visible light may be used as the cathode 104. In the latter case, a conductive oxide film that transmits visible light may be provided on the metal-containing film.

[0016] 3.Hole injection layer The hole injection layer 112 has a function of promoting hole injection from the anode 102 to the EL layer 110. The hole injection layer 112 can be made of a compound that is easy to inject holes into, that is, is easily oxidized (electron donating). In other words, a compound with a shallow highest occupied molecular orbital (HOMO) level can be used. For example, aromatic amines such as benzidine derivatives and triarylamines, carbazole derivatives, thiophene derivatives, phthalocyanine derivatives such as copper phthalocyanine, and the like can be used. Alternatively, polymer materials such as polythiophene, polyaniline, and derivatives thereof can be used, and an example thereof is poly(ethylenedioxythiophene) / poly(styrenesulfonic acid). A mixture of an electron donating compound such as the aromatic amines or carbazole derivatives or aromatic hydrocarbons described above 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 having strong electron-withdrawing groups such as cyano groups. The hole injection layer 112 may have a single layer structure or may be made of multiple layers containing different materials.

[0017] 4.Hole transport layer The hole transport layer 114 is provided so as to be in contact with the hole injection layer 112. The hole transport layer 114 has a function of transporting holes injected into the hole injection layer 112 to the light emitting layer 120 side, and may be made of the same or similar material as that 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 about 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] 5.Electron Blocking Layer The electron blocking layer 116 is provided so as to be in contact with the hole transport layer 114. The electron blocking layer 116 has a function of preventing electrons injected from the cathode 104 from passing through the light emitting layer 120 and being injected into the hole transport layer 114 without contributing to recombination in the light emitting layer 120, thereby confining the electrons within the light emitting layer 120, and also prevents the excitation energy obtained in the light emitting layer 120 from being transferred to the molecules in the hole transport layer 114. This makes it possible to prevent a decrease in luminous efficiency.

[0019] For the electron blocking layer 116, it is preferable to use a material having a hole transporting property higher than or equal to the electron transporting property, a lowest unoccupied molecular orbital (LUMO) level shallower than the molecules in the light emitting layer 120, and a large band gap. Specifically, the difference between the LUMO level of the molecules in the electron blocking layer 116 and that of the molecules in the light emitting layer 120 is preferably 0.2 eV, 0.3 eV, or 0.5 eV or more. Also, the difference between the band gap of the molecules in the electron blocking layer 116 and that of the molecules in the light emitting layer 120 is preferably 0.2 eV, 0.3 eV, or 0.5 eV or more. Specifically, aromatic amine derivatives, carbazole derivatives, 9,10-dihydroacridine derivatives, benzofuran derivatives, benzothiophene derivatives, and the like can be used for the electron blocking layer 116. The electron blocking layer 116 may also have a single layer structure, or may be composed of multiple layers containing different materials.

[0020] 6. Buffer layer The buffer layer 118 is a functional layer for adjusting the carrier balance of the light-emitting element 100, and is provided so as to be in contact with the electron blocking layer 116. As will be described later, by adjusting the carrier balance using the buffer layer 118, it is possible to improve the reliability without impairing the characteristics of the light-emitting element 100. Furthermore, when the light-emitting element 100 is applied to a display device, it is possible to provide the display device at a lower cost.

[0021] The buffer layer 118 contains the host material contained in the light-emitting layer 120 described below. In other words, the material contained in the buffer layer 118 is the same as the host material contained in the light-emitting layer 120. Preferably, the buffer layer 118 is made of the host material and does not substantially contain other components. The thickness of the buffer layer 118 is relatively small, for example, from 2.0 nm to 10 nm, or from 2.0 nm to 8.5 nm.

[0022] 7.Emitting layer The light-emitting layer 120 contains a host material as a main component and also contains a light-emitting material that is responsible for emitting light. The volume ratio of the host material to the light-emitting material (light-emitting material / host material) may be, for example, 0.30 or more and 0.6 or less. As the host material, various compounds can be used depending on the emission wavelength of the light-emitting material. For example, as the host material, in addition to zinc or aluminum-based metal complexes, oxadiazole derivatives, triazole derivatives, benzimidazole derivatives, quinoxaline derivatives, dibenzoquinoxaline derivatives, dibenzothiophene derivatives, dibenzofuran derivatives, pyrimidine derivatives, triazine derivatives, pyridine derivatives, bipyridine derivatives, phenanthroline derivatives, aromatic amine derivatives, carbazole derivatives, etc. can be used.

