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

Abstract

To provide a light-emitting element with high reliability 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 and a light-emitting layer. The electron block layer includes an electron block material. The light-emitting layer exists on the electron block layer, is in contact with the electron block layer, and includes a host material and a first light-emitting material exhibiting thermal-active delay fluorescence. The light-emitting element is configured so that the resistance thereof becomes 101.2% or more and 102.5% or less of the resistance in a case where the electron block material is replaced by the host material. The resistance is the resistance between the anode and the cathode when current flows at a constant current density between the anode and the cathode.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 that exhibits high efficiency and a display device having the light-emitting element. [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 and an emitting layer. The electron blocking layer includes an electron blocking material. The emitting layer is located on the electron blocking layer, is in contact with the electron blocking layer, and includes a host material and a first emitting material that exhibits thermally activated delayed fluorescence. The light-emitting device is configured so that the resistance is 101.0% or more and 104.0% or less of the resistance when the electron blocking material is replaced with the host material. The resistance is the resistance between the anode and the cathode when a current is passed between the anode and the cathode at a constant current density.

[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 which has an electroluminescent element. At least one of the red-emitting pixel, the green-emitting pixel, and the blue-emitting pixel has a pixel electrode, an electron blocking layer, an emitting layer, and a cathode. The electron blocking layer is located on the pixel electrode and includes an electron blocking material. The emitting layer is located on the electron blocking layer, is in contact with the electron blocking layer, and includes a host material and a first emitting material that exhibits thermally activated delayed fluorescence. The cathode is located on the emitting layer. The at least one electroluminescent element is configured to have a resistance that is 101.0% or more and 104.0% or less of the resistance when the electron blocking material is replaced with the host material. The resistance is the resistance between the anode and the cathode when a current is passed between the anode and the cathode at a constant current density. [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. 1 is a schematic end view of a display device according to an embodiment of the present invention. [Diagram 5] 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 6] 4 shows voltage-current density curves of the light-emitting elements of Example and Comparative Examples 1 and 2. [Figure 7] 2 shows current density-normalized external quantum efficiency curves of the light-emitting elements of Example and Comparative Examples 1 and 2. [Figure 8] 4 shows voltage-capacity curves of the light-emitting elements of Example and Comparative Examples 1 and 2. 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 FIG. 1 shows a schematic end view of a light-emitting element 100. 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 118 that emits light, as well as an electron blocking layer 116 that is disposed between the emitting layer 118 and the anode 102 and in contact with the emitting layer 118. The EL layer 110 may further include a hole injection layer 112, a hole transport layer 114, a hole blocking layer 120, an electron transport layer 122, an electron injection layer 124, and the like. By creating a voltage difference between the anode 102 and the cathode 104, holes and electrons are injected from the anode 102 and the cathode 104, respectively, into the EL layer 110, and these carriers are recombined in the emitting layer 118. The light-emitting material present in the light-emitting layer 118 is excited by the recombination of holes and electrons, and the energy released when this excited state returns to the ground state can be extracted as light. Each component will be described below.

[0014] 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 film of a conductive oxide that transmits visible light may be further 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 118 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 118 and being injected into the hole transport layer 114 without contributing to recombination in the light emitting layer 118, thereby confining the electrons within the light emitting layer 118, and also prevents the excitation energy obtained in the light emitting layer 118 from being transferred to the molecules in the hole transport layer 114. This makes it possible to prevent a decrease in luminous efficiency.

[0019] The electron blocking layer 116 includes an electron blocking material. Preferably, the electron blocking layer 116 is made of an electron blocking material. As the electron blocking material, 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 host material in the light-emitting layer 118, and a large band gap is preferably used. By setting the LUMO level of the electron blocking material shallower than that of the host material, it is possible to prevent electrons from penetrating the light-emitting layer 118 and increase the efficiency of the light-emitting element 100.

