Light-emitting element and display device

Abstract

A quantum dot layer (30) contains first quantum dots (QD1), second quantum dots (QD2) positioned between the first quantum dots (QD1) and a cathode (CA), and third quantum dots (QD3) at least partially positioned between the first and second quantum dots (QD1, QD2). Each of the first to third quantum dots (QD1-QD3) has a core (CR1-CR3) composed of the same material and a shell (SH1-SH3) containing a metal element. The shell of the third quantum dots (QD3) contains at least a first chalcogen element. When x is the composition ratio of the first chalcogen element with respect to the metal element in the shell (SH1) of the first quantum dots (QD1), y is the composition ratio of the first chalcogen element with respect to the metal element in the shell (SH2) of the second quantum dots (QD2), and z is the composition ratio of the first chalcogen element with respect to the metal element in the shell (SH3) of the third quantum dots (QD3), x, y, and z satisfy the relationships of 0 ≤ x < z and 0 ≤ y < z.

Description

Light-emitting element and display device 【0001】 The present disclosure relates to a light-emitting element and a display device. 【0002】 In the light-emitting layer of the light-emitting device disclosed in Patent Document 1, it is composed of a first light-emitting layer formed of first quantum dots and a second light-emitting layer formed of second quantum dots. The surface of the shell portion of the first quantum dots is coated with a surfactant, and the thickness of the shell portion is 3 to 5 ML based on the constituent molecules of the shell portion. The surface of the shell portion of the second quantum dots is coated with two types of surfactants having hole transportability and electron transportability, and the thickness of the shell portion is less than 3 ML based on the constituent molecules of the shell portion. 【0003】 WO2015 / 056750A1 【0004】 In order to improve the EQE (External Quantum Efficiency) of the light-emitting element, it is required to improve both the PL (Photo Luminescence) quantum yield of the light-emitting layer and the charge injection efficiency into the light-emitting layer. 【0005】 A light-emitting element according to one aspect of the present disclosure includes an anode and a cathode, and a quantum dot layer located between the anode and the cathode. The quantum dot layer includes first quantum dots, second quantum dots located on the cathode side of the first quantum dots, and third quantum dots at least partially located between the first and second quantum dots. Each of the first to third quantum dots has a core composed of the same material and a shell containing a metal element. The shell of the third quantum dots contains at least a first chalcogen element. Let the composition ratio of the first chalcogen element to the metal element in the shell of the first quantum dots be x, the composition ratio of the first chalcogen element to the metal element in the shell of the second quantum dots be y, and the composition ratio of the first chalcogen element to the metal element in the shell of the third quantum dots be z. The configuration is such that 0 ≤ x < z and 0 ≤ y < z. 【0006】 A display device according to one aspect of the present disclosure has a configuration including a light-emitting element according to one aspect of the present disclosure. 【0007】 According to one aspect of this disclosure, the EQE of a light-emitting element can be improved. 【0008】 This is a cross-sectional view showing an example of the configuration of a light-emitting element according to one embodiment of the present disclosure. This is a band diagram showing an example of the band structure of the light-emitting element shown in Figure 1. This shows a list of various examples of quantum dot materials. This shows various examples of quantum dots. This shows the PL quantum yield of the quantum dot layer according to the embodiment of the present disclosure and the light-emitting layer of the comparative example. This shows the carrier balance factor of the light-emitting element according to the embodiment and comparative example of the present disclosure. This shows the EQE of the light-emitting element according to the embodiment and comparative example of the present disclosure. This shows the PL quantum yield, carrier factor and EQE shown in Figures 5 to 7 together. This is a cross-sectional view showing an example of the configuration of a third quantum dot according to one embodiment of the present disclosure. This is a band diagram showing an example of the band structure of a light-emitting element comprising a quantum dot layer including the third quantum dot shown in Figure 9. This is a cross-sectional view showing an example of the configuration of a third quantum dot according to one embodiment of the present disclosure. This is a band diagram showing an example of the band structure of a light-emitting element comprising a quantum dot layer including the third quantum dot shown in Figure 11. This is a cross-sectional view showing an example of the configuration of a third quantum dot according to one embodiment of the present disclosure. This is a cross-sectional view showing an example of the configuration of a light-emitting element according to one embodiment of the present disclosure. This is a band diagram showing an example of the band structure of the light-emitting element shown in Figure 14. This is a cross-sectional view showing an example of the configuration of a light-emitting element according to one embodiment of the present disclosure. This is a band diagram showing an example of the band structure of the quantum dot layer shown in Figure 16. An example of the configuration of a display device according to one embodiment of this disclosure is shown. This is a band diagram showing the band structure of a comparative example light-emitting element. This is a band diagram showing the band structure of a comparative example light-emitting element. This is a band diagram showing the band structure of a comparative example light-emitting element. 【0009】[Embodiment 1] (Configuration of a light-emitting element) Figure 1 is a cross-sectional view showing an example of the configuration of a light-emitting element according to one embodiment of the present disclosure. As shown in Figure 1, the light-emitting element ED according to the present disclosure comprises an anode 10 and a cathode 50, and a quantum dot layer 30 located between the anode 10 and the cathode 50. The quantum dot layer 30 includes a first quantum dot QD1, a second quantum dot QD2 located closer to the cathode 50 than the first quantum dot QD1, and a third quantum dot QD3, at least a portion of which is located between the first quantum dot QD1 and the second quantum dot QD2. In other words, the first to third quantum dots QD1 to QD3 overlap each other in a plan view. In the present disclosure, when an element X overlaps with another element Y in a plan view, it means that at least a portion of element X overlaps with at least a portion of element Y in a plan view. In the present disclosure, a plan view means a plan view seen in the film thickness direction (Z direction), that is, in the direction from the cathode 50 toward the anode 10. 