Light-emitting element, display device, and production method for light-emitting element

Abstract

A light-emitting layer of this light-emitting element includes: a first quantum dot layer including first quantum dots and a first inorganic oxide covering the periphery of the first quantum dots; and a second quantum dot layer including second quantum dots and a second inorganic oxide covering the periphery of the second quantum dots. M is a metal element, and A and B are group 16 elements. A first shell of the first quantum dot contains a substance having a composition of MnB1-xAx, and a second shell of the second quantum dot contains a substance having a composition of MmB1-yAy.

Description

Light-emitting element, display device, and method for manufacturing the same

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

[0002] In a light-emitting device having a light-emitting layer containing quantum dots (semiconductor nanoparticles) as a light-emitting material, there is a problem that a difference in the concentration of holes and electrons occurs in the light-emitting layer, resulting in a decrease in light-emitting efficiency. Patent Document 1 discloses a light-emitting device in which electron leakage is reduced and light-emitting efficiency is improved by providing a difference in the film thickness of the shell of the quantum dots in the light-emitting layer.

[0003] International Publication No. 2015 / 056750

[0004] In the light-emitting device described in Patent Document 1, it is difficult to efficiently inject carriers into quantum dots with thick shells, making it difficult to efficiently improve the luminous efficiency. Furthermore, in the light-emitting device described in Patent Document 1, if the shells of the quantum dots deteriorate, not only will the luminous efficiency of the quantum dots decrease, but the effect obtained by differences in the shell thicknesses of the quantum dots will also decrease.

[0005] a second quantum dot layer including a second quantum dot having a second core emitting light of the same color as the first core and a second shell surrounding the second core, and a second inorganic oxide covering the second quantum dot, the second quantum dot layer being located closer to the cathode than the first quantum dot layer; wherein M is a metal element, A is a first Group 16 element, B is a second Group 16 element located in a period later than the first Group 16 element in the periodic table, x and y are real numbers satisfying 0≦x<y≦1, n is a real number satisfying 0<n, and m is a real number satisfying 0<m; and the first shell has a composition of M ｎ B １－ｘ A ｘ and the second shell comprises a material having a composition of Mｍ B １－ｙ A ｙ This includes substances that are

[0006] a second quantum dot layer including: a first quantum dot layer including: a first quantum dot having a first core and a first shell surrounding the first core; and a first inorganic oxide covering the first quantum dot; and a second quantum dot layer including: a second quantum dot emitting light of the same color as the first core; a second quantum dot having a second core and a second shell surrounding the second core; and a second inorganic oxide covering the second quantum dot; and the second quantum dot layer being located closer to the cathode than the first quantum dot layer; wherein M is a metal element, A is a first Group 16 element, B is a second Group 16 element located in a period later than the first Group 16 element in the periodic table, x and y are real numbers satisfying 0≦x<y≦1, n is a real number satisfying 0<n, and m is a real number satisfying 0<m; and the first shell has a composition of M ｎ B １－ｘ A ｘ and the second shell comprises a material having a composition of M ｍ B １－ｙ A ｙ This includes substances that are

[0007] The luminous efficiency of the light-emitting element is improved.

[0008] FIG. 1 is a schematic side cross-sectional view of a display device according to embodiment 1. FIG. 2 is a schematic view of a display device according to embodiment 1. FIG. 3 is a schematic view showing another example of quantum dots according to embodiment 1. FIG. 4 is a flowchart showing a method for manufacturing a display device according to embodiment 1. FIG. 5 is a flowchart showing a method for forming a light-emitting layer according to embodiment 1. FIG. 6 is a schematic side cross-sectional view of a display device according to embodiment 2. FIG. 7 is a schematic view showing an inorganic oxide filling spaces between quantum dots according to embodiment 2. FIG. 8 is a schematic side cross-sectional view of a display device according to embodiment 3. FIG. 9 is a schematic side cross-sectional view of a display device according to embodiment 4. FIG. 10 is a schematic side cross-sectional view of a display device according to embodiment 5. FIG. 11 is a schematic side cross-sectional view of a display device according to embodiment 6.

[0009] [Embodiment 1] <Display Device> Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. Note that in each drawing, similar configurations are assigned the same reference numerals, and their description will be omitted. Furthermore, in this disclosure, for simplicity of illustration, components assigned the same reference numerals may be shown at different scales or with different hatching depending on the drawing. However, the components shown in each drawing of the present disclosure are merely examples, and the scales are not limited to those shown in the drawings. Furthermore, in this disclosure, even for components with different hatching, components assigned the same reference numerals have similar configurations as described above.

[0010] 2 is a schematic diagram 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, a smartphone, or the like. The display device 1 includes a display unit DA including a plurality of sub-pixels X, and a driver circuit DR that drives the plurality of sub-pixels X. Each of the plurality of sub-pixels X includes a light-emitting element 2 and a pixel circuit PC that drives the light-emitting element 2. The display device 1 performs display on the display unit DA by controlling light emission from each of the plurality of light-emitting elements 2 formed in the display unit DA via the driver circuit DR and the pixel circuit PC.

[0011] The structure of the display unit DA of the display device 1, particularly the structure of the light-emitting element 2, 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 according to an embodiment of the present disclosure, particularly showing a cross section perpendicular to the display surface of the display device 1 and passing through the light-emitting element 2. Note that each schematic cross-sectional view and each process cross-sectional view of the display device in the present disclosure shows a cross section corresponding to the cross section of the display device 1 shown in Fig. 1.

[0012] 1, the display device 1 according to this embodiment includes a display section DA that includes the above-described plurality of light-emitting elements 2 and a substrate 3, and in particular includes the plurality of light-emitting elements 2 on the substrate 3. The display device 1 has a structure in which the layers of the light-emitting elements 2 are stacked on the substrate 3 on which, for example, TFTs (Thin Film Transistors) (not shown) are formed as pixel circuits PC. In this specification, the direction from the light-emitting elements 2 of the display device 1 to the substrate 3 is referred to as the "downward direction," and the direction opposite to the downward direction is referred to as the "upward direction."

[0013] The light-emitting element 2 includes an anode 21, a hole transport layer 22, a light-emitting layer 23, an electron transport layer 24, and a cathode 25, in this order from the substrate 3 side. In other words, the cathode 25 faces the anode 21, and the light-emitting layer 23 is located between the anode 21 and the cathode 25. The anode 21 is electrically connected to the TFT of the substrate 3.

[0014] <Outline of Light-Emitting Element> The structure of each layer of the light-emitting element 2 will be described in more detail below.

[0015] The anode 21 and the cathode 25 comprise conductive materials and are electrically connected to the hole transport layer 22 and the electron transport layer 24, respectively.

[0016] At least one of the anode 21 and the cathode 25 is a transparent electrode that transmits visible light. Examples of transparent electrodes include ITO (indium tin oxide), IZO (indium zinc oxide), ZnO, AZO (aluminum-doped zinc oxide, also known as ZAO), BZO (boron-doped zinc oxide), and FTO (fluorine-doped tin oxide). Either the anode 21 or the cathode 25 may contain a metal material. As the metal material, Al, Cu, Au, Ag, or Mg, or an alloy thereof, which have high visible light reflectance, are preferred. The anode 21 and the cathode 25 may be formed by sputtering or the like, or may be patterned by dry etching or the like.

[0017] The hole transport layer 22 is adjacent to the anode 21 side of the light-emitting layer 23 and contains a hole transport material. The hole transport layer 22 transports holes from the anode 21 to the light-emitting layer 23. In the present disclosure, "two components adjacent" may refer to the two components being in direct contact with each other, or may refer to the two components being in close proximity to each other. For example, in the present disclosure, "two components adjacent" may refer to the shortest distance between the two components being 4 nm or less, or a component having a size of 4 nm or less may be located between the two components. In this embodiment, the hole transport layer 22 may contain an organic material as a hole transport material. For example, the hole transport layer 22 may contain at least one of polyvinylcarbazole (PVK) and [N,N'-bis(4-butylphenyl)-N,N'-bis(phenyl)-benzidine] (TPD).

[0018] However, this embodiment is not limited to this, and the material of the hole transport layer 22 can be an organic or inorganic material that has been conventionally used in light-emitting devices containing quantum dots. For example, the organic material of the hole transport layer 22 can be a conductive compound such as 4,4'-bis(carbazol-9-yl)biphenyl (CBP), polyphenylene vinylene (PPV), a composite of poly(3,4-ethylenedioxythiophene) (PEDOT) and polystyrene sulfonic acid (PSS) (PEDOT-PSS), or poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4'-(N-4-sec-butylphenyl)diphenylamine)]) (TFB). The inorganic material of the hole transport layer 22 can be molybdenum oxide, NiO, Cr ２ O ３ , MgO, MgZnO, LaNiO ３ , MoO ３ , or W.O. ３ In particular, as the material for the hole transport layer 22, a material having a large electron affinity and ionization potential is suitable.

