Light-emitting element and display device

Abstract

This light-emitting element comprises a charge transport layer between an anode and a light-emitting layer, or between the light-emitting layer and a cathode. The charge transport layer includes a first layer, a second layer, and a third layer in this order from the anode side. The second layer has a lower ionization potential and a lower electron affinity than the first layer and the third layer. Alternatively, the second layer has a higher ionization potential and a higher electron affinity than the first layer and the third layer.

Description

Light-emitting element, display device

[0001] This disclosure relates to a light-emitting element and a display device equipped with the light-emitting element.

[0002] Patent Document 1 discloses a light-emitting device having a light-emitting region corresponding to a light-emitting layer containing quantum dots (semiconductor nanoparticles) as a light-emitting material. In particular, Patent Document 1 describes an invention that improves luminescence efficiency by improving the efficiency of hole-electron recombination in the light-emitting region by alternately stacking a light-emitting layer and an inorganic compound layer in the light-emitting region.

[0003] Japanese Patent Application Publication No. 2007-73315

[0004] In light-emitting devices equipped with quantum dots as an emissive layer, holes or electrons injected into the emissive layer may flow out from the emissive layer to the electrode side, and the recombination of holes or electrons at a location different from the emissive layer reduces the luminous efficiency of the light-emitting device. In the light-emitting device described in Patent Document 1, even if the efficiency of hole-electron recombination in the emissive region can be improved, it is difficult to reduce the recombination of holes and electrons outside the emissive region.

[0005] A light-emitting element according to one aspect of the present disclosure comprises an anode, a cathode facing the anode, a light-emitting layer located between the anode and the cathode, and a charge transport layer located between the anode and the light-emitting layer, or between the cathode and the light-emitting layer, wherein the charge transport layer includes a first layer, a second layer, and a third layer stacked from the anode toward the cathode, and the ionization potential of the first layer is IP1, the ionization potential of the second layer is IP2, the ionization potential of the third layer is IP3, the electron affinity of the first layer is EA1, the electron affinity of the second layer is EA2, and the electron affinity of the third layer is EA3, satisfying either condition 1: IP2 < IP1, IP2 < IP3, EA2 < EA1, and EA2 < EA3, and condition 2: IP2 > IP1, IP2 > IP3, EA2 > EA1, and EA2 > EA3.

[0006] According to one aspect of this disclosure, the probability of hole-electron recombination in a portion of the light-emitting element that is different from the light-emitting layer is reduced, thereby improving the luminescence efficiency of the light-emitting element.

[0007] This is a schematic side cross-sectional view and band diagram of the light-emitting element of a display device according to Embodiment 1. This is a schematic diagram of the display device according to Embodiment 1. This is a schematic side cross-sectional view and band diagram of the light-emitting element of a display device according to a comparative embodiment. This is a band diagram showing how holes and electrons are transported in the light-emitting elements of the embodiment and the comparative embodiment, respectively. This is a graph showing the light emission characteristics of the light-emitting elements of the embodiment and the comparative example, respectively. This is a schematic side cross-sectional view and band diagram of the light-emitting element of a display device according to Embodiment 2. This is a schematic side cross-sectional view and band diagram of the light-emitting element of a display device according to Embodiment 3. This is a schematic side cross-sectional view and band diagram of the light-emitting element of a display device according to Embodiment 4. This is a schematic side cross-sectional view and band diagram of the light-emitting element of a display device according to Embodiment 5. This is a schematic side cross-sectional view and band diagram of the light-emitting element of a display device according to Embodiment 6. This is a schematic side cross-sectional view and band diagram of the light-emitting element of a display device according to Embodiment 7. This is a schematic side cross-sectional view of a display device according to Embodiment 8. This is a band diagram of each layer of the hole transport layer according to Embodiment 8. This is a schematic side cross-sectional view of a display device according to Embodiment 9. This is a band diagram of each layer of the electron transport layer according to Embodiment 9.

[0008] [Embodiment 1] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. In each drawing, similar components are denoted by the same reference numerals and their descriptions are omitted. Also, in this disclosure, for the sake of simplicity of illustration, the scale may differ in the drawings for components denoted by the same reference numerals, and different hatching may be applied. However, the components shown in each drawing of this disclosure are merely examples, and their scale is not limited to that shown in the drawing. Furthermore, even for components denoted by different hatching in this disclosure, components denoted by the same reference numerals have the same configuration as described above.

[0009] <Overview of the Display Device> Figure 2 is a schematic diagram of the display device 1 according to this embodiment. The display device 1 is a device that can be used, for example, in a television or smartphone display. The display device 1 comprises a display unit DA including a plurality of subpixels and a driver circuit DR that drives the plurality of subpixels. Each of the plurality of subpixels comprises a light-emitting element, which will be described later, and a pixel circuit PC that drives the light-emitting element.

[0010] In particular, the display device 1 includes a red subpixel XR, a green subpixel XG, and a blue subpixel XB in the display unit DA. The red subpixel XR is equipped with a red light-emitting element 2R that emits red light, the green subpixel XG is equipped with a green light-emitting element 2G that emits green light, and the blue subpixel XB is equipped with a blue light-emitting element 2B that emits blue light, each as a light-emitting element.

[0011] In this embodiment, blue light refers to light having a central emission wavelength in the wavelength band of 380 nm to 500 nm. Green light refers to light having a central emission wavelength in the wavelength band of more than 500 nm to 600 nm. Red light refers to light having a central emission wavelength in the wavelength band of more than 600 nm to 780 nm.

[0012] The display device 1 performs display on the display unit DA by controlling the light emitted from each of the multiple light-emitting elements formed on the display unit DA via the driver circuit DR and the pixel circuit PC. In particular, the display device 1 may perform full-color display by controlling the red light from the red subpixel XR, the green light from the green subpixel XG, and the blue light from the blue subpixel XB, respectively.

[0013] The structure of the display unit DA of the display device 1 will be described in more detail with reference to Figure 1. Figure 1 shows a schematic side cross-sectional view F1 of the display device 1 according to this embodiment, and a band diagram B1 of each layer in the light-emitting element 2, which will be described later, shown in the schematic side cross-sectional view F1.

[0014] The schematic side cross-sectional views of the display device 1 according to this disclosure, including the schematic side cross-sectional view F1 in Figure 1, all show a cross-section perpendicular to the display surface of the display device 1 and passing through one of the red subpixel XR, green subpixel XG, and blue subpixel XB, unless otherwise specified. In other words, the light-emitting element 2 shown in the schematic side cross-sectional view of the display device 1 according to this disclosure is one of the red light-emitting element 2R, green light-emitting element 2G, and blue light-emitting element 2B. In the schematic side cross-sectional view of the display device 1 according to this disclosure, the planar direction DP parallel to the upper surface of the substrate 3, which will be described later, is defined as the left-right direction with respect to the plane of the paper, and the film thickness direction DT parallel to the thickness direction of the substrate 3 is defined as the up-down direction with respect to the plane of the paper.

[0015] The band diagrams of the light-emitting element 2 according to this disclosure, including band diagram B1, are all illustrated assuming that the vacuum level is located above. The band diagrams of the light-emitting element 2 according to this disclosure show the band gap of the material contained in each layer from the hole injection layer 22 to the electron transport layer 25, which will be described later. Furthermore, the band diagrams of the light-emitting element 2 according to this disclosure show the Fermi level of the material contained in each electrode, including the anode 21 and cathode 26, which will be described later. In addition, for some of the band gaps shown in the band diagrams of this disclosure, including band diagram B1, the Fermi level of the material contained in the layer having that band gap is shown by a dotted line.

[0016] The display device 1 comprises a plurality of light-emitting elements 2 and a substrate 3, and in particular, a plurality of light-emitting elements 2 are provided on the substrate 3. The display device 1 has a structure in which each layer of light-emitting elements 2 is stacked on a substrate 3 on which, for example, a TFT (Thin Film Transistor) (not shown) is formed as a pixel circuit PC. In this specification, the direction from the light-emitting elements 2 to the substrate 3 of the display device 1 is described as the "downward direction," and the direction opposite to the downward direction is described as the "upward direction."

[0017] The display device 1 may be manufactured by preparing a substrate 3 and forming a light-emitting element 2 on the substrate 3. The substrate 3 may be prepared, for example, by forming a plurality of pixel circuits on a base material. The light-emitting element 2 may be formed by sequentially depositing each layer of the light-emitting element 2, as described later, from the substrate 3 side using a method described later.

[0018] <Light-emitting element: Overview> The light-emitting element 2 includes, in order from the substrate 3 side, an anode 21, a hole injection layer 22, a hole transport layer 23, a light-emitting layer 24, an electron transport layer 25, and a cathode 26, all stacked together. In particular, in this embodiment, the light-emitting element 2 includes an anode 21, a cathode 26 facing the anode 21, and a light-emitting layer 24 located between the anode 21 and the cathode 26. The light-emitting element 2 also includes a hole transport layer 23 as a charge transport layer located between the anode 21 and the light-emitting layer 24, and an electron transport layer 25 as a charge transport layer located between the cathode 26 and the light-emitting layer 24. However, the light-emitting element 2 according to this embodiment is not limited to the above configuration. For example, the light-emitting element 2 may include, in order from the substrate 3 side, a cathode 26, an electron transport layer 25, a light-emitting layer 24, a hole transport layer 23, a hole injection layer 22, and an anode 21, all stacked together.

[0019] <Light-emitting element: Electrode> At least one of the anode 21 and cathode 26 is a transparent electrode that transmits visible light. Examples of transparent electrodes include ITO, InZnO, and SnO. ２ Alternatively, FTO or the like may be used. In addition, either the anode 21 or the cathode 26 may be a reflective electrode. The reflective electrode may contain a metallic material with high reflectivity of visible light, and this metallic material may be, for example, Al, Ag, Cu, or Au alone or an alloy thereof.

[0020] The anode 21 and cathode 26 may be formed by creating a thin film of a conductive material using a method such as sputtering, and then patterning the thin film. In particular, the electrodes connected to the drive circuit of the substrate 3, such as the anode 21 in this embodiment, may be patterned for each subpixel. On the other hand, the cathode 26 in this embodiment may be formed in common for multiple subpixels.

[0021] <Light-emitting element: Hole injection layer> The hole injection layer 22 is a layer that injects holes from the anode 21 into the light-emitting layer 24. The material of the hole injection layer 22 can be an organic or inorganic material with hole transport properties, which has been conventionally used in light-emitting elements including quantum dots. The hole injection layer 22 contains nickel oxide (NiO) nanoparticles: [2-(9H-carbazole-9-yl)ethyl]phosphonic acid (2PACz), [2-(3,6-dimethoxy-9H-carbazole-9-yl)ethyl]phosphonic acid (MeO-2PACz), [2-(3,6-dimethyl-9H-carbazole-9-yl)ethyl]phosphonic acid (Me-2PACz), [2-(3,6-dichloro-9H-carbazole-9-yl)ethyl]phosphonic acid (Cl-2PACz), and [2-(3,6-dibromo-9H-carbazole-9-yl)ethyl]phosphonic acid (Br-2PA It may also contain self-assembled monolayers such as Cz), [4-(9H-carbazole-9-yl)butyl]phosphonic acid (4PACz), [4-(3,6-dimethoxy-9H-carbazole-9-yl)butyl]phosphonic acid (MeO-4PACz), [4-(3,6-dimethyl-9H-carbazole-9-yl)butyl]phosphonic acid (Me-4PACz), [4-(3,6-dichloro-9H-carbazole-9-yl)butyl]phosphonic acid (Cl-4PACz), and [4-(3,6-dibromo-9H-carbazole-9-yl)butyl]phosphonic acid (Br-4PACz). Other materials for the hole injection layer 22 include, for example, a composite of poly(3,4-ethylenedioxythiophene) (PEDOT) and polystyrene sulfonic acid (PSS) (abbreviated as "PEDOT:PSS"), CuSCN (copper thiocyanate), and molybdenum oxide (MoO2). ３ ), tungsten oxide (WO ３ ), vanadium oxide (V ２ O ５ ), cuprous oxide (Cu ２Examples include 0) and cupric oxide (CuO). Furthermore, the hole implantation layer 22 may also contain bulk NiO (nickel oxide) as a material, rather than nanoparticles. These materials may be used individually or mixed in combination of two or more as appropriate. In this disclosure, the chemical formulas are representative examples. Also, in this disclosure, the composition ratios described in the chemical formulas do not necessarily have to be stoichiometric measurements where the actual compound composition matches the chemical formula.