【0010】 Each of the first to third quantum dots QD1 to QD3 has a core CR1 to CR3 made of the same material and a shell SH1 to SH3 containing a metal element. Let x be the composition ratio of the first chalcogen element to the metal element in the shell SH1 of the first quantum dot QD1, let y be the composition ratio of the first chalcogen element to the metal element in the shell SH2 of the second quantum dot QD2, and let z be the composition ratio of the first chalcogen element to the metal element in the shell SH3 of the third quantum dot QD3, such that 0 ≤ x < z and 0 ≤ y < z, where z > 0. When x = 0, the shell SH1 of the first quantum dot QD1 does not contain the first chalcogen element. When y = 0, the shell SH2 of the second quantum dot QD2 does not contain the first chalcogen element. Since z > 0, the shell SH3 of the third quantum dot QD3 contains the first chalcogen element. 【0011】 Taking into account manufacturing variations and measurement accuracy, for example, if the difference between two composition ratios is less than 0.05, the two composition ratios may be considered identical. In this case, the difference between x and z is preferably 0.05 or more, and the difference between y and z is preferably 0.05 or more. 【0012】One or more of the shells SH1 to SH3 may optionally contain multiple types of metallic elements. When shells SH1 to SH3 contain multiple types of metallic elements, the composition ratio for the metallic elements is the composition ratio for the metallic element with the highest concentration among those multiple types of metallic elements. Here, concentration is the ratio of the number of elements or amount of substance to the volume or mass of the shell. The metallic element with the highest concentration among the one or more metallic elements contained in each of the shells SH1 to SH3 of the first to third quantum dots QD1 to QD3 may be the same or different from each other. If they are different, one may be selected from the metallic elements with the highest concentration in each of the shells SH1 to SH3, and the composition ratio for the metallic elements is the composition ratio for the selected metallic element. For example, if the shells SH1 to SH3 of the first to third quantum dots QD1 to QD3 contain both zinc and cadmium, the highest concentration of the metal element in the shells SH1 and SH2 of the first and second quantum dots QD1 and QD2 may be zinc, and the highest concentration of the metal element in the shell SH3 of the third quantum dot QD3 may be cadmium. In this case, zinc may be selected, and the composition ratio of the first chalcogen element to zinc in each of the shells SH1 to SH3 may be compared. Alternatively, cadmium may be selected, and the composition ratio of the first chalcogen element to cadmium in each of the shells SH1 to SH3 may be compared. The highest concentration metal element may be selected from the group consisting of, for example, zinc (Zn), cadmium (Cd), aluminum (Al), gallium (Ga), magnesium (Mg), copper (Cu), and silver (Ag). Hereafter, for the sake of simplicity, metal elements other than the highest concentration metal element will be ignored. 【0013】Each of the first to third quantum dots QD1 to QD3 is a light-emitting quantum dot, and the quantum dot layer 30 is a so-called light-emitting layer. The quantum dot layer 30 may optionally contain a ligand material that can coordinate to the surfaces of the first to third quantum dots QD1 to QD3, and may also contain a matrix material located between the first quantum dot QD1 and the third quantum dot QD3 and / or between the second quantum dot QD2 and the third quantum dot QD3. The ligand material may be an organic ligand such as a carboxylic acid, alkylthiol, or alkylamine, or a halogen such as fluorine, chlorine, bromine, or iodine. In this disclosure, the matrix material means a member that contains and holds other materials, and can be rephrased as a substrate, base material, or filler. The inorganic matrix material may be solid at room temperature. Compared to organic ligands, halogens and inorganic matrix materials have higher resistance to device driving and can suppress the degradation of quantum dots. 【0014】 The first chalcogen element is an element selected from the group of chalcogen elements, i.e., the group 16 elements. The first chalcogen element may be a nonmetallic element or a metallic element. The group 16 elements include oxygen (O), sulfur (S), selenium (Se), tellurium (Te), and polonium (Po). In this disclosure, the notation of group numbers of elements using Roman numerals is based on the old IUPAC (International Union of Pure and Applied Chemistry) system or the old CAS (Chemical Abstracts Service) system, and the notation of group numbers of elements using Arabic numerals is based on the new IUPAC system. 【0015】 For the sake of simplicity, let's designate the quantum dot QD1 closer to the anode 10 as the first quantum dot QD1 and the quantum dot QD2 closer to the cathode 50 as the second quantum dot QD2. In other words, the second quantum dot QD2 is closer to the cathode 50 than the first quantum dot QD1. 【0016】The particle size, core material, core particle size, shell material, shell thickness, and shell composition ratio of the first to third quantum dots QD1 to QD3 may be determined using FIB (Focused Ion Beam)-TEM (Transmission Electron Microscope)-EDX (Energy Dispersive X-ray Spectroscopy). Here, "particle size" simply refers to the particle size of the entire quantum dot, which is the sum of the core particle size and twice the shell thickness. The particle size of a quantum dot is the diameter of a circle having an area equal to the cross-sectional area of ​​the quantum dot. 【0017】 The core material and shell material can be estimated from the wavelength distribution in EDX, respectively. 【0018】 The core particle size and shell thickness of the first to third quantum dots QD1 to QD3 may be calculated from cross-sectional photographs. For quantum dots in which the core contains an intrinsic element, for example, quantum dots with a core / shell ratio of InP / ZnS, it is easy to distinguish between the core and the shell. In the cross-section of such a quantum dot, the region where the core-intrinsic element exists can be estimated as the core, and the region where the core-intrinsic element does not exist can be estimated as the shell. The diameter of a circle with an area equal to the area of ​​the region estimated to be the core may be used as the core particle size. The thickness of the region estimated to be the shell may be measured at several locations and the average thickness may be used as the shell thickness. 【0019】The core particle sizes of the first to third quantum dots (QD1 to QD3) can be calculated from the core material and emission wavelength. The emission wavelength of a quantum dot depends on the energy difference between the HOMO (Highest Occupied Molecular Orbital) and LUMO (Lowest Unoccupied Molecular Orbital) of the quantum dot's core. Due to quantum effects, the energy difference between the HOMO and LUMO of the quantum dot's core depends on the core material and core particle size. The shell thickness of a quantum dot is half the difference between the overall particle size of the quantum dot and the core particle size. Based on the core particle size and shell thickness, the core and shell regions can be estimated, respectively. This method of calculating core particle size and shell thickness can be applied to quantum dots where it is easy to distinguish between the core and shell, as well as those where it is difficult. However, in quantum dots where the core does not contain any intrinsic elements, for example, a quantum dot where the core is made of ZnSe and the shell is a multi-shell having layers made of ZnSeS and layers made of ZnS, it is difficult to distinguish between the core and the shell. 