[0019] The electron transport layer 24 is a layer containing an electron transport material that transports electrons from the cathode 25 to the light-emitting layer 23. The material of the electron transport layer 24 can be an organic or inorganic material that has been conventionally used in light-emitting devices containing quantum dots. For example, the electron transport layer 24 can be made of zinc oxide (ZnO), zinc magnesium oxide (ZnMgO), titanium oxide (TiO), and tungsten oxide (WO ３ ), or may contain an inorganic nanoparticle material that is a nanoparticle of these inorganic materials. Alternatively, the electron transport layer 24 may contain an organic material as the electron transport material, such as tris(8-quinolinol)aluminum complex (Alq3), bathocuproine (BCP), or (2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole) (t-Bu-PBD). Note that, as the inorganic material of the electron transport layer 24, metal oxides such as ZnO, ZAO, ITO, InGaZnO, or electride may be used. In particular, a material with a small electron affinity is suitable as the material of the electron transport layer 24.

[0020] In this embodiment, the hole transport layer 22 and the electron transport layer 24 can be formed by vacuum deposition, sputtering, or a coating method using a colloidal solution using the above-mentioned materials. The light-emitting element 2 may also include a hole injection layer between the anode 21 and the hole transport layer 22, or an electron injection layer between the cathode 25 and the electron transport layer 24. The light-emitting element 2 may also include an intermediate layer between the hole transport layer 22 and the light-emitting layer 23, or between the electron transport layer 24 and the light-emitting layer 23. The hole injection layer, electron injection layer, and intermediate layer may all be formed by the same method as the hole transport layer 22 or the electron transport layer 24.

[0021] <Light-Emitting Layer: Quantum Dots: Overview> The light-emitting layer 23 according to this embodiment includes a first quantum dot layer 31 and a second quantum dot layer 32 stacked in this order from the substrate 3 side. In other words, the second quantum dot layer 32 is located closer to the cathode 25 than the first quantum dot layer 31. The first quantum dot layer 31 includes first quantum dots 41 and a first inorganic oxide 51 that covers the peripheries of the first quantum dots 41. The second quantum dot layer 32 includes second quantum dots 42 and a second inorganic oxide 52 that covers the peripheries of the second quantum dots 42. The light-emitting layer 23 may also include an organic ligand dispersed in at least one of the first quantum dot layer 31 and the second quantum dot layer 32, and the organic ligand may contribute to the injection of holes or electrons into the quantum dots.

[0022] The first quantum dot 41 includes a first core 41C and a first shell 41S formed around the first core 41C. The second quantum dot 42 includes a second core 42C and a second shell 42S formed around the second core 42C. Each of the first quantum dot 41 and the second quantum dot 42 is, for example, a luminescent semiconductor nanoparticle that emits light upon recombination of injected electrons and holes. Because the light emitted from each of the first quantum dot 41 and the second quantum dot 42 has a narrow spectrum due to the quantum confinement effect, it is possible to obtain light emission with a relatively deep chromaticity.

[0023] In this embodiment, the center of the quantum dot in the cross section may be considered as a part of the core. Alternatively, the composition of the material contained in the core may be confirmed in the cross section, and then the part not containing the element contained in the core may be considered as the shell. In this embodiment, the boundary between the core and the shell of the quantum dot may be confirmed.

[0024] In the present disclosure, the particle sizes of the quantum dots and cores may be measured by performing cross-sectional observation of the light-emitting layer 23 in the thickness direction DT. For example, in this cross-sectional observation, the particle size of each quantum dot may be considered to be the same as the diameter of a circle having the same area as the cross-sectional area of ​​the quantum dot. Therefore, the shape of the quantum dots is not necessarily limited to a spherical shape. The particle size of the quantum dots may be measured by measuring the particle size of 20 quantum dots and calculating the average. The cross-sectional observation may be performed by analyzing cross-sectional images obtained by imaging using a TEM. The particle size of the core may be measured by a method in which the quantum dots in the above method are replaced with cores. Alternatively, the particle size of the core may be calculated from the wavelength of light emitted by the quantum dots.

[0025] The thickness of the shell may be half the value obtained by subtracting the diameter of the core from the diameter of the quantum dot. In the present disclosure, the shell may be considered to be located in a region of the cross section of the quantum dot where the distance from the outer periphery of the quantum dot is smaller than the thickness of the shell.

[0026] The particle size of the quantum dots contained in the light-emitting layer 23 may be about 1 to 100 nm. ２ Each of the light-emitting layers 23 may contain one or more first quantum dots 41 or second quantum dots 42. In this case, the light-emitting layer 23 generally contains quantum dots at a concentration sufficient to function as a light-emitting layer of a light-emitting element.

[0027] In this embodiment, when observing a cross section of the light-emitting layer 23 along the planar direction DP, a portion where the ratio of the first quantum dots 41 to all quantum dots is 70% or more may be defined as the first quantum dot layer 31. Also, in this embodiment, when observing a cross section of the light-emitting layer 23 along the planar direction DP, a portion where the ratio of the second quantum dots 42 to all quantum dots is 70% or more may be defined as the second quantum dot layer 32. In this disclosure, "all quantum dots" or a "proportion" thereof refers to all quantum dots in an arbitrary cross section of the light-emitting layer 23 having a length of about 100 nm along the planar direction DP, and the number ratio thereamong. In other words, even when this disclosure states that "all quantum dots have a specific configuration," it is not necessary to confirm that all quantum dots in the light-emitting layer 23 have the specific configuration.

[0028] However, in this embodiment, the first quantum dot layer 31 may contain quantum dots different from the first quantum dots 41, for example, the second quantum dots 42, in an amount that is less than 30% of all quantum dots contained in the first quantum dot layer 31. Also, in this embodiment, the second quantum dot layer 32 may contain quantum dots different from the second quantum dots 42, for example, the first quantum dots 41, in an amount that is less than 30% of all quantum dots contained in the second quantum dot layer 32. For example, each of the first quantum dot layer 31 and the second quantum dot layer 32 may contain quantum dots different from both the second quantum dots 42 and the first quantum dots 41. Furthermore, as will be described later, the proportion of the second quantum dots 42 among all quantum dots contained in the first quantum dot layer 31 may be smaller than the proportion of the second quantum dots 42 among all quantum dots contained in the second quantum dot layer 32.

[0029] The first quantum dot layer 31 and the second quantum dot layer 32 may have thicknesses T31 and T32, respectively, along the thickness direction DT of the light-emitting layer 23. At least one of the first quantum dot layer 31 and the second quantum dot layer 32 may be a continuous film in plan view. In this case, the thickness T31 or the thickness T32 may be the average value or the maximum value of the thickness of the continuous portion of the first quantum dot layer 31 or the second quantum dot layer 32 in the thickness direction DT.

[0030] However, at least one of the first quantum dot layer 31 and the second quantum dot layer 32 may be formed in an island shape in a planar view. In the present disclosure, the term "island shape" may refer to a region in which the first quantum dot layer 31 or the second quantum dot layer 32 is partially present in a planar view of the light-emitting element 2. In this case, for example, the sum of the areas of the regions in a planar view may be 50% or more of the area of ​​the anode 21 or the cathode 25 located in each subpixel in a planar view. In this case, the region may be considered to be a layer consisting of the first quantum dot layer 31 or the second quantum dot layer 32. In this case, the thickness T31 or the thickness T32 may be the average value or the maximum value of the thickness of each portion of the first quantum dot layer 31 or the second quantum dot layer 32 in the thickness direction DT.

[0031] The light-emitting layer 23 may include at least one first quantum dot layer 31 and at least one second quantum dot layer 32. In other words, the light-emitting layer 23 may include multiple first quantum dot layers 31 or multiple second quantum dot layers 32. In this case, the thickness T31 may be the thickness of all of the first quantum dot layers 31, and the thickness T32 may be the thickness of all of the second quantum dot layers 32. The number of first quantum dot layers 31 may be calculated by dividing the thickness T31 by the particle diameter of the first quantum dots 41, and the number of second quantum dot layers 32 may be calculated by dividing the thickness T32 by the particle diameter of the second quantum dots 42.

[0032] <Light-emitting layer: quantum dot: core> For example, recombination of electrons and holes in the first quantum dot 41 occurs mainly in the first core 41C, and recombination of electrons and holes in the second quantum dot 42 occurs mainly in the second core 42C. Therefore, light emitted from the first quantum dot 41 and the second quantum dot 42 is obtained mainly from the first core 41C and the second core 42C, respectively.

[0033] The wavelength of the light emitted from each of the first core 41C and the second core 42C can be controlled by controlling the particle size of each of the first core 41C and the second core 42C. Therefore, by controlling the particle size of each of the first core 41C and the second core 42C, the wavelength of the light emitted by the display device 1 can be controlled.

[0034] In the present disclosure, the second core 42C emits light of the same color as the first core 41C. In the present disclosure, for example, light with a central wavelength greater than 600 nm and equal to or less than 780 nm is defined as red light, light with a central wavelength greater than 500 nm and equal to or less than 600 nm is defined as green light, and light with a central wavelength greater than or equal to 380 nm and equal to or less than 500 nm is defined as blue light. In this case, "light of the same color" may refer to, for example, light whose central wavelengths fall within the wavelength range of the same color as described above. Alternatively, "light of the same color" may refer to light whose central wavelengths differ by less than 5 nm. Alternatively, in the present disclosure, it may be assumed that the particle diameter of the first core 41C and the particle diameter of the second core 42C are the same and the composition of the material contained in the first core 41C and the composition of the material contained in the second core 42C are the same. In this case, the light emitted by the first core 41C and the light emitted by the second core 42C may be considered to be the same color. For example, when the difference between the particle size of the first core 41C and the particle size of the second core 42C is 1 nm or less, the particle size of the first core 41C and the particle size of the second core 42C may be considered to be the same.