[0023] As the light-emitting material, a material exhibiting thermally activated delayed fluorescence (TADF) (thermally activated delayed fluorescent material) is used. In the thermally activated delayed fluorescent material, the difference between the triplet excited energy level and the singlet excited energy level is small, for example, 5 meV to 20 meV. Therefore, the triplet excited state of the light-emitting material generated by carrier recombination can undergo intersystem crossing to the singlet excited state by extremely small thermal energy at room temperature or lower. As a result, the rate of thermal deactivation of the triplet excited state is relatively decreased, and radiative deactivation from the singlet excited state is promoted. This makes it possible to dramatically improve the efficiency of the light-emitting element 100. Since light emission occurs by such a mechanism, the thermally activated delayed fluorescent material exhibits light emission with a spectrum similar to that of normal fluorescence, but with a remarkably long life. The fluorescence life of the thermally activated delayed fluorescent material is 10 -6 seconds (1 ns) or more, preferably 10 -3seconds (1μs) or more.

[0024] There is no restriction on the color of the light emitted by the thermally activated delayed fluorescent material, and a thermally activated delayed fluorescent material that emits blue light, green light, or red light can be used.Preferably, the thermally activated delayed fluorescent material emits green light or red light, and more preferably emits red light.Here, the blue light emission is light emission whose maximum emission peak wavelength is located in the range of 400 nm to 500 nm, the green light emission is light emission whose maximum emission peak wavelength is located in the range of 500 nm to 650 nm, and the red light emission is light emission whose maximum emission peak wavelength is located in the range of 650 nm to 750 nm.

[0025] Examples of the thermally activated delayed fluorescent material include fullerene and its derivatives, acridine derivatives such as proflavine, and eosin. Examples of the metal-containing porphyrin include magnesium, zinc, cadmium, tin, platinum, indium, or palladium. Examples of the metal-containing porphyrin include protoporphyrin-tin fluoride complex, mesoporphyrin-tin fluoride complex, hematoporphyrin-tin fluoride complex, coproporphyrin tetramethyl ester-tin fluoride complex, octaethylporphyrin-tin fluoride complex, etioporphyrin-tin fluoride complex, and octaethylporphyrin-platinum chloride complex.

[0026] Furthermore, a compound in which an electron donor component and an electron acceptor component are linked may be used. Examples of the electron donor component and the electron acceptor component include a π-electron rich heteroaromatic ring and a π-electron deficient heteroaromatic ring, 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. Examples of such compounds include 2-(biphenyl-4-yl)-4,6-bis(12-phenylindolo[2,3-a]carbazol-11-yl)-1,3,5-triazine, 9-(4,6-diphenyl-1,3,5-triazin-2-yl)-9'-phenyl-9H,9'H-3,3'-bicarbazole, 9-[4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl]-9'-phenyl-9H,9'H-3,3'-bicarbazole, and 2-[4-(10H-phenoxazin-10-yl)phenyl]-4,6-diphenyl-1,3,5-triazine.

[0027] The light-emitting layer 120 may further include, as the light-emitting material, a fluorescent material (hereinafter also referred to as a second light-emitting material) capable of receiving the excited singlet energy of the thermally activated delayed fluorescent material to form a singlet excited state. The second light-emitting material is selected so that the energy level of its singlet excited state is lower than that of the thermally activated delayed fluorescent material, that is, so that the band gap is smaller than that of the thermally activated delayed fluorescent material. The second light-emitting material does not exhibit thermally activated delayed fluorescence in the light-emitting element 100, and therefore exhibits a relatively short fluorescence lifetime (for example, 1 ps or more and less than 1 ns). Specific examples of the second light-emitting material include fluorescent materials such as coumarin derivatives, pyran derivatives, quinocridone derivatives, tetracene derivatives, pyrene derivatives, anthracene derivatives, and pyran derivatives. In general, the emission spectrum exhibited by the thermally activated delayed fluorescent material is broad and has low color purity. In contrast, the above-mentioned fluorescent materials provide an emission spectrum with a relatively narrow half-width, and therefore can emit light with high color purity. Therefore, by further adding the second light-emitting material to the light-emitting layer 120, it is possible to provide the light-emitting device 100 having not only high luminous efficiency due to the thermally activated delayed fluorescent material but also excellent color purity.

[0028] 8. Hole Block Layer The hole blocking layer 122 has a function of preventing holes injected from the anode 102 from passing through the light-emitting layer 120 and being injected into the electron transport layer 124 without contributing to recombination, thereby confining the holes within the light-emitting layer 120, and also prevents the excitation energy obtained in the light-emitting layer 120 from being transferred to molecules in the electron transport layer 124. This makes it possible to prevent a decrease in luminous efficiency.

