Light-emitting element and display device

Abstract

This light-emitting element (3) comprises an anode (31), a cathode (36), a light-emitting layer (33), a first electron transport layer (34), and a well layer (35). The light-emitting layer is located between the anode and the cathode and includes quantum dots (4). The first electron transport layer and the well layer are located between the cathode and the light-emitting layer. The well layer is joined with the first electron transport layer and includes a material having a Fermi level lower than that of the material of the first electron transport layer.

Description

Light-emitting element, display device

[0001] The present disclosure relates to a light-emitting element and a display device including the light-emitting element.

[0002] Patent Document 1 discloses a method for improving the luminous efficiency and durability of a stacked organic EL light-emitting element.

[0003] International Publication No. 2020 / 009039

[0004] In the light-emitting device described in Patent Document 1, the barrier to hole injection from the anode to the light-emitting layer is still large, and there is a possibility that an excess of electrons may occur in the light-emitting layer. The excess of electrons in the light-emitting layer may cause a decrease in the luminous efficiency of the light-emitting device and may also cause deterioration of the light-emitting layer and surrounding layers due to Auger electrons.

[0005] A light-emitting element according to one aspect of the present disclosure includes an anode, a cathode facing the anode, a light-emitting layer located between the anode and the cathode and including quantum dots, a first electron transport layer located between the cathode and the light-emitting layer, and a well layer located between the cathode and the light-emitting layer, in contact with the first electron transport layer, and including a material having a lower Fermi level than a material of the first electron transport layer.

[0006] According to the configuration according to one aspect of the present disclosure, excess electrons in the light-emitting layer are reduced, and the light-emitting efficiency or the reliability of each layer of the light-emitting element is improved.

[0007] FIG. 1 is a schematic side cross-sectional view of a display device according to embodiment 1. FIG. 2 is a schematic plan view of a display device according to embodiment 1. FIG. 3 is a band diagram of each layer of a light-emitting element according to embodiment 1 before bonding the layers. FIG. 4 is a band diagram of each layer of a light-emitting element according to embodiment 1 after bonding the layers. FIG. 5 is a schematic side cross-sectional view of a display device according to embodiment 2. FIG. 6 is a band diagram of each layer of a light-emitting element according to embodiment 2 before bonding the layers. FIG. 7 is a band diagram of each layer of a light-emitting element according to embodiment 2 after bonding the layers.

[0008] [Embodiment 1] <Display Device: Overview> Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. Note that in each drawing, similar components are denoted by the same reference numerals and description thereof will be omitted.

[0009] FIG. 2 is a schematic plan view of a display device 1 according to this embodiment. The display device 1 is a device that can be used, for example, as a display for a television or a smartphone. The display device 1 includes a display unit DA and a frame unit NA formed around the periphery of the display unit DA. The display device 1 displays an image on the display unit DA by controlling light emission from each of a plurality of light-emitting elements (described later) formed in the display unit DA. Drivers and the like for driving each of the plurality of light-emitting elements of the display unit DA may be formed in the frame unit NA.

[0010] <Display Device: Stacked Structure> The display device 1, and in particular the display unit DA, will be described in more detail with reference to Fig. 1. Fig. 1 is a schematic side cross-sectional view of the display device 1, and is a cross-sectional view taken along line II in Fig. 2, in other words, a side cross-section passing through the display unit DA.

[0011] The display device 1 includes a substrate 2 and a light-emitting element 3 located on the substrate 2. The light-emitting element 3 includes, in order from the substrate 2 side, an anode 31, a hole transport layer 32, a light-emitting layer 33, a first electron transport layer 34, a well layer 35, and a cathode 36. However, without being limited thereto, the light-emitting element 3 may include, in order from the substrate 2 side, the cathode 36, the well layer 35, the first electron transport layer 34, the light-emitting layer 33, the hole transport layer 32, and the anode 31. Although FIG. 1 illustrates only one light-emitting element 3, the display device 1 may include a light-emitting element 3 in each of a plurality of sub-pixels on the substrate 2.

[0012] <Display Device: Substrate> The substrate 2 may be, for example, a rigid substrate such as a glass substrate, or a flexible substrate such as a film substrate. The substrate 2 may have a pixel circuit for each sub-pixel, which is individually driven by the above-mentioned driver or the like.

[0013] <Display device: Light-emitting element: Electrode> The anode 31 and the cathode 36 contain a conductive material. At least one of the anode 31 and the cathode 36 is a light-transmitting electrode. One of the anode 31 and the cathode 36 may be a light-reflecting electrode. In this embodiment, an example will be described in which the anode 31 is a light-transmitting electrode and the cathode 36 is a light-reflecting electrode.