[0035] <Light-emitting layer: quantum dot: shell> Each of the first shell 41S and the second shell 42S has a function of suppressing the occurrence of defects or dangling bonds, etc. in the first core 41C and the second core 42C, and reducing the recombination of carriers that undergo a deactivation process. In the present disclosure, each of the first shell 41S and the second shell 42S contains a substance having a metal element and a Group 16 element.

[0036] In the present disclosure, M is a metal element, A is a first Group 16 element, and B is a second Group 16 element located in a period after the first Group 16 element in the periodic table. Also, x and y are real numbers satisfying 0≦x<y≦1, n is 0<n, and m is 0<m.

[0037] In the present disclosure, the first shell 41S has a composition of M ｎ B １－ｘ A ｘ The second shell 42S contains a material having a composition of M ｍ B １－ｙ A ｙ This may be confirmed, for example, as follows. As will be described later, the elements contained in each of the first shell 41S and the second shell 42S may be confirmed by performing EDX (energy dispersive X-ray spectroscopy) on a cross section of the light-emitting layer 23 in the thickness direction DT. When the elements contained in the first shell 41S and the second shell 42S each contain three or more elements, and when three elements M, A, and B are selected from these elements, it is sufficient to confirm that the selection can be made so as to satisfy all of the following (1) to (3): (1) M is a metal element, A is a Group 16 element, and B is a Group 16 element located in a period later than A in the periodic table. (2) The composition of the substance contained in the first shell 41S is M ｎ B １－ｘ A ｘ The composition of the material contained in the second shell 42S is M ｍ B １－ｙ A ｙ (3) x, y, n, and m satisfy 0≦x<y≦1, 0<n, and 0<m. Each of the first shell 41S and the second shell 42S may contain 50% by weight or more of the above-mentioned substance. The difference between x and y may be 0.05 or more.

[0038] In general, the band gap of a shell containing a Group 16 element tends to become smaller as the period of the Group 16 element on the periodic table increases. Therefore, the band gap of the second shell 42S in the present disclosure is larger than the band gap of the first shell 41S. As described above, the second quantum dot layer 32 is located closer to the cathode 25 than the first quantum dot layer 31. Therefore, the light-emitting layer 23 includes at least one second quantum dot 42 closer to the cathode 25 than at least one first quantum dot 41. In other words, the light-emitting layer 23 includes, as a quantum dot, a second quantum dot 42 having a second shell 42S with a larger band gap than the first shell 41S of the first quantum dot 41, located closer to the cathode 25 than the first quantum dot 41.

[0039] In general, in light-emitting devices that include quantum dots in the light-emitting layer, an excess of electrons tends to occur, in which the electron concentration in the light-emitting layer is higher than the hole concentration when the light-emitting device is driven. The excess of electrons in the light-emitting layer reduces the probability of recombination between holes and electrons in the quantum dots, thereby reducing the light-emitting efficiency of the light-emitting device. Furthermore, the excess of electrons in the light-emitting layer increases the probability of processes that do not contribute to light emission, such as Auger electrons, due to interactions between excess electrons, thereby reducing the light-emitting efficiency of the light-emitting device. In particular, Auger electrons have high energy, which can cause deterioration of various parts of the light-emitting layer or layers adjacent to the light-emitting layer.

[0040] As described above, the light-emitting layer 23 of the light-emitting element 2 according to this embodiment includes, as quantum dots, second quantum dots 42 having second shells 42S with a larger band gap than the first shells 41S of the first quantum dots 41, located closer to the cathode 25 than the first quantum dots 41. Generally, the larger the band gap of the shell, the lower the efficiency of carrier injection into the core of the quantum dot having the shell. Therefore, in the light-emitting layer 23, the efficiency of hole injection from the anode 21 to the first quantum dots 41 is higher than the efficiency of electron injection from the cathode 25 to the second quantum dots 42.

[0041] Therefore, with the above-described configuration, the light-emitting element 2 according to this embodiment reduces the excess electrons in the light-emitting layer 23, improving light-emitting efficiency. Furthermore, the light-emitting element 2 reduces the probability of Auger electron generation processes occurring in the light-emitting layer 23, reducing deterioration of the light-emitting layer 23 and layers located near the light-emitting layer 23. The display device 1 including the light-emitting element 2 achieves power saving and a long life.

[0042] From the viewpoint of improving the efficiency of hole injection into the quantum dots contained in the first quantum dot layer 31, the proportion of the first quantum dots 41 among all the quantum dots contained in the first quantum dot layer 31 may be 70% or more. From the viewpoint of reducing the efficiency of electron injection into the quantum dots contained in the second quantum dot layer 32, the proportion of the second quantum dots 42 among all the quantum dots contained in the second quantum dot layer 32 may be 70% or more. With the above configuration, the light-emitting element 2 further reduces the excess electrons in the light-emitting layer 23, further improving the light-emitting efficiency.

[0043] Furthermore, the proportion of the second quantum dots 42 among all the quantum dots contained in the first quantum dot layer 31 may be smaller than the proportion of the second quantum dots 42 among all the quantum dots contained in the second quantum dot layer 32. In this case, the light-emitting element 2 further reduces the excess electrons in the light-emitting layer 23, further improving the light-emitting efficiency.

[0044] The first quantum dot layer 31 may include only the first quantum dots 41 as quantum dots, and the second quantum dot layer 32 may include only the second quantum dots 42 as quantum dots. In this case, the electron excess in the light-emitting layer 23 of the light-emitting element 2 is further reduced, and the light-emitting efficiency is further improved.

[0045] <Other Examples of Light-Emitting Layer: Quantum Dots: Shell> Another example of the quantum dots contained in the light-emitting layer 23 according to this embodiment, particularly another example of the shells the quantum dots have, will be described in detail with reference to FIG. 3. FIG. 3 is a schematic diagram showing another example of the quantum dots according to this embodiment. As other examples of the first quantum dots 41, FIG. 3 also shows a schematic diagram F31 of a first quantum dot 41A and a schematic diagram F32 of a first quantum dot 41B. Schematic diagram F31 and schematic diagram F32 show cross sections passing through the centers of the first quantum dots 41A and 41B, respectively.

[0046] 3, the first quantum dot 41A has the first core 41C and a first shell 41SA located around the first core 41C. The first shell 41SA has a first portion 61 and a second portion 62. In particular, the second portion 62 contains a substance having a different composition from the substance contained in the first portion 61, and is located closer to the outer periphery than the first portion 61.

[0047] In this case, the composition of the material contained in the first shell 41SA may be the average value of the entire composition of the first shell 41SA, including the first portion 61 and the second portion 62. For example, the compositions of the first portion 61 and the second portion 62 are calculated. Next, the volume ratio between the first portion 61 and the second portion 62 is calculated by observing a cross section of the first quantum dot 41A. Next, the average value of the composition of the first portion 61 and the second portion 62 is calculated by weighting it by the volume ratio between the first portion 61 and the second portion 62. In this manner, the composition of the material contained in the first shell 41SA may be confirmed.

[0048] However, the method for determining the composition of the material contained in the first shell 41SA is not limited to the above. For example, the band gap in the portion of the shell closer to the outer periphery of the quantum dot generally contributes more strongly to carrier injection into the quantum dot. Therefore, the composition of the material contained in the first shell 41SA may be the average value of the composition of the portion of the first shell 41SA within a predetermined distance from the outer periphery. For example, the composition of the material contained in the first shell 41SA may be the average value of the composition of the portion of the first shell 41SA within 1 nm from the outer periphery. Alternatively, the composition of the material contained in the first shell 41SA may be the composition of the second portion 62, which is located closer to the outer periphery than the first portion 61.

[0049] As shown in schematic diagram F32 of FIG. 3 , the first quantum dot 41B has the first core 41C described above and a first shell 41SB located around the first core. The first shell 41SB has a first portion 61, a second portion 62, and a third portion 63. As described above, the second portion 62 contains a substance having a different composition from the substance contained in the first portion 61 and is located closer to the outer periphery than the first portion 61. The third portion 63 is located between the first portion 61 and the second portion 62. Furthermore, the composition of the substance contained in the third portion 63 continuously changes from the composition of the substance contained in the first portion 61 to the composition of the substance contained in the second portion 62 from the side of the first portion 61 to the side of the second portion 62. For example, the composition of the material contained in the third part 63 continuously changes from the composition of the material contained in the first part 61 to the composition of the material contained in the second part 62 along the direction DC, which is the direction from the center 41CC of the first core 41C toward the outer periphery of the first core 411C.

[0050] Even in this case, the composition of the material contained in first shell 41SB may be the average value of the composition of the entire first shell 41SB, including first portion 61, second portion 62, and third portion 63. Alternatively, the composition of the material contained in first shell 41SB may be the average value of the composition of a portion of first shell 41SB within a predetermined distance from the outer periphery.

[0051] Like the first quantum dot 41A or 41B described above, the first quantum dot 41 according to this embodiment may have a composition that varies depending on the position of the first shell 41S. This improves the degree of freedom in designing the shells of the quantum dots included in the light-emitting layer 23 of the light-emitting element 2, making it easier to achieve a configuration that is more suitable for the shells of the quantum dots included in the light-emitting layer 23. In this embodiment, the second shell 42S of the second quantum dot 42 may have a first portion 61 and a second portion 62, and may further have a third portion 63, similar to the first shell 41SA or 41SB, except for the composition.