[0029] For the hole blocking layer 122, it is preferable to use a material having a higher or equal electron transporting property than a hole transporting property, a deeper HOMO level than the molecules in the light emitting layer 120, and a larger band gap. Specifically, the difference between the HOMO level of the molecules in the hole blocking layer 122 and that of the molecules in the light emitting layer 120 is preferably 0.2 eV, 0.3 eV, or 0.5 eV or more. Also, the difference between the band gap of the molecules in the hole blocking layer 122 and that of the molecules in the light emitting layer 120 is preferably 0.2 eV, 0.3 eV, or 0.5 eV or more. Specifically, metal complexes having a relatively large band gap (for example, 2.8 eV or more), such as phenanthroline derivatives, oxadiazole derivatives, triazole derivatives, and bis(2-methyl-8-quinolinolato)(4-hydroxy-biphenylyl)aluminum, may be used. The hole blocking layer 122 may also have a single layer structure, or may be composed of multiple layers containing different materials.

[0030] 9.Electron transport layer The electron transport layer 124 has a function of transporting electrons injected from the cathode 104 through the electron injection layer 126 to the light emitting layer 120. The electron transport layer 124 may be made of a compound that is easily reduced (electron accepting). In other words, a compound with a shallow LUMO level may be used. For example, metal complexes containing a ligand having benzoquinolinol as a basic skeleton, such as tris(8-quinolinolato)aluminum and tris(4-methyl-8-quinolinolato)aluminum, and metal complexes containing a ligand having oxadiazole or thiazole as a basic skeleton may be used. In addition to these metal complexes, compounds having an electron-deficient heteroaromatic ring, such as oxadiazole derivatives, thiazole derivatives, triazole derivatives, and phenanthroline derivatives, may be used. The electron transport layer 124 may also have a single layer structure, or may be made of multiple layers containing different materials.

[0031] 10.Electron injection layer The electron injection layer 126 may be made of a compound that promotes electron injection from the cathode 104 to the electron transport layer 124. For example, a mixture of a compound that can be used in the electron transport layer 124 and an electron donor such as lithium or magnesium may be used. Alternatively, an inorganic compound such as lithium fluoride or calcium fluoride may be used.

[0032] As described above, in the light-emitting device 100, between the light-emitting layer 120 containing a host material and a thermally activated delayed fluorescent material and the electron blocking layer 116, the buffer layer 118 containing the host material or made of the host material is provided. As shown in the examples, by adopting this configuration, it is possible to maintain an appropriate carrier balance in the light-emitting device 100 without depending heavily on the material of the electron blocking layer 116, and as a result, the design freedom of the display device including the light-emitting device 100 is improved. As will be described later, such characteristics contribute to reducing the manufacturing cost of the display device. In addition, the above configuration can improve the light-emitting life (i.e., reliability) of the light-emitting device 100 without impairing the characteristics of the light-emitting device 100, so that a highly reliable display device can be provided at low cost.

[0033] <Second embodiment> In this embodiment, a display device 200 including the light emitting device 100 described in the first embodiment will be described. Descriptions of configurations that are the same as or similar to those described in the first embodiment may be omitted.

[0034] 1.Overall structure A schematic top view of the display device 200 is shown in FIG. 2. As shown in FIG. 2, the display device 200 includes a substrate 202, on which various insulating films, semiconductor films, and conductive films are patterned and laminated. By appropriately laminating these films, a plurality of pixels 210 and driving circuits (scanning line driving circuit 204, signal line driving circuit 206) for driving the pixels 210 are formed on the substrate 202. An opposing substrate (not shown in FIG. 2) is provided on the pixels 210, the scanning line driving circuit 204, and the signal line driving circuit 206. The substrate 202 and the opposing substrate are fixed by an adhesive such as a sealant, thereby sealing and protecting the pixels 210, the scanning line driving circuit 204, and the signal line driving circuit 206. A plurality of terminals 208 formed of a conductive film are provided on the substrate 202, and the terminals 208 are electrically connected to an external circuit (not shown) via a connector such as a flexible printed circuit (FPC) board. Various signals and power sources for displaying images are supplied from an external circuit to the scanning line driver circuit 204 and the signal line driver circuit 206 via terminals 208. Note that either or both of the scanning line driver circuit 204 and the signal line driver circuit 206 do not need to be formed directly on the substrate 202, and a driver circuit formed on a substrate (such as a semiconductor substrate) different from the substrate 202 may be provided on the substrate 202 or a connector.

[0035] Each pixel 210 is formed with a pixel circuit, and further includes any one of the three primary color light-emitting elements (i.e., red light-emitting element, green light-emitting element, and blue light-emitting element). Based on various signals supplied from an external circuit, a signal for operating the pixel circuit is generated by the scanning line driving circuit 204 and the signal line driving circuit 206. This causes the light-emitting element connected to the pixel circuit to emit light, and each pixel 210 functions as a minimum unit for providing color information. As a result, full color display is possible. Here, the red light-emitting element, the green light-emitting element, and the blue light-emitting element refer to elements that have emission peak wavelengths in the ranges of 650 nm to 750 nm, 500 nm to 650 nm, and 400 nm to 500 nm, respectively.