[0020] The electron blocking material has a structure different from that of the host material in the light-emitting layer 118 described later. The electron blocking layer 116 is configured so that the resistance of the light-emitting element 100 is 101.0% to 104.0%, 101.2% to 103.7%, or 101.2% to 102.5% of the resistance of a reference element obtained by replacing the electron blocking material of the light-emitting element 100 with the host material in the light-emitting layer 118. The reference element is different from the light-emitting element 100 only in the electron blocking material, and the other configurations are all the same. Here, the resistance of the light-emitting element is the resistance between the anode and the cathode when a current flows between the anode and the cathode at a constant current density. That is, if the current I flows when a voltage V is applied between the anode and the cathode, the resistance R is calculated by V / I. The current density at this time is the current density in the saturation region. The saturation region is a region in which the current efficiency does not increase with an increase in current density in a current density-current efficiency plot of the light-emitting element, and is, for example, a region of 10 mA / cm 2 By configuring the light emitting device 100 and the electron blocking layer 116 in this way, it is possible to achieve an increase in external quantum efficiency and an improvement in reliability (light emission life) without an increase in driving voltage, as shown in the examples.

[0021] The difference in LUMO level between the electron blocking material and the host material is preferably greater than 0 eV and less than or equal to 0.1 eV. As shown in the examples, by setting the difference in LUMO level between the electron blocking material and the host material to be relatively small in this way, not only can the electron penetration be effectively prevented but also a good carrier balance can be ensured. In addition, the HOMO level of the electron blocking material is deeper than the HOMO level of the host material, and the difference is preferably 0.1 eV or more and 0.3 eV or less. By selecting the electron blocking material and the host material in this way, holes can be efficiently transported to the light-emitting layer 118 while preventing energy transfer from the light-emitting layer 118.

[0022] Specific examples of the electron blocking material include aromatic amine derivatives, carbazole derivatives, 9,10-dihydroacridine derivatives, benzofuran derivatives, and benzothiophene derivatives.

[0023] 6.Emitting layer The light-emitting layer 118 is provided on the electron blocking layer 116 so as to be in contact with the electron blocking layer 116. The light-emitting layer 118 contains a host material having a structure different from that of the electron blocking 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, and the like can be used.

[0024] 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 -3 seconds (1μs) or more.

[0025] There is no restriction on the emission color of 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 based on the emission color required for the light-emitting element 100. Preferably, the thermally activated delayed fluorescent material emits green light or red light, and more preferably emits green light. Here, blue light emission is light emission whose maximum emission peak wavelength is located in the range of 400 nm to 500 nm, green light emission is light emission whose maximum emission peak wavelength is located in the range of 500 nm to 650 nm, and red light emission is light emission whose maximum emission peak wavelength is located in the range of 650 nm to 750 nm.

[0026] 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.

[0027] 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.

[0028] The light-emitting layer 118 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 118, 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.

[0029] 7. Hole Block Layer The hole blocking layer 120 has a function of preventing holes injected from the anode 102 from passing through the light emitting layer 118 and being injected into the electron transport layer 122 without contributing to recombination, thereby confining the holes within the light emitting layer 118, and also prevents the excitation energy obtained in the light emitting layer 118 from being transferred to molecules in the electron transport layer 122. This makes it possible to prevent a decrease in luminous efficiency.

[0030] For the hole blocking layer 120, 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 118, and a larger band gap. Specifically, the difference between the HOMO level of the molecules in the hole blocking layer 120 and that of the molecules in the light emitting layer 118 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 120 and that of the molecules in the light emitting layer 118 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 120 may also have a single layer structure, or may be composed of multiple layers containing different materials.

[0031] 8.Electron transport layer The electron transport layer 122 has a function of transporting electrons injected from the cathode 104 through the electron injection layer 124 to the light emitting layer 118. The electron transport layer 122 can be made of a compound that is easily reduced (electron accepting). In other words, a compound with a shallow LUMO level can 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 can 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, can be used. The electron transport layer 122 can also have a single layer structure, or can be made of multiple layers containing different materials.

[0032] 9.Electron injection layer The electron injection layer 124 may be made of a compound that promotes electron injection from the cathode 104 to the electron transport layer 122. For example, a mixture of a compound that can be used in the electron transport layer 122 and an electron donor such as lithium or magnesium may be used. Alternatively, an inorganic compound such as lithium fluoride or calcium fluoride may be used.

