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

Abstract

A light-emitting element (10) according to the present disclosure comprises: an anode (E1) and a cathode (E2); and a light-emitting layer (EL) that is located between the anode (E1) and the cathode (E2). The light-emitting layer (EL) includes: first quantum dots (Q1) and second quantum dots (Q2); and a matrix material (MX) that is located between the first quantum dots (Q1) and the second quantum dots (Q2). The matrix material (MX) is a p-type conductive metal oxide.

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, and a method for manufacturing a light-emitting element.

[0002] Patent Document 1 discloses a configuration in which an inorganic oxide layer is provided between a plurality of light-emitting layers.

[0003] Special Publication No. 2010-500733 (published January 7, 2010)

[0004] In a light-emitting device having a light-emitting layer containing quantum dots, there is a demand for an improvement in the carrier balance in the light-emitting layer.

[0005] A light-emitting element according to one aspect of the present disclosure comprises an anode, a cathode, and a light-emitting layer located between the anode and the cathode, the light-emitting layer having first quantum dots and second quantum dots, and a matrix material located between the first quantum dots and the second quantum dots, the matrix material being a p-type conductive metal oxide.

[0006] A display device according to one aspect of the present disclosure includes a light-emitting element according to the present disclosure.

[0007] A method for manufacturing a light-emitting element according to one aspect of the present disclosure is a method for manufacturing a light-emitting element including an anode, a cathode, and a light-emitting layer located between the anode and the cathode, and includes the steps of applying a solution containing a precursor of a matrix material that is a p-type conductive metal oxide and a plurality of quantum dots to a base layer, and heating the applied solution.

[0008] The carrier balance in the light-emitting layer can be improved.

[0009] 1 is a cross-sectional view showing an example of the configuration of a light-emitting device according to an embodiment of the present disclosure; FIG. 2 is a graph showing the rate of change in carrier density with respect to temperature of a metal oxide semiconductor exhibiting NTC characteristics by changing the B constant, which indicates thermistor characteristics, and the activation energy of conduction; FIG. 3 is a schematic diagram showing an example of the band structure of the light-emitting layer EL shown in FIG. 1; FIG. 4 is a cross-sectional view showing an example of a method for manufacturing a light-emitting device according to an embodiment of the present disclosure; FIG. 5 is a cross-sectional view showing an example of the configuration of a light-emitting device according to Example 1 of the present disclosure; FIG. 6 is a graph showing the external quantum efficiency with respect to the current density when the temperature is kept constant for a light-emitting device according to Example 1 of the present disclosure; FIG. 7 is a graph showing the external quantum efficiency with respect to the current density as the temperature increases for a light-emitting device according to Example 1 of the present disclosure; FIG. 8 is a graph showing the external quantum efficiency (EQE) with respect to the current density as the temperature increases for a light-emitting device according to Comparative Example 1 of the present disclosure; FIG. 9 is a graph showing the temperature with respect to the current density for a light-emitting device according to Example 1 of the present disclosure; FIG. 10 is a cross-sectional view showing an example of the configuration of a light-emitting device according to an embodiment of the present disclosure; FIG. 11 is a schematic diagram showing an example of the band structure of a light-emitting layer of a light-emitting device according to an embodiment of the present disclosure; FIG. 12 is a cross-sectional view showing an example of the configuration of a light-emitting device obtained by combining Embodiment 4 with Embodiment 1; FIG. 13 is a cross-sectional view showing an example of the configuration of a light-emitting device obtained by combining Embodiment 4 with Embodiment 2; 1 is a cross-sectional view illustrating an example of a configuration of a display device according to an embodiment of the present disclosure.

[0010] [Embodiment 1] Fig. 1 is a cross-sectional view showing an example of the configuration of a light-emitting device according to an embodiment of the present disclosure. As shown in Fig. 1, a light-emitting device 10 according to the present disclosure includes an anode E1, a cathode E2, and an emitting layer EL located between the anode E1 and the cathode E2. The emitting layer EL includes first quantum dots Q1, second quantum dots Q2, and a matrix material MX located between the first quantum dots Q1 and the second quantum dots Q2. The matrix material MX is a p-type conductive metal oxide.