[0052] <Specific Examples of Light-Emitting Layer: Quantum Dots: Shell> The compositions of the materials contained in the first shell 41S and the second shell 42S will be described with reference to Table 1 below.

[0053] In Table 1, the "Core" column shows examples of materials contained in the first core 41C and the second core 42C. In Table 1, the first core 41C and the second core 42C are assumed to contain the same material. In addition, in Table 1, the "First Shell" column shows examples of materials contained in the first shell 41S when the first core 41C and the second core 42C contain the materials listed in the "Core" column. Furthermore, in Table 1, the "Second Shell" column shows examples of materials contained in the second shell 42S when the first core 41C and the second core 42C contain the materials listed in the "Core" column and the first shell 41S contains the materials listed in the "First Shell" column. However, the materials contained in the first core 41C and the second core 42C are not limited to the examples shown in Table 1, and may be CuInGaS, InZnP, CdSeTe, CuInZnS, CuInS, CuGaS, AgInS, or ZnAgInS. Note that the chemical formulas of the compounds of the materials contained in the first core 41C and the second core 42C are all representative examples, and the composition ratios described in the chemical formulas may be stoichiometric, but are not necessarily stoichiometric.

[0054] The entry (x=0) in the "First Shell" column indicates that the composition of the material contained in the first shell 41S is M. ｎ B １－ｘ A ｘ In the case where the second shell 42S is expressed as follows, x=0. The expression (y=1) in the "Second shell" column indicates that the composition of the material contained in the second shell 42S is M. ｍ B １－ｙ A ｙ In particular, when the composition of the material contained in the first shell 41S is M ｎ B １－ｘ A ｘ and when x=0, the composition of the material contained in the first shell 41S is M ｎ B. Therefore, the material contained in the first shell 41S contains only a single metal element and a single Group 16 element. Therefore, with the above configuration, the light-emitting element 2 has a simpler configuration of the first shell 41S, and the first quantum dots 41 can be synthesized more easily.

[0055] The composition of the material contained in the first shell 41S is M ｎ B １－ｘ A ｘand the composition of the material contained in the second shell 42S is M ｍ B １－ｙ A ｙ Assume that. When the first shell 41S and the second shell 42S each contain the material shown in Table 1, n is 1 and m is 1. In this case, the ratio of Group 16 element atoms to metal atoms in the substances contained in the first shell 41S and the second shell 42S is approximately the same. Therefore, with the above configuration, the light-emitting element 2 has a simpler configuration for the first shell 41S and the second shell 42S, or makes it easier to synthesize the first quantum dots 41 and the second quantum dots 42.

[0056] In particular, in this embodiment, the second Group 16 element may be a Group 16 element located in the period next to the first Group 16 element in the periodic table. For example, when the first Group 16 element is sulfur, the second Group 16 element may be selenium. Furthermore, when the first Group 16 element is selenium, the second Group 16 element may be tellurium. Furthermore, when the first Group 16 element is tellurium, the second Group 16 element may be polonium.

[0057] With the above configuration, the light-emitting element 2 reduces excessive differences in carrier injection efficiency between the first quantum dot 41 and the second quantum dot 42 while differentiating the band gap between the first shell 41S and the second shell 42S. Furthermore, in this embodiment, the difference in atomic radius between the first Group 16 element and the second Group 16 element can be reduced. Therefore, in the light-emitting layer 23 according to this embodiment, defects are unlikely to occur inside each shell even when the values ​​of x and y are adjusted, thereby increasing the quantum yield of the first quantum dot 41 and the second quantum dot 42. Furthermore, in this embodiment, the carrier balance of the light-emitting element 2 can be easily adjusted by adjusting the values ​​of x and y. Therefore, the light-emitting element 2 has improved luminous efficiency with the above configuration.

[0058] In particular, in this embodiment, the first Group 16 element may be sulfur and the second Group 16 element may be selenium. In this case, the metal element contained in the first shell 41S and the second shell 42S may be zinc. With the above configuration, the light-emitting device 2 improves the quantum yield of the first quantum dots 41 and the second quantum dots 42, even if the first shell 41S and the second shell 42S do not contain cadmium, an element that has a high impact on the human body and the environment. Therefore, with the above configuration, the light-emitting device 2 improves its luminous efficiency.

[0059] In particular, the composition of the material contained in the first shell 41S is ZnSe. １－ｘ S ｘ and when x = 0, the first shell 41S contains ZnSe. In this case, the band gap of the first shell 41S in the light-emitting element 2 can be particularly reduced, improving the efficiency of hole injection from the anode 21 to the first quantum dots 41 in the light-emitting layer 23. Therefore, with the above configuration, the light-emitting element 2 further reduces the excess electrons in the light-emitting layer 23, further improving the light-emitting efficiency.

[0060] <Light-emitting layer: inorganic oxide> The first inorganic oxide 51 covers the periphery of the first quantum dots 41, and the second inorganic oxide 52 covers the periphery of the second quantum dots 42. Therefore, the first inorganic oxide 51 and the second inorganic oxide 52 protect the first quantum dots 41 and the second quantum dots 42 from moisture, air, heat, and the like, which may deteriorate the quantum dots. Furthermore, even if the first quantum dots 41 and the second quantum dots 42 have organic ligands, the first inorganic oxide 51 and the second inorganic oxide 52 fix the positions of the organic ligands. Therefore, the first inorganic oxide 51 and the second inorganic oxide 52 can reduce deterioration of the first quantum dots 41 and the second quantum dots 42 due to detachment of the organic ligands from the first quantum dots 41 and the second quantum dots 42.

[0061] Therefore, in the light-emitting layer 23, the first shell 41S of the first quantum dot 41 and the second shell 42S of the second quantum dot 42 are protected by the first inorganic oxide 51 and the second inorganic oxide 52, respectively. Therefore, in the light-emitting layer 23, deterioration of the first shell 41S and the second shell 42S can be reduced, and thus a decrease in the effect achieved by the difference in band gap between the first shell 41S and the second shell 42S described above is suppressed. Therefore, the light-emitting device 2 according to this embodiment improves the reliability of the first quantum dot 41 and the second quantum dot 42, making it easier to reduce electron excess in the light-emitting layer 23 and therefore easier to improve luminous efficiency.

[0062] The first inorganic oxide 51 and the second inorganic oxide 52 may be located, for example, around the entire periphery of the first quantum dot 41 and the second quantum dot 42, respectively. For example, as shown in FIG. 1 , in any cross section passing through any first quantum dot 41, the first inorganic oxide 51 may be located around the entire periphery of the first quantum dot 41. Furthermore, in any cross section passing through any second quantum dot 42, the second inorganic oxide 52 may be located around the entire periphery of the second quantum dot 42. In the present disclosure, "the inorganic oxide is located around the entire periphery of the quantum dot" may mean that the inorganic oxide is located around 90% or more of the periphery of the quantum dot. Furthermore, as shown in FIG. 1 , the surface of the first quantum dot 41 and the first inorganic oxide 51 may be in contact, and the surface of the second quantum dot 42 and the second inorganic oxide 52 may be in contact. Alternatively, as will be described later, for example, MPS may be bonded to the first quantum dots 41 or the second quantum dots 42 via a thiol group, and silicon oxide converted from MPS or TMOS may become the first inorganic oxide 51 or the second inorganic oxide 52. In this case, the first quantum dots 41 and the first inorganic oxide 51, or the second quantum dots 42 and the second inorganic oxide 52, may be bonded via a carbon chain.

[0063] The band gap of the first inorganic oxide 51 may be larger than the band gap of the first shell 41S in order to improve the carrier confinement effect in the first quantum dot 41. Furthermore, the band gap of the second inorganic oxide 52 may be larger than the band gap of the second shell 42S in order to improve the carrier confinement effect in the second quantum dot 42.

[0064] The first inorganic oxide 51 and the second inorganic oxide 52 may have, for example, insulating properties in order to improve the protective effect of the first quantum dots 41 and the second quantum dots 42 by the first inorganic oxide 51 and the second inorganic oxide 52. The first inorganic oxide 51 and the second inorganic oxide 52 may contain, for example, at least one selected from the group including silicon oxide, boron oxide, phosphorus oxide, germanium oxide, titanium oxide, tellurium oxide, aluminum oxide, bismuth oxide, vanadium oxide, antimony oxide, and lead oxide.

[0065] In particular, at least one of the first inorganic oxide 51 and the second inorganic oxide 52 may contain silicon oxide. The first inorganic oxide 51 or the second inorganic oxide 52 containing silicon oxide becomes denser, and therefore the first inorganic oxide 51 or the second inorganic oxide 52 better protects the first quantum dots 41 or the second quantum dots 42. When at least one of the first inorganic oxide 51 or the second inorganic oxide 52 contains silicon oxide, at least one of the first inorganic oxide 51 or the second inorganic oxide 52 containing silicon oxide can be formed from MPS or TMOS, as described below. For example, suppose that MPS, described below, binds to the first quantum dots 41 or the second quantum dots 42 via a thiol group, and silicon oxide converted from MPS or TMOS becomes the first inorganic oxide 51 or the second inorganic oxide 52. In this case, the first quantum dots 41 and the first inorganic oxide 51, or the second quantum dots 42 and the second inorganic oxide 52, can form a structure bonded via a carbon chain. Therefore, the first inorganic oxide 51 or the second inorganic oxide 52 having the above-mentioned configuration can prevent the thiol groups from being detached from the first quantum dots 41 or the second quantum dots 42, thereby reducing deterioration of the first quantum dots 41 or the second quantum dots 42.