[0033] In a light-emitting element containing a light-emitting material exhibiting activated delayed fluorescence, it is relatively difficult to achieve an appropriate carrier balance, and is particularly affected by the electron blocking layer. Therefore, if the electron blocking material is not appropriately selected, carriers (electrons) tend to penetrate the light-emitting layer, shifting the recombination region toward the anode, resulting in an increase in the rate of non-radiative recombination and a decrease in luminous efficiency. This has a significant impact on the reliability of the light-emitting element.

[0034] On the other hand, in order to prevent the penetration of carriers, it is necessary to provide an electron injection barrier between the electron blocking layer and the light-emitting layer. That is, the light-emitting device is designed so that the LUMO level of the electron blocking layer is higher than that of the light-emitting layer. However, as shown in the examples, if the difference in LUMO level becomes too large (for example, 0.2 eV), electrons are accumulated between the electron blocking layer and the light-emitting layer immediately after the light-emitting device starts to operate, and as a result, a large internal electric field is generated in the light-emitting device, hindering carrier injection. This not only causes an increase in the driving voltage, but also promotes non-radiative recombination of the accumulated carriers, resulting in a decrease in luminous efficiency and reliability.

[0035] In contrast, in the light-emitting device 100, the electron blocking material is configured so that the resistance of the light-emitting device 100 is 101.2% to 103.7%, or 101.2% to 102.5%, of the resistance of the reference device. The light-emitting device 100 can also be configured so that the difference in LUMO level between the electron blocking layer 116 and the light-emitting layer 118 is 0.1 eV or less. In this way, by providing an extremely small energy barrier against electron injection from the light-emitting layer 118 to the electron blocking layer 116, the accumulation of carriers and the generation of an internal electric field caused by the accumulation of carriers are effectively suppressed, even compared to a light-emitting device without an energy barrier. Therefore, the light-emitting device 100 can be driven at a lower driving voltage. Furthermore, since non-radiative recombination caused by the accumulation of carriers is suppressed, the light-emitting device can exhibit higher luminous efficiency and reliability. Therefore, by applying the embodiments of the present invention, a highly efficient and highly reliable light-emitting device can be provided.

[0036] <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.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] FIG. 4 shows a schematic diagram of an end surface along the dashed line AA′ in FIG. 3A. In FIG. 4, the end surfaces of a red light-emitting pixel 210-1, a green light-emitting pixel 210-2, and a blue light-emitting pixel 210-3 arranged in succession 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. 4, 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. Here, an example is shown in which light from the light-emitting layer 118 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 118 is extracted through the substrate 202.

[0041] 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.

[0042] 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.

[0043] 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. 4, the pixel electrode 246 is shared by the light emitting element 100 and the capacitor element 240 .

[0044] 4. Light-emitting element 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. The light-emitting element 100 may be disposed in all of the red light-emitting pixel 210-1, the green light-emitting pixel 210-2, and the blue light-emitting pixel 210-3, or may be disposed in some of the pixels 210. For example, the light-emitting element 100 may be disposed in the red light-emitting pixel 210-1 and the green light-emitting pixel 210-2, and a light-emitting element having a different structure from the light-emitting element 100 may be disposed in the blue light-emitting pixel 210-3. Alternatively, the light-emitting element 100 may be disposed in one of the red light-emitting pixel 210-1 and the green light-emitting pixel 210-2, and a light-emitting element having a different structure from the light-emitting element 100 may be disposed in the other one and the blue light-emitting pixel 210-3. Here, the light-emitting element having a different structure from the light-emitting element 100 may be, for example, a light-emitting element containing a phosphorescent material or a fluorescent material (not exhibiting thermally activated delayed fluorescence) as a light-emitting material. Alternatively, as a light-emitting element having a structure different from that of the light-emitting element 100, a light-emitting element including thermally activated delayed fluorescence in an emitting layer but not having an electron blocking layer, or a light-emitting element having an electron blocking material identical to a host material may be used. Alternatively, a light-emitting element having an electron blocking layer and a host material different from each other but in which the resistance of the light-emitting element 100 deviates from the above-mentioned range with respect to the resistance of a reference element obtained by replacing the electron blocking layer with a host material may be used. Alternatively, a light-emitting element having an electron blocking layer and a host material different from each other but in which the relationship of the HOMO levels or the relationship of the LUMU levels therebetween deviates from the above-mentioned range may be used.