[0022] <Light-emitting element: Hole transport layer> The hole transport layer 23 has the function of transporting holes injected from the hole injection layer 22 to the light-emitting layer 24. The hole transport layer 23 according to this embodiment includes a first layer 31, a second layer 32, and a third layer 33 stacked in order from the anode 21 toward the cathode 26. Each of the first layer 31, the second layer 32, and the third layer 33 contains a hole transport material that has the function of transporting holes injected from the hole injection layer 22. The band gaps of the first layer 31, the second layer 32, and the third layer 33 as shown in the band diagram in this disclosure correspond to the band gaps of the hole transport material contained in each of the first layer 31, the second layer 32, and the third layer 33.

[0023] In this embodiment, the ionization potential of the second layer 32 is smaller than that of the first layer 31 and the third layer 33. Also, the electron affinity of the second layer 32 is smaller than that of the first layer 31 and the third layer 33. In this disclosure, the ionization potential of the first layer 31 is denoted as IP1, the ionization potential of the second layer 32 as IP2, and the ionization potential of the third layer 33 as IP3. Also in this disclosure, the electron affinity of the first layer 31 is denoted as EA1, the electron affinity of the second layer 32 as EA2, and the electron affinity of the third layer 33 as EA3. In this case, the display device 1 according to this embodiment satisfies the following condition 1.

[0024] Condition 1: IP2 < IP1, IP2 < IP3, EA2 < EA1, and EA2 < EA3.

[0025] In the present disclosure, the ionization potential of a certain layer of the light-emitting element 2 refers to the energy difference (absolute value) between the upper end of the valence band (VBM) of the material included in the layer and the vacuum level. For example, in the band diagram of the light-emitting element 2 in the present disclosure, the lower the lower end of the band gap of a certain layer is located downward toward the paper surface, the larger the ionization potential of the layer becomes.

[0026] In the present disclosure, the electron affinity of a certain layer of the light-emitting element 2 refers to the energy difference (absolute value) between the lower end of the conduction band (CBM) of the material included in the layer and the vacuum level. For example, in the band diagram of the light-emitting element 2 in the present disclosure, the lower the upper end of the band gap of a certain layer is located downward toward the paper surface, the larger the electron affinity of the layer becomes.

[0027] The specific configuration of each layer included in the hole transport layer 23 will be described in detail later.

[0028] <Light-Emitting Element: Electron Transport Layer> The electron transport layer 25 is a layer that transports electrons injected from the cathode 26 to the light-emitting layer 24. The electron transport layer 25 includes, as an electron transport material, for example, zinc oxide (ZnO) doped with at least one of Li, Mg, Al, Ti, Ga, Zr, zinc oxide (ZnO), titanium oxide (TiO ２ ) or nanoparticles of zirconium oxide (ZrO ２ ). However, the electron transport material included in the electron transport layer 25 is not limited to nanoparticles. For example, in the electron transport layer 25, as the electron transport material, in a light-emitting element including quantum dots or the like, an organic or inorganic material having electron transport properties that has been conventionally adopted can be used. The electron transport material may include, for example, 2,2',2''-(1,3,5-benzenetriyl)-tris(1-phenyl-1H-benzimidazole) (abbreviation "TPBi"), 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (abbreviation "BCP"), 4,7-diphenyl-1,10-phenanthroline (abbreviation "Bphen"), etc. Alternatively, the electron transport layer 25 includes, as the electron transport material, bulk zinc oxide (ZnO), zinc oxide (ZnO), titanium oxide (TiO ２ ) or zirconium oxide (ZrO ２It may contain ( ). The bulk zinc oxide (ZnO) may be doped with at least one of Li, Mg, Al, Ti, Ga, and Zr. The electron transport material may contain only one kind of the materials described above, or may contain two or more kinds as appropriate.

[0029] Each of the hole injection layer 22, the hole transport layer 23, and the electron transport layer 25 may be formed by coating, for example, a dispersion in which the material is dispersed, by a spin coating method, a slit coating method, or the like. Each of the hole injection layer 22, the hole transport layer 23, and the electron transport layer 25 may be formed commonly for a plurality of sub-pixels.

[0030] <Light-emitting element: Light-emitting layer> The light-emitting layer 24 contains a plurality of quantum dots 41 as a light-emitting material. The quantum dots 41 are light-emitting quantum dots that emit light by recombination of holes and electrons injected into the light-emitting layer 24. The light-emitting element 2 emits light by extracting the light from the quantum dots 41 of the light-emitting layer 24 to the outside. The light emitted by the quantum dots 41 becomes light with a small half-value width due to the quantum confinement effect of the holes and electrons injected into the quantum dots 41. Therefore, the light-emitting element 2 improves the chromaticity of the display in the display device 1.

[0031] The quantum dots 41 may be various light-emitting quantum dots including conventionally known quantum dots. For example, the quantum dots 41 may have a core / shell structure including a core that emits light by recombination of holes and electrons and a shell that is located around the core and protects the core. The light-emitting layer 24 may contain a ligand such as an organic ligand that coordinates to the outermost surface of each of the quantum dots 41 and reduces the aggregation of the quantum dots 41 in the light-emitting layer 24.

[0032] The quantum dot 41 may include materials used for conventionally known core and shell materials in the core and shell materials, respectively. For example, the quantum dot 41 may have a core / shell structure of a group I-III-V chalcopyrite material / ZnS including InP / ZnS, CdSe / ZnS, CdSe / ZnSe, CdSe / CdS, ZnSe / ZnS, or CuInGaS (CIGS). In addition, the quantum dot 41 may contain InZnP, CdSeTe, or ZnSeTe. Furthermore, the core of the quantum dot 41 may contain CuInZnS, CuInS, CuGaS, AgInS, or ZnAgInS. The shell may be formed from multiple layers containing multiple different materials.

[0033] The light-emitting layer 24 may include an adduct that surrounds at least one quantum dot 41 or fills the space between at least two quantum dots 41. The adduct may have properties such as a semiconductor or an insulator. The adduct may include any of a metal oxide, a metalloid oxide, or a metal sulfide.

[0034] The adduct is an oxide, for example, silicon oxide (SiO₂). ２ ), titanium oxide (TiO ２ ), aluminum oxide (Al ２ O ３ ), boron oxide (B ２ O ３ ), phosphorus oxide (P ２ O ５ ), germanium oxide (GeO ２ ), hafnium oxide (HfO ２ ), zinc oxide (ZnO), zirconium oxide (ZrO ２ ), tellurium oxide (TeO ２ ), bismuth oxide (Bi ２ O ３ ), vanadium oxide (V ２ O ５ ), antimony oxide (Sb ２ O ５The adduct may contain at least one of the following: ), lead oxide (PbO), and copper oxide (CuO). Here, the adduct may be formed as a polymer. The adduct may also contain, as a sulfide, for example, zinc sulfide (ZnS) and magnesium zinc sulfide (ZnMgS, ZnMgS ２ ) may contain at least one of the following. Note that the chemical formula of the compound is a representative example, and the composition ratio indicated in the chemical formula may be stoichiometric, or not necessarily stoichiometric.

[0035] In the case where the adduct surrounds at least one quantum dot 41, the quantum dot 41 may be considered to be covered by the adduct if, in the cross-section of the light-emitting layer 24 passing through the quantum dot 41, the adduct occupies more than 80% of the area around the quantum dot 41. The adduct is, for example, mainly composed of silicon oxide and may be arranged in contact with the surface of the quantum dot 41, or with an organic ligand between the adduct and the quantum dot 41.

[0036] If the adduct is formed to fill the space between at least two quantum dots 41, the adduct is 1000 nm in a direction perpendicular to the film thickness direction DT of the light-emitting layer 24 at any position in the film thickness direction DT of the light-emitting layer 24. ２It may be formed as a continuous film having the above area. In addition, in the light-emitting layer 24, the quantum dots 41 may be embedded in the continuous film of the adduct. For example, if 60% or more of the surface of 80% or more of the quantum dots 41 constituting the light-emitting layer 24 is in contact with the continuous film of the adduct, then the quantum dots contained in the light-emitting layer 24 can be said to be embedded in the adduct. Furthermore, when we say that the adduct fills the space between at least two quantum dots 41, the space between the two quantum dots 41 may be filled only with the adduct, or it may be filled with the adduct and other materials. In addition to the adduct, a material such as a ligand different from the adduct may be included between the two quantum dots 41. The material such as the ligand may be an organic ligand that coordinates to the quantum dot 41, or it may be an organic ligand that is separated from the quantum dot 41. When the light-emitting layer 24 contains an organic ligand, for example, the weight ratio of the organic ligand to the total weight of the light-emitting layer 24 may be less than 5%. The weight ratio of the organic ligand can be measured, for example, using TOF-SIMS (time-of-flight secondary ion mass spectrometry). If the organic ligand is dispersed in the light-emitting layer, it may contribute to the injection of holes and electrons into the quantum dot 41.

[0037] In forming the light-emitting layer 24, first, a quantum dot dispersion is prepared. The quantum dot dispersion is a dispersion in which quantum dots 41 and an adduct precursor are dispersed in a solvent. The precursor includes a material that is converted into an adduct through hydrolysis and dehydration condensation by performing a specific operation such as heating or light irradiation. The precursor may have a coordinating functional group that forms a coordination bond with the surface of the quantum dots 41 in the quantum dot dispersion. In this case, the precursor may coordinate to the quantum dots in the quantum dot dispersion. The quantum dot dispersion may also contain a halide having a halogen atom (for example, zinc chloride (ZnCl)). ２The material may also contain halogen atoms. By mixing a material having halogen atoms into the quantum dot dispersion, the halogen atoms can coat the surface of the quantum dots 41 independently of the adduct, thereby reducing defects on the surface of the quantum dots 41. Therefore, the presence of halogen atoms in the adduct increases the coverage of the quantum dots in the light-emitting layer of each light-emitting device, thereby improving its light-emitting properties. For example, if the adduct contains zinc sulfide, the precursor may contain zinc xanthogenic acid, zinc thioureate, and zinc dithiocarboxylate, etc.

[0038] Next, the quantum dot dispersion is applied to the lower layer of the light-emitting layer 24 to convert the precursor into an adduct. In the case of a precursor containing zinc xanthogenic acid, for example, the zinc xanthogenic acid is decomposed to form zinc sulfide as an adduct. The conversion of the precursor into an adduct occurs sequentially around the quantum dots in the quantum dot dispersion. By this method, a light-emitting layer 24 having an adduct can be formed.

[0039] However, the light-emitting material included in the light-emitting layer 24 according to this embodiment is not limited to quantum dots 41. For example, the light-emitting layer 24 may include various light-emitting materials, including conventionally known materials, as long as it includes a material that emits light through the recombination of holes and electrons injected into the light-emitting layer 24. For example, the light-emitting layer 24 may include organic light-emitting materials such as organic fluorescent materials or organic phosphorescent materials as the light-emitting material.

[0040] The light-emitting layer 24 is formed in an island-like manner for each subpixel in a planar view. The light-emitting layer 24 also contains a light-emitting material that emits light of the corresponding subpixel's emission color. The light-emitting layer 24 may be formed, for example, by coating a thin film containing a light-emitting material that emits light of a specific emission color, and then patterning it so that the light-emitting material remains only on the subpixels corresponding to that emission color. The deposition and patterning of the light-emitting material in the formation of the light-emitting layer 24 may be repeated as many times as there are emission colors of the light-emitting elements 2 included in the display device 1.

[0041] <Light-emitting element in comparative form> The effects of the light-emitting element 2 according to this embodiment will be explained in more detail in comparison with the problems that arise in the light-emitting element in the comparative form. Figure 3 shows a schematic side cross-sectional view F3 of the display device 1A in the comparative form, and a band diagram B3 of each layer in the light-emitting element 2A shown in the schematic side cross-sectional view F3.

[0042] The comparative display device 1A differs from the display device 1 according to this embodiment in that it includes a light-emitting element 2A instead of the light-emitting element 2. The comparative display device 2A has the same configuration as the light-emitting element 2 according to this embodiment, except that it includes a hole transport layer 23A instead of the hole transport layer 23.