[0066] Furthermore, both the first inorganic oxide 51 and the second inorganic oxide 52 may contain silicon oxide. With the above configuration, the first inorganic oxide 51 and the second inorganic oxide 52 better protect both the first quantum dots 41 and the second quantum dots 42. In this case, the concentration of silicon atoms per unit volume in the first quantum dot layer 31 may be lower than the concentration of silicon atoms per unit volume in the second quantum dot layer 32.

[0067] In general, the lattice constant of a shell containing a Group 16 element tends to increase as the period of the Group 16 element on the periodic table increases. Therefore, in the present disclosure, the lattice constant of the second shell 42S is smaller than that of the first shell 41S. Assume that MPS is bonded to the first quantum dots 41 or the second quantum dots 42 via thiol groups, and silicon oxide converted from MPS or TMOS becomes the first inorganic oxide 51 or the second inorganic oxide 52. Additionally, assume that the first quantum dots 41 and the first inorganic oxide 51, or the second quantum dots 42 and the second inorganic oxide 52, form a structure in which they are bonded via carbon chains. In this case, the density of MPS bonded to the second quantum dots 42 is greater than the density of MPS bonded to the first quantum dots 41. That is, in this embodiment, the concentration of the first inorganic oxide 51 in the first quantum dot layer 31 can be made higher than the concentration of the second inorganic oxide 52 in the second quantum dot layer 32.

[0068] Assume that the surface of the first quantum dot 41 or the second quantum dot 42 is in contact with the first inorganic oxide 51 or the second inorganic oxide 52. In this case, the first inorganic oxide 51 or the second inorganic oxide 52 is formed to have the same lattice constant as the underlying first shell 41S or the second shell 42S, at least near the surface of the first quantum dot 41 or the second quantum dot 42. Therefore, in this embodiment, the above configuration allows the concentration of the first inorganic oxide 51 in the first quantum dot layer 31 to be higher than the concentration of the second inorganic oxide 52 in the second quantum dot layer 32. In general, inorganic oxides have a wide band gap and low electrical conductivity, which reduces the efficiency of electron injection into the second quantum dots 42 compared to the first quantum dots 41. Therefore, the above configuration further reduces the electron excess in the light-emitting layer 23 of the light-emitting device 2, further improving the light-emitting efficiency.

[0069] <Additional Note> The light-emitting element 2 according to this embodiment is not limited to the layer structure described above. For example, the light-emitting element 2 may include at least one of a hole injection layer or an electron blocking layer located between the anode 21 and the hole transport layer 22, and an electron injection layer or a hole blocking layer located between the cathode 25 and the electron transport layer 24. Furthermore, the light-emitting element 2 according to this embodiment may have a reversed stacking order with respect to the substrate 3. For example, the light-emitting element 2 may include, in order from the substrate 3 side, the cathode 25, the electron transport layer 24, the light-emitting layer 23, the hole transport layer 22, and the anode 21.

[0070] The specific structure of each layer of the light-emitting element 2 according to this embodiment may be confirmed by observing a cross section of each layer of the light-emitting element 2 and performing EDX (energy dispersive X-ray spectroscopy) on the cross section. The cross-sectional observation may be, for example, Method 1: cross-sectional observation using a SEM (scanning electron microscope) or Method 2: cross-sectional observation using a TEM (transmission electron microscope). In particular, when the cross-section is observed using a TEM and EDX is performed on the cross section, FIB (focused ion beam) processing may be used in combination. The confirmation methods are prioritized in the order of Method 1 and Method 2. If confirmation is achieved using the first method, confirmation using the second method may be omitted.

[0071] <Method of Manufacturing Display Device: Up to Formation of Hole Transport Layer> A method of manufacturing the display device 1 according to this embodiment will be described with reference to Fig. 4. Fig. 4 is a flowchart showing an example of a method of manufacturing the display device 1 according to this embodiment.

[0072] In the method for manufacturing the display device 1 according to this embodiment, first, a substrate 3 is prepared (step S1). The substrate 3 may be manufactured by, for example, forming a plurality of the pixel circuits PC described above on a glass substrate.

[0073] Next, the light-emitting element 2 is formed on the substrate 3. In the process of forming the light-emitting element 2, for example, first, the anode 21 is formed on the substrate 3 by any of the methods described above (step S2). For example, if the substrate 3 has a plurality of pixel circuits PC, the anode 21 may be formed in an island shape for each pixel circuit PC.

[0074] Next, by any of the methods described above, the hole transport layer 22 is formed on the anode 21 (step S3). The hole transport layer 22 may be formed in common for the plurality of anodes 21, or may be formed in an island shape for each anode 21. When the hole transport layer 22 is formed in an island shape, the material of the hole transport layer 22 may differ depending on the emission color of the sub-pixel.

[0075] <Display Device Manufacturing Method: Formation of Light-Emitting Layer: Formation of First Quantum Dot Layer> Next, the light-emitting layer 23 is formed (step S4). The method for forming the light-emitting layer 23 will be described in more detail with reference to Fig. 5. Fig. 5 is a flowchart showing an example of the method for forming the light-emitting layer 23 according to this embodiment.

[0076] In the method for forming the light-emitting layer 23 according to this embodiment, the first quantum dot layer 31 is formed first. In forming the first quantum dot layer 31, the first quantum dots 41 are first synthesized (step S41). The synthesis of the first quantum dots 41 may be performed, for example, by synthesizing a plurality of first cores 41C and then forming a first shell 41S around each of the first cores 41C. Here, the first shell 41S is formed using a material having at least the above-mentioned metal element and a second Group 16 element. The material may contain the first Group 16 element. This results in a composition of M. ｎ B １－ｘ A ｘ A first quantum dot 41 is synthesized that includes a first shell 41S having a material:

[0077] The first core 41C and the first shell 41S may be synthesized by various methods, including conventionally known methods such as a heating method, a hot injection method, a microwave-assisted method, a continuous flow method, etc. The first quantum dots 41 may be synthesized in a solvent such as ethanol in which an organic ligand is dispersed, in which case the organic ligand may be coordinated to the outermost surface of the first quantum dots 41. Alternatively, the first quantum dots 41 may be isolated by adding ethyl acetate to a dispersion liquid in which the first quantum dots 41 are dispersed to precipitate the first quantum dots 41, recovering the precipitate by centrifugation, and washing the precipitate.

[0078] Next, a first quantum dot dispersion liquid is prepared (step S42). The first quantum dot dispersion liquid contains a plurality of first quantum dots 41 and a first precursor, which is a precursor of the first inorganic oxide 51, dispersed in a solvent. The first precursor contains a material that is converted into the first inorganic oxide 51 by a treatment such as heating, which will be described later. In particular, when the first inorganic oxide 51 contains silicon oxide, the first precursor may contain tetramethyl orthosilicate (TMOS) represented by the following formula (1):

[0079] In addition, the first precursor may be tetraethyl orthosilicate (TEOS), tetraisopropyl orthosilicate, tetrapropyl orthosilicate, tetrabutyl orthosilicate, trimethoxymethylsilane, triethoxymethylsilane, or the like. Trimethoxy(propyl)silane, Triethoxy(propyl)silane, Triethoxy(propyl)silane, Butyltrimethoxysilane, Butyltriethoxysilane, Triethoxy(isobutyl)silane, Cyclopentyltrimethoxysilane , Hexyltrimethoxysilane, Hexyltriethoxysilane, Decyltrimethoxysilane, Decyltriethoxysilane, Hexadecyltrimethoxysilane, Hexadecyltriethoxysilane, Octadecyltriethoxy silane (octadecyltriethoxysilane), Octadecyltrimethoxysilane (octadecyltrimethoxysilane), Trimethoxyphenylsilane (trimethoxyphenylsilane), Triethoxyphenylsilane (triethoxyphenylsilane), Allyltrimethoxysilane (allyltrimethoxysilane), Allyltriethoxysilane (allyltriethoxysilane), Vinyltrimethoxysilane (vinyltrimethoxysilane),and vinyltriethoxysilane.

[0080] Furthermore, when the first inorganic oxide 51 contains silicon oxide, the first precursor may contain 3-(mercaptopropyl)trimethoxysilane (MPS) represented by the following formula (2).

[0081] In addition, the second precursor may include at least one of (3-mercaptopropyl)triethoxysilane, 3-aminopropyltrimethoxysilane (APS), 3-aminopropyltriethoxysilane, 4-aminobutyltrimethoxysilane, and 4-aminobutyltriethoxysilane.

[0082] The first quantum dot dispersion may include a halide such as zinc fluoride, zinc chloride, zinc bromide, zinc iodide, indium fluoride, indium chloride, indium bromide, or indium iodide.

[0083] The first quantum dot dispersion may be prepared by, for example, stirring the first dispersion and the second dispersion. The first dispersion may be prepared by dispersing the first quantum dots 41 synthesized in step S41 in a first solvent. The second dispersion may be prepared by dispersing the first precursor in a second solvent having a polarity different from that of the first solvent. The second dispersion may contain the above-mentioned halide.