[0045] Since the structure of the light-emitting layer 118 determines the color of emitted light, the light-emitting layer 118 is separated between the red light-emitting pixel 210-1, the green light-emitting pixel 210-2, and the blue light-emitting pixel 210-3. In contrast, some or all of the functional layers except the light-emitting layer 118 may be provided so as to be continuous across the adjacent pixels 210 so as to be shared between the adjacent pixels 210, or may be provided so as to be separated between the adjacent pixels 210. For example, as shown in FIG. 4, the electron blocking layer 116 may be formed so as to be shared by all the pixels 210 and to be continuous across all the pixels. Alternatively, in addition to the electron blocking layer 116, the hole injection layer 112, the hole transport layer 114, the hole blocking layer 120, the electron transport layer 122, the electron injection layer 124, 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.

[0046] 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.

[0047] 6. Variations In the example shown in FIG. 4, 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. 5, 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.

[0048] As described in the first embodiment, the light-emitting device 100 contains a thermally activated delayed fluorescent material and therefore exhibits extremely high efficiency. In addition, by configuring the electron blocking layer 116 so that the resistance change when the electron blocking material is replaced with the host material in the light-emitting device 100 and the LUMO levels of the electron blocking material and the host material satisfy the relationship described in the first embodiment, it is possible to reduce the driving voltage and increase the luminous efficiency and reliability. Therefore, by applying the embodiment of the present invention, it is possible to provide a display device that has both high efficiency and reliability. EXAMPLES

[0049] The light-emitting devices of the Example and Comparative Examples 1 and 2 were prepared, and their characteristics were evaluated. 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. In all the light-emitting devices, the light-emitting layer contained a thermally activated delayed fluorescent material that emits green light, and the materials constituting the functional layer were the same among all the light-emitting devices, except for the electron blocking material. The thickness of the functional layer constituting the EL layer is as shown in Table 1. Here, the electron blocking material of the light-emitting device of Comparative Example 2 is the same as the host material of the light-emitting layer. Therefore, the LUMO level is the same between the electron blocking layer and the light-emitting layer. On the other hand, in both the light-emitting devices of Comparative Example 1 and the Example, the LUMO level of the electron blocking layer was shallower than that of the light-emitting device itself, and the difference was 0.2 eV and 0.1 eV, respectively.

[0050] [Table 1]

[0051] FIG. 6 shows the voltage-current density curve of the fabricated light-emitting element. From FIG. 6, it can be seen that, in the element of the Example, the voltage-current density characteristic is shifted to the lower voltage side, in comparison with Comparative Example 2, although an electron injection barrier exists between the electron blocking layer and the light-emitting layer. In particular, it was found that the element of the Example can be driven at a lower voltage than Comparative Examples 1 and 2 in the high current density region. Furthermore, looking at the current density-normalized external quantum efficiency curve (FIG. 7), in the low current density region, the efficiency is low in Comparative Example 2, in which no electron injection barrier exists between the electron blocking layer and the light-emitting layer. Furthermore, the normalized external quantum efficiency in the low current density region does not match the order of the magnitude of the electron injection barrier between the electron blocking layer and the light-emitting layer, and the light-emitting element of the Example (ΔLUMO=0.1 eV) shows the highest efficiency.

[0052] The characteristics of these light-emitting devices are summarized in Table 2. As shown in Table 2, the light-emitting device of the example exhibits the lowest driving voltage, and also has high current efficiency and reliability.