[0011] Generally, the carrier balance of the light-emitting layer tends to be biased toward an excess of electrons and a deficiency of holes. A deterioration in carrier balance leads to heat generation due to non-radiative recombination. In contrast, with the above configuration, the matrix material MX has p-type conductivity, and the hole density in the matrix material MX increases as the temperature of the light-emitting layer EL increases. Therefore, as the temperature of the light-emitting layer EL increases, the hole density of the light-emitting layer EL increases, improving the carrier balance. The improvement in carrier balance reduces non-radiative recombination, reduces heat generation, and improves the internal quantum efficiency (IQE) of the light-emitting element 10. In other words, the p-type conductive matrix material MX forms a feedback loop of deterioration in carrier balance, temperature increase, increased hole density, and improved carrier balance.

[0012] In the present disclosure, "quantum dots" refers to dots having a maximum width of 100 nm or less. The shape of the quantum dots is not particularly limited as long as it satisfies the above-mentioned maximum width, and is not limited to a spherical three-dimensional shape (circular cross-sectional shape). For example, the quantum dots may have a polygonal cross-sectional shape, a rod-like three-dimensional shape, a branch-like three-dimensional shape, a three-dimensional shape with an uneven surface, or a combination thereof. In this embodiment, the quantum dots are semiconductor microparticles having a particle size of, for example, 100 nm or less, and may include II-VI group semiconductor compounds such as MgS, MgSe, MgTe, CaS, CaSe, CaTe, SrS, SrSe, SrTe, BaS, BaSe, BaTe, ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe, and HgTe, and / or crystals of III-V group semiconductor compounds such as GaAs, GaP, InN, InAs, InP, and InSb, and / or crystals of Group IV semiconductor compounds such as Si and Ge. Furthermore, the quantum dots may have a core / shell structure in which the above-mentioned semiconductor crystal serves as a core and the core is overcoated with a shell material having a large band gap. The shell does not necessarily need to completely cover the core; it may be formed on at least a portion of the core.

[0013] Additionally, the first quantum dot Q1 and the second quantum dot Q2 contained in the light-emitting layer EL are luminescent quantum dots that can emit light by recombination of holes and electrons. Hereinafter, the plurality of quantum dots including the first quantum dot Q1 and the second quantum dot Q2 may be collectively referred to as "quantum dots QD." The first quantum dot Q1 is any one of the plurality of quantum dots QD, and the second quantum dot Q2 is any one of the plurality of quantum dots QD other than the first quantum dot Q1. The second quantum dot Q2 may be identical to or different from the first quantum dot Q1 in terms of size, shape, composition, and the like.

[0014] In the present disclosure, "carrier balance" refers to the balance of carriers in the light-emitting layer EL, i.e., the balance between holes and conduction electrons. The carrier balance is at its best when the abundance of holes and conduction electrons in the light-emitting layer EL is balanced at a 1:1 ratio. Hereinafter, conduction electrons may be simply referred to as "electrons."

[0015] In the present disclosure, the term "matrix material" refers to a material that contains and holds other substances, and may be referred to as a substrate, a base material, or a filler. The matrix material MX may be solid at room temperature. The matrix material MX may be a material that contains and holds a plurality of quantum dots QD. The matrix material MX may be a component of the light-emitting layer EL that contains a plurality of quantum dots QD.

[0016] The matrix material MX may be filled between the quantum dots QDs. Filling the matrix material MX between the quantum dots QDs means that the matrix material MX fills the region between two adjacent quantum dots QDs. By filling the region between at least two adjacent quantum dots QDs with the matrix material MX, a desired effect is achieved in at least the region between the two quantum dots QDs where the matrix material MX is filled. In the cross section of the light-emitting layer EL, the region between two adjacent quantum dots QDs may be a region surrounded by two straight lines (common circumtangents) tangent to the peripheries of the two adjacent quantum dots QDs and the opposing peripheries of the two adjacent quantum dots QDs.

[0017] The matrix material MX may fill regions (spaces) in the light-emitting layer EL other than the quantum dot group. Here, three or more quantum dots QD are collectively referred to as a quantum dot group. The matrix material MX may fill regions (spaces) in the light-emitting layer EL other than the multiple quantum dots QD.

[0018] The outer edges (top and bottom surfaces) of the light-emitting layer EL may be covered with a matrix material MX. Alternatively, the light-emitting layer EL may be configured such that a portion of the matrix material MX extends from the outer edge thereof, and the quantum dots QD are positioned away from the outer edge. The outer edge of the light-emitting layer EL does not need to be formed solely from the matrix material MX, and some of the quantum dots QD may be exposed from the matrix material MX. The matrix material MX may refer to the portion of the light-emitting layer EL excluding the quantum dots QD.