[0036] There is no restriction on the arrangement of the pixels 210. For example, as shown in FIG. 3A, a stripe arrangement may be adopted in which a red light-emitting pixel 210-1, a green light-emitting pixel 210-2, and a blue light-emitting pixel 210-3 that respectively provide red, green, and blue colors are arranged in order in the row direction, and pixels 210 that provide the same emission color are arranged in the same column. Alternatively, although not shown, various arrangements such as a mosaic arrangement in which a red light-emitting pixel 210-1, a green light-emitting pixel 210-2, and a blue light-emitting pixel 210-3 are arranged in order in both the row direction and the column direction, a delta arrangement, a pentile arrangement, and the like may be adopted. Alternatively, as shown in FIG. 3B, a plurality of pixels 210 may be arranged such that one or more red light-emitting pixels 210-1 and one or more green light-emitting pixels 210-2 are sandwiched between adjacent blue light-emitting pixels 210-3. In this case, by arranging the multiple pixels 210 so that the area of ​​the blue light-emitting pixel 210-3, in which the blue light-emitting element with the lowest light-emitting efficiency is arranged, is larger than the area of ​​the other pixels 210, the burden on the blue light-emitting element can be reduced and the reliability of the display device 200 can be improved.

[0037] The light-emitting element 100 described in the first embodiment is disposed in at least one of the red light-emitting pixel 210-1, the green light-emitting pixel 210-2, and the blue light-emitting pixel 210-3. For example, as shown in FIG. 4, the light-emitting element 100 may be disposed in the red light-emitting pixel 210-1, and a light-emitting element having a structure different from that of the light-emitting element 100 may be disposed in the green light-emitting pixel 210-2 and the blue light-emitting pixel 210-3. For example, a light-emitting element not having a buffer layer 118 may be disposed in the green light-emitting pixel 210-2 and the blue light-emitting pixel 210-3. In this case, the electron blocking layer 116 may also be provided in the green light-emitting pixel 210-2 and the blue light-emitting pixel 210-3, but the light-emitting layer 120 in the green light-emitting pixel 210-2 and the blue light-emitting pixel 210-3 is electrically and physically in contact with the electron blocking layer 116. Alternatively, a light-emitting element including a fluorescent material or a phosphorescent material (not exhibiting activated delayed fluorescence) in the light-emitting layer 120 may be disposed in the green light-emitting pixel 210-2 and the blue light-emitting pixel 210-3.

[0038] FIG. 5 shows a schematic diagram of an end surface along the dashed line AA′ in FIG. 3A. In FIG. 5, the end surfaces of the red light-emitting pixel 210-1, the green light-emitting pixel 210-2, and the blue light-emitting pixel 210-3, which are successively arranged, are shown. The configuration of the pixel circuit formed in each pixel 210 can be determined arbitrarily, and a known configuration may be applied. In the example shown in FIG. 5, one transistor 220 and a capacitance element (auxiliary capacitance element) 240 connected to the transistor 220 are shown as part of the elements constituting the pixel circuit. Furthermore, a light-emitting element connected to the pixel circuit is provided in each pixel 210. In the example shown in FIG. 5, the combination shown in FIG. 4 is applied. That is, the light-emitting element 100 is arranged in the red light-emitting pixel 210-1, and light-emitting elements not having a buffer layer 118 are arranged in the green light-emitting pixel 210-2 and the blue light-emitting pixel 210-3. Here, an example is shown in which light from the light-emitting layer 120 is extracted through the opposing substrate 250 in all the light-emitting elements, but the display device 200 may be configured so that light from the light-emitting layer 120 is extracted through the substrate 202.

[0039] 2. Substrate and opposing substrate The substrate 202 and the counter substrate 250 are provided to provide physical strength to the display device 200 and to protect the pixels 210, the scanning line driving circuit 204, and the signal line driving circuit 206. The substrate 202 and the counter substrate 250 may be substrates containing inorganic materials such as crystalline semiconductor substrates, glass substrates, and quartz substrates, or may contain polymers such as polyimide, polyamide, and polycarbonate. The substrate 202 and the counter substrate 250 may be flexible or may not be flexible. In the former case, the substrate 202 and / or the counter substrate 250 may be flexible enough to be elastically deformed, or may be highly flexible enough to be plastically deformed. When light emitted from the light emitting element is taken out through the counter substrate 250, at least the counter substrate 250 is configured to transmit visible light. Conversely, when light emitted from the light emitting element is taken out through the substrate 202, at least the substrate 202 is configured to transmit visible light.