[0084] By stirring the first dispersion and the second dispersion, for example, at least a portion of the organic ligands coordinated to the first quantum dots 41 in the first dispersion may be replaced with the first precursor containing MPS, for example. As a result, when the first dispersion and the second dispersion are stirred, the dispersibility of the first quantum dots 41 in the solvent may change, and an intermediate layer containing the first quantum dots 41 and the first precursor may be formed between the first dispersion and the second dispersion.

[0085] Next, an intermediate layer may be extracted from the mixture of the first dispersion and the second dispersion, and then the intermediate layer may be centrifuged to extract a precipitate containing the first quantum dots 41 and the first precursor. The precipitate may then be washed and dispersed in a solvent such as toluene to prepare a first quantum dot dispersion containing a plurality of first quantum dots 41 and the first precursor dispersed in the solvent.

[0086] Note that steps S41 and S42 do not have to be performed after step S3, in other words, they do not have to be performed after the formation of the hole transport layer 22. In particular, in this embodiment, it is sufficient that the preparation of the first quantum dot dispersion liquid is completed by the time step S3 is completed, in other words, by the time the formation of the hole transport layer 22 is completed.

[0087] Following the preparation of the first quantum dot dispersion, the first quantum dot dispersion is applied onto the hole transport layer 22 (step S43). In particular, in step S43, the first quantum dot dispersion is applied onto the hole transport layer 22 by any method including a conventionally known application method such as spin coating.

[0088] Next, the applied first quantum dot dispersion liquid is heated (step S44). In step S44, for example, each portion of the substrate 3 containing the applied first quantum dot dispersion liquid is heated, for example, at 80°C to 200°C for 10 minutes to 60 minutes. As a result, the solvent in the first quantum dot dispersion liquid volatilizes, and the first precursor in the first quantum dot dispersion liquid is converted into the first inorganic oxide 51.

[0089] For example, when the first precursor contains TMOS and MPS, the methoxy groups of TMOS and MPS are converted into water (H ２O), which is replaced by a hydroxyl group (OH-), and methanol is produced as a by-product. Next, the two hydroxyl groups undergo dehydration condensation with each other, resulting in dehydration condensation between TMOS and MPS, between two TMOSs, and between two MPSs. This converts TMOS and MPS into silicon oxide.

[0090] The reaction of the first precursor occurs sequentially around the first quantum dots 41. Therefore, in step S44, the first inorganic oxide 51 is formed around the first quantum dots 41, and the first inorganic oxide 51 eventually covers the periphery of the first quantum dots 41. In this way, the first quantum dot layer 31 is formed on the hole transport layer 22.

[0091] <Display Device Manufacturing Method: Formation of Light-Emitting Layer: Formation of Second Quantum Dot Layer> In the method for forming the light-emitting layer 23 according to this embodiment, the second quantum dot layer 32 is formed after the formation of the first quantum dot layer 31. In forming the second quantum dot layer 32, the second quantum dots 42 are first synthesized (step S45). The synthesis of the second quantum dots 42 may be performed using the same method as the synthesis of the first quantum dots 41, except for the material of the first shell 41S. The second shell 42S is formed using a material containing at least the above-described metal element and a first Group 16 element. The material may also contain a second Group 16 element. However, the ratio of the concentration of the second Group 16 element to the concentration of the first Group 16 element in the material used to form the second shell 42S is smaller than the ratio of the concentration of the second Group 16 element to the concentration of the first Group 16 element in the material used to form the first shell 41S. This results in a composition of M ｍ B １－ｙ A ｙ A second quantum dot 42 is synthesized that includes a second shell 42S having a material that is

[0092] Next, a second quantum dot dispersion is prepared (step S46). The second quantum dot dispersion contains a plurality of second quantum dots 42 and a second precursor, which is a precursor of the second inorganic oxide 52, dispersed in a solvent. The second quantum dot dispersion may be prepared by the same method as the first precursor, except that the first quantum dots 41 are replaced with the second quantum dots 42 and the first precursor is replaced with the second precursor. The second precursor may contain the same material as the first precursor, for example, TMOS or MPS.

[0093] Note that steps S45 and S46 do not have to be performed after step S44, in other words, they do not have to be performed after the formation of the first quantum dot layer 31. In particular, in this embodiment, it is sufficient that the preparation of the second quantum dot dispersion liquid is completed by the time step S44 is completed, in other words, by the time the formation of the first quantum dot layer 31 is completed.

[0094] Following the preparation of the second quantum dot dispersion, the second quantum dot dispersion is applied (step S47). In particular, in step S47, the second quantum dot dispersion is applied above the previously formed first quantum dot layer 31, particularly onto the upper surface of the first quantum dot layer 31 in this embodiment, by any method including a conventionally known application method such as spin coating.

[0095] Next, the applied second quantum dot dispersion liquid is heated (step S48). Step S48 may be performed by the same method as above, except that the heating target is each portion on the substrate 3 containing the applied second quantum dot dispersion liquid. As a result, the second precursor in the second quantum dot dispersion liquid is sequentially converted around the second quantum dots 42. Therefore, in step S48, the second inorganic oxide 52 is formed around the second quantum dots 42, and thus the second inorganic oxide 52 covers the second quantum dots 42. In this manner, the second quantum dot layer 32 is formed on the first quantum dot layer 31, thereby completing the formation of the light-emitting layer 23 according to this embodiment.

[0096] Alternatively, in step S42 of the manufacturing method of this embodiment, a first structure dispersion liquid may be prepared that contains a plurality of dispersed first quantum dot structures, each having a plurality of first quantum dots 41 and a first inorganic oxide 51 located around the first quantum dots 41. Furthermore, in step S46, a second structure dispersion liquid may be prepared that contains a plurality of dispersed second quantum dot structures, each having a plurality of second quantum dots 42 and a second inorganic oxide 52 located around the second quantum dots 42.

[0097] The first structure dispersion liquid and the second structure dispersion liquid may be prepared by heating the above-described first quantum dot dispersion liquid and second quantum dot dispersion liquid before coating to convert the first precursor and the second precursor in each dispersion liquid. In the manufacturing method of this embodiment, the first quantum dot layer 31 may be formed by coating and heating the first structure dispersion liquid, and the second quantum dot layer 32 may be formed by coating and heating the second structure dispersion liquid.

[0098] 4 , after the light-emitting layer 23 is formed, the electron transport layer 24 is formed on the light-emitting layer 23 by any of the methods described above (step S5). The electron transport layer 24 may be formed by the same method as the hole transport layer 22, except for the formation position and the material used.

[0099] Next, the cathode 25 is formed on the electron transport layer 24 by the various methods described above (step S6). The cathode 25 may be formed in common for a plurality of anodes 21. In this manner, the display device 1 including the light-emitting element 2 on the substrate 3 is manufactured.

[0100] As described above, the above method makes it possible to manufacture a light-emitting element 2 that reduces the excess electrons in the light-emitting layer 23 and improves luminous efficiency, and a display device 1 that includes the light-emitting element 2. Furthermore, the above method forms the light-emitting layer 23 by forming a first quantum dot layer 31, which includes applying a first quantum dot dispersion, followed by forming a second quantum dot layer 32, which includes applying a second quantum dot dispersion above the first quantum dot layer 31. Compared to forming the light-emitting layer 23 all at once, the above method makes it possible to more simply form the first quantum dot layer 31 and the second quantum dot layer 32, which have different substance compositions contained in the shells of the quantum dots.

[0101] 6 is a schematic side cross-sectional view of a display device 1 according to this embodiment. The display device 1 according to this embodiment has the same configuration as the display device 1 according to the previous embodiment, except for the configuration of the light-emitting element 2. 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 configurations of the first inorganic oxide 51 and the second inorganic oxide 52 in the light-emitting layer 23.

[0102] In this embodiment, at least one of the first inorganic oxide 51 and the second inorganic oxide 52 fills at least one of the space between two first quantum dots 41, the space between two second quantum dots 42, or the space between one first quantum dot 41 and one second quantum dot 42.

[0103] The material filling the spaces between the quantum dots will be described in more detail with further reference to FIG. 7 . Schematic diagrams F71 and F72 in FIG. 7 are schematic diagrams illustrating the material filling the spaces between the quantum dots contained in the light-emitting layer 23. In particular, schematic diagrams F71 and F72 are diagrams illustrating two examples of a set P of two quantum dots and the region (space) K between them, as shown in FIG. 6 . In particular, schematic diagrams F71 and F72 are diagrams illustrating sets P1 and P2, which are examples of sets of quantum dots 40A and 40B, respectively. Each of quantum dots 40A and 40B is either a first quantum dot 41 or a second quantum dot 42.

[0104] In the present disclosure, the term "a member fills the spaces between the first quantum dots 41" means that the member fills at least the region K between the quantum dots 40A and 40B, as shown in the schematic diagram F71 of the set P1 in Fig. 7. Region K is a region surrounded by two lines (common circumscribing lines) tangent to the peripheries of the quantum dots 40A and 40B and the opposing peripheries of the quantum dots 40A and 40B in the cross section of the light-emitting layer 23. Therefore, as shown in the schematic diagram F72 of the set P2 in Fig. 7, region K can exist even if the quantum dots 40A and 40B are close to each other, and the member fills region K.