[0019] The matrix material MX may be formed to fill spaces formed between the quantum dots QDs. The quantum dots QDs may be embedded in the matrix material MX at intervals. The matrix material MX may be partially or completely filled between the quantum dots QDs.

[0020] The matrix material MX may include a continuous film having an area of ​​1000 nm or more along a plane direction perpendicular to the film thickness direction. A continuous film means a film that is not separated in one plane by a material other than the material that constitutes the continuous film.

[0021] The matrix material MX may be the same material as the shells contained in each of the multiple quantum dots QD. In this case, the average distance between adjacent cores (core-to-core distance) may be 3 nm or more, or may be 5 nm or more. Alternatively, the average distance between adjacent cores may be 0.5 times or more the average core diameter. The core-to-core distance is the average distance between adjacent cores in a space containing 20 cores. The core-to-core distance should be kept wider than the distance when shells are in contact with each other. The average core diameter is the average core diameter of 20 cores in a cross-sectional observation of a space containing 20 cores. The core diameter can be the diameter of a circle having the same area as the core area in cross-sectional observation.

[0022] The concentration of the matrix material MX in the light-emitting layer EL may be 0.1% or more and 79.0% or less. This concentration may be measured, for example, from the area ratio in image processing during cross-sectional observation. When the quantum dots QD have a core-shell structure, the concentration of the shell may be 0.1% or more and 39% or less. When the shell and the matrix material MX are made of the same material (same composition) and cannot be distinguished from each other, the concentration of the combined region of the shell and the matrix material MX may be 0.1% or more and 99.9% or less. In this way, when the shell and the matrix material MX cannot be distinguished from each other, the shell may be part of the matrix material MX.

[0023] The light-emitting layer EL may be composed of a plurality of quantum dots QD and a matrix material MX. When the light-emitting layer EL is analyzed, the intensity of carbon detected by the chain structure may be less than noise. The structure of the matrix material MX may be observed in a cross-section of the light-emitting layer EL with a width of about 100 nm, as long as it is clear that the structure is as described above, and it is not necessary for the structure to be observed throughout the entire light-emitting layer EL. The matrix material MX may contain a substance different from the main material, for example, as an additive.

[0024] The matrix material MX is not limited to the same material as the shell material of the quantum dots QD, and inorganic materials with a band gap at least larger than that of the core material can be used for the matrix material MX. Metal oxide semiconductors with excess oxygen typically exhibit p-type conductivity. For example, nickel oxide with excess oxygen exhibits p-type conductivity because Ni defects act as acceptors and generate holes. The composition of the metal oxide contained in the matrix material MX may be oxygen-excessive relative to the stoichiometric ratio. The carrier density of semiconductors typically increases with increasing temperature. The hole density of the matrix material MX may increase with increasing temperature from 20°C to 100°C. The temperature change rate of the carrier density of the light-emitting layer EL may be 1.3% / °C to 7.0% / °C.

[0025] The metal oxide contained in the matrix material MX may be nickel oxide (NiO). The band gap of nickel oxide is 3.7 eV, which is larger than the band gap of quantum dots that emit red light (2.0 eV), the band gap of quantum dots that emit green light (2.3 eV), and the band gap of quantum dots that emit blue light (2.8 eV). When the density of nickel (Ni) in the matrix material MX is 5×10 ２２ [ / cm ３ ] or less.

[0026] The light-emitting layer EL may exhibit negative temperature coefficient (NTC) thermistor characteristics, and the B constant, which indicates the thermistor characteristics of the light-emitting layer EL, may be 1300 to 6000. The matrix material MX may exhibit NTC thermistor characteristics, causing the light-emitting layer EL to exhibit NTC thermistor characteristics. Among metal oxide semiconductors, semiconductors having a specific acceptor level are known to exhibit NTC characteristics, in which electrical resistance decreases with increasing temperature. In particular, materials with a large B constant, which indicates the magnitude of resistance change, are used as thermistors in temperature sensors, etc. The B constant can be calculated using the following formula (1):

[0027] B = ln{(R / R ０ ) ÷ (1 / T - 1 / T ０ ) ) ) (1) where R and R ０ are the temperatures T and T ０ is the electrical resistance value when

[0028] The activation energy of the light-emitting layer EL may be 80 meV to 300 meV. The color of the light emitted by the light-emitting layer EL is arbitrary, and may be any of red, green, and blue, for example.