[0043] The hole transport layer 23A in the comparative configuration consists of only one continuous layer containing the hole transport material. For example, the hole transport layer 23A consists of a single hole transport material. Therefore, as shown in band diagram B3, the band gap of the hole transport layer 23A is substantially the same from the anode 21 side to the cathode 26 side.

[0044] <Carrier Recombination Outside the Light-Emitting Layer> The problems that arise in the light-emitting element 2A of the comparative configuration, and the method of solving these problems by the light-emitting element 2 of this embodiment, will be explained with reference to Figure 4. Figure 4 is a band diagram showing how holes and electrons are transported in the light-emitting elements of this embodiment and the comparative configuration, respectively. In particular, Figure 4 shows the band diagram B41 of the light-emitting element 2A of the comparative configuration and the band diagram B42 of the light-emitting element 2 of this embodiment. In each band diagram of Figure 4, holes H and electrons E in the hole transport layer are shown together.

[0045] In the comparative configuration, when the light-emitting element 2A is driven by applying a voltage to the anode 21, holes are injected from the anode 21 into the hole injection layer 22, and electrons are injected from the cathode 26 into the electron transport layer 25. The holes injected into the hole injection layer 22 and transported to the light-emitting layer 24 via the hole transport layer 23A, and the electrons injected into the electron transport layer 25 and transported to the light-emitting layer 24, recombine in the quantum dots 41 of the light-emitting layer 24, causing the quantum dots 41 to emit light.

[0046] However, if the recombination of holes and electrons occurs in a layer that does not contain luminescent material, such as the hole transport layer 23A, then this recombination does not contribute to the luminescence of the light-emitting layer 24. In particular, in the light-emitting element 2A, if the electron concentration becomes higher than the hole concentration in the light-emitting layer 24, electrons tend to flow out to the hole transport layer 23. As described above, the hole transport layer 23A of the light-emitting element 2A in the comparative configuration consists of a single layer and has a substantially the same band gap from the anode 21 side to the cathode 26 side. Therefore, the barrier to electron transport within the hole transport layer 23A is very small. As a result, in the hole transport layer 23A of the light-emitting element 2A in the comparative configuration, as shown in the band diagram B41, electrons E may flow out into the hole transport layer 23A and be widely distributed from the anode 21 side to the cathode 26 side of the hole transport layer 23A. Therefore, in the comparative configuration, the probability of electrons E remaining in the hole transport layer 23A recombining with holes H transported by the hole transport layer 23A increases.

[0047] Furthermore, the energy generated by the recombination of electrons and holes in the hole transport layer 23A does not contribute to the light emission of the quantum dot 41, and may be converted into factors that can degrade the hole transport layer 23A, such as heat. If the hole transport material of the hole transport layer 23A is degraded due to the recombination of electrons and holes in the hole transport layer 23A, the efficiency of hole transport from the anode 21 to the light-emitting layer 24 will decrease, and an excess of electrons may occur in the light-emitting layer 24.

[0048] Therefore, in the comparative form of the light-emitting element 2A, a decrease in the luminous efficiency of the light-emitting element 2A or a shortening of the lifespan of the light-emitting element 2A may occur.

[0049] <Reducing Carrier Recombination Outside the Emitting Layer> Generally, the hole injection barrier from the first layer to the second layer corresponds to the energy obtained by subtracting the ionization potential of the first layer from the ionization potential of the second layer. Therefore, in order to promote hole injection from the first layer to the second layer, the ionization potential of the second layer must be smaller than that of the first layer. Also, in order to reduce the efficiency of hole injection from the first layer to the second layer, the ionization potential of the second layer must be larger than that of the first layer.

[0050] Generally, the electron injection barrier from the first layer to the second layer corresponds to the energy obtained by subtracting the electron affinity of the second layer from the electron affinity of the first layer. Therefore, in order to promote electron injection from the first layer to the second layer, the electron affinity of the second layer must be greater than that of the first layer. Conversely, in order to reduce electron injection from the first layer to the second layer, the electron affinity of the second layer must be smaller than that of the first layer.

[0051] As described above, in this embodiment, the ionization potential of the second layer 32 is smaller than that of the first layer 31 and the third layer 33. Therefore, in the hole transport layer 23 of the light-emitting element 2, the injection of holes injected into the first layer 31 into the second layer 32 is promoted, while the injection of holes from the second layer 32 into the third layer 33 is reduced. As a result, as shown in band diagram B42, when the light-emitting element 2 is driven in this embodiment, holes H tend to accumulate in the second layer 32 of the hole transport layer 23 more than in the first layer 31 and the third layer 33.

[0052] On the other hand, the electron affinity of the second layer 32 is smaller than that of the first layer 31 and the third layer 33. Therefore, in the hole transport layer 23 of the light-emitting element 2, the outflow of electrons that have flowed out into the third layer 33 to the second layer 32 is reduced, while the outflow of electrons from the second layer 32 to the third layer 33 is promoted. As a result, as shown in the band diagram B42, when the light-emitting element 2 is driven in this embodiment, electrons E that have flowed out into the hole transport layer 23 tend to accumulate more in the first layer 31 and the third layer 33 than in the second layer 32 of the hole transport layer 23.

[0053] Therefore, in the hole transport layer 23 of the light-emitting element 2 according to this embodiment when it is driven, the positions where holes tend to accumulate and the positions where electrons tend to accumulate in the film thickness direction DT are different from each other. For this reason, the light-emitting element 2 according to this embodiment reduces the proximity of holes and electrons in the hole transport layer 23 when it is driven, and reduces the probability of recombination between holes and electrons in the hole transport layer 23.

[0054] As described above, the light-emitting element 2 according to this embodiment reduces the probability of recombination of holes and electrons in the hole transport layer 23 and increases the efficiency of hole transport from the anode 21 to the light-emitting layer 24. This increases the probability of recombination of holes and electrons in the light-emitting layer 24, thereby increasing the luminous efficiency of the light-emitting element 2. Furthermore, by reducing the probability of recombination of holes and electrons in the hole transport layer 23, the light-emitting element 2 according to this embodiment reduces the degradation of the hole transport material in the hole transport layer 23, thereby extending its lifespan. The display device 1 equipped with the light-emitting element 2 according to this embodiment achieves energy saving or a longer lifespan. The reduction in the probability of recombination of holes and electrons in the hole transport layer 23 by the hole transport layer 23 according to this embodiment is more strongly exhibited when there is a tendency for an excess of electrons to occur in the light-emitting layer 24 when the light-emitting element 2 is driven.

[0055] <Specific Example of Hole Transport Layer> To explain a more detailed example of the hole transport layer 23 according to this embodiment, we will return to Figure 1. In at least one of the first layer 31 and the second layer 32, and the second layer 32 and the third layer 33 according to this embodiment, the difference in ionization potential may be 0.1 eV or less. This makes it easier for holes to accumulate in the second layer 32 of the hole transport layer 23 than in the first layer 31 and the third layer 33, and increases the efficiency of hole transport from the hole injection layer 22 to the light-emitting layer 24 via the hole transport layer 23.

[0056] Furthermore, in at least one of the electron affinity differences between the first layer 31 and the second layer 32, and between the second layer 32 and the third layer 33 in this embodiment, the difference in electron affinity may be greater than 0.1 eV. This allows the light-emitting element 2 to retain electrons more easily in the first layer 31 and the third layer 33 than in the second layer 32 of the hole transport layer 23, thereby further reducing the probability of recombination between holes and electrons in the hole transport layer 23.

[0057] As shown in the schematic side cross-sectional view F1, the film thicknesses of the first layer 31, the second layer 32, and the third layer 33 in the film thickness direction DT are denoted as film thickness T31, film thickness T32, and film thickness T33, respectively. In this embodiment, film thickness T32 may be greater than film thicknesses T31 and T33. This corresponds to the case where the film thickness of the layer with the smaller ionization potential is greater than the film thickness of the other layer. In other words, if IP2 < IP1 and IP2 < IP3 are satisfied, the film thickness T32 of the second layer 32 may be greater than the film thickness T31 of the first layer 31 and the film thickness T33 of the third layer 33. Also, if IP2 > IP1 and IP2 > IP3 are satisfied, the film thickness T32 of the second layer 32 may be smaller than the film thickness T31 of the first layer 31 and the film thickness T33 of the third layer 33.

[0058] As a result, the light-emitting element 2 can reduce the proportion of the first layer 31 and the third layer 33, which are layers where electrons that have flowed out into the hole transport layer 23 tend to accumulate, in the total volume of the hole transport layer 23 compared to the proportion of the second layer 32. Furthermore, with the above configuration, the light-emitting element 2 can reduce the film thickness of the third layer 33, which reduces the transport of holes from the hole injection layer 22 to the second layer 32 and from the second layer 32 to the light-emitting layer 24. Therefore, with the above configuration, the light-emitting element 2 improves the hole transport efficiency in the hole transport layer 23 while further reducing the probability of recombination between holes and electrons in the hole transport layer 23.

[0059] Furthermore, among the first layer 31, the third layer 33, and the second layer 32, the film thickness of the layer with the smaller ionization potential may be 2 nm or more. In other words, if IP2 < IP1 and IP2 < IP3 are satisfied, the film thickness T32 of the second layer 32 may be 2 nm or more. Also, if IP2 > IP1 and IP2 > IP3 are satisfied, the film thickness T31 of the first layer 31 and the film thickness T33 of the third layer 33 may be 2 nm or more. Moreover, among the first layer 31, the third layer 33, and the second layer 32, the film thickness of the layer with the larger ionization potential may be 2 nm or less. In other words, if IP2 < IP1 and IP2 < IP3 are satisfied, the film thickness T31 of the first layer 31 and the film thickness T33 of the third layer 33 may be 2 nm or less. Also, if IP2 > IP1 and IP2 > IP3 are satisfied, the film thickness T32 of the second layer may be 2 nm or less.

[0060] For example, when IP2 < IP1 and IP2 < IP3 are satisfied, the film thickness T32 may be 2 nm or more from the viewpoint of further reducing the probability of recombination of holes and electrons in the hole transport layer 23. Also, from the viewpoint of improving the hole transport efficiency in the hole transport layer 23, the film thicknesses T31 and T33 may be 2 nm or less. Furthermore, from the viewpoint of reducing the overall electrical resistance of the light-emitting element 2, the film thickness T32 may be 30 nm or less. Also, from the viewpoint of improving the film formation properties of the first layer 31 and the third layer 33, the film thicknesses T31 and T33 may be 1 nm or more.

[0061] In this embodiment, the work function of the second layer 32 may be greater than the work function of the first layer 31 and the work function of the third layer 33. This corresponds to the work function of the layer with the smaller ionization potential being greater than the work function of the other layer. In other words, if IP2 < IP1 and IP2 < IP3 are satisfied, the work function of the second layer 32 may be greater than the work functions of the first layer 31 and the third layer 33. Also, if IP2 > IP1 and IP2 > IP3 are satisfied, the work function of the second layer 32 may be smaller than the work functions of the first layer 31 and the third layer 33.

[0062] In this disclosure, the work function of a certain layer of the light-emitting element 2 refers to the energy difference (absolute value) between the Fermi level and the vacuum level of the material contained in that layer. For example, in the band diagram of the light-emitting element 2 in this disclosure, the lower the Fermi level of a certain layer is located toward the plane of the paper, the larger the work function of that layer.

[0063] Generally, the Fermi level of a semiconductor changes depending on the concentration and polarity of the dopant in the semiconductor. For example, generally, the higher the concentration of p-type dopant in a semiconductor, the closer the Fermi level of the semiconductor tends to be to the lower end of the band gap in the band diagram of this disclosure. Therefore, in order for the work functions of the first layer 31, the second layer 32, and the third layer 33 in this embodiment to satisfy the above-mentioned relationship, it is conceivable to make the concentration of p-type dopant in the second layer 32 higher than the concentration of p-type dopant in the first layer 31 and the third layer 33.

[0064] As shown in band diagram B1, the Fermi level 32F of the second layer 32 is located below the Fermi level 31F of the first layer 31 and the Fermi level 33F of the third layer 33. As mentioned above, this corresponds to the fact that the work function of the second layer 32 is larger than the work function of the first layer 31 and the work function of the third layer 33.

[0065] The concentration of p-type dopant in the second layer 32, which has a work function greater than that of the first layer 31 and the third layer 33, tends to be higher than the concentration of p-type dopant in the first layer 31 and the third layer 33. As a result, holes are more likely to accumulate in the second layer 32 than in the first layer 31 and the third layer 33. Conversely, electrons are more likely to accumulate in the first layer 31 and the third layer 33 than in the second layer 32. Therefore, with the above configuration, the light-emitting element 2 further reduces the probability of recombination between holes and electrons in the hole transport layer 23.