[0105] The expression "a material fills the spaces between the first quantum dots 41" does not necessarily mean that the region K between the quantum dots 40A and 40B is made entirely of the material. For example, the region K between the quantum dots 40A and 40B may contain a material, such as an organic ligand, that is different from the material of the material. In this case, in the light-emitting layer 23, from the viewpoint of improving the reliability of the light-emitting layer 23, the weight ratio of the organic ligand to the total weight including the region K may be less than 5%, for example.

[0106] In this embodiment, at least one of the first inorganic oxide 51 and the second inorganic oxide 52 fills the space between any two of the quantum dots contained in the light-emitting layer 23. Therefore, the light-emitting device 2 according to this embodiment improves the protective effect of the first inorganic oxide 51 or the second inorganic oxide 52 on the first quantum dots 41 or the second quantum dots 42, and reduces deterioration of the first quantum dots 41 or the second quantum dots 42.

[0107] 6 , the first inorganic oxide 51 may form the lower surface of the light-emitting layer 23 in the thickness direction DT, or the second inorganic oxide 52 may form the upper surface of the light-emitting layer 23 in the thickness direction DT. In other words, a portion of the first inorganic oxide 51 may be located closer to the anode 21 than all of the first quantum dots 41 contained in the first quantum dot layer 31. Furthermore, a portion of the second inorganic oxide 52 may be located closer to the cathode 25 than all of the second quantum dots 42 contained in the second quantum dot layer 32.

[0108] The light-emitting layer 23 is formed by depositing a layer of 1000 nm in a plane direction DP perpendicular to the thickness direction DT at any position in the thickness direction DT of the light-emitting layer 23. ２ In the light-emitting layer 23, at least one of the first quantum dots 41 and the second quantum dots 42 may be encapsulated in a continuous film of at least one of the first inorganic oxide 51 and the second inorganic oxide 52.

[0109] For example, assume that 60% or more of the surface of 80% or more of the quantum dots contained in the light-emitting layer 23 is in contact with a continuous film of at least one of the first inorganic oxide 51 and the second inorganic oxide 52. In this case, it can be said that the quantum dots contained in the light-emitting layer 23 are encapsulated in at least one of the first inorganic oxide 51 and the second inorganic oxide 52. In this way, the light-emitting layer 23 containing quantum dots encapsulated in at least one of the first inorganic oxide 51 and the second inorganic oxide 52 further enhances the protection effect of the quantum dots, improves the light-emitting properties, or extends the lifetime.

[0110] The display device 1 according to this embodiment may be manufactured by the same method as the display device 1 according to the previous embodiment, with some exceptions. In particular, in this embodiment, the concentration of the first precursor in the first quantum dot dispersion and the concentration of the second precursor in the second quantum dots 42 may be higher than in the previous embodiment.

[0111] 8 is a schematic side cross-sectional view of a display device 1 according to this embodiment. The display device 1 according to this embodiment has the same configuration as the display device 1 according to the previous embodiments, except for the configuration of the light-emitting element 2. In particular, the light-emitting element 2 according to this embodiment has the same configuration as the light-emitting element 2 according to embodiment 1, except for the number of layers and thickness T31 of the first quantum dot layer 31 in the light-emitting layer 23 being larger.

[0112] For example, in the light-emitting layer 23 according to this embodiment, the number of first quantum dot layers 31 is greater than the number of second quantum dot layers 32. For example, the light-emitting layer 23 may include two first quantum dot layers 31, or may include three or more first quantum dot layers 31. Alternatively, the total thickness T31 of the first quantum dot layers 31 included in the light-emitting layer 23 is greater than the total thickness T32 of the second quantum dot layers 32 included in the light-emitting layer 23.

[0113] In the light-emitting layer 23 including the first quantum dot layer 31, which has a larger number of layers or a larger film thickness than the second quantum dot layer 32, the efficiency of hole injection into the first quantum dots 41 is further increased. Therefore, the light-emitting element 2 according to this embodiment further reduces the excess electrons in the light-emitting layer 23, further improving the light-emitting efficiency. In particular, the light-emitting element 2 according to this embodiment more significantly reduces the excess electrons when the hole concentration in the light-emitting layer 23 is low, including when the efficiency of hole transport from the anode 21 to the light-emitting layer 23 is low.

[0114] The display device 1 according to this embodiment may be manufactured by a method identical to the manufacturing method of the display device 1 according to the above-described embodiments, with some exceptions. In particular, in this embodiment, the concentration of the first quantum dot dispersion applied in step S43 may be greater than the concentration of the second quantum dot dispersion applied in step S47. Alternatively, in this embodiment, steps S43 and S44 may be alternately performed multiple times to form multiple first quantum dot layers 31.

[0115] 9 is a schematic side cross-sectional view of a display device 1 according to this embodiment. The display device 1 according to this embodiment has the same configuration as the display device 1 according to the previous embodiments, except for the configuration of the light-emitting element 2. In particular, the light-emitting element 2 according to this embodiment has the same configuration as the light-emitting element 2 according to embodiment 1, except for the number of layers and thickness T32 of the second quantum dot layer 32 in the light-emitting layer 23 being larger.

[0116] For example, in the light-emitting layer 23 according to this embodiment, the number of second quantum dot layers 32 is greater than the number of first quantum dot layers 31. For example, the light-emitting layer 23 may include two second quantum dot layers 32, or may include three or more second quantum dot layers 32. Alternatively, the total thickness T32 of the second quantum dot layers 32 included in the light-emitting layer 23 is greater than the total thickness T31 of the first quantum dot layers 31 included in the light-emitting layer 23.

[0117] In the light-emitting layer 23 including the second quantum dot layer 32, which has a larger number of layers or a larger film thickness than the first quantum dot layer 31, the efficiency of electron injection into the second quantum dots 42 is further reduced. Therefore, the light-emitting layer 23 reduces the probability of occurrence of processes that do not contribute to light emission, such as the generation of Auger electrons caused by the injection of multiple electrons into the second quantum dots 42. Therefore, the light-emitting device 2 according to this embodiment further improves the light-emitting efficiency. In particular, the light-emitting device 2 according to this embodiment more significantly improves the light-emitting efficiency when the efficiency of electron injection into each quantum dot is excessively high due to, for example, a high halogen concentration in the light-emitting layer 23 and a low electron level of each quantum dot.

[0118] The display device 1 according to this embodiment may be manufactured by a method identical to the manufacturing method of the display device 1 according to the above-described embodiments, with some exceptions. In particular, in this embodiment, the concentration of the second quantum dot dispersion applied in step S47 may be greater than the concentration of the first quantum dot dispersion applied in step S43. Alternatively, in this embodiment, steps S47 and S48 may be alternately performed multiple times to form multiple second quantum dot layers 32.

[0119] 10 is a schematic side cross-sectional view of a display device 1 according to this embodiment. The display device 1 according to this embodiment has the same configuration as the display devices 1 according to the previous embodiments, except for the configuration of the light-emitting element 2. In particular, 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 light-emitting layer 23 further includes a third quantum dot layer 33.

[0120] The third quantum dot layer 33 is located between the first quantum dot layer 31 and the second quantum dot layer 32. The third quantum dot layer 33 includes a mixture of first quantum dots 41 and second quantum dots 42. The third quantum dot layer 33 also includes a first inorganic oxide 51 covering the peripheries of the first quantum dots 41 and a second inorganic oxide 52 covering the peripheries of the second quantum dots 42. The third quantum dot layer 33 may have a thickness T33 along the thickness direction DT of the light-emitting layer 23.

[0121] In the present disclosure, it is assumed that it has been confirmed that both the first quantum dots 41 and the second quantum dots 42 are contained in a cross section taken along the planar direction DP at any position in the thickness direction DT of the light-emitting layer 23. Here, if the proportion of the first quantum dots 41 and the proportion of the second quantum dots 42 among the 20 or more quantum dots in the cross section are both higher than 30%, the light-emitting layer 23 can be considered to contain a mixture of the first quantum dots 41 and the second quantum dots 42 in the portion including the cross section.

[0122] The light-emitting device 2 according to this embodiment includes a third quantum dot layer 33 between the first quantum dot layer 31 and the second quantum dot layer 32. The third quantum dot layer 33 includes both first quantum dots 41 with improved hole injection efficiency and second quantum dots 42 that reduce excessive electron injection. Therefore, the light-emitting device 2 according to this embodiment achieves both improved hole injection into each quantum dot in the light-emitting layer 23 and reduced electron excess in the light-emitting layer 23.

[0123] The display device 1 according to this embodiment may be manufactured by a method identical to the manufacturing method of the display device 1 according to the above-described embodiments, with some exceptions. In particular, in this embodiment, the formation of the third quantum dot layer 33 may be performed between step S44 and step S45. The third quantum dot layer 33 may be formed by applying and heating a third quantum dot dispersion liquid containing the first quantum dots 41, the second quantum dots 42, the first precursor, and the second precursor dispersed in a solvent. Alternatively, the third quantum dot layer 33 may be formed by applying and heating a mixture of the first structure dispersion liquid and the second structure dispersion liquid described above.

[0124] 11 is a schematic side cross-sectional view of a display device 1 according to this embodiment. The display device 1 according to this embodiment has the same configuration as the display devices 1 according to the previous embodiments, except for the configuration of the light-emitting element 2. In particular, 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 light-emitting layer 23 further includes a fourth quantum dot layer 34.