[0014] The anode 31 includes a light-transmitting conductive material, and may include, for example, indium tin oxide (ITO), indium zinc oxide (IZO), aluminum-doped zinc oxide (AZO), or gallium-doped zinc oxide (GZO). The anode 31 may be electrically connected to a pixel circuit on the substrate 2, and the display device 1 may apply a voltage to the anode 31 via a driver, a pixel circuit, or the like. When the display device 1 includes a plurality of light-emitting elements 3, the anode 31 may be formed in an island shape in a planar view for each light-emitting element 3. The anode 31 may be formed by depositing a material by a sputtering method or the like and then patterning the film by dry etching.

[0015] The cathode 36 faces the anode 31 via the hole transport layer 32 and the well layer 35, and may include, for example, a metal material having light reflectivity, such as Al, Cu, Au, or Ag. The cathode 36 may be electrically connected to an auxiliary electrode (not shown) or the like, and a predetermined voltage may be applied to the cathode 36. When the display device 1 includes a plurality of light-emitting elements 3, the cathode 36 may be formed in common for the plurality of light-emitting elements 3. The cathode 36 may be formed by depositing a metal thin film by, for example, a vapor deposition method or the like.

[0016] The display device 1 controls the voltage applied to the anode 31 and applies a predetermined voltage to the cathode 36, thereby generating a potential difference between the anode 31 and the cathode 36 and driving the light-emitting element 3. When the light-emitting element 3 is driven, the anode 31 injects holes into a hole transport layer 32 (described later), and the cathode 36 injects electrons into a well layer 35 (described later).

[0017] <Display Device: Light-Emitting Element: Hole Transport Layer> The hole transport layer 32 is a charge transport layer that is joined to the anode 31 and has the function of transporting holes injected from the anode 31 to the light-emitting layer 33 described below. As the material for the hole transport layer 32, a material having hole transport properties that has been conventionally employed in light-emitting elements including quantum dots can be used. The hole transport layer 32 may include, for example, at least one of PEDOT:PSS (polyethylenedioxythiophene / polystyrene sulfonate), PVK (poly-N-vinylcarbazole), TFB (poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4'-(N-(4-sec-butylphenyl)diphenylamine))], and poly-TPD (N,N'-bis(4-butylphenyl)-N,N'-bis(phenyl)-benzidine). The hole transport layer 32 may also include, for example, an inorganic semiconductor material such as NiO.

[0018] <Display Device: Light-Emitting Element: Well Layer> The well layer 35 is in contact with the cathode 36 and is a charge transport layer that has a function of transporting electrons injected from the cathode 36 to the first electron transport layer 34 described below. In this embodiment, the material of the well layer 35 includes, for example, a p-type semiconductor.

[0019] The material of the well layer 35 may include, for example, a dichalcogenide compound having at least one of Mo and W and at least one of S, Se, and Te. The dichalcogenide compound is, for example, MoS ２ , W.S. ２ , MoSe ２ , WSe ２ , MoTe ２ , or WTe ２ may include:

[0020] The material of the well layer 35 may include a III-V compound having at least one of In and Sb, such as InN, InP, InAs, InSb, AlSb, or GaSb.

[0021] The material of the well layer 35 may include a compound having at least one of Si and Ge. The compound may include, for example, a semiconductor in which elemental Si or elemental Ge is p-type doped. Alternatively, the compound may include, for example, SiC.

[0022] The material of the well layer 35 may include a first perovskite compound having a first metal element, a second metal element different from the first metal element, and a halogen element. The first metal element may be a Group I element, and the second metal element may be a Group VI element. The material of the well layer 35 may include a second perovskite compound having formamidinium or methylammonium, a third metal element, and a halogen element. The third metal element may be a Group IV element. The halogen element may be any of Cl, Br, and I.

[0023] The first perovskite compound is, for example, CsPbCl ３ , CsSnCl ３ , CsPbBr ３ , CsSnBr ３ , CsPbI ３ , or CsSnI ３ The second perovskite compound may include, for example, FASnCl ３ , MASnCl ３ , MAPbCl ３ , FAPbCl ３ , FASnBr ３ , MAPbBr ３ , FAPbBr ３ , MASnBr ３ , MAPbI ３ , FAPbI ３ , MASnI ３ , or FASnI ３ In the chemical formula of the second perovskite compound described above, "FA" represents formamidinium, and "MA" represents methylammonium.