[0066] In this embodiment, the hole transport material contained in the hole transport layer 23 may include poly-TPD (N,N'-bis(4-butylphenyl)-N,N'-bis(phenyl)-benzidine) in the first layer 31 and the third layer 33, and TFB (poly(2,7-(9,9-di-n-octylfluorene)-(1,4-phenylene-((4-2-butylphenyl)imino)-1,4-phenylene) in the second layer 32. Alternatively, the first layer 31 and the third layer 33 may contain CBP (4,4'-bis(carbazole- The first and third layers may contain poly-TPD, and the second layer 32 may contain CBP. Alternatively, the first and third layers may contain F8, and the second layer 32 may contain TFB. Alternatively, the first and third layers may contain F8, and the second layer 32 may contain poly-TPD. Alternatively, the first and third layers may contain F8, and the second layer 32 may contain TFB. Alternatively, the first and third layers may contain PEDOT / PFI (perfluorinated ionomer), and the second layer 32 may contain TFB.

[0067] The hole transport material described above has relatively high hole transport properties. Furthermore, when the materials contained in the first layer 31 and the third layer 33 and the second layer 32 are the above combination, the ionization potential and electron affinity can be made more different between the second layer 32 and the first layer 31 and the third layer 33. Therefore, with the above configuration, the light-emitting element 2 improves the efficiency of hole transport in the hole transport layer 23 while further reducing the probability of recombination between holes and electrons in the hole transport layer 23. Moreover, the hole transport material described above is commonly used as the material for the hole transport layer when luminescent quantum dots are used as the light-emitting material in the light-emitting layer. For this reason, the light-emitting element 2 further improves its luminescence efficiency.

[0068] <Characteristics of the Light-Emitting Element> The characteristics of the light-emitting element 2 according to this embodiment will be explained by comparing the characteristics of the light-emitting elements of the example and the comparative example. The light-emitting element according to the example has the same configuration as the light-emitting element 2 according to this embodiment. The light-emitting element according to the comparative example has the same configuration as the light-emitting element 2A according to the comparative example.

[0069] Here, in the hole transport layer 23A of the light-emitting element according to the comparative example, the film thickness in the film thickness direction DT is 10 nm, and the hole mobility is 1 × 10 ３ cm ２ Assume that the non-luminescent recombination rate is 1 ns. Also, assume that a voltage of 2 V is applied to the light-emitting element of the comparative example. In this case, the time required for a hole to move through the hole transport layer 23A from the end on the hole injection layer 22 side to the end on the light-emitting layer 24 side is 0.5 ns. From the above, when the light-emitting element of the comparative example is driven, the ratio of non-luminescent recombination occurring in the hole transport layer 23A to the holes injected into the hole transport layer 23A can be estimated to be about 33%.

[0070] On the other hand, in the hole transport layer 23 of the light-emitting element according to the embodiment, the positions of holes and electrons in the film thickness direction DT can be separated between the first layer 31 and the third layer 33 and the second layer 32. For this reason, it is easy to make the non-luminescent recombination rate in the hole transport layer 23 of the light-emitting element according to the embodiment 10 times or more longer than the non-luminescent recombination rate in the hole transport layer 23 of the light-emitting element according to the comparative example.

[0071] For example, in the hole transport layer 23 of the light-emitting element according to the embodiment, the film thickness in the film thickness direction DT is 10 nm, and the hole mobility is 1 × 10 ３ cm ２ Let's assume that the non-luminescent recombination rate is 10 ns. Also, let's assume that a voltage of 2 V is applied to the light-emitting element according to the embodiment. Even in this case, the time required for a hole to move through the hole transport layer 23 from the end on the hole injection layer 22 side to the end on the light-emitting layer 24 side is 0.5 ns. From the above, when the light-emitting element according to the embodiment is driven, the ratio of non-luminescent recombination occurring in the hole transport layer 23 to the holes injected into the hole transport layer 23 can be estimated to be reduced to about 5%.

[0072] The light emission characteristics of the light-emitting elements in the examples and comparative examples will be described with reference to Figure 5. Figure 5 is a graph showing the light emission characteristics of the light-emitting elements in the examples and comparative examples. In the graph of Figure 5, the light emission characteristics of the light-emitting elements in the examples are shown by a solid line, and the light emission characteristics of the light-emitting elements in the comparative examples are shown by a dotted line. In the graph of Figure 5, the horizontal axis represents the voltage applied to each light-emitting element (unit: V), and the vertical axis represents EQE (external quantum efficiency, unit: %).

[0073] As shown in Figure 5, the light-emitting elements of the example and comparative example each emit light when a driving voltage of approximately 1.8V or higher is applied. As is clear from Figure 5, when a driving voltage of approximately 1.8V or higher is applied, the EQE of the light-emitting element of the example is higher than that of the light-emitting element of the comparative example. This is thought to be due to a reduction in the probability of non-luminescent recombination between holes and electrons in the hole transport layer. In particular, the maximum value of the EQE of the light-emitting element of the example improves from approximately 10%, which is the maximum value of the EQE of the light-emitting element of the comparative example, to approximately 14%.

[0074] [Embodiment 2] <Modification of Hole Transport Layer> Figure 6 shows a schematic side cross-sectional view F6 of the display device 1 according to this embodiment, and a band diagram B6 of each layer in the light-emitting element 2 shown in the schematic side cross-sectional view F6. The light-emitting element 2 according to this embodiment has the same configuration as the light-emitting element 2 according to the previous embodiment, except for the configuration of the hole transport layer 23. The hole transport layer 23 according to this embodiment differs from the hole transport layer 23 according to the previous embodiment in that the band gaps and film thickness in the film thickness direction DT of the first layer 31, the second layer 32, and the third layer 33 are different.

[0075] In this embodiment, the ionization potential of the second layer 32 is greater than that of the first layer 31 and the third layer 33. Furthermore, the electron affinity of the second layer 32 is greater than that of the first layer 31 and the third layer 33. In other words, the display device 1 according to this embodiment satisfies the following condition 2.

[0076] Condition 2: IP2 > IP1, IP2 > IP3, EA2 > EA1, and EA2 > EA3.

[0077] Therefore, in the hole transport layer 23 of the light-emitting element 2 according to this embodiment, the injection of holes injected into the first layer 31 into the second layer 32 is reduced, while the injection of holes from the second layer 32 into the third layer 33 is promoted. For this reason, when the light-emitting element 2 is driven in this embodiment, holes tend to accumulate more in the first layer 31 and the third layer 33 than in the second layer 32 of the hole transport layer 23.

[0078] Furthermore, in the hole transport layer 23 of the light-emitting element 2, the outflow of electrons that have flowed out into the third layer 33 to the second layer 32 is promoted, while the outflow of electrons from the second layer 32 to the third layer 33 is reduced. For this reason, when the light-emitting element 2 is driven in this embodiment, electrons that have flowed out into the hole transport layer 23 tend to accumulate more in the second layer 32 than in the first layer 31 and the third layer 33 of the hole transport layer 23.

[0079] Therefore, in the hole transport layer 23 during operation of the light-emitting element 2 according to this embodiment, the positions where holes tend to accumulate and the positions where electrons tend to accumulate in the film thickness direction DT are different from each other. As a result, the light-emitting element 2 increases the probability of recombination between holes and electrons in the light-emitting layer 24, thereby increasing the luminous efficiency, for the same reasons as explained in the previous embodiment. Furthermore, the light-emitting element 2 according to this embodiment reduces the deterioration of the hole transport material in the hole transport layer 23, and consequently extends its lifespan, for the same reasons as explained in the previous embodiment. The display device 1 equipped with the light-emitting element 2 according to this embodiment achieves energy saving or a longer lifespan.

[0080] In this embodiment, film thicknesses T31 and T33 may be greater than film thickness T32. This corresponds to the film thickness of the first layer 31, the third layer 33, and the second layer 32 having a smaller ionization potential being greater than the film thickness of the other. With the above configuration, the light-emitting element 2 improves the hole transport efficiency in the hole transport layer 23 while further reducing the probability of recombination between holes and electrons in the hole transport layer 23, for the same reasons as explained in the previous embodiment.

[0081] In this embodiment, as shown in band diagram B6, the Fermi level 32F of the second layer 32 may be located lower in band diagram B6 than the Fermi level 31F of the first layer 31 and the Fermi level 33F of the third layer 33. This corresponds to the fact that the work function in the layer with the smaller ionization potential among the first layer 31, the third layer 33, and the second layer 32 is larger than the work function in the other layer. With the above configuration, for the same reasons mentioned above, the light-emitting element 2 further reduces the probability of recombination between holes and electrons in the hole transport layer 23.

[0082] [Embodiment 3] <Reduction of recombination in the electron transport layer> Figure 7 shows a schematic side cross-sectional view F7 of the display device 1 according to this embodiment, and a band diagram B7 of each layer in the light-emitting element 2 shown in the schematic side cross-sectional view F7. The light-emitting element 2 according to this embodiment has the same configuration as the light-emitting element 2 according to the above-described embodiments, except for the configuration of the hole transport layer 23 and the electron transport layer 25. The hole transport layer 23 according to this embodiment may include, for example, only one of the first layer 31, second layer 32, and third layer 33 according to the above-described embodiments. The electron transport layer 25 according to this embodiment includes a first layer 51, a second layer 52, and a third layer 53 stacked in order from the anode 21 toward the cathode 26.

[0083] In this embodiment, the ionization potential of the second layer 52 is smaller than that of the first layer 51 and the third layer 53. Furthermore, the electron affinity of the second layer 52 is smaller than that of the first layer 51 and the third layer 53.

[0084] Even if recombination of holes and electrons occurs in the electron transport layer 25, this recombination does not contribute to the emission of light from the light-emitting layer 24. In particular, in the light-emitting element 2, if the concentration of holes becomes higher than the concentration of electrons in the light-emitting layer 24, electrons tend to flow out to the electron transport layer 25. Furthermore, if the electron transport material of the electron transport layer 25 deteriorates due to the recombination of electrons and holes in the electron transport layer 25, the efficiency of electron transport from the cathode 26 to the light-emitting layer 24 decreases, and an excess of holes in the light-emitting layer 24 may occur.

[0085] In this embodiment, the ionization potential of the second layer 52 is smaller than that of the first layer 51 and the third layer 53. Therefore, in the electron transport layer 25 of the light-emitting element 2, the injection of holes that have flowed out into the first layer 51 into the second layer 52 is promoted, while the outflow of holes from the second layer 52 to the third layer 53 is reduced. For this reason, when the light-emitting element 2 is driven in this embodiment, holes that have flowed out into the electron transport layer 25 tend to accumulate more in the second layer 52 than in the first layer 51 and the third layer 53 of the electron transport layer 25.

[0086] On the other hand, the electron affinity of the second layer 52 is smaller than that of the first layer 51 and the third layer 53. Therefore, in the electron transport layer 25 of the light-emitting element 2, the injection of electrons injected into the third layer 53 into the second layer 52 is reduced, while the injection of electrons from the second layer 52 into the third layer 53 is promoted. For this reason, when the light-emitting element 2 is driven in this embodiment, electrons injected into the electron transport layer 25 tend to accumulate more in the first layer 51 and the third layer 53 than in the second layer 52 of the electron transport layer 25.

[0087] Therefore, in the electron transport layer 25 of the light-emitting element 2 according to this embodiment, the positions where holes tend to accumulate and the positions where electrons tend to accumulate in the film thickness direction DT are different from each other. For this reason, the light-emitting element 2 according to this embodiment reduces the proximity of holes and electrons in the electron transport layer 25 during operation, and reduces the probability of recombination between holes and electrons in the electron transport layer 25.

[0088] As described above, the light-emitting element 2 according to this embodiment reduces the probability of recombination of holes and electrons in the electron transport layer 25 and increases the efficiency of electron transport from the cathode 26 to the light-emitting layer 24. This increases the probability of recombination of holes and electrons in the light-emitting layer 24, thereby increasing the luminous efficiency of the light-emitting element 2. Furthermore, by reducing the probability of recombination of holes and electrons in the electron transport layer 25, the light-emitting element 2 according to this embodiment reduces the degradation of the electron transport material of the electron transport layer 25, thereby extending its lifespan. The display device 1 equipped with the light-emitting element 2 according to this embodiment achieves energy saving or a longer lifespan. The reduction in the probability of recombination of holes and electrons in the electron transport layer 25 by the electron transport layer 25 according to this embodiment is more strongly exhibited when there is a tendency for excess holes to occur in the light-emitting layer 24 during operation of the light-emitting element 2.