【0020】 The composition ratio of the first chalcogen element to the metal element in each of the shells SH1 to SH3 of the first to third quantum dots QD1 to QD3 may be the average value of the composition ratio in the shell. Specifically, the composition ratio may be calculated at multiple locations in the region estimated to be a shell, for example, five locations, and the average value of the calculated composition ratio may be adopted. Alternatively, the composition ratio of the first chalcogen element to the metal element in each of the shells SH1 to SH3 of the first to third quantum dots QD1 to QD3 may be the maximum value of the composition ratio in the shell. Specifically, the composition ratio may be calculated at multiple locations in the region estimated to be a shell, for example, five locations, and the maximum value among the calculated composition ratio may be adopted. In cases where there are no particular limitations on the calculation of the composition ratio, it is preferable to adopt the average value of the composition ratio in the shell. 【0021】The light-emitting element ED may optionally further include a hole functional layer 20 located between the anode 10 and the quantum dot layer 30. The hole functional layer 20 may include one or more hole injection layers, hole transport layers, and electron blocking layers. The light-emitting element ED may optionally further include an electronic functional layer 40 located between the quantum dot layer 30 and the cathode 50. The electronic functional layer 40 may include one or more electron injection layers, electron transport layers, and hole blocking layers. 【0022】 The core particle sizes of the first to third quantum dots QD1 to QD3 may be approximately the same. Multiple quantum dots with the same core material and approximately the same core particle size will have approximately the same emission wavelength. For example, it is preferable that the emission wavelengths of the first to third quantum dots QD1 to QD3 fall within the wavelength range of 600 to 700 nm, which corresponds to red light. For example, it is also preferable that the emission wavelengths of the first to third quantum dots QD1 to QD3 fall within the wavelength range of 500 to 600 nm, which corresponds to green light, or within the wavelength range of 400 to 500 nm, which corresponds to blue light. 【0023】 (Band Structure of Light-Emitting Device) Figure 2 is a band diagram showing an example of the band structure of the light-emitting device shown in Figure 1. In Figure 2, the Fermi levels of the anode 10 and cathode 50 are shown by solid lines, and the band gaps between the hole functional layer 20 and the electron functional layer 40, respectively, of the first to third quantum dots QD1 to QD3 of the quantum dot layer 30 (cores CR1 to CR3 and shells SH1 to SH3) are shown by rectangles. The top of the rectangle represents the LUMO, and the bottom of the rectangle represents the HOMO. Electron affinity is related to the energy difference between the vacuum level and the LUMO, and ionization potential is related to the energy difference between the vacuum level and the HOMO. In this disclosure, an energy level X being shallower than another energy level Y means that the absolute value of the energy difference from the vacuum level to level X is smaller than the absolute value of the energy difference from the vacuum level to level Y. Furthermore, the statement that level X is deeper than level Y means that the absolute value of the energy difference from the vacuum level to level X is greater than the absolute value of the energy difference from the vacuum level to level Y. 【0024】As shown in Figure 2, the shell SH3 of the third quantum dot QD3 has a shallower LUMO than the shell SH1 of the first quantum dot QD1 and the shell SH2 of the second quantum dot QD2. 【0025】 The shell SH1 of the first quantum dot QD1 may have the same LUMO as the shell SH2 of the second quantum dot QD2, and / or the same HOMO. Let x be the composition ratio of the first chalcogen element to the metal element in the shell SH1 of the first quantum dot QD1, and let y be the composition ratio of the first chalcogen element to the metal element in the shell SH2 of the second quantum dot QD2, then x = y. Note that if the difference between x and y is less than, for example, 0.05, then x = y may be considered. 【0026】 The configurations of the first and second quantum dots QD1 and QD2 may be the same. In this disclosure, "the configurations of two quantum dots are the same" means that the core material and shell material of the two quantum dots are the same, the core particle size is substantially the same, and the difference in the composition ratio of the first chalcogen element to the metal element in the shell is less than 0.05. In this disclosure, "the materials are the same" means that the constituent elements are the same, and it is not necessary that the composition ratio of each constituent element is the same. When the configurations of the first and second quantum dots QD1 and QD2 are the same, the number of materials used in the manufacture of the light-emitting element ED can be reduced, thereby simplifying the manufacturing process and lowering manufacturing costs. 【0027】 (Quantum Dot Composition) The shells SH1 and SH2 of the first and second quantum dots QD1 and QD2, respectively, may contain the second chalcogen element. The shell SH3 of the third quantum dot QD3 may optionally contain the second chalcogen element. The second chalcogen element is a different element from the first chalcogen element. The second chalcogen element is a single element different from the first chalcogen element, selected from the group consisting of chalcogen elements, i.e., group 16 elements. The second chalcogen element may be a nonmetallic element or a metallic element. 【0028】The second chalcogen element may have a larger period than the first chalcogen element. For example, when the first chalcogen element is oxygen in the second period, the second chalcogen element may be sulfur in the third period, selenium in the fourth period, tellurium in the fifth period, or polonium in the sixth period. For example, when the first chalcogen element is sulfur, the second chalcogen element may be selenium, tellurium, or polonium. 【0029】 When there are multiple chalcogen elements in at least one of the shells SH1 to SH3 of the first to third quantum dots QD1 to QD3, the chalcogen element with the smallest period among the multiple chalcogen elements may be designated as the first chalcogen element, and the chalcogen element with the second smallest period may be designated as the second chalcogen element. Chalcogen elements present at the impurity level may be ignored. For example, when each of the shells SH1 to SH3 consists of zinc selenide sulfide (ZnSeS) contaminated with oxygen (O), the oxygen may be ignored, and the first chalcogen element may be sulfur and the second chalcogen element as selenium. For example, chalcogen elements whose composition ratio to metal elements in each of the shells SH1 to SH3 is less than 0.05 may be ignored. 