[0040] 3. Pixel circuit As described above, a known configuration can be applied to the pixel circuit, so detailed description will be omitted. In the example shown in FIG. 5, a transistor 220 functioning as a driving transistor is provided on a substrate 202. The transistor 220 may be provided directly on the substrate 202, or may be formed on the substrate 202 via an undercoat 212 that prevents diffusion of impurities contained in the substrate 202. The transistor 220 shown in FIG. 5 is composed of a semiconductor film 222, a gate insulating film 224 on the semiconductor film 222, a gate electrode 226 on the gate insulating film 224, an interlayer insulating film 228 on the gate electrode 226, and a pair of terminals 230 and 232 provided on the interlayer insulating film 228 and electrically connected to the semiconductor film 222. The transistor 220 shown here is a top-gate transistor, but there is no restriction on the structure of the transistor 220, and a bottom-gate transistor or a transistor having gate electrodes above and below the semiconductor film may be used as the transistor 220.

[0041] A planarization film 236 is provided on the transistor 220 to absorb unevenness caused by elements such as the transistor 220 included in the pixel circuit and provide a flat surface. A capacitance electrode 242, a capacitance insulating film 244 on the capacitance electrode 242, and a pixel electrode 246 can be arranged on the planarization film 236, and these can form a capacitance element 240. Here, the pixel electrode 246 functions as the anode 102 of the light-emitting element 100. An opening for exposing the terminal 232 is provided in the planarization film 236, and the pixel electrode 246 is electrically connected to the terminal 232 through this opening, either directly or via a connection electrode 234 covering this opening. A partition wall 238, which is an insulating film, is provided to cover the end of the pixel electrode 246, and the EL layer 110 is arranged to cover the pixel electrode 246 and the partition wall 238. This electrically insulates adjacent light-emitting elements 100, and prevents the EL layer 110 from being cut by the end of the pixel electrode 246. In the example shown in FIG. 5, the pixel electrode 246 is shared by the light emitting element 100 and the capacitor element 240 .

[0042] 4. Light-emitting element As described above, the light-emitting element 100 is disposed in the red light-emitting pixel 210-1, and light-emitting elements not having the buffer layer 118 are disposed in the green light-emitting pixel 210-2 and the blue light-emitting pixel 210-3. Therefore, all or a part of the functional layers other than the light-emitting layer 120 and the buffer layer 118 can be provided so as to be shared by all the pixels 210 and to be continuous across all the pixels 210. For example, as shown in FIG. 5, the electron blocking layer 116 can be formed so as to be shared by all the pixels 210 and to be continuous across all the pixels. As shown in FIG. 5, in addition to the electron blocking layer 116, the hole injection layer 112, the hole transport layer 114, the hole blocking layer 122, the electron transport layer 124, the electron injection layer 126, and the cathode 104 may also be provided so as to be shared by all the pixels 210 and to be continuous across all the pixels.

[0043] Usually, each functional layer constituting the EL layer 110 is formed by using a vapor deposition method. For this reason, a metal mask is used to selectively arrange the functional layer in a predetermined region, but an increase in the number of metal masks directly leads to an increase in the manufacturing cost of the display device. However, in the display device 200, all or a part of the functional layers other than the light-emitting layer 120 and the buffer layer 118 can be provided so as to be continuous across all pixels so as to be shared by all pixels 210. For this reason, for example, the electron blocking layer 116 can be formed on all pixels 210 using a single metal mask, and there is no need to paint different layers using multiple metal masks. This prevents an increase in the manufacturing cost of the display device and makes it possible to provide a display device at a lower cost.

[0044] 5. Other configurations As an optional configuration, one or more cap layers 130 may be provided on the cathode 104 to resonate the light extracted from the cathode 104 to improve color purity and brightness in the front direction. Furthermore, a protective film 132 may be provided on the light emitting element to prevent impurities such as water and oxygen from entering the EL layer 110. The protective film 132 may be formed, for example, of a film containing silicon nitride, a film containing a polymer such as an acrylic resin or an epoxy resin, or a laminate of these.

[0045] 6. Variations In the example shown in FIG. 5, the configuration (number of layers and / or thickness) of the hole transport layer 114 is the same in all the pixels 210, but the configuration of the hole transport layer 114 may be different between the pixels 210. For example, as shown in FIG. 6, the number of layers and / or thickness of the hole transport layer 114 may be different between the red light-emitting pixel 210-1, the green light-emitting pixel 210-2, and the blue light-emitting pixel 210-3. It is preferable to configure the pixels 210 so that the number of layers and / or thickness of the hole transport layer 114 of the pixel 210 that gives a longer wavelength is larger. This allows a more effective resonator structure to be formed using the EL layer 110. For example, the first hole transport layer 114-1 may be formed continuously over all the pixels 210, and the second hole transport layer 114-2 may be further formed on the first hole transport layer 114-1 in the red light-emitting pixel 210-1 and the green light-emitting pixel 210-2. In this case, the pixels 210 may be configured so that the thickness of the second hole transport layer 114-2 in the red light-emitting pixel 210-1 is greater than that in the green light-emitting pixel 210-2. Although not shown, the number of stacked hole transport layers 114 may be changed for each pixel 210.