[0089] In this embodiment, the difference in electron affinity between the first layer 51 and the second layer 52, and between the second layer 52 and the third layer 53, may be 0.1 eV or less. This makes it easier for electrons to remain in the first layer 51 and the third layer 53 of the electron transport layer 25 than in the second layer 52, and increases the efficiency of electron transport from the cathode 26 to the light-emitting layer 24 via the electron transport layer 25.

[0090] Furthermore, in at least one of the ionization potential differences between the first layer 51 and the second layer 52, and between the second layer 52 and the third layer 53, the difference in ionization potential may be greater than 0.1 eV. This allows the light-emitting element 2 to retain holes more easily in the second layer 52 than in the first layer 51 and the third layer 53 of the electron transport layer 25, thereby further reducing the probability of recombination between holes and electrons in the electron transport layer 25.

[0091] As shown in the schematic side cross-sectional view F1, the film thicknesses of the first layer 51, the second layer 52, and the third layer 53 in the film thickness direction DT are denoted as film thickness T51, film thickness T52, and film thickness T53, respectively. In this embodiment, film thickness T52 may be smaller than film thicknesses T51 and T53. This corresponds to the film thickness of the layer with the smaller ionization potential being smaller than the film thickness of the other layer. In other words, if IP2 < IP1 and IP2 < IP3 are satisfied, the film thickness T52 of the second layer 52 may be smaller than the film thickness T51 of the first layer 51 and the film thickness T53 of the third layer 53. Also, if IP2 > IP1 and IP2 > IP3 are satisfied, the film thickness T52 of the second layer 52 may be larger than the film thickness T51 of the first layer 51 and the film thickness T53 of the third layer 53.

[0092] As a result, the light-emitting element 2 can reduce the proportion of the second layer 52, which is a layer where holes that have flowed out into the electron transport layer 25 tend to accumulate, within the total volume of the electron transport layer 25, compared to the proportion of the first layer 51 and the third layer 53. Furthermore, with the above configuration, the light-emitting element 2 reduces the thickness of the second layer 52, which reduces the transport of electrons from the first layer 51 to the third layer 53. Therefore, with the above configuration, the light-emitting element 2 improves the electron transport efficiency in the electron transport layer 25 while further reducing the probability of recombination between holes and electrons in the electron transport layer 25.

[0093] Furthermore, among the first layer 51, the third layer 53, and the second layer 52, the film thickness of the layer with the larger ionization potential may be 2 nm or more. In other words, if IP2 < IP1 and IP2 < IP3 are satisfied, the film thickness T51 of the first layer 51 and the film thickness T53 of the third layer 53 may be 2 nm or more. If IP2 > IP1 and IP2 > IP3 are satisfied, the film thickness T52 of the second layer 52 may be 2 nm or more. Moreover, among the first layer 51, the third layer 53, and the second layer 52, the film thickness of the layer with the smaller ionization potential may be 2 nm or less. In other words, if IP2 < IP1 and IP2 < IP3 are satisfied, the film thickness T32 of the second layer 52 may be 2 nm or less. Also, if IP2 > IP1 and IP2 > IP3 are satisfied, the film thickness T51 of the first layer 51 and the film thickness T53 of the third layer 53 may be 2 nm or less.

[0094] For example, when IP2 < IP1 and IP2 < IP3 are satisfied, the film thickness T51 and T53 may be 2 nm or more from the viewpoint of further reducing the probability of hole-electron recombination in the electron transport layer 25. Also, from the viewpoint of improving the electron transport efficiency in the electron transport layer 25, the film thickness T52 may be 2 nm or less. Furthermore, from the viewpoint of improving the film formation properties of the second layer 52, the film thickness T52 may be 30 nm or more. Also, from the viewpoint of reducing the overall electrical resistance of the light-emitting element 2, the film thickness T51 and T53 may be 1 nm or less.

[0095] In this embodiment, as shown in the band diagram B7, the Fermi level 51F of the first layer 51 and the Fermi level 53F of the third layer 53 may be located lower on the band diagram B7 than the Fermi level 52F of the second layer 52. This corresponds to the work function in the layer with the larger ionization potential being greater than the work function in the other layer.

[0096] The concentrations of n-type dopants in the first layer 51 and the third layer 53, which have work functions greater than those of the second layer 52, tend to be lower than those of n-type dopants in the second layer 52. As a result, holes are more likely to accumulate in the second layer 52 than in the first layer 51 and the third layer 53. Conversely, electrons are more likely to accumulate in the first layer 51 and the third layer 53 than in the second layer 52. Therefore, with the above configuration, the light-emitting element 2 further reduces the probability of recombination between holes and electrons in the electron transport layer 25.

[0097] In this embodiment, the electron transport material included in the electron transport layer 25 may be such that the first layer 51 and the third layer 53 contain ZnO and the second layer 52 contains TPBi. Alternatively, the first layer 51 and the third layer 53 may contain ZnMgO and the second layer 52 may contain TPBi. Alternatively, the first layer 51 and the third layer 53 may contain ZnO and the second layer 52 may contain BCP. Alternatively, the first layer 51 and the third layer 53 may contain ZnMgO and the second layer 52 may contain BCP. Alternatively, the first layer 51 and the third layer 53 may contain ZnO and the second layer 52 may contain PBD. Alternatively, the first layer 51 and the third layer 53 may contain ZnMgO and the second layer 52 may contain PBD.

[0098] The electron transport material described above has relatively high electron transport properties. Furthermore, when the materials contained in the first layer 51 and the third layer 53 and the second layer 52 are the above combination, the ionization potential and electron affinity can be made more different between the second layer 52 and the first layer 51 and the third layer 53. Therefore, with the above configuration, the light-emitting element 2 improves the efficiency of electron transport in the electron transport layer 25 while further reducing the probability of hole-electron recombination in the electron transport layer 25. Moreover, the electron transport material described above is commonly used as the material for the electron transport layer when luminescent quantum dots are used as the light-emitting material in the light-emitting layer. For this reason, the light-emitting element 2 further improves its luminescence efficiency.

[0099] [Embodiment 4] <Modification of the electron transport layer> Figure 8 shows a schematic side cross-sectional view F8 of the display device 1 according to this embodiment, and a band diagram B8 of each layer in the light-emitting element 2 shown in the schematic side cross-sectional view F8. The light-emitting element 2 according to this embodiment has the same configuration as the light-emitting element 2 according to the previous embodiment, except for the configuration of the electron transport layer 25. The electron transport layer 25 according to this embodiment differs from the electron transport layer 25 according to the previous embodiment in that the band gaps and film thickness in the film thickness direction DT of the first layer 51, the second layer 52, and the third layer 53 are different.

[0100] In this embodiment, the ionization potential of the second layer 52 is greater than that of the first layer 51 and the third layer 53. Furthermore, the electron affinity of the second layer 52 is greater than that of the first layer 51 and the third layer 53.

[0101] Therefore, in the electron transport layer 25 of the light-emitting element 2 according to this embodiment, the outflow of holes that have flowed out into the first layer 51 to the second layer 52 is reduced, while the outflow of holes from the second layer 52 to the third layer 53 is promoted. For this reason, when the light-emitting element 2 is driven in this embodiment, holes tend to accumulate more in the first layer 51 and the third layer 53 of the electron transport layer 25 than in the second layer 52.

[0102] Furthermore, in the electron transport layer 25 of the light-emitting element 2, the injection of electrons injected into the third layer 53 into the second layer 52 is promoted, while the injection of electrons from the second layer 52 into the third layer 53 is reduced. For this reason, when the light-emitting element 2 is driven in this embodiment, electrons injected into the electron transport layer 25 tend to accumulate more in the second layer 52 than in the first layer 51 and the third layer 53 of the electron transport layer 25.

[0103] Therefore, in the electron transport layer 25 during operation of the light-emitting element 2 according to this embodiment, the positions where holes tend to accumulate and the positions where electrons tend to accumulate in the film thickness direction DT are different from each other. As a result, the light-emitting element 2 increases the probability of recombination between holes and electrons in the light-emitting layer 24, thereby increasing the luminous efficiency, for the same reasons as explained in the previous embodiment. Furthermore, the light-emitting element 2 according to this embodiment reduces the degradation of the electron transport material in the electron transport layer 25, and consequently extends its lifespan, for the same reasons as explained in the previous embodiment. The display device 1 equipped with the light-emitting element 2 according to this embodiment achieves energy saving or a longer lifespan.

[0104] In this embodiment, film thicknesses T51 and T53 may be smaller than film thickness T52. This corresponds to the film thickness of the layer with the smaller ionization potential being smaller than the film thickness of the other layer, among the first layer 51 and the third layer 53 and the second layer 52. With the above configuration, the light-emitting element 2 improves the electron transport efficiency in the electron transport layer 25 while further reducing the probability of recombination between holes and electrons in the electron transport layer 25, for the same reasons as explained in the previous embodiment.

[0105] In this embodiment, as shown in the band diagram B8, the Fermi level 32F of the second layer 52 may be located lower in the band diagram B8 than the Fermi level 31F of the first layer 51 and the Fermi level 33F of the third layer 53. This corresponds to the work function in the layer with the larger ionization potential (the first layer 51, the third layer 53, and the second layer 52) being larger than the work function in the other layer. With this configuration, for the same reasons mentioned above, the light-emitting element 2 further reduces the probability of hole-electron recombination in the electron transport layer 25.

[0106] [Embodiment 5] <Fourth Hole Transport Layer> Figure 9 shows a schematic side cross-sectional view F9 of the display device 1 according to this embodiment, and a band diagram B9 of each layer in the light-emitting element 2 shown in the schematic side cross-sectional view F9. The light-emitting element 2 according to this embodiment has the same configuration as the light-emitting element 2 according to Embodiment 1, except that the hole transport layer 23 further comprises a fourth layer 34.

[0107] The fourth layer 34 is located on the cathode 26 side of the hole transport layer 23 than the third layer 33, and may, for example, be in contact with the cathode 26 side of the third layer 33. In particular, the fourth layer 34 may form the cathode 26 side end of the hole transport layer 23, and may, for example, be in contact with the anode 21 side of the light-emitting layer 24.

[0108] The ionization potential of the fourth layer 34 is smaller than that of the first layer 31 and the third layer 33. Also, the electron affinity of the fourth layer 34 is smaller than that of the first layer 31 and the third layer 33. For the same reasons as described above, holes tend to accumulate more easily in the fourth layer 34 than in the first layer 31 and the third layer 33, and electrons tend to accumulate more easily in the first layer 31 and the third layer 33 than in the fourth layer 34.

[0109] Therefore, in the hole transport layer 23 according to this embodiment, electrons tend to accumulate in the first layer 31 and the third layer 33, while holes tend to accumulate in the second layer 32 and the fourth layer 34. As a result, in the hole transport layer 23 according to this embodiment, the positions of holes injected from the hole injection layer 22 and electrons flowing out from the light-emitting layer 24 in the film thickness direction DT can be made to be more different. For the same reasons as described above, the light-emitting element 2 according to this embodiment reduces the probability of recombination of electrons and holes in the hole transport layer 23 and reduces the deterioration of the hole transport material in the hole transport layer 23.

[0110] The film thickness T34 of the fourth layer 34 in the film thickness direction DT may be greater than that of film thicknesses T31 and T33. The work function of the fourth layer 34 may be greater than that of film thicknesses T31 and T33. In particular, the fourth layer 34 may have the same configuration as the second layer 32. In this case, the method for forming the second layer 32 and the method for forming the fourth layer 34 in the manufacturing method of the light-emitting element 2 can be the same, further simplifying the manufacturing method of the light-emitting element 2.

[0111] [Embodiment 6] <Fourth layer of electron transport layer> Figure 10 shows a schematic side cross-sectional view F10 of the display device 1 according to this embodiment, and a band diagram B10 of each layer in the light-emitting element 2 shown in the schematic side cross-sectional view F10. The light-emitting element 2 according to this embodiment has the same configuration as the light-emitting element 2 according to Embodiment 3, except that the electron transport layer 25 further comprises a fourth layer 54.