[0125] The fourth quantum dot layer 34 is located between the first quantum dot layer 31 and the second quantum dot layer 32. The fourth quantum dot layer 34 includes third quantum dots 43 and a third inorganic oxide 53 that covers the periphery of the third quantum dots 43. The third quantum dots 43 have third cores 43C that emit light of the same color as the first cores 41C and the second cores 42C, and a third shell 43S that is located around the third cores 43C. The fourth quantum dot layer 34 may have a thickness T34 along the thickness direction DT of the light-emitting layer 23.

[0126] The third core 43C may contain the same material as the first core 41C or the second core 42C, or may contain a material different from both the first core 41C and the second core 42C, as long as it emits light of the same color as the first core 41C and the second core 42C. The third shell 43S has a metal element, a first Group 16 element, and a second Group 16 element located in a period later than the first Group 16 element. In particular, in the present disclosure, z is a real number satisfying x<z<y, and p is a real number satisfying 0<p. In this case, the third shell 43S has a composition of M ｐ B １－ｚ A ｚ The third shell 43S may contain 50 wt % or more of the substance. The third inorganic oxide 53 may have the same structure as either the first inorganic oxide 51 or the second inorganic oxide 52.

[0127] The light-emitting layer 23 according to this embodiment includes, as quantum dots, third quantum dots 43 having a third shell whose band gap is between the first shell 41S and the second shell 42S, between the first quantum dots 41 and the second quantum dots 42. Therefore, the light-emitting element 2 according to this embodiment achieves both improved hole injection into each quantum dot in the light-emitting layer 23 and reduced electron excess in the light-emitting layer 23.

[0128] The display device 1 according to this embodiment may be manufactured by a method identical to the manufacturing method of the display device 1 according to the above-described embodiments, with some exceptions. In particular, in this embodiment, the formation of the fourth quantum dot layer 34 may be performed between step S44 and step S45. The fourth quantum dot layer 34 may be formed by applying and heating a fourth quantum dot dispersion liquid containing the third quantum dots 43 and a third precursor, which is a precursor of the third inorganic oxide 53, dispersed in a solvent. The third precursor may have the same composition as either the first precursor or the second precursor.

[0129] 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.

[0130] REFERENCE SIGNS LIST 1 Display device 2 Light-emitting element 3 Substrate 21 Anode 23 Light-emitting layer 31 First quantum dot layer 32 Second quantum dot layer 33 Third quantum dot layer 34 Fourth quantum dot layer 41 First quantum dot 42 Second quantum dot 43 Third quantum dot 51 First inorganic oxide 52 Second inorganic oxide 53 Third inorganic oxide 61 First part 62 Second part 63 Third part

Claims

1. An optical element comprising: an anode; a cathode facing the anode; and a light-emitting layer located between the anode and the cathode, wherein the light-emitting layer comprises: a first quantum dot layer including first quantum dots each having a first core and a first shell surrounding the first core, and a first inorganic oxide covering the first quantum dot; and a second quantum dot layer including second quantum dots each having a second core emitting light of the same color as the first core and a second shell surrounding the second core, and a second inorganic oxide covering the second quantum dot, the second quantum dot layer being located closer to the cathode than the first quantum dot layer, wherein M is a metal element, A is a first Group 16 element, B is a second Group 16 element located in a period later than the first Group 16 element in the periodic table, x and y are real numbers satisfying 0≦x<y≦1, n is a real number satisfying 0<n, and m is a real number satisfying 0<m, and the first shell has a composition of M ｎ B １－ｘ A ｘ and the second shell comprises a material having a composition of M ｍ B １－ｙ A ｙ A light-emitting element comprising a material which is 2. The light-emitting element described in claim 1, wherein the proportion of the first quantum dots among all quantum dots contained in the first quantum dot layer is 70% or more, and the proportion of the second quantum dots among all quantum dots contained in the second quantum dot layer is 70% or more.

3. A light-emitting element according to claim 1 or 2, wherein the proportion of the second quantum dots among all quantum dots contained in the first quantum dot layer is smaller than the proportion of the second quantum dots among all quantum dots contained in the second quantum dot layer.

4. A light-emitting device according to any one of claims 1 to 3, wherein the first quantum dot layer contains only the first quantum dots as quantum dots, and the second quantum dot layer contains only the second quantum dots as quantum dots.

5. A light-emitting element according to any one of claims 1 to 4, wherein at least one of the first inorganic oxide and the second inorganic oxide fills at least one of the gaps between two of the first quantum dots, between two of the second quantum dots, or between one of the first quantum dots and one of the second quantum dots.

6. The light-emitting element according to any one of claims 1 to 5, wherein at least one of the first inorganic oxide and the second inorganic oxide contains silicon oxide.

7. The light-emitting device according to claim 6, wherein both the first inorganic oxide and the second inorganic oxide contain silicon oxide.

8. The light-emitting device according to claim 7, wherein the concentration of silicon atoms per unit volume in the first quantum dot layer is lower than the concentration of silicon atoms per unit volume in the second quantum dot layer.

9. A light-emitting element described in any one of claims 1 to 8, wherein at least one of the first shell and the second shell has a first portion and a second portion that contains a substance having a composition different from that of the substance contained in the first portion and is located closer to the outer periphery than the first portion.

10. A light-emitting element as described in claim 9, wherein at least one of the first shell and the second shell has a third portion between the first portion and the second portion, in which the composition of the material contained therein changes continuously from the composition of the material contained in the first portion to the composition of the material contained in the second portion from the side of the first portion to the side of the second portion.

11. The light-emitting element according to any one of claims 1 to 10, wherein n is 1 and m is 1.

12. The light-emitting device according to any one of claims 1 to 11, wherein the second Group 16 element is a Group 16 element located in the period next to the first Group 16 element in the periodic table.

13. A light-emitting element according to any one of claims 1 to 12, wherein the metal element is zinc, the first Group 16 element is sulfur, and the second Group 16 element is selenium.

14. The light-emitting element according to any one of claims 1 to 13, wherein x is 0.

15. The light-emitting element according to any one of claims 1 to 13, wherein the x and y are real numbers that satisfy the relationship 0<x<y<1.

16. The light-emitting device according to any one of claims 1 to 15, wherein the thickness of the first quantum dot layer is greater than the thickness of the second quantum dot layer.

17. A light-emitting device according to any one of claims 1 to 15, comprising at least one first quantum dot layer and at least one second quantum dot layer, wherein the number of first quantum dot layers is greater than the number of second quantum dot layers.

18. A light-emitting device according to any one of claims 1 to 15, wherein the thickness of the first quantum dot layer is smaller than the thickness of the second quantum dot layer.

19. A light-emitting device according to any one of claims 1 to 15, comprising at least one first quantum dot layer and at least one second quantum dot layer, wherein the number of first quantum dot layers is less than the number of second quantum dot layers.

20. A light-emitting element described in any one of claims 1 to 19, wherein the light-emitting layer includes the first quantum dots, the second quantum dots, the first inorganic oxide covering the periphery of the first quantum dots, and the second inorganic oxide covering the periphery of the second quantum dots, and also includes a third quantum dot layer located between the first quantum dot layer and the second quantum dot layer and containing a mixture of the first quantum dots and the second quantum dots.

21. The light-emitting layer includes third quantum dots each having a third core emitting light of the same color as the first core and the second core and a third shell surrounding the third core, a third inorganic oxide covering the third quantum dots, and a fourth quantum dot layer located between the first quantum dot layer and the second quantum dot layer, wherein z is a real number satisfying x<z<y and p is a real number satisfying 0<p, and the third shell has a composition of M ｐ B １－ｚ A ｚ 20. The light-emitting device of claim 1, comprising a material in which:

22. A display device comprising a substrate and a light-emitting element according to any one of claims 1 to 21 on the substrate.

23. A method for manufacturing a light-emitting device, comprising: forming an anode; forming a cathode facing the anode; and forming a light-emitting layer located between the anode and the cathode, wherein the formation of the light-emitting layer comprises: forming a first quantum dot layer including first quantum dots each having a first core and a first shell located around the first core, and a first inorganic oxide covering the periphery of the first quantum dot; and forming a second quantum dot layer including second quantum dots each having a second core emitting light of the same color as the first core and a second shell located around the second core, and a second inorganic oxide covering the periphery of the second quantum dot, the second quantum dot layer being located closer to the cathode than the first quantum dot layer, wherein M is a metal element, A is a first Group 16 element, B is a second Group 16 element located in a period later than the first Group 16 element in the periodic table, x and y are real numbers satisfying 0≦x<y≦1, n is a real number satisfying 0<n, and m is a real number satisfying 0<m, and the first shell has a composition of M ｎ B １－ｘ A ｘ and the second shell comprises a material having a composition of M ｍ B １－ｙ A ｙ A method for manufacturing a light-emitting device comprising the material.

24. The method for manufacturing a light-emitting element described in claim 23, wherein forming the first quantum dot layer includes: applying a first quantum dot dispersion liquid in which the first quantum dots and a first precursor of the first inorganic oxide are dispersed in a solvent; and converting the first precursor in the first quantum dot dispersion liquid to the first inorganic oxide; and forming the second quantum dot layer includes applying a second quantum dot dispersion liquid in which the second quantum dots and a second precursor of the second inorganic oxide are dispersed in a solvent, above the previously formed first quantum dot layer; and converting the second precursor in the second quantum dot dispersion liquid to the second inorganic oxide.

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