[0046] 5, the red light-emitting pixel 210-1 is provided with a light-emitting element 100, and the green light-emitting pixel 210-2 and the blue light-emitting pixel 210-3 are provided with light-emitting elements that do not have a buffer layer 118. The configuration of the display device 200 is not limited to this, and as shown in FIG. 7, the red light-emitting pixel 210-1 and the green light-emitting pixel 210-2 may be provided with a light-emitting element 100, and the blue light-emitting pixel 210-3 may be provided with a light-emitting element having a structure different from that of the light-emitting element 100. For example, the light-emitting element provided in the blue light-emitting pixel 210-3 may be a light-emitting element that does not have a buffer layer 118, or a light-emitting element that includes a fluorescent material (that does not exhibit activated delayed fluorescence) in the light-emitting layer 120 may be provided.

[0047] The red light-emitting pixel 210-1 and the green light-emitting pixel 210-2 may be provided with buffer layers 118 having different structures. Alternatively, the red light-emitting pixel 210-1 and the green light-emitting pixel 210-2 may be provided with buffer layers 118 having the same structure. In this case, the buffer layer 118 is shared by the red light-emitting pixel 210-1 and the green light-emitting pixel 210-2 and is provided so as to be continuous between the red light-emitting pixel 210-1 and the green light-emitting pixel 210-2. The buffer layer 118 contains the host material of the light-emitting layer 120 of the red light-emitting pixel 210-1 or the host material of the light-emitting layer 120 of the green light-emitting pixel 210-2, or is made of the host material of the light-emitting layer 120 of the red light-emitting pixel 210-1 or the host material of the light-emitting layer 120 of the green light-emitting pixel 210-2.

[0048] Alternatively, as shown in FIG. 8, the light-emitting element 100 may be provided in all the pixels 210. In this case, a buffer layer 118 having a different structure may be provided for each of the red light-emitting pixel 210-1, the green light-emitting pixel 210-2, and the blue light-emitting pixel 210-3. Alternatively, the buffer layer 118 having the same structure may be provided so as to be shared by all the pixels 210 and continuous with all the pixels 210. In this case, the buffer layer 118 includes the host material of the light-emitting layer 120 of the red light-emitting pixel 210-1, the host material of the light-emitting layer 120 of the green light-emitting pixel 210-2, or the host material of the light-emitting layer 120 of the blue light-emitting pixel 210-3, or is made of the host material of the light-emitting layer 120 of the red light-emitting pixel 210-1, the host material of the light-emitting layer 120 of the green light-emitting pixel 210-2, or the host material of the light-emitting layer 120 of the blue light-emitting pixel 210-3.

[0049] As described above, by disposing the light-emitting element 100 including the buffer layer 118 in the display device 200, a part or all of the functional layers such as the electron blocking layer 116 can be provided so as to be shared across the pixels 210. This contributes to reducing the number of metal masks used in manufacturing the display device and, therefore, reducing manufacturing costs.

[0050] In general, in a light-emitting element containing a thermally activated delayed fluorescent material, it is relatively difficult to adjust the carrier balance, and the margin for obtaining an appropriate carrier balance with respect to the element structure is narrow. In particular, when the configuration of the electron blocking layer 116 changes, the carrier balance changes significantly, and as a result, various characteristics such as the luminous efficiency and the driving voltage may be significantly reduced. However, as shown in the examples, by providing the buffer layer 118, the influence of the electron blocking layer 116 on the carrier balance can be suppressed and an appropriate carrier balance can be easily ensured, so that the characteristics of the light-emitting element can be improved or maintained even if the configuration of the electron blocking layer 116 changes. Furthermore, the reliability of the light-emitting element is also improved. For this reason, by applying the embodiments of the present invention, it is possible to provide a display device with high efficiency and high reliability at low cost. EXAMPLES

[0051] A number of red light-emitting devices (Examples) with different thicknesses of the buffer layer (BL) and two red light-emitting devices (Comparative Examples) without a buffer layer were prepared. In all the light-emitting devices, ITO was used as the anode and a co-evaporation film of silver and magnesium was used as the cathode. The size of the light-emitting region was 2.0 mm x 2.0 mm. The light-emitting layer contained a host material and a red-emitting thermally activated delayed fluorescent material, and lithium fluoride was used as the electron injection layer. All the light-emitting devices were the same except for the configuration of the electron blocking layer and the thickness of the buffer layer. The thicknesses of the functional layers constituting the EL layer are as shown in Table 1. Here, the light-emitting device of Comparative Example 1 is an element having an electron blocking layer optimized as a red light-emitting device, and the light-emitting devices of Comparative Example 2 and the Examples are elements having an electron blocking layer used in a light-emitting device containing a green-emitting thermally activated delayed fluorescent material.