[0112] The fourth layer 54 is located on the cathode 26 side of the electron transport layer 25 than the third layer 53, and may, for example, be in contact with the cathode 26 side of the third layer 53. In particular, the fourth layer 54 may form the cathode 26 side end of the electron transport layer 25, and may, for example, be in contact with the anode 21 side of the cathode 26.

[0113] The ionization potential of the fourth layer 54 is smaller than that of the first layer 51 and the third layer 53. Also, the electron affinity of the fourth layer 54 is smaller than that of the first layer 51 and the third layer 53. For the same reasons as described above, holes tend to accumulate more easily in the fourth layer 54 than in the first layer 51 and the third layer 53, and electrons tend to accumulate more easily in the first layer 51 and the third layer 53 than in the fourth layer 54.

[0114] Therefore, in the electron transport layer 25 according to this embodiment, electrons tend to accumulate in the first layer 51 and the third layer 53, while holes tend to accumulate in the second layer 52 and the fourth layer 54. As a result, in the electron transport layer 25 according to this embodiment, the positions of electrons injected from the cathode 26 and holes flowing out from the light-emitting layer 24 in the film thickness direction DT can be made to be more different. For the same reasons as described above, the light-emitting element 2 according to this embodiment reduces the probability of electron-hole recombination in the electron transport layer 25 and reduces the degradation of the electron transport material in the electron transport layer 25.

[0115] The film thickness T34 of the fourth layer 54 in the film thickness direction DT may be smaller than that of film thicknesses T31 and T33. The work function of the fourth layer 54 may be smaller than that of film thicknesses T31 and T33. In particular, the fourth layer 54 may have the same configuration as the second layer 52. In this case, the method for forming the second layer 52 and the method for forming the fourth layer 54 in the manufacturing method of the light-emitting element 2 can be the same, further simplifying the manufacturing method of the light-emitting element 2.

[0116] [Embodiment 7] <Laminated structure of hole transport layer and electron transport layer> Figure 11 shows a schematic side cross-sectional view F11 of the display device 1 according to this embodiment, and a band diagram B11 of each layer in the light-emitting element 2 shown in the schematic side cross-sectional view F11. The light-emitting element 2 according to this embodiment has the same configuration as the light-emitting element 2 according to Embodiment 1, except that the electron transport layer 25 has the same configuration as the electron transport layer 25 according to Embodiment 3.

[0117] Therefore, for the same reasons as described above, the light-emitting element 2 according to this embodiment reduces the probability of electron-hole recombination in the hole transport layer 23 and reduces the degradation of the hole transport material in the hole transport layer 23. In addition, for the same reasons as described above, the light-emitting element 2 according to this embodiment reduces the probability of electron-hole recombination in the electron transport layer 25 and reduces the degradation of the electron transport material in the electron transport layer 25. Thus, the light-emitting element 2 according to this embodiment further improves the luminous efficiency and improves the reliability of both the hole transport layer 23 and the electron transport layer 25.

[0118] [Embodiment 8] <Specific Example of Display Device> Figure 12 is a schematic side cross-sectional view of the display device 1 according to this embodiment. Figure 12 particularly shows a side cross-section of the display device 1 in a plane passing through the red light-emitting element 2R, the green light-emitting element 2G, and the blue light-emitting element 2B of the display device 1.

[0119] For example, the anode 21 according to this embodiment comprises anode 21R, anode 21G, and anode 21B arranged in an island-like configuration on the substrate 3. In a plan view of the substrate 3, anodes 21R, 21G, and 21B each overlap with the red subpixel XR, green subpixel XG, and blue subpixel XB, respectively. Anodes 21R, 21G, and 21B may each have the same configuration as the anode 21 according to the respective embodiments described above.

[0120] For example, the hole transport layer 23 according to this embodiment comprises hole transport layer 23R, hole transport layer 23G, and hole transport layer 23B arranged in an island-like configuration on the substrate 3. In a plan view of the substrate 3, hole transport layer 23R, hole transport layer 23G, and hole transport layer 23B each overlap with the red subpixel XR, green subpixel XG, and blue subpixel XB, respectively.

[0121] The hole transport layer 23R includes a first layer 31R, a second layer 32R, and a third layer 33R stacked in order from the anode 21R side. The hole transport layer 23G includes a first layer 31G, a second layer 32G, and a third layer 33G stacked in order from the anode 21G side. The hole transport layer 23B includes a first layer 31B, a second layer 32B, and a third layer 33B stacked in order from the anode 21B side.

[0122] Details of the hole transport layer 23R, hole transport layer 23G, and hole transport layer 23B according to this embodiment will be described later.

[0123] For example, the light-emitting layer 24 according to this embodiment comprises a red light-emitting layer 24R, a green light-emitting layer 24G, and a blue light-emitting layer 24B arranged in an island-like configuration on the substrate 3. The red light-emitting layer 24R, the green light-emitting layer 24G, and the blue light-emitting layer 24B each overlap with the red subpixel XR, the green subpixel XG, and the blue subpixel XB, respectively, in a plan view of the substrate 3. The red light-emitting layer 24R, the green light-emitting layer 24G, and the blue light-emitting layer 24B each include a red quantum dot 41R that emits red light, a green quantum dot 41G that emits green light, and a blue quantum dot 41B that emits blue light, respectively. The red quantum dot 41R, the green quantum dot 41G, and the blue quantum dot 41B may all have the same configuration as the quantum dot 41 according to each of the embodiments described above, except for the emission color.

[0124] Furthermore, the display device 1 according to this embodiment includes a bank BK on the substrate 3. The bank BK partitions the anode 21, hole transport layer 23, and light-emitting layer 24 for each subpixel. The bank BK may include, for example, a light-transmitting resin material.

[0125] On the other hand, the hole injection layer 22, electron transport layer 25, and cathode 26 according to this embodiment may be commonly located for multiple subpixels in a plan view of the substrate 3. In particular, a portion of the hole injection layer 22 may be located on the bank BK.

[0126] Each of the red light-emitting element 2R, green light-emitting element 2G, and blue light-emitting element 2B includes a layer on the substrate 3 that is located in a position overlapping with each of the red subpixel XR, green subpixel XG, and blue subpixel XB in a plan view of the substrate 3.

[0127] For example, the red light-emitting element 2R comprises an anode 21R, a hole injection layer 22, a hole transport layer 23R, a red light-emitting layer 24R, an electron transport layer 25, and a cathode 26, which overlap with the red subpixel XR in a plan view of the substrate 3. The green light-emitting element 2G comprises an anode 21G, a hole injection layer 22, a hole transport layer 23G, a green light-emitting layer 24G, an electron transport layer 25, and a cathode 26, which overlap with the green subpixel XG in a plan view of the substrate 3. Furthermore, the blue light-emitting element 2B comprises an anode 21B, a hole injection layer 22, a hole transport layer 23B, a blue light-emitting layer 24B, an electron transport layer 25, and a cathode 26, which overlap with the blue subpixel XB in a plan view of the substrate 3.

[0128] Display device 1 extracts red light from the red quantum dot 41R of the red light-emitting layer 24R of the red light-emitting element 2R located in the red subpixel XR. Display device 1 also extracts green light from the green quantum dot 41G of the green light-emitting layer 24G of the green light-emitting element 2G located in the green subpixel XG. Furthermore, display device 1 extracts blue light from the blue quantum dot 41B of the blue light-emitting layer 24B of the blue light-emitting element 2B located in the blue subpixel XB. As a result, display device 1 achieves full-color display.

[0129] <Differences in the hole transport layer due to subpixels> The hole transport layer 23 according to this embodiment will be described in more detail with reference to Figure 13 in conjunction with Figure 12. Figure 13 shows the band diagrams B13G for each layer of the hole transport layer 23G, and the band diagrams B13B for each layer of the hole transport layer 23B according to this embodiment.

[0130] In this embodiment, the ionization potential of the second layer 32R is smaller than that of the first layer 31R and the third layer 33R. Also, the electron affinity of the second layer 32R is smaller than that of the first layer 31R and the third layer 33R. Furthermore, the ionization potential of the second layer 32G is smaller than that of the first layer 31G and the third layer 33G. Also, the electron affinity of the second layer 32G is smaller than that of the first layer 31G and the third layer 33G. Moreover, the ionization potential of the second layer 32B is smaller than that of the first layer 31B and the third layer 33B. Also, the electron affinity of the second layer 32B is smaller than that of the first layer 31B and the third layer 33B.

[0131] In other words, the relative magnitudes of the ionization potential and electron affinity of each layer of the hole transport layer 23R, hole transport layer 23G, and hole transport layer 23B are all the same as the relative magnitudes of the ionization potential and electron affinity of each layer of the hole transport layer 23 according to Embodiment 1. To put it another way, each of the red light-emitting element 2R, green light-emitting element 2G, and blue light-emitting element 2B according to this embodiment has the same configuration as the light-emitting element 2 according to Embodiment 1, except for the lower band gap level and film thickness of each second layer of the hole transport layer 23.

[0132] Therefore, the display device 1 according to this embodiment reduces the probability of recombination of holes and electrons in the hole transport layer 23 in each of the red light-emitting element 2R, green light-emitting element 2G, and blue light-emitting element 2B, thereby improving luminescence efficiency. Furthermore, the display device 1 according to this embodiment improves the reliability of the hole transport layer 23 in each of the red light-emitting element 2R, green light-emitting element 2G, and blue light-emitting element 2B.

[0133] In this embodiment, the difference in ionization potential between the second layer 32R and at least one of the first layer 31R and the third layer 33R is smaller than the difference in ionization potential between the second layer 32G and at least one of the first layer 31G and the third layer 33G. Also in this embodiment, the difference in ionization potential between the second layer 32G and at least one of the first layer 31G and the third layer 33G is smaller than the difference in ionization potential between the second layer 32B and at least one of the first layer 31B and the third layer 33B.

[0134] Generally, the band gap of light-emitting materials containing quantum dots increases as the wavelength of the light emitted by the material decreases. Therefore, in light-emitting devices, the efficiency of carrier injection into the light-emitting layer may decrease as the wavelength of the emitted light decreases. In particular, if there is a tendency for electron excess in the light-emitting layer in such devices, this excess may become more pronounced as the wavelength of the light emitted by the device decreases.

[0135] With the above configuration, in this embodiment, holes are more likely to accumulate in the second layer 32G of the hole transport layer 23G compared to the second layer 32R of the hole transport layer 23R, and consequently, the hole transport efficiency in the hole transport layer 23G is improved compared to the hole transport layer 23R. Furthermore, with the above configuration, in this embodiment, holes are more likely to accumulate in the second layer 32B of the hole transport layer 23B compared to the second layer 32G of the hole transport layer 23G, and consequently, the hole transport efficiency in the hole transport layer 23B is improved compared to the hole transport layer 23G.

[0136] Therefore, the display device 1 according to this embodiment improves the hole injection efficiency into the light-emitting layer 24 as the emission wavelength of the light-emitting elements becomes shorter. Thus, the display device 1 reduces the difference in luminous efficiency among the light-emitting elements, simplifies the control content of each light-emitting element, or improves the display quality.

[0137] In this embodiment, the film thickness of the second layer 32R in the film thickness direction DT is smaller than the film thickness of the second layer 32G in the film thickness direction DT. This corresponds to the fact that among the first layer 31R and the third layer 33R and the second layer 32R, the film thickness of the layer with the smaller ionization potential is smaller than the film thickness of the first layer 31G and the third layer 33G and the second layer 32G, which has the smaller ionization potential. Also in this embodiment, the film thickness of the second layer 32G in the film thickness direction DT is smaller than the film thickness of the second layer 32B in the film thickness direction DT. This corresponds to the fact that among the first layer 31G and the third layer 33G and the second layer 32G, the film thickness of the layer with the smaller ionization potential is smaller than the film thickness of the first layer 31B and the third layer 33B and the second layer 32B, which has the smaller ionization potential.

[0138] Here, let RIP1 be the ionization potential of the first layer of the red light-emitting element 2R, RIP2 be the ionization potential of the second layer of the red light-emitting element 2R, and RIP3 be the ionization potential of the third layer of the red light-emitting element 2R. Also, let GIP1 be the ionization potential of the first layer of the green light-emitting element 2G, GIP2 be the ionization potential of the second layer of the green light-emitting element 2G, and GIP3 be the ionization potential of the third layer of the green light-emitting element 2G. Furthermore, let BIP1 be the ionization potential of the first layer of the blue light-emitting element 2B, BIP2 be the ionization potential of the second layer of the blue light-emitting element 2B, and BIP3 be the ionization potential of the third layer of the blue light-emitting element 2B.