[0024] <Display Device: Light-Emitting Element: First Electron Transport Layer> The first electron transport layer 34 is a charge transport layer that is in contact with the well layer 35 and has the function of transporting electrons injected from the well layer 35 to the light-emitting layer 33 (described below). Materials conventionally used in quantum dot-containing light-emitting elements and the like can be used as the material for the first electron transport layer 34. The first electron transport layer 34 may contain, for example, at least one of ZnO, ZnMgO, and TPBi. When the first electron transport layer 34 contains MgZnO, the amount of Mg added may be 10% of the total. In this embodiment, the material for the first electron transport layer 34 may contain an n-type semiconductor.

[0025] The hole transport layer 32, the first electron transport layer 34, and the well layer 35 may each be formed by a deposition method, a sputtering method, or a coating method using a colloidal solution.

[0026] <Display Device: Light-Emitting Element: Light-Emitting Layer> The light-emitting layer 33 includes, as a light-emitting material, a plurality of quantum dots 4. The quantum dots 4 emit light due to excitons generated by recombination between holes injected from the hole transport layer 32 into the light-emitting layer 33 and electrons injected from the first electron transport layer 34 into the light-emitting layer 33.

[0027] The quantum dots 4 may be, for example, conventionally known luminescent quantum dots, or may be, for example, core / shell quantum dots. The cores of the quantum dots 4 may contain, for example, CdSe, InP, perovskite, or chalcogenide. In order to reduce the burden on the environment, the material of the quantum dots 4 may not contain Cd. The material of the shells of the quantum dots 4 may have a wider band gap than the cores in order to enhance the exciton confinement effect. The shells of the quantum dots 4 may contain, for example, ZnS.

[0028] The wavelength of light emitted by the light-emitting element 3 can be changed by changing the particle size of the quantum dots 4. The wavelength of light emitted by the quantum dots 4 may differ depending on the sub-pixel in which the light-emitting element 3 having the light-emitting layer 33 containing the quantum dots 4 is formed. This allows the display device 1 to extract light of different wavelengths from each of the multiple light-emitting elements 3, in other words, to perform color display.

[0029] The light-emitting layer 33 may be formed by forming a thin film containing the quantum dots 4 by applying a solution in which the quantum dots 4 are dispersed, and patterning the thin film by photolithography, lift-off, etc. Alternatively, the light-emitting layer 33 may be formed by applying the solution in which the quantum dots 4 are dispersed in different layers by an inkjet method, etc.

[0030] The light-emitting element 3 may extract light from the light-emitting layer 33 to the outside from the light-transmitting electrode of the anode 31 and the cathode 36. When the light-transmitting electrode of the anode 31 and the cathode 36 is located closer to the substrate 2 than the light-emitting layer 33, the display device 1 may extract light emitted from the light-emitting element 3 and transmitted through the substrate 2 to the outside.

[0031] <Display Device: Light-Emitting Element: Supplementary Notes> The light-emitting element 3 may include a capping layer above the cathode 36 to improve the light extraction efficiency from the light-emitting element 3. The display device 1 may also include a sealing layer on the substrate 2 to seal the light-emitting element 3.

[0032] The display device 1 may be manufactured by sequentially depositing the layers of the light-emitting element 3 on the substrate 2 by the above-described method.

[0033] <Display Device: Light-Emitting Element: Band Diagram> A portion of each layer of the light-emitting element 3 according to this embodiment, particularly the first electron transport layer 34 and the well layer 35, undergoes band gap curvature due to their mutual junction. Furthermore, Fermi level pinning occurs in the cathode 36 of the light-emitting element 3 according to this embodiment due to their junction with the well layer 35. These will be described with reference to FIGS. 3 and 4 .

[0034] Fig. 3 is a band diagram of each layer of the light-emitting element 3 before the layers are bonded together. Fig. 4 is a band diagram of each layer of the light-emitting element 3 after the layers are bonded together. In the band diagrams of the light-emitting element of the present disclosure, including Fig. 3 and Fig. 4, the Fermi level of each electrode and the band gap of each layer between the two electrodes are shown. In addition, in the band diagram of the light-emitting element of the present disclosure, the side of higher energy of electrons is the upper side as viewed from the page.