【0030】 The smaller the period to which a chalcogen element belongs, the smaller the bond distance with the metal element, which affects the size of the band gap of the metal chalcogenide. Therefore, when the second chalcogen element has a larger period than the first chalcogen element, and the sum of the composition ratios of the first and second chalcogen elements is constant, the larger the composition ratio of the first chalcogen element, the larger the band gap of the metal chalcogenide. 【0031】Figure 3 shows a list of various examples of quantum dot materials. In the list shown in Figure 3, the "Core" column shows the materials for the cores CR1 to CR3 of the first to third quantum dots QD1 to QD3, the "Small Bandgap Shell" column shows the materials for the shells SH1 and SH2 of the first and second quantum dots QD1 and QD2, and the "Large Bandgap Shell" column shows the material for the shell SH3 of the third quantum dot QD3. Furthermore, each row shows the material combination of cores CR1 to CR3 and shells SH1 to SH3. In this list, 0 < p < q < 1, and p and q represent the composition ratio of the first chalcogen element to the metal element in the corresponding shell. In this list, the first chalcogen element is sulfur and the second chalcogen element is selenium. Note that the chemical formulas of the materials listed in the "Core" column are representative examples, and the composition ratios may differ. 【0032】 In each of the first to third quantum dots QD1 to QD3, the composition ratio of the metal element to the first and second chalcogen elements may be 1:1. As shown in Figure 3, for example, the total composition ratio of sulfur and selenium to zinc, cadmium, or gallium may be 1:1. However, if the composition ratio is, for example, 1:0.95 or more and 1:1.05 or less, it may be considered as 1:1. 【0033】The constituent elements and compositional ratios of the shells SH1 to SH3 of the first quantum dot QD1 to the third quantum dot QD3 may be uniform or variable within the shell. Each of the shells SH1 to SH3 of the first to third quantum dots QD1 to QD3 may include a region where the compositional ratio of the first chalcogen element to the metal element is at its maximum value, and may also include other regions where the compositional ratio of the first chalcogen element to the metal element is smaller than the above-mentioned maximum value. Specifically, the shell SH1 of the first quantum dot QD1 contains a first region where the composition ratio x of the first chalcogen element to the metal element in the shell SH1 of the first quantum dot QD1 is at its maximum value, the shell SH2 of the second quantum dot QD2 contains a second region where the composition ratio y of the first chalcogen element to the metal element in the shell SH2 of the second quantum dot QD2 is at its maximum value, and the shell SH3 of the third quantum dot QD3 contains a third region where the composition ratio z of the first chalcogen element to the metal element in the shell SH3 of the third quantum dot QD3 is at its maximum value. 【0034】Figure 4 shows various examples of quantum dots. In this example, the calculation of the composition ratio of the first chalcogen element to the metal element uses the maximum value of the composition ratio in the shell, rather than the average value of the composition ratio in the shell. The first to third quantum dots QD1 to QD3 may each have a configuration like any of the quantum dots QDA to QDC shown in Figure 4, or a configuration other than that. As shown in Figure 4, the shell SHA of quantum dot QDA may include a region LCA where the composition ratio of the first chalcogen element to the metal element is the maximum value a, and other regions OTA where the composition ratio of the first chalcogen element to the metal element is less than the maximum value a. The region OTA with a composition ratio of less than a may surround the core CRA and may have an arbitrary cross-sectional shape such as a polygon. A part of the region OTA with a composition ratio of less than a may be exposed on the surface of the shell SHA. The region LCA with a composition ratio a may have an arbitrary cross-sectional shape. The shell SHB of a quantum dot QDB may include a region LCB where the composition ratio of the first chalcogen element to the metal element is the maximum value b, and other region OTBs where the composition ratio of the first chalcogen element to the metal element is less than the maximum value b. The region OTBs with a composition ratio of less than b may be covered by the region LCB with a composition ratio b. The shell SHC of a quantum dot QDC may include a region LCC where the composition ratio of the first chalcogen element to the metal element is the maximum value c, and other region OTCs where the composition ratio of the first chalcogen element to the metal element is less than the maximum value c. The cross-section of the shell SHC may include a region consisting only of the region LCC with a composition ratio c, extending from a portion of the surface of the core CRC to a portion of the surface of the shell SHC, and a region consisting only of the region OTCs with a composition ratio of less than c, extending from another portion of the surface of the core CRC to another portion of the surface of the shell SHC. In this disclosure, the expression "composition ratio less than the maximum value" includes the case where the composition ratio is 0. 【0035】 The particle sizes of the first to third quantum dots QD1 to QD3 may be approximately the same. Note that "approximately the same particle size" means that the difference in particle size is 1 nm or less. 【0036】The shell SH3 of the third quantum dot QD3 can have a larger band gap than the shell SH1 of the first quantum dot QD1 and the shell SH2 of the second quantum dot QD2. As a result, the shell SH3 of the third quantum dot QD3 has a shallower LUMO and a deeper HOMO than the shell SH1 of the first quantum dot QD1 and the shell SH2 of the second quantum dot QD2. 【0037】 A preferred combination is one in which the highest concentration of the metal element in the shells SH1 to SH3 of the first to third quantum dots QD1 to QD3 is zinc, and the first chalcogen element, which is present in at least the shell SH3 of the third quantum dot QD3 and optionally also present in the shells SH1 and SH2 of the first and second quantum dots QD1 and QD2, is sulfur. If the first and second quantum dots QD1 and QD2 contain a second chalcogen element, a preferred combination is one in which the metal element is zinc, the first chalcogen element is sulfur, and the second chalcogen element is selenium. 【0038】 (Method for manufacturing a light-emitting element) Referring again to Figure 1, the light-emitting element ED according to the present disclosure can be manufactured as follows. 【0039】 First, a first dispersion containing multiple first quantum dots QD1, a second dispersion containing multiple second quantum dots QD2, and a third dispersion containing multiple third quantum dots QD3 are manufactured. An anode 10 and a hole functional layer 20 are formed on a substrate in this order. The first dispersion is then applied to the hole functional layer 20 and dried or fired. Subsequently, the third dispersion is applied to the film containing the first quantum dots QD1 and dried or fired. Subsequently, the second dispersion is applied to the film containing the third quantum dots QD3 and dried or fired. This forms a quantum dot layer 30 on the hole functional layer 20. Next, an electronic functional layer 40 and a cathode 50 are formed on the quantum dot layer 30. 