[0139] In this case, the following relationship may hold true between the red light-emitting element 2R and the green light-emitting element 2G.

[0140] Assume that RIP2 < RIP1 and RIP2 < RIP3, and that GIP2 < GIP1 and GIP2 < GIP3. In this case, the film thickness of the second layer 32R of the red light-emitting element 2R may be smaller than the film thickness of the second layer 32G of the green light-emitting element 2G.

[0141] Assume that RIP2 < RIP1 and RIP2 < RIP3, and that GIP2 > GIP1 and GIP2 > GIP3. In this case, the film thickness of the second layer 32R of the red light-emitting element 2R may be smaller than the film thickness of the first layer 31G of the green light-emitting element 2G and the third layer 33G of the green light-emitting element 2G.

[0142] Assume that RIP2 > RIP1 and RIP2 > RIP3, and that GIP2 < GIP1 and GIP2 < GIP3. In this case, the film thickness of the first layer 31R of the red light-emitting element 2R and the third layer 33R of the red light-emitting element 2R may be smaller than the film thickness of the second layer 32G of the green light-emitting element 2G.

[0143] Assume that RIP2 > RIP1 and RIP2 > RIP3, and that GIP2 > GIP1 and GIP2 > GIP3. In this case, the film thickness of the first layer 31R of the red light-emitting element 2R and the third layer 33R of the red light-emitting element 2R may be smaller than the film thickness of the first layer 31G of the green light-emitting element 2G and the third layer 33G of the green light-emitting element 2G.

[0144] In addition to satisfying the above relationship between the red light-emitting element 2R and the green light-emitting element 2G, the following may also be true regarding the relationship between the green light-emitting element 2G and the blue light-emitting element 2B.

[0145] Assume that GIP2 < GIP1 and GIP2 < GIP3, and that BIP2 < BIP1 and BIP2 < BIP3. In this case, the film thickness of the second layer 32G of the green light-emitting element 2G may be smaller than the film thickness of the second layer 32B of the blue light-emitting element 2B.

[0146] Assume that GIP2 < GIP1 and GIP2 < GIP3, and that BIP2 > BIP1 and BIP2 > BIP3. In this case, the film thickness of the second layer 32G of the green light-emitting element 2G may be smaller than the film thickness of the first layer 31B of the blue light-emitting element 2B and the third layer 33B of the blue light-emitting element 2B.

[0147] Assume that GIP2 > GIP1 and GIP2 > GIP3, and that BIP2 < BIP1 and BIP2 < BIP3. In this case, the film thickness of the first layer 31G of the green light-emitting element 2G and the third layer 33G of the green light-emitting element 2G may be smaller than the film thickness of the second layer 32B of the blue light-emitting element 2B.

[0148] Assume that GIP2 > GIP1 and GIP2 > GIP3, and that BIP2 > BIP1 and BIP2 > BIP3. In this case, the film thickness of the first layer 31G of the green light-emitting element 2G and the third layer 33G of the green light-emitting element 2G may be smaller than the film thickness of the first layer 31B of the blue light-emitting element 2B and the third layer 33B of the blue light-emitting element 2B.

[0149] With the above configuration, in this embodiment, the concentration of holes in the hole transport layer 23G is improved compared to the hole transport layer 23R, and consequently, the hole transport efficiency in the hole transport layer 23G is improved compared to the hole transport layer 23R. Furthermore, with the above configuration, in this embodiment, the concentration of holes in the hole transport layer 23B is improved compared to the hole transport layer 23G, and consequently, the hole transport efficiency in the hole transport layer 23B is improved compared to the hole transport layer 23G.

[0150] Therefore, the display device 1 according to this embodiment further improves the hole injection efficiency into the light-emitting layer 24 as the emission wavelength of the light-emitting elements becomes shorter. Thus, the display device 1 reduces the difference in luminous efficiency among the light-emitting elements, further simplifies the control content of each light-emitting element, or further improves the display quality.

[0151] Furthermore, in the film thickness direction DT, the film thicknesses of the hole transport layer 23R, hole transport layer 23G, and hole transport layer 23B may be approximately the same. This allows the display device 1 to have a substantially uniform film thickness of the hole transport layer 23 regardless of the subpixel. Therefore, the display device 1 can achieve more uniform characteristics for each light-emitting element, or the manufacturing method can be simplified.

[0152] The hole transport layer 23 according to this embodiment may be formed by repeatedly applying a dispersion containing a hole transport material to each subpixel by inkjet printing or the like, and then drying the dispersion. Alternatively, the hole transport layer 23 may be formed by repeatedly applying a dispersion containing a hole transport material and patterning it, thereby forming each layer for each subpixel. In the hole transport layer 23 formation process, the film thickness of each layer of the hole transport layer 23 may be adjusted by adjusting the concentration of the hole transport material in the dispersion to be applied.

[0153] [Embodiment 9] <Other Specific Examples of Display Devices> Figure 14 is a schematic side cross-sectional view of the display device 1 according to this embodiment. Figure 14 particularly shows a side cross-section of the display device 1 in a plane passing through the red light-emitting element 2R, the green light-emitting element 2G, and the blue light-emitting element 2B of the display device 1.

[0154] The hole transport layer 23 of the display device 1 according to this embodiment differs in configuration from the hole transport layer 23 of the display device 1 according to the previous embodiment in that it is commonly located for multiple subpixels in a plan view of the substrate 3. Furthermore, the electron transport layer 25 of the display device 1 according to this embodiment differs in configuration from the electron transport layer 25 of the display device 1 according to the previous embodiment in that it is formed in an island-like manner for each subpixel. In this embodiment, the bank BK may be located on the hole transport layer 23 in addition to the substrate 3. The bank BK on the hole transport layer 23 may partition the light-emitting layer 24 and the electron transport layer 25 for each subpixel.

[0155] For example, the electron transport layer 25 according to this embodiment comprises electron transport layer 25R, electron transport layer 25G, and electron transport layer 25B arranged in an island-like configuration on the substrate 3. In a plan view of the substrate 3, electron transport layer 25R, electron transport layer 25G, and electron transport layer 25B each overlap with the red subpixel XR, green subpixel XG, and blue subpixel XB, respectively.

[0156] The electron transport layer 25R includes a first layer 51R, a second layer 52R, and a third layer 53R stacked in order from the anode 21R side. The electron transport layer 25G includes a first layer 51G, a second layer 52G, and a third layer 53G stacked in order from the anode 21G side. The electron transport layer 25B includes a first layer 51B, a second layer 52B, and a third layer 53B stacked in order from the anode 21B side.

[0157] Except as described above, the display device 1 according to this embodiment has the same configuration as the display device 1 according to the previous embodiment.

[0158] <Differences in the electron transport layer due to subpixels> The electron transport layer 25 according to this embodiment will be described in more detail with reference to Figure 15 in conjunction with Figure 14. Figure 15 shows the band diagrams B15G for each layer of the electron transport layer 25G, the band diagrams B15B for each layer of the electron transport layer 25B according to this embodiment.

[0159] In this embodiment, the ionization potential of the second layer 52R is smaller than that of the first layer 51R and the third layer 53R. Also, the electron affinity of the second layer 52R is smaller than that of the first layer 51R and the third layer 53R. Furthermore, the ionization potential of the second layer 52G is smaller than that of the first layer 51G and the third layer 53G. Also, the electron affinity of the second layer 52G is smaller than that of the first layer 51G and the third layer 53G. Moreover, the ionization potential of the second layer 52B is smaller than that of the first layer 51B and the third layer 53B. Also, the electron affinity of the second layer 52B is smaller than that of the first layer 51B and the third layer 53B.

[0160] In other words, the relative magnitudes of the ionization potential and electron affinity of each layer of the electron transport layer 25R, electron transport layer 25G, and electron transport layer 25B are all the same as the relative magnitudes of the ionization potential and electron affinity of each layer of the electron transport layer 25 according to Embodiment 3. To put it another way, each of the red light-emitting element 2R, green light-emitting element 2G, and blue light-emitting element 2B according to this embodiment has the same configuration as the light-emitting element 2 according to Embodiment 3, except for the upper level of the band gap and the film thickness of each second layer of the electron transport layer 25.

[0161] Therefore, the display device 1 according to this embodiment reduces the probability of hole-electron recombination in the electron transport layer 25 in each of the red light-emitting element 2R, green light-emitting element 2G, and blue light-emitting element 2B, thereby improving luminous efficiency. Furthermore, the display device 1 according to this embodiment improves the reliability of the electron transport layer 25 in each of the red light-emitting element 2R, green light-emitting element 2G, and blue light-emitting element 2B.

[0162] In this embodiment, the difference in electron affinity between the second layer 52R and at least one of the first layer 51R and the third layer 53R is greater than the difference in electron affinity between the second layer 52G and at least one of the first layer 51G and the third layer 53G. Also in this embodiment, the difference in electron affinity between the second layer 52G and at least one of the first layer 51G and the third layer 53G is greater than the difference in electron affinity between the second layer 52B and at least one of the first layer 51B and the third layer 53B.

[0163] With the above configuration, in this embodiment, the electron transport efficiency from the third layer 53G to the first layer 51G of the electron transport layer 25G is higher than the electron transport efficiency from the first layer 51R to the third layer 53R of the electron transport layer 25R. Therefore, the electron transport efficiency in the electron transport layer 25G is improved compared to the electron transport layer 25R. Also, with the above configuration, in this embodiment, the electron transport efficiency from the third layer 53B to the first layer 51B of the electron transport layer 25B is higher than the electron transport efficiency from the first layer 51G to the third layer 53G of the electron transport layer 25G. Therefore, the electron transport efficiency in the electron transport layer 25B is improved compared to the electron transport layer 25G.

[0164] Therefore, the display device 1 according to this embodiment improves the electron injection efficiency into the light-emitting layer 24 as the emission wavelength of the light-emitting elements becomes shorter. Thus, the display device 1 reduces the difference in luminous efficiency among the light-emitting elements, simplifies the control content of each light-emitting element, or improves the display quality.

[0165] In this embodiment, the film thickness of the second layer 52R in the film thickness direction DT is greater than the film thickness of the second layer 52G in the film thickness direction DT. This corresponds to the fact that, among the first layer 51R and the third layer 53R and the second layer 52R, the film thickness of the layer with the smaller ionization potential is greater than the film thickness of the first layer 51G and the third layer 53G and the second layer 52G, which has the smaller ionization potential. Also in this embodiment, the film thickness of the second layer 52G in the film thickness direction DT is greater than the film thickness of the second layer 52B in the film thickness direction DT. This corresponds to the fact that, among the first layer 51G and the third layer 53G and the second layer 52G, the film thickness of the layer with the smaller ionization potential is greater than the film thickness of the first layer 51B and the third layer 53B and the second layer 52B, which has the smaller ionization potential.

[0166] In other words, the following may hold true regarding the relationship between the red light-emitting element 2R and the green light-emitting element 2G.

[0167] Assume that RIP2 < RIP1 and RIP2 < RIP3, and that GIP2 < GIP1 and GIP2 < GIP3. In this case, the film thickness of the second layer 52R of the red light-emitting element 2R may be greater than the film thickness of the second layer 52G of the green light-emitting element 2G.

[0168] Assume that RIP2 < RIP1 and RIP2 < RIP3, and that GIP2 > GIP1 and GIP2 > GIP3. In this case, the film thickness of the second layer 52R of the red light-emitting element 2R may be greater than the film thickness of the first layer 51G of the green light-emitting element 2G and the third layer 53G of the green light-emitting element 2G.

[0169] Assume that RIP2 > RIP1 and RIP2 > RIP3, and that GIP2 < GIP1 and GIP2 < GIP3. In this case, the film thickness of the first layer 51R of the red light-emitting element 2R and the third layer 53R of the red light-emitting element 2R may be greater than the film thickness of the second layer 52G of the green light-emitting element 2G.

[0170] Assume that RIP2 > RIP1 and RIP2 > RIP3, and that GIP2 > GIP1 and GIP2 > GIP3. In this case, the film thickness of the first layer 51R of the red light-emitting element 2R and the third layer 53R of the red light-emitting element 2R may be greater than the film thickness of the first layer 51G of the green light-emitting element 2G and the third layer 53G of the green light-emitting element 2G.