[0053] [Table 2]

[0054] These results are believed to be explained by the behavior of the generation of the internal electric field in the EL layer. In the voltage-capacity plot (FIG. 8) obtained by impedance spectroscopy, a large capacity was observed in the light-emitting device of Comparative Example 1 from the low voltage region, and it is believed that carriers were accumulated in the EL layer from the early stage of driving the light-emitting device, generating an internal electric field. On the other hand, the results of FIG. 8 show that in Comparative Example 2, in which there is no electron injection barrier between the electron blocking layer and the light-emitting layer, the generation of capacitance in the low voltage region is suppressed compared to Comparative Example 1, but what is interesting is that the light-emitting device of the embodiment, in which a small electron injection barrier (Δ0.1 eV) is provided, can more effectively suppress the generation of the internal electric field. From the above, it is believed that the accumulation of carriers is suppressed by restricting the electron injection barrier between the electron blocking layer and the light-emitting layer, i.e., the difference in the LUMO levels between these layers, to 0.1 eV or less, and as a result, non-radiative recombination is suppressed, the normalized external quantum efficiency is increased, and the driving voltage is reduced and the reliability is improved.

[0055] The above results show that, by applying the embodiments of the present invention, it is possible to provide a light-emitting element with extremely low power consumption and improved reliability, and a display device including this light-emitting element.

[0056] 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.

[0057] Even if there are other effects and advantages different from those brought about by the aspects of each of the above-mentioned embodiments, those that are clear from the description in this specification or that can be easily predicted by a person skilled in the art are naturally understood to be brought about by the present invention. [Explanation of symbols]

[0058] 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: light-emitting layer, 120: hole blocking layer, 122: electron transport layer, 124: electron injection layer, 130: cap layer, 132: protective film, 200: display device, 202: substrate, 204: scanning line driving circuit, 206: signal line driving circuit, 20 8: 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 an electron blocking layer comprising an electron blocking material; a light-emitting layer located on the electron blocking layer, in contact with the electron blocking layer, the light-emitting layer including a host material and a first light-emitting material exhibiting thermally activated delayed fluorescence; a resistance of the electron blocking material is 101.0% or more and 104.0% or less of a resistance when the electron blocking material is substituted for the host material; The resistance is a resistance between the anode and the cathode when a current is passed between the anode and the cathode at a constant current density.

2. The current density is 10 mA / cm 2 The light-emitting device according to claim 1 ,

3. The light-emitting device according to claim 1 , wherein a difference in the lowest unoccupied molecular orbital level between the electron blocking material and the host material is 0.1 eV or less.

4. The light-emitting device according to claim 1 , wherein a difference in the highest occupied molecular orbital level between the electron blocking material and the host material is 0.1 eV or more and 0.3 eV or less.

5. 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.

6. 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.

7. The light-emitting device according to claim 1 , wherein the electron blocking layer is made of the electron blocking material.

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

9. a red light-emitting pixel, a green light-emitting pixel, and a blue light-emitting pixel, each of which has an electroluminescent element; At least one of the red light emitting pixel, the green light emitting pixel, and the blue light emitting pixel is an electroluminescent element, Pixel electrodes, an electron blocking layer located on the pixel electrode and comprising an electron blocking material; an emitting layer located on and in contact with the electron blocking layer, the emitting layer including a host material and a first emitting material exhibiting thermally activated delayed fluorescence; A cathode on the light-emitting layer, the at least one electroluminescent device is configured to have a resistance that is 101.0% or more and 104.0% or less of a resistance when the electron blocking material is substituted for the host material; The resistance is a resistance between the pixel electrode and the cathode when a current with a constant current density is caused to flow between the pixel electrode and the cathode.

10. The current density is 10 mA / cm 2 The display device according to claim 9 ,

11. 10. The display device according to claim 9, wherein a difference in the lowest unoccupied molecular orbital level between the electron blocking material and the host material is 0.1 eV or less.

12. 10. The display device according to claim 9, wherein a difference in the highest occupied molecular orbital level between the electron blocking material and the host material is 0.1 eV or more and 0.3 eV or less.

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

14. the light-emitting layer further comprises a second light-emitting material; The display device of claim 9 , 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.

15. The display device according to claim 9 , wherein the electron blocking layer is made of the electron blocking material.

16. The display device of claim 9 , wherein the first light-emitting material emits red or green light.

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