[0052] [Table 1]

[0053] FIG. 9A shows the voltage-current density curve of the fabricated light-emitting device. Comparing Comparative Example 1 and Comparative Example 2, it can be seen that the voltage-current density curve shifts to the high voltage side when the material of the electron blocking layer is changed. This suggests that the change in electron blocking property generates an internal electric field due to carriers (electrons) accumulated at the interface between the electron blocking layer and the light-emitting layer, changing the carrier balance and increasing the driving voltage of the light-emitting device. However, as in the light-emitting device of the example, the provision of a buffer layer causes carriers to diffuse. As a result, it was found that the voltage-current density curve shifts to the low voltage side and voltage-current density characteristics close to those of Comparative Example 1 can be obtained. This suggests that the carrier balance can be improved by the buffer layer even when a non-optimized electron blocking layer is used.

[0054] This is also suggested by Figures 9B and 10A. Figure 9B is a plot showing the relationship between the driving voltage and the thickness of the buffer layer of the light-emitting elements of Comparative Example 2 and the Example, and it can be seen that the driving voltage decreases as the thickness of the buffer layer increases. Figure 10A is a plot showing the relationship between the current efficiency and the thickness of the buffer layer of the light-emitting elements of Comparative Example 2 and the Example, and it can be seen that the current efficiency increases as the thickness of the buffer layer increases. It is also suggested that the effect of the buffer layer saturates as the thickness approaches 8 nm.

[0055] FIG. 10B shows a plot of the normalized external quantum efficiency versus the current density. As is clear from the comparison between Comparative Example 1 and Comparative Example 2, when the buffer layer is not optimized, the normalized external quantum efficiency is significantly reduced, especially in the low current density region. This suggests that when the current density is low, that is, when the driving voltage is low, the carrier balance is greatly disrupted, and the contribution of non-radiative recombination increases. Non-radiative recombination corresponds to, for example, the generation of exciplexes and light absorption by polarons. In contrast, by providing a buffer layer and increasing its thickness, the plot approaches that of Comparative Example 1, and it can be seen from FIG. 10B that the normalized external quantum efficiency is improved, especially in the low current density region. This also shows that even when a non-optimized electron blocking layer is used, the effect of the buffer layer can be suppressed by providing the buffer layer, and characteristics similar to or better than those of a light-emitting device including an optimized electron blocking layer can be obtained.

[0056] FIG. 11A shows the voltage-capacity plots of the light-emitting devices of Comparative Examples 1 and 2 obtained by impedance spectroscopy. From the comparison between Comparative Examples 1 and 2, it can be seen that when the electron blocking layer is not optimized, the capacity at low voltage increases. This indicates that when the electron blocking layer is not optimized, carriers are accumulated in the EL layer, especially at the interface between the electron blocking layer and the light-emitting layer, from the early stage of driving, and a relatively large internal electric field is generated. In contrast, as can be seen from FIG. 11B, in the light-emitting device of the embodiment, as the thickness of the buffer layer increases, the increase in capacity in the low voltage region is eliminated, and the plot tends to approach that of the light-emitting device of Comparative Example 1. These results also indicate that the buffer layer improves the carrier balance, suppresses the generation of an internal electric field, and reduces the driving voltage. Furthermore, the capacity in the high voltage region increases conversely. This suggests that the carriers used for recombination increase, improving efficiency. In other words, even if the structure of the electron blocking layer changes, the effect of the change can be suppressed by the buffer layer.

[0057] As described above, the light-emitting element containing the thermally activated delayed fluorescent material can obtain extremely high luminous efficiency, but it is relatively difficult to establish an optimal carrier balance and is easily affected by the carrier block layer (particularly the electron block layer). In contrast, the above results show that by applying the embodiment of the present invention, the influence of the electron block layer can be suppressed and an appropriate carrier balance can be ensured without significantly impairing reliability. This means that the electron block layer can be made common even between pixels with different luminescent colors, which makes it possible to reduce the number of metal masks during the manufacture of a display device equipped with a light-emitting element containing a thermally activated delayed fluorescent material. Therefore, it can be said that the embodiment of the present invention contributes to providing a display device with high efficiency and reliability at low cost.

[0058] The above-described embodiments of the present invention may be combined as appropriate as long as they are not mutually inconsistent. In addition, those in which a person skilled in the art appropriately adds or removes components or modifies designs, or adds or omits steps or modifies conditions, based on the light-emitting device or display device of each embodiment are included in the scope of the present invention as long as they include the gist of the present invention.