[0035] 3 , before the layers of the light-emitting element 3 are bonded together, the Fermi level 35F of the material of the well layer 35 is lower than the Fermi level 34F of the material of the first electron transport layer 34. In other words, the well layer 35 contains a material whose Fermi level is lower than that of the material of the first electron transport layer 34. In this embodiment, the material of the first electron transport layer 34 contains an n-type semiconductor, so the Fermi level 34F is located above the middle of the band gap. Furthermore, the material of the well layer 35 contains a p-type semiconductor, so the Fermi level 35F is located below the middle of the band gap. Therefore, the Fermi level 35F of the material of the well layer 35 is even lower than the Fermi level 34F of the material of the first electron transport layer 34.

[0036] 3 , before the layers of the light-emitting element 3 are bonded together, the band gap of the material of the first electron transport layer 34 is wider than the band gap of the material of the well layer 35. In other words, the well layer 35 contains a material with a narrower band gap than the material of the first electron transport layer 34. Furthermore, before the layers of the light-emitting element 3 are bonded together, the electron affinity of the material of the first electron transport layer 34 is smaller than the electron affinity of the material of the well layer 35. In other words, the well layer 35 contains a material with a higher electron affinity than the material of the first electron transport layer 34.

[0037] 3, before the layers of the light-emitting element 3 are bonded together, the Fermi level of the cathode 36 is located below the band gap of the well layer 35. In other words, the work function of the cathode 36 is greater than the ionization energy of the well layer 35.

[0038] <Display Device: Light-Emitting Element: Bandgap Curvature and Fermi Level Pinning> In this embodiment, the Fermi level 35F of the material of the well layer 35 is lower than the Fermi level 34F of the material of the first electron transport layer 34. Therefore, when the layers of the light-emitting element 3 are joined, and in particular, the first electron transport layer 34 is joined to the well layer 35, charge transfer occurs between the first electron transport layer 34 and the well layer 35 so that the Fermi levels 34F and 35F approximately coincide with each other. In particular, in this embodiment, electrons move from the first electron transport layer 34 to the well layer 35, and holes move from the well layer 35 to the first electron transport layer 34.

[0039] As a result, a local electric field is generated near the boundary between the first electron transport layer 34 and the well layer 35 due to the charge imbalance. Therefore, as shown in Fig. 4, the band gap is curved near the boundary between the first electron transport layer 34 and the well layer 35 that are joined. In particular, a part of the band gap of the first electron transport layer 34 on the well layer 35 side is curved upward, and a part of the band gap of the well layer 35 on the first electron transport layer 34 side is curved downward. Therefore, as shown in Fig. 4, a quantum well 35W having a lower energy level than the surrounding area is formed on the first electron transport layer 34 side of the upper end of the band gap of the well layer 35.

[0040] In this embodiment, the junction between the well layer 35 and the cathode 36 is a Schottky junction. Therefore, by junction between the well layer 35 and the cathode 36, the Fermi level of the cathode 36 is pinned toward the valence band side of the middle of the band gap of the well layer 35, in other words, downward, as shown in FIG. 4 , and in other words, the work function of the cathode 36 becomes high.

[0041] In the light-emitting element 3 according to this embodiment, the well layer 35 has the quantum well 35W, which makes it easier for electrons injected from the cathode 36 to accumulate in the quantum well 35W. Therefore, the light-emitting element 3 can reduce the excess of electrons in the light-emitting layer 33.

[0042] Generally, in field injection light-emitting elements, the carrier balance in the light-emitting layer tends to be electron-excessive due to the difference in mobility between electrons and holes. Furthermore, electron excess in the light-emitting layer is a contributing factor to deterioration of the light-emitting layer or the functional layers around the light-emitting layer due to the generation of Auger electrons. Therefore, a light-emitting element 3 that can reduce the electron excess in the light-emitting layer 33 can efficiently achieve improved light-emitting efficiency or longer life. A display device 1 equipped with the light-emitting element 3 can achieve power savings or a longer life.

[0043] The well layer 35 includes a material having a narrower band gap than the material of the first electron transport layer 34 and a higher electron affinity than the material of the first electron transport layer 34. The material of the well layer 35 also includes a p-type semiconductor. This causes the Fermi level 35F of the material of the well layer 35 to be lower than the Fermi level 34F of the material of the first electron transport layer 34. Therefore, with the above configuration, the quantum well 35W of the well layer 35 of the light-emitting element 3 can be made deeper, and the excess electrons in the light-emitting layer 33 can be further reduced. Furthermore, since the well layer 35 includes a material having a narrower band gap than the material of the first electron transport layer 34, the carrier mobility in the well layer 35 is improved, the resistance of the light-emitting element 3 as a whole is reduced, and the light-emitting efficiency is improved. However, this embodiment is not limited to this. For example, the material of the well layer 35 may include an n-type semiconductor.