【0040】Alternatively, the cathode 50, the electron functional layer 40, the quantum dot layer 30, the hole functional layer 20, and the anode 10 may be formed on the substrate in this order. In this case, the second dispersion is applied onto the electron functional layer 40 and dried or fired. Subsequently, the third dispersion is applied onto the film containing the second quantum dot QD2 and dried or fired. Subsequently, the first dispersion is applied onto the film containing the third quantum dot QD3 and dried or fired. 【0041】 By repeating the application in this manner, the quantum dot layer 30 is formed such that at least a part of the third quantum dot QD3 is positioned between the first quantum dot QD1 and the second quantum dot QD2. The quantum dot layer 30 includes at least one set of quantum dots including a set of the first to third quantum dots QD1 to QD3. Preferably, in the quantum dot layer 30, 20% or more, 50% or more, or 80% or more of the sets of quantum dots arranged substantially along the thickness direction (z-axis direction) include three quantum dots satisfying the conditions of the first to third quantum dots QD1 to QD3. 【0042】 (Example 1) As the first to third quantum dots QD1 to QD3 according to Example 1 of the present disclosure, luminescent quantum dots in which the materials and particle diameters of the cores CR1 to CR3 are the same as each other, the thicknesses of the shells SH1 to SH3 are the same as each other, the shells SH1 to SH3 have a single-layer structure, and the composition ratios within the shells SH1 to SH3 are each uniform were used. The first and second quantum dots QD1 and QD2 are the same as each other, and the materials of the shells SH1 and SH2 are ZnSe １－ｐ S ｐ It was. The material of the shell SH3 of the third quantum dot QD3 was ZnSe １－ｑ S ｑ It was. 0 < p < q < 1. 【0043】 The light-emitting device ED according to Example 1 of the present disclosure was fabricated, a current was applied, and the EQE was measured. At the same time, a sample in which only the quantum dot layer 30 was formed on the substrate was fabricated, and external light was irradiated to measure the PL quantum yield. Since the EQE of the light-emitting device is the product of the PL quantum yield, the carrier balance factor, and the light extraction efficiency, the carrier balance factor was determined with the light extraction efficiency being 0.2. 【0044】(Comparative Examples 1 to 3) Light-emitting elements of Comparative Examples 1 to 3 were fabricated, and the PL quantum yield, carrier balance factor, and EQE were determined in the same manner. 【0045】 FIGS. 19 to 21 are band diagrams showing the band structures of the light-emitting elements of the comparative examples. As shown in FIGS. 19 to 21, the anode 110, hole functional layer 120, electron functional layer 140, and cathode 150 of Comparative Examples 1 to 3 are the same as the anode 10, hole functional layer 20, electron functional layer 40, and cathode 50 of Example 1. As shown in FIG. 19, the light-emitting layer 130 of Comparative Example 1 included only the same quantum dots as the first and second quantum dots QD1 and QD2 according to Example 1. As shown in FIG. 20, the light-emitting layer 230 of Comparative Example 2 included only the same quantum dots as the third quantum dot QD3. As shown in FIG. 21, the light-emitting layer 330 of Comparative Example 3 had a layer located on the anode 110 side and including only the same quantum dots as the third quantum dot QD3, and a layer located on the cathode 150 side and including only the same quantum dots as the first and second quantum dots QD1 and QD2. 【0046】 (Effect) FIG. 5 shows the PL quantum yields of the quantum dot layer according to the example of the present disclosure and the light-emitting layers of the comparative examples. As shown in FIG. 5, only the light-emitting layer 130 according to Comparative Example 1 had a low PL quantum yield, and the quantum dot layer 30 according to Example 1 and the light-emitting layers 230 and 330 of Comparative Examples 2 and 3 had a similarly high PL quantum yield. The PL quantum yield increases as the excited electrons are confined in individual quantum dots. Conversely, as the excited electrons move between the quantum dots, the probability of returning to the ground state without contributing to light emission tends to be high, and the PL quantum yield decreases. Therefore, the difference in the PL quantum yield is presumed to reflect the difference in the mobility of the excited electrons. 【0047】As shown in Figure 19, the light-emitting layer 130 of Comparative Example 1 contains only quantum dots with a small absolute value of the LUMO energy difference between the shell and the core, making it easy for photo-excited electrons to move between the quantum dots. In contrast, as shown in Figure 2, the quantum dot layer 30 of Example 1 contains a third quantum dot with a large LUMO energy difference between the shell SH3 and the core CR3, making it difficult for photo-excited electrons to move between the first and third quantum dots QD1, QD3 and between the second and third quantum dots QD2, QD3. Similarly, as shown in Figures 20 and 21, the light-emitting layers 230 and 330 of Comparative Examples 2 and 3 also contain quantum dots with a large LUMO energy difference between the shell and the core, making it difficult for photo-excited electrons to move between the quantum dots. 【0048】 Figure 6 shows the carrier balance factors of light-emitting devices according to the examples and comparative examples of this disclosure. The carrier balance factor represents the agreement rate between the number of holes injected from anodes 10 and 110 into the quantum dot layer 30 or light-emitting layers 130, 230, and 330, and the number of electrons injected from cathodes 50 and 150 into the quantum dot layer 30 or light-emitting layers 130, 230, and 330. As shown in Figure 6, the carrier balance factor is highest in Comparative Example 1 and decreases in the order of Example 1, Comparative Example 2, and Comparative Example 3. 【0049】Comparing Figures 2 and 19-21, the band structures of the quantum dot layer 30 in Example 1 and the light-emitting layers 130 and 230 in Comparative Examples 1 and 2 are symmetrical in the film thickness direction (Z direction), while the band structure of the light-emitting layer 330 in Comparative Example 3 is asymmetrical. For this reason, it is presumed that the carrier balance factor of Comparative Example 3 was the smallest. Compared with Comparative Example 1 (Figure 19), Comparative Example 2 (Figure 20) has a larger band gap of the quantum dot shells in the light-emitting layer 230, making it difficult to inject both holes and electrons into the light-emitting layer 230. It is presumed that the carrier balance factor decreased significantly due to the slight difference in the number of holes and electrons. In Example 1 (Figure 2), the shells SH1 and SH2 of the first and second quantum dots QD1 and QD2 and the shell SH3 of the third quantum dot QD3 cooperate to form a stepped band structure. Therefore, it is easy to inject charge into the third quantum dot QD3 via the first and second quantum dots QD1 and QD2, and it is estimated that the decrease in carrier balance was small in Example 1 (Figure 2). 