[0059] Even if there are other effects and advantages different from those brought about by the aspects of each of the above-mentioned embodiments, those which are clear from the description in this specification or which 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]

[0060] 100: light-emitting element, 102: anode, 104: cathode, 110: EL 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: buffer layer, 120: light-emitting layer, 122: hole blocking layer, 124: electron transport layer, 126: electron injection layer, 130: cap layer, 132: protective film, 200: display device, 202: substrate, 204: scanning line driving circuit, 206: signal line driving circuit Circuit, 208: terminal, 210: pixel, 210-1: red light emitting pixel, 210-2: green light emitting pixel, 210-3: blue light emitting pixel, 212: undercoat, 220: transistor, 222: semiconductor film, 224: gate insulating film, 226: gate electrode, 228: interlayer insulating film, 230: terminal, 232: terminal, 234: connection electrode, 236: planarizing film, 238: partition wall, 240: capacitance element, 242: capacitance electrode, 244: capacitance insulating film, 246: pixel electrode, 250: opposing substrate

Claims

1. an anode, a cathode, and an electroluminescent layer between the anode and the cathode; The electroluminescent layer is Electron blocking layer, a buffer layer on the electron blocking layer; and a light-emitting layer located on the buffer layer and including a host material and a first light-emitting material exhibiting thermally activated delayed fluorescence; The buffer layer is a light-emitting element containing the host material.

2. The light-emitting element according to claim 1 , wherein an energy difference between a triplet excited state and a singlet excited state of the first light-emitting material is equal to or greater than 5 meV and equal to or less than 20 meV.

3. the light-emitting layer further comprises a second light-emitting material; The light-emitting device of claim 1 , wherein an energy level of a singlet excited state of the second light-emitting material is lower than an energy level of the singlet excited state of the first light-emitting material.

4. The light-emitting device according to claim 1 , wherein the buffer layer is made of the host material.

5. The light-emitting element of claim 1 , wherein the first light-emitting material emits red or green light.

6. The light-emitting device according to claim 1 , wherein the buffer layer is in contact with the electron blocking layer and the light-emitting layer.

7. The light-emitting device according to claim 1 , wherein the buffer layer has a thickness of 2.0 nm or more and 10.0 nm or less.

8. A display device comprising red, green and blue light emitting pixels, Each of the red light emitting pixel, the green light emitting pixel, and the blue light emitting pixel is Pixel electrodes, an electroluminescent layer located on the pixel electrode, the electroluminescent layer including an electron blocking layer and a light emitting layer on the electron blocking layer; and a cathode on the electroluminescent layer; the electroluminescent layer of the red-emitting pixel comprises a first host material and a first red-emitting material exhibiting thermally activated delayed fluorescence; the red-emitting pixel further includes a first buffer layer between the electron blocking layer and the light-emitting layer, the first buffer layer including the first host material; the electron blocking layer is shared by the red, green, and blue light-emitting pixels and is continuous across the red, green, and blue light-emitting pixels.

9. The display device according to claim 8 , wherein an energy difference between a triplet excited state and a singlet excited state of the first red light-emitting material is 5 meV to 20 meV.

10. the light-emitting layer of the red-emitting pixel further comprises a second red-emitting material; The display device of claim 8 , wherein an energy level of a singlet excited state of the second red light-emitting material is lower than an energy level of the singlet excited state of the first red light-emitting material.

11. The display device according to claim 8 , wherein the first buffer layer is made of the first host material.

12. The display device according to claim 8 , wherein in the red light-emitting pixel, the first buffer layer is in contact with the electron blocking layer and the light-emitting layer.

13. The display device according to claim 8 , wherein the first buffer layer has a thickness of not less than 2.0 nm and not more than 10.0 nm.

14. The display device according to claim 8 , wherein the electron blocking layer is in contact with the light-emitting layer in the green light-emitting pixel and the blue light-emitting pixel.

15. the light-emitting layer of the green light-emitting pixel includes a second host material and a first green light-emitting material that exhibits thermally activated delayed fluorescence; The display device according to claim 8 , wherein the green light-emitting pixel further comprises a second buffer layer between the electron blocking layer and the light-emitting layer, the second buffer layer including the second host material.

16. The display device according to claim 15 , wherein an energy difference between a triplet excited state and a singlet excited state of the first green light-emitting material is 5 meV to 20 meV.

17. the light-emitting layer of the green light-emitting pixel further comprises a second green light-emitting material; 16. The display of claim 15, wherein an energy level of a singlet excited state of the second green light-emitting material is lower than an energy level of the singlet excited state of the first green light-emitting material.

18. The display device according to claim 15 , wherein the second buffer layer is made of the second host material.

19. The display device according to claim 15 , wherein in the green light-emitting pixel, the second buffer layer is in contact with the electron blocking layer and the light-emitting layer.

20. The display device according to claim 15 , wherein the second buffer layer has a thickness of 2.0 nm to 10.0 nm.

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