[0044] In this embodiment, the well layer 35 does not have to be in direct contact with the cathode 36. For example, an electron injection layer or the like may be formed between the well layer 35 and the cathode 36. Furthermore, in this embodiment, the well layer 35 is bonded to the first electron transport layer 34 on the side of the cathode 36, but this is not limiting. For example, the well layer 35 may be located between the first electron transport layer 34 and the light-emitting layer 33, or may be bonded to the first electron transport layer 34 on the side of the light-emitting layer 33. Even in this case, the quantum well 35W is formed on the first electron transport layer 34 side of the well layer 35.

[0045] [Embodiment 2] <Second electron transport layer> A display device 5 according to this embodiment will be described with reference to Fig. 5. Fig. 5 is a schematic cross-sectional side view of the display device 5, taken at the same position as the cross-sectional side view shown in Fig. 1.

[0046] The display device 5 according to this embodiment has the same configuration as the display device 1 according to the previous embodiment, except that it includes a light-emitting element 6 instead of the light-emitting element 3. The light-emitting element 6 according to this embodiment has the same configuration as the light-emitting element 3 according to the previous embodiment, except that it includes a second electron transport layer 61 between the well layer 35 and the cathode 36.

[0047] The second electron transport layer 61 is a charge transport layer that is in contact with the cathode 36 and has a function of transporting electrons from the cathode 36 to the well layer 35. In this embodiment, the well layer 35 is in contact with the second electron transport layer 61 and has a function of transporting electrons from the second electron transport layer 61 to the first electron transport layer 34. In other words, the second electron transport layer 61 is in contact with the well layer 35 on the side opposite to the first electron transport layer 34 with respect to the well layer 35. The second electron transport layer 61 contains a material having a higher Fermi level than the material of the well layer 35.

[0048] From the viewpoint of reducing manufacturing costs or simplifying the manufacturing process, the second electron transport layer 61 may contain the same material as the first electron transport layer. In this embodiment, the second electron transport layer 61 contains an n-type semiconductor. The second electron transport layer 61 may have a bandgap equal to or larger than the bandgap of the material of the first electron transport layer 34, or may contain a material with a wider bandgap than the material of the well layer 35.

[0049] The Fermi level and band gap of each layer of the light-emitting element 6 according to this embodiment will be described with reference to Fig. 6 and Fig. 7. Fig. 6 is a band diagram of each layer of the light-emitting element 6 before bonding the layers. Fig. 7 is a band diagram of each layer of the light-emitting element 6 after bonding the layers.

[0050] 6 , before the layers of the light-emitting element 6 are bonded together, the Fermi level 35F of the material of the well layer 35 is lower than the Fermi level 61F of the material of the second electron transport layer 61. In this embodiment, the material of the second electron transport layer 61 includes an n-type semiconductor, and therefore the Fermi level 61F is located on the upper end side of the band gap rather than the middle. Therefore, the Fermi level 35F of the material of the well layer 35 is even lower than the Fermi level 61F of the material of the second electron transport layer 61.

[0051] 6 , before the layers of the light-emitting element 6 are bonded together, the band gap of the material of the second electron transport layer 61 is wider than the band gap of the material of the well layer 35. Furthermore, before the layers of the light-emitting element 6 are bonded together, the electron affinity of the material of the second electron transport layer 61 is smaller than the electron affinity of the material of the well layer 35. As shown in FIG. 6 , before the layers of the light-emitting element 6 are bonded together, the Fermi level of the cathode 36 is located in the middle of the band gap of the second electron transport layer 61.

[0052] The layers of the light-emitting element 6 are joined together, and in particular, the second electron transport layer 61 is joined to the well layer 35, causing charge transfer between the second electron transport layer 61 and the well layer 35 so that the Fermi levels 61F and 35F approximately coincide. As a result, as shown in Fig. 7 , the band gap is curved near the boundary between the joined second electron transport layer 61 and the well layer 35. In particular, a portion of the band gap of the second electron transport layer 61 on the well layer 35 side is curved upward, and a portion of the band gap of the well layer 35 on the second electron transport layer 61 side is curved downward. Therefore, as shown in Fig. 7 , a quantum well 35W having a lower level than the surrounding area is formed on the second electron transport layer 61 side of the upper end of the band gap of the well layer 35.