【0050】 The light-emitting elements of Comparative Examples 1 to 3 have the same configuration as the light-emitting element ED of Example 1, except for the light-emitting layers 130, 230, and 330. Therefore, the light extraction efficiency of Comparative Examples 1 to 3 is the same as that of Example 1. The EQE of a light-emitting element is the product of the PL quantum yield, the carrier balance factor, and the light extraction efficiency. Therefore, the EQE of Example 1 and Comparative Examples 1 to 3 is proportional to the PL quantum yield and the carrier balance factor, and from the measurement results shown in Figures 5 and 6, it was predicted that the EQE of Example 1 would be the highest. 【0051】 Figure 7 shows the EQE of light-emitting elements according to the embodiments and comparative examples of this disclosure. As shown in Figure 7, the measured EQE of Example 1 was the highest. It has been demonstrated that the configuration according to this disclosure has a high PL quantum yield and a high carrier balance factor, thereby exhibiting a high EQE. 【0052】 Figure 8 summarizes the PL quantum yield, carrier factor, and EQE shown in Figures 5 to 7. 【0053】[Embodiment 2] Figure 9 is a cross-sectional view showing an example of the configuration of a third quantum dot according to one embodiment of the present disclosure. In this example, the calculation of the composition ratio of the first chalcogen element to the metal element is based on the maximum value of the composition ratio in the shell, rather than the average value of the composition ratio in the shell. As shown in Figure 9, the shell SH3 of the third quantum dot QD3 according to Embodiment 2 of the present disclosure is a multilayer shell. The outermost shell of the shell SH3 of the third quantum dot QD3, which is furthest from the core CR3, contains a third region LC3 where the composition ratio of the first chalcogen element to the metal element is at its maximum value. 【0054】 Figure 10 is a band diagram showing an example of the band structure of a light-emitting element comprising a quantum dot layer including the third quantum dot shown in Figure 9. As shown in Figure 10, the shell SH3 of the third quantum dot QD3 makes it difficult for photoexcited electrons to move, and the shell SH1 and SH2 of the first and second quantum dots QD1 and QD2 and the shell SH3 of the third quantum dot QD3 cooperate to form a stepped band structure. Therefore, according to the configuration of this embodiment 2, similar to the configuration of embodiment 1 described above, the light-emitting element ED has a high PL quantum yield and a high carrier balance factor, and thereby exhibits a high EQE. 【0055】 [Embodiment 3] Figure 11 is a cross-sectional view showing an example of the configuration of a third quantum dot according to one embodiment of the present disclosure. In this example, the calculation of the composition ratio of the first chalcogen element to the metal element is shown using the maximum value of the composition ratio in the shell, rather than the average value of the composition ratio in the shell. As shown in Figure 11, the shell SH3 of the third quantum dot QD3 according to Embodiment 3 of the present disclosure is a gradient shell in which the composition ratio changes depending on the distance from the core CR3. The third part LC3, in which the composition ratio of the first chalcogen element to the metal element is the maximum value, may be included in the surface portion of the shell SH3 that is furthest from the core CR. 【0056】Figure 12 is a band diagram showing an example of the band structure of a light-emitting element comprising a quantum dot layer including the third quantum dot shown in Figure 11. As shown in Figure 12, the shell SH3 of the third quantum dot QD3 makes it difficult for photoexcited electrons to move, and the shell SH1 and SH2 of the first and second quantum dots QD1 and QD2 and the shell SH3 of the third quantum dot QD3 cooperate to form a stepped band structure. Therefore, according to the configuration of this embodiment 3, similar to the configurations of embodiments 1 and 2 described above, the light-emitting element ED has a high PL quantum yield and a high carrier balance factor, thereby exhibiting a high EQE. 【0057】 [Embodiment 4] Figure 13 is a cross-sectional view showing an example of the configuration of a third quantum dot according to one embodiment of the present disclosure. In this example, the calculation of the composition ratio of the first chalcogen element to the metal element is shown using the maximum value of the composition ratio in the shell, rather than the average value of the composition ratio in the shell. As shown in Figure 13, the shell SH3 of the third quantum dot QD3 according to Embodiment 3 of the present disclosure includes a third part LC3 where the composition ratio of the first chalcogen element to the metal element is at its maximum value, and a fourth part LC4 with the same composition, and the third part LC3 and the fourth part LC4 are separated from each other. In other words, the shell SH3 of the third quantum dot QD3 has two or more separate parts where the composition ratio of the first chalcogen element to the metal element is at its maximum value z. 【0058】 The light-emitting element ED, which includes a quantum dot layer 30 containing the third quantum dot QD3 shown in Figure 13, also has a high PL quantum yield and a high carrier balance factor, similar to the light-emitting elements EDs according to embodiments 1 to 3 described above, and thereby exhibits a high EQE. 【0059】 [Embodiment 5] Figure 14 is a cross-sectional view showing an example of the configuration of a light-emitting element according to one embodiment of the present disclosure. As shown in Figure 14, the quantum dot layer 30 according to Embodiment 5 of the present disclosure includes a fourth quantum dot QD4 and a fifth quantum dot QD5 between a second quantum dot QD2 and a cathode 50, with at least a portion of the fourth quantum dot QD4 located between the second quantum dot QD2 and the fifth quantum dot QD5. The fifth quantum dot QD5 overlaps with the second quantum dot QD2 in a plan view. 【0060】 The fourth and fifth quantum dots QD4 and QD5, respectively, have cores CR4 and CR5 made of the same material as the cores CR1 to CR3 of the first to third quantum dots QD1 to QD3, and shells SH4 and SH5 containing the same metal elements as the shells SH1 to SH3 of the first to third quantum dots QD1 to QD3. Let y be the composition ratio of the first chalcogen element to the metal element in the shell SH2 of the second quantum dot QD2, let v be the composition ratio of the first chalcogen element to the metal element in the shell SH4 of the fourth quantum dot QD4, and let w be the composition ratio of the first chalcogen element to the metal element in the shell SH5 of the fifth quantum dot QD5, such that y < v and w < v. Here, y ≥ 0, v > 0, and w ≥ 0. 【0061】 The configurations of the second and fifth quantum dots QD2 and QD5 may be the same. And / or the configurations of the third and fourth quantum dots QD3 and QD4 may be the same. By reducing the number of manufacturing materials for the light-emitting element ED, the manufacturing process can be simplified and manufacturing costs can be reduced. 