[0171] In addition to satisfying the above relationship between the red light-emitting element 2R and the green light-emitting element 2G, the following may also be true regarding the relationship between the green light-emitting element 2G and the blue light-emitting element 2B.

[0172] Assume that GIP2 < GIP1 and GIP2 < GIP3, and that BIP2 < BIP1 and BIP2 < BIP3. In this case, the film thickness of the second layer 52G of the green light-emitting element 2G may be greater than the film thickness of the second layer 52B of the blue light-emitting element 2B.

[0173] Assume that GIP2 < GIP1 and GIP2 < GIP3, and that BIP2 > BIP1 and BIP2 > BIP3. In this case, the film thickness of the second layer 52G of the green light-emitting element 2G may be greater than the film thickness of the first layer 51B of the blue light-emitting element 2B and the third layer 53B of the blue light-emitting element 2B.

[0174] Assume that GIP2 > GIP1 and GIP2 > GIP3, and that BIP2 < BIP1 and BIP2 < BIP3. In this case, the film thickness of the first layer 51G of the green light-emitting element 2G and the third layer 53G of the green light-emitting element 2G may be greater than the film thickness of the second layer 52B of the blue light-emitting element 2B.

[0175] Assume that GIP2 > GIP1 and GIP2 > GIP3, and that BIP2 > BIP1 and BIP2 > BIP3. In this case, the film thickness of the first layer 51G of the green light-emitting element 2G and the third layer 53G of the green light-emitting element 2G may be greater than the film thickness of the first layer 51B of the blue light-emitting element 2B and the third layer 53B of the blue light-emitting element 2B.

[0176] With the above configuration, in this embodiment, the electron transport efficiency in the electron transport layer 25G is improved compared to the electron transport layer 25R. Furthermore, with the above configuration, in this embodiment, the electron transport efficiency in the electron transport layer 25B is improved compared to the electron transport layer 25G.

[0177] Therefore, the display device 1 according to this embodiment further improves the electron injection efficiency into the light-emitting layer 24 as the emission wavelength of the light-emitting elements becomes shorter. Thus, the display device 1 reduces the difference in luminous efficiency among the light-emitting elements, further simplifies the control content of each light-emitting element, or further improves the display quality.

[0178] Furthermore, in the film thickness direction DT, the film thicknesses of electron transport layer 25R, electron transport layer 25G, and electron transport layer 25B may be approximately the same. This allows the display device 1 to have a substantially uniform film thickness of electron transport layer 25 regardless of the subpixel. Therefore, the display device 1 can achieve more uniform characteristics for each light-emitting element, or the manufacturing method can be simplified.

[0179] The electron transport layer 25 according to this embodiment may be formed by repeatedly applying a dispersion containing an electron transport material to each subpixel by inkjet printing or the like, and then drying the dispersion. Alternatively, the electron transport layer 25 may be formed by repeatedly applying a dispersion containing an electron transport material and patterning it, thereby forming each layer for each subpixel. In the electron transport layer 25 formation process, the film thickness of each layer of the electron transport layer 25 may be adjusted by adjusting the concentration of the electron transport material in the dispersion to be applied.

[0180] This disclosure is not limited to the embodiments described above, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of this disclosure. Furthermore, new technical features can be formed by combining the technical means disclosed in each embodiment.

[0181] For example, in the light-emitting element 2 according to Embodiment 7, the hole transport layer 23 may include the fourth layer 34 according to Embodiment 5 on the cathode 26 side of the third layer 33. Also, in the light-emitting element 2 according to Embodiment 7, the electron transport layer 25 may include the fourth layer 54 according to Embodiment 6 on the cathode 26 side of the third layer 53.

[0182] Furthermore, for example, in the light-emitting element 2 according to Embodiment 7, the second layer 32 of the hole transport layer 23 may have a larger ionization potential and a larger electron affinity compared to the first layer 31 and the third layer 33. Also, in the light-emitting element 2 according to Embodiment 7, the second layer 52 of the electron transport layer 25 may have a larger ionization potential and a larger electron affinity compared to the first layer 51 and the third layer 53.

[0183] In addition, in each light-emitting element of the display device 1 according to Embodiment 8, the hole transport layer 23 may include the fourth layer described above on the cathode 26 side of the third layer. Also, in each light-emitting element of the display device 1 according to Embodiment 9, the electron transport layer 25 may include the fourth layer described above on the cathode 26 side of the third layer. Furthermore, in each light-emitting element of the display device 1 according to Embodiment 8, the electron transport layer 25 may include the first layer, second layer, and third layer of the electron transport layer 25 according to Embodiment 9.

[0184] 1 Display device 2 Light-emitting element 2B Blue light-emitting element 2G Green light-emitting element 2R Red light-emitting element 3 Substrate 21 Anode 23 Hole transport layer 24 Light-emitting layer 25 Electron transport layer 26 Cathode 31 First layer 32 Second layer 33 Third layer 51 First layer 52 Second layer 53 Third layer

Claims

1. A light-emitting element comprising: an anode; a cathode facing the anode; a light-emitting layer located between the anode and the cathode; and a charge transport layer located between the anode and the light-emitting layer, or between the cathode and the light-emitting layer, wherein the charge transport layer includes a first layer, a second layer, and a third layer stacked from the anode toward the cathode, and the ionization potential of the first layer is IP1, the ionization potential of the second layer is IP2, the ionization potential of the third layer is IP3, the electron affinity of the first layer is EA1, the electron affinity of the second layer is EA2, and the electron affinity of the third layer is EA3, and the light-emitting element satisfies either of the following conditions: Condition 1: IP2 < IP1, IP2 < IP3, EA2 < EA1, and EA2 < EA3, and Condition 2: IP2 > IP1, IP2 > IP3, EA2 > EA1, and EA2 > EA3.

2. The light-emitting element according to claim 1, wherein the charge transport layer is a hole transport layer located between the anode and the light-emitting layer.

3. The light-emitting element according to claim 2, wherein the difference in ionization potential between the first layer and the second layer, and between the second layer and the third layer, is 0.1 eV or less.

4. The light-emitting element according to claim 2 or 3, wherein the difference in electron affinity between the first layer and the second layer, and between the second layer and the third layer, is greater than 0.1 eV.

5. The light-emitting element according to any one of claims 2 to 4, wherein the film thickness of the second layer is greater than the film thicknesses of the first and third layers when IP2 < IP1 and IP2 < IP3, and the film thickness of the second layer is smaller than the film thicknesses of the first and third layers when IP2 > IP1 and IP2 > IP3.

6. The light-emitting element according to any one of claims 2 to 5, wherein the film thickness of the second layer is 2 nm or more when IP2 < IP1 and IP2 < IP3, and the film thickness of the first layer and the third layer is 2 nm or more when IP2 > IP1 and IP2 > IP3.

7. The light-emitting element according to any one of claims 2 to 6, wherein the film thickness of the layer with the larger ionization potential among the first layer, the third layer, and the second layer is 2 nm or less.

8. The light-emitting element according to any one of claims 2 to 7, wherein the work function of the second layer is greater than the work functions of the first and third layers when IP2 < IP1 and IP2 < IP3, and the work function of the second layer is smaller than the work functions of the first and third layers when IP2 > IP1 and IP2 > IP3.

9. The light-emitting element according to any one of claims 2 to 8, wherein the light-emitting layer includes light-emitting quantum dots as a light-emitting material.

10. A light-emitting element according to any one of claims 2 to 9, wherein the first layer and the third layer contain poly-TPD and the second layer contains TFB, or the first layer and the third layer contain CBP and the second layer contains poly-TPD, or the first layer and the third layer contain CBP and the second layer contains TFB, or the first layer and the third layer contain F8 and the second layer contains poly-TPD, or the first layer and the third layer contain F8 and the second layer contains TFB, or the first layer and the third layer contain PEDOT / PFI and the second layer contains TFB.

11. The light-emitting element according to claim 1, wherein the charge transport layer is an electron transport layer located between the cathode and the light-emitting layer.

12. The light-emitting element according to claim 11, wherein the difference in electron affinity between the first layer and the second layer, and between the second layer and the third layer, is 0.1 eV or less.

13. The light-emitting element according to claim 11 or 12, wherein the difference in ionization potential between the first layer and the second layer, and between the second layer and the third layer, is greater than 0.1 eV.

14. The light-emitting element according to any one of claims 11 to 13, wherein the film thickness of the second layer is smaller than the film thicknesses of the first and third layers when IP2 < IP1 and IP2 < IP3, and the film thickness of the second layer is greater than the film thicknesses of the first and third layers when IP2 > IP1 and IP2 > IP3.

15. The light-emitting element according to any one of claims 11 to 14, wherein the film thickness of the first layer and the third layer is 2 nm or more when IP2 < IP1 and IP2 < IP3, and the film thickness of the second layer is 2 nm or more when IP2 > IP1 and IP2 > IP3.

16. The light-emitting element according to any one of claims 11 to 15, wherein the film thickness of the second layer is 2 nm or less when IP2 < IP1 and IP2 < IP3, and the film thickness of the first layer and the third layer is 2 nm or less when IP2 > IP1 and IP2 > IP3.

17. The light-emitting element according to any one of claims 11 to 16, wherein the work function of the second layer is smaller than that of the first and third layers when IP2 < IP1 and IP2 < IP3, and the work function of the second layer is larger than that of the first and third layers when IP2 > IP1 and IP2 > IP3.

18. The light-emitting element according to any one of claims 11 to 17, wherein the light-emitting layer includes light-emitting quantum dots as a light-emitting material.

19. A light-emitting element according to any one of claims 11 to 18, wherein the first layer and the third layer contain ZnO and the second layer contains TPBi, or the first layer and the third layer contain ZnMgO and the second layer contains TPBi, or the first layer and the third layer contain ZnO and the second layer contains BCP, or the first layer and the third layer contain ZnMgO and the second layer contains BCP, or the first layer and the third layer contain ZnO and the second layer contains PBD, or the first layer and the third layer contain ZnMgO and the second layer contains PBD.

20. A display device comprising a substrate, a red light-emitting element, a green light-emitting element, and a blue light-emitting element on the substrate, wherein the red light-emitting element, the green light-emitting element, and the blue light-emitting element are each light-emitting elements according to any one of claims 2 to 10.

21. The display device according to claim 20, wherein the difference in ionization potential between the first layer and the second layer and between the second layer and the third layer in the red light-emitting element is smaller than the difference in ionization potential between the first layer and the second layer and between the second layer and the third layer in the green light-emitting element, and the difference in ionization potential between the first layer and the second layer and between the second layer and the third layer in the green light-emitting element is smaller than the difference in ionization potential between the first layer and the second layer and between the second layer and the third layer in the blue light-emitting element.

22. The display device according to claim 20, wherein, among the first layer, the third layer, and the second layer in the red light-emitting element, the film thickness of the layer with the smaller ionization potential is smaller than the film thickness of the first layer, the third layer, and the second layer in the green light-emitting element, and the film thickness of the layer with the smaller ionization potential in the green light-emitting element is smaller than the film thickness of the first layer, the third layer, and the second layer in the blue light-emitting element, which has the smaller ionization potential.

23. A display device comprising a substrate, a red light-emitting element, a green light-emitting element, and a blue light-emitting element on the substrate, wherein the red light-emitting element, the green light-emitting element, and the blue light-emitting element are each light-emitting elements according to any one of claims 11 to 19.

24. The display device according to claim 23, wherein the difference in electron affinity between the first layer and the second layer and between the second layer and the third layer in the red light-emitting element is greater than the difference in electron affinity between the first layer and the second layer and between the second layer and the third layer in the green light-emitting element, and the difference in electron affinity between the first layer and the second layer and between the second layer and the third layer in the green light-emitting element is greater than the difference in electron affinity between the first layer and the second layer and between the second layer and the third layer in the blue light-emitting element.

25. The display device according to claim 23, wherein, among the first layer, the third layer, and the second layer in the red light-emitting element, the film thickness of the layer with the smaller ionization potential is greater than the film thickness of the first layer, the third layer, and the second layer in the green light-emitting element, and the film thickness of the layer with the smaller ionization potential in the green light-emitting element is greater than the film thickness of the first layer, the third layer, and the second layer in the blue light-emitting element, which has the smaller ionization potential.

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