[0053] 7 , a quantum well 35W is also formed at the upper end of the band gap of the well layer 35 on the side of the first electron transport layer 34. In other words, in this embodiment, the well layer 35 has quantum wells 35W on both the side of the first electron transport layer 34 and the side of the second electron transport layer 61. Therefore, the light-emitting element 6 can further reduce the excess electrons in the light-emitting layer 33.

[0054] In this embodiment, the second electron transport layer 61 contains a material that has a bandgap equal to or larger than the bandgap of the material of the first electron transport layer 34 and that is wider than the material of the well layer 35. This makes the Fermi level 35F of the material of the well layer 35 even lower than the Fermi level 61F of the material of the second electron transport layer 61. Therefore, with the above configuration, the light-emitting element 3 can have a deeper quantum well 35W on the second electron transport layer 61 side of the well layer 35, and can further reduce the electron excess in the light-emitting layer 33.

[0055] In this embodiment, the junction between the second electron transport layer 61 and the cathode 36 is a Schottky junction. Therefore, by junction between the second electron transport layer 61 and the cathode 36, the Fermi level of the cathode 36 is pinned toward the valence band side of the middle of the band gap of the second electron transport layer 61, in other words, toward the lower side, as shown in Fig. 7 , and in other words, the work function of the cathode 36 is lowered. As a result, the light-emitting element 6 reduces the efficiency of electron injection from the cathode 36 to the second electron transport layer 61, and can further reduce the excess electrons in the light-emitting layer 33.

[0056] The present disclosure is not limited to the above-described embodiments, 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 the present disclosure. Furthermore, new technical features can be formed by combining the technical means disclosed in each embodiment.

[0057] 1, 5 Display device 3, 6 Light-emitting element 4 Quantum dot 31 Anode 33 Light-emitting layer 34 First electron transport layer 35 Well layer 36 Cathode 61 Second electron transport 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 including quantum dots; a first electron transport layer located between the cathode and the light-emitting layer; and a well layer located between the cathode and the light-emitting layer, in contact with the first electron transport layer, and including a material having a lower Fermi level than a material of the first electron transport layer.

2. The light-emitting device according to claim 1, wherein the well layer contains a material having a narrower band gap than the material of the first electron transport layer.

3. The light-emitting device according to claim 1, wherein the well layer contains a material having a larger electron affinity than the material of the first electron transport layer.

4. The light-emitting device according to any one of claims 1 to 3, wherein the material of the well layer contains a p-type semiconductor.

5. The light-emitting device according to claim 4, wherein the material of the well layer contains a dichalcogenide compound having at least one of Mo and W, and at least one of S, Se, and Te.

6. The light-emitting device according to claim 4 or 5, wherein the material of the well layer contains a III-V compound containing at least one of In and Sb.

7. The light-emitting device according to any one of claims 4 to 6, wherein the material of the well layer includes a compound containing at least one of Si and Ge.

8. A light-emitting device according to any one of claims 4 to 7, wherein the material of the well layer contains a first perovskite compound having a first metal element, a second metal element different from the first metal element, and a halogen element.

9. The light-emitting device according to claim 8, wherein the first metal element is a Group I element, and the second metal element is a Group VI element.

10. A light-emitting device according to any one of claims 4 to 9, wherein the material of the well layer contains a second perovskite compound having formamidinium or methylammonium, a third metal element, and a halogen element.

11. The light-emitting device according to claim 10, wherein the third metallic element is a Group IV element.

12. The light-emitting device according to any one of claims 8 to 11, wherein the halogen element is any one of Cl, Br and I.

13. The light-emitting device according to any one of claims 1 to 12, wherein the material of the well layer contains an n-type semiconductor.

14. A light-emitting device according to any one of claims 1 to 13, further comprising a second electron transport layer located between the cathode and the light-emitting layer, in contact with the well layer on the side opposite the first electron transport layer, the second electron transport layer containing a material having a higher Fermi level than a material of the well layer.

15. The light-emitting device of claim 14, wherein said second electron-transporting layer comprises the same material as said first electron-transporting layer.

16. The light-emitting element according to claim 14 or 15, wherein the first electron transport layer is located between the light-emitting layer and the well layer, and the second electron transport layer is located between the well layer and the cathode.

17. The light-emitting device of claim 16, wherein said second electron transport layer is in contact with said cathode.

18. The light-emitting device according to claim 17, wherein the second electron transport layer contains a material having a band gap equal to or larger than the band gap of the material of the first electron transport layer and wider than the band gap of the material of the well layer.

19. A display device comprising a light-emitting element according to any one of claims 1 to 18.

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