【0062】 Figure 15 is a band diagram showing an example of the band structure of the light-emitting device shown in Figure 14. In Figure 15, the Fermi levels of the anode 10 and cathode 50 are shown by solid lines, and the band gaps between the hole functional layer 20 and the electron functional layer 40, respectively, for the first to fifth quantum dots QD1 to QD5 of the quantum dot layer 30 (cores CR1 to CR5 and shells SH1 to SH5) are shown by rectangles. The top edge of the rectangle represents the LUMO, and the bottom edge of the rectangle represents the HOMO. 【0063】As shown in Figure 15, the shells SH3 and SH4 of the third and fourth quantum dots QD3 and QD4 make it difficult for photoexcited electrons to move, the shells SH1 and SH2 of the first and second quantum dots QD1 and QD2 and the shell SH3 of the third quantum dot QD3 cooperate to form a stepped band structure, and the shells SH2 and SH5 of the second and fifth quantum dots QD2 and QD5 and the shell SH3 of the third quantum dot QD3 cooperate to form a stepped band structure. Therefore, according to the configuration of this embodiment 5, similar to the configurations of embodiments 1 to 4 described above, the light-emitting element ED has a high PL quantum yield and a high carrier balance factor, thereby exhibiting a high EQE. Furthermore, while maintaining a high PL quantum yield, the quantum dot layer 30 can be made thicker compared to the configurations of embodiments 1 to 4 described above. Depending on the thickness of the quantum dot layer 30, the hole functional layer 20 and the electronic functional layer 40 may penetrate the quantum dot layer 30, especially when using small-particle quantum dots, thus reducing the possibility of a short circuit in the light-emitting element ED. 【0064】 [Embodiment 6] Figure 16 is a cross-sectional view showing an example of the configuration of a light-emitting element according to one embodiment of the present disclosure. As shown in Figure 16, the quantum dot layer 30 according to Embodiment 6 of the present disclosure comprises sixth and seventh quantum dots QD6 and QD7 that overlap each other when viewed from a direction (X direction) perpendicular to the film thickness direction (Z direction). At least a portion of the first quantum dot QD1 is located between the sixth quantum dot QD6 and the seventh quantum dot QD7. 【0065】 The sixth and seventh quantum dots QD6 and QD7, respectively, have cores CR6 and CR7 made of the same material as the cores CR1 to CR3 of the first to third quantum dots QD1 to QD3, and shells SH6 and SH7 containing the same metal elements as the shells SH1 to SH3 of the first to third quantum dots QD1 to QD3. Let x be the composition ratio of the first chalcogen element to the metal element in the shell SH1 of the first quantum dot QD1, let t be the composition ratio of the first chalcogen element to the metal element in the shell SH6 of the sixth quantum dot QD6, and let u be the composition ratio of the first chalcogen element to the metal element in the shell SH7 of the seventh quantum dot QD7, such that x < t and x < u. Here, x ≥ 0, t > 0, and u > 0. 【0066】 The quantum dot layer 30 according to Embodiment 6 of this disclosure may further include eighth and ninth quantum dots QD8 and QD9 that overlap each other when viewed from a direction (X direction) perpendicular to the film thickness direction (Z direction). At least a portion of the second quantum dot QD2 is located between the eighth quantum dot QD8 and the ninth quantum dot QD9. 【0067】 The eighth and ninth quantum dots QD8 and QD9, respectively, have cores CR8 and CR9 made of the same material as the cores CR1 to CR3 of the first to third quantum dots QD1 to QD3, and shells SH8 and SH9 containing the same metal elements as the shells SH1 to SH3 of the first to third quantum dots QD1 to QD3. Let y be the composition ratio of the first chalcogen element to the metal element in the shell SH1 of the second quantum dot QD2, let r be the composition ratio of the first chalcogen element to the metal element in the shell SH8 of the eighth quantum dot QD8, and let s be the composition ratio of the first chalcogen element to the metal element in the shell SH9 of the ninth quantum dot QD9, such that y < r and y < s. Here, y ≥ 0, r > 0, and s > 0. 【0068】 Figure 17 is a band diagram showing an example of the band structure of the quantum dot layer shown in Figure 16. As shown in Figure 17, the shells SH6 and SH7 of the sixth and seventh quantum dots QD6 and QD7 make charge transfer difficult in the direction perpendicular to the film thickness direction (Z direction) (X direction). Similarly, the shells SH8 and SH9 of the eighth and ninth quantum dots QD8 and QD9 also make charge transfer difficult in the direction perpendicular to the film thickness direction (Z direction) (X direction). Therefore, according to the configuration of this embodiment 6, the light-emitting element ED has a PL quantum yield that is equal to or higher than that of the configurations of embodiments 1 to 5 described above, and thereby exhibits a high EQE. 【0069】 The quantum dot layer 30 according to this embodiment 6 can be manufactured by, for example, preparing a fourth dispersion containing a first quantum dot QD1, a sixth quantum dot QD6, and a seventh quantum dot QD7, and a fifth dispersion containing a second quantum dot QD2, an eighth quantum dot QD8, and a ninth quantum dot QD9, and using the fourth dispersion instead of the first dispersion and the fifth dispersion instead of the second dispersion in one example of the manufacturing method described in Embodiment 1 above. 【0070】 Any two or more of the configurations of the sixth to ninth quantum dots (QD6 to QD9) may be the same as those of each other. Any one or more of the configurations of the sixth to ninth quantum dots (QD6 to QD9) may be the same as that of the third quantum dot (QD3). By reducing the number of manufacturing materials for the light-emitting element (ED), the manufacturing process can be simplified and manufacturing costs can be reduced. 【0071】 [Embodiment 7] Figure 18 shows an example of the configuration of a display device according to one embodiment of the present disclosure. As shown in Figure 1, the display device DP according to the present disclosure comprises one or more light-emitting elements ED according to the present disclosure. The display device DP may also comprise a pixel PX including the light-emitting elements ED and a pixel circuit PC for controlling the light-emitting elements ED, and a drive circuit DC for driving the pixel PX. 【0072】 This disclosure is not limited to the embodiments described above, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of this disclosure. Furthermore, new technical features can be formed by combining the technical means disclosed in each embodiment. 【0073】 10 Anode 20 Hole Functional Layer 30 Quantum Dot Layer 40 Electronic Functional Layer 50 Cathode CR1 Core of the first quantum dot CR2 Core of the second quantum dot CR3 Core of the third quantum dot CR4 Core of the fourth quantum dot CR5 Core of the fifth quantum dot DA Display Area DC Driving Circuit DP Display Device ED Light-emitting element NA Frame Area PX Subpixel QD1 First quantum dot QD2 Second quantum dot QD3 Third quantum dot QD4 Fourth quantum dot QD5 Fifth quantum dot SH1 Shell of the first quantum dot SH2 Shell of the second quantum dot SH3 Shell of the third quantum dot SH4 Shell of the fourth quantum dot SH5 Shell of the fifth quantum dot

Claims

1. A light-emitting element comprising an anode and a cathode, and a quantum dot layer located between the anode and the cathode, wherein the quantum dot layer includes a first quantum dot, a second quantum dot located closer to the cathode than the first quantum dot, and a third quantum dot whose part is located between the first and second quantum dots, and each of the first to third quantum dots has a core made of the same material and a shell containing a metal element, the shell of the third quantum dot contains at least a first chalcogen element, the composition ratio of the first chalcogen element to the metal element in the shell of the first quantum dot is x, the composition ratio of the first chalcogen element to the metal element in the shell of the second quantum dot is y, and the composition ratio of the first chalcogen element to the metal element in the shell of the third quantum dot is z, such that 0 ≤ x < z and 0 ≤ y < z.

2. The light-emitting element according to claim 1, wherein the core particle sizes of the first to third quantum dots are the same.

3. The light-emitting element according to claim 1 or 2, wherein the shell of the third quantum dot has a shallower LUMO than the shells of the first quantum dot and the second quantum dot.

4. The light-emitting element according to any one of claims 1 to 3, wherein x = y.

5. The light-emitting element according to any one of claims 1 to 4, wherein each of the shells of the first and second quantum dots contains a second chalcogen element.

6. The light-emitting element according to claim 5, wherein the second chalcogen element has a longer period than the first chalcogen element.

7. The light-emitting element according to claim 5 or 6, wherein the first chalcogen element is the chalcogen element with the smallest period among a plurality of chalcogen elements contained in at least one of the shells of the first to third quantum dots, and the second chalcogen element is the chalcogen element with the second smallest period among the plurality of chalcogen elements.

8. The light-emitting element according to any one of claims 5 to 7, wherein in each of the shells of the first to third quantum dots, the composition ratio of the metal element to the first and second chalcogen elements is 1:

1.

9. The light-emitting element according to any one of claims 1 to 8, wherein the shell of the first quantum dot includes a first region where x is the maximum value, the shell of the second quantum dot includes a second region where y is the maximum value, and the shell of the third quantum dot includes a third region where z is the maximum value.

10. The light-emitting element according to claim 9, wherein the shell of the third quantum dot is a multilayer shell, and the third portion is included in the outermost shell of the multilayer shell of the third quantum dot.

11. The light-emitting element according to claim 9, wherein the shell of the third quantum dot is a gradient shell whose composition ratio changes depending on the distance from the core of the third quantum dot, and the third portion is included on the surface of the shell of the third quantum dot.

12. The light-emitting element according to claim 9, wherein the shell of the third quantum dot includes a fourth portion having the same composition as the third portion, and the third portion and the fourth portion are separated.

13. The light-emitting element according to any one of claims 1 to 12, wherein the particle sizes of the first to third quantum dots are the same.

14. The light-emitting element according to claim 3, wherein the shell of the third quantum dot has a larger band gap than the shells of the first quantum dot and the second quantum dot.

15. The light-emitting element according to any one of claims 1 to 14, wherein the quantum dot layer includes a fourth quantum dot and a fifth quantum dot between the second quantum dot and the cathode, at least a portion of the fourth quantum dot is located between the second quantum dot and the fifth quantum dot, the configurations of the second and fifth quantum dots are the same, and the configurations of the third and fourth quantum dots are the same.

16. The light-emitting element according to any one of claims 1 to 14, wherein the quantum dot layer includes a fourth quantum dot and a fifth quantum dot between the second quantum dot and the cathode, at least a portion of the fourth quantum dot is located between the second quantum dot and the fifth quantum dot, and v is the composition ratio of the first chalcogen element to the metal element in the shell of the fourth quantum dot, and w is the composition ratio of the first chalcogen element to the metal element in the shell of the fifth quantum dot, such that y < v and w < v.

17. The light-emitting element according to any one of claims 1 to 16, wherein the metallic element is zinc and the first chalcogen element is sulfur.

18. The light-emitting element according to any one of claims 5 to 7, wherein the metal element is zinc, the first chalcogen element is sulfur, and the second chalcogen element is selenium.

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

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