[0029] The light-emitting layer EL may further include a metal compound MY in contact with the matrix material MX. The metal compound MY is different from the matrix material MX. When nickel oxide is in direct contact with the quantum dots, hydroxyl groups (—OH) present in small amounts on the surface of the nickel oxide may cause deactivation of the quantum dots. When the matrix material MX includes nickel oxide, the metal compound MY may be in contact with (at least a portion of) the first quantum dot Q1 and may be located in a portion of the periphery of the first quantum dot Q1 to reduce deactivation of the first quantum dot Q1. The metal compound MY may also be in contact with the second quantum dot Q2 and other quantum dots and may be located in a portion of the periphery of each of them.

[0030] The metal compound MY (e.g., metal sulfide) may be in the form of small pieces (e.g., a size smaller than the quantum dots QD), a shell shape (a coating of the quantum dots QD), or an inorganic medium shape (layer shape). The matrix material MX may be in contact with a portion of the surface of the metal compound MY, or may be in contact with the entire surface of the metal compound MY (e.g., the matrix material MX may encase the metal compound MY). The metal compound MY may be in contact with a portion of the surface of the quantum dots QD, or may be in contact with the entire surface of the quantum dots QD. Note that even if the metal compound MY is not in contact with the quantum dots QD, the carrier balance improvement effect in the light-emitting layer EL can be obtained.

[0031] The metal compound MY has a larger band gap than the first quantum dot Q1. When the size, shape, composition, etc. of the second quantum dot Q2 are different from the size, shape, composition, etc. of the first quantum dot Q1, the metal compound MY may have a larger band gap than the second quantum dot Q2 and other quantum dots. The metal compound MY may include a metal sulfide. The metal sulfide is, for example, zinc sulfide (ZnS).

[0032] In the light-emitting layer EL, the volume occupancy of the metal compound bodies MY is smaller than the volume occupancy of the matrix material MX. The first quantum dots Q1 and the second quantum dots Q2 may be in contact with the matrix material MX. In the present disclosure, the volume occupancy may be estimated from the area occupancy in a cross section, or the area occupancy in a cross section may be used instead of the volume occupancy.

[0033] Optionally, the light-emitting element 10 may include a first charge functional layer F1 located between the anode E1 and the light-emitting layer EL, which may include one or more of a hole injection layer, a hole transport layer, and an electron blocking layer. Optionally, the light-emitting element 10 may include a second charge functional layer F2 located between the cathode E2 and the light-emitting layer EL, which may include one or more of an electron injection layer, an electron transport layer, and a hole blocking layer. The light-emitting layer EL may have a plurality of quantum dots QDs, including a first quantum dot Q1 and a second quantum dot Q2, and the number of quantum dots QDs may be three or more.

[0034] (NTC Thermistor Characteristics) Fig. 2 is a graph showing the rate of change in carrier density with temperature in a metal oxide semiconductor exhibiting NTC characteristics, where the B constant and the activation energy of conduction, which indicate the thermistor characteristics, are changed. The rate of change in carrier density shown in Fig. 2 is the carrier density [ / cm ３ ] at 300 [K] to the carrier density [ / cm ３ ] and is expressed in dimensionless units (au). Figure 2 is a semi-logarithmic graph in which the vertical axis is logarithmic. In Figure 2, the thick solid line indicates when the B constant is 1300 and the activation energy is 80 [meV], the thin solid line indicates when the B constant is greater than 1300 and the activation energy is greater than 80 [meV], and the dashed line indicates when the B constant is less than 1300 and the activation energy is less than 80 [meV].

[0035] As mentioned above, metal oxide semiconductors with specific acceptor levels are known to exhibit NTC characteristics. Such oxide semiconductors have a high conduction activation energy, and as shown in FIG. 2 , the rate of change of carrier density near room temperature is large, being approximately 1 [a.u.] or greater than 1 [a.u.]. When the light-emitting layer EL (particularly the matrix material MX) exhibits NTC characteristics, the increase in hole density in the light-emitting layer EL with increasing temperature is greater than when the light-emitting layer EL does not exhibit NTC characteristics. This results in a more improved carrier balance, a more reduced non-radiative recombination, a more reduced heat generation, and an improved IQE. In addition, the NTC characteristics reduce electrical resistance and Joule heat generation with increasing temperature, and increase electrical resistance and Joule heat generation with decreasing temperature. Therefore, temperature increases and decreases feed back to each other, forming an additional negative feedback loop. This additional negative feedback loop can further stabilize the carrier balance and IQE.

[0036] To achieve the above-described IQE improvement, it is necessary to select a metal oxide with an appropriate level difference between the top of the valence band (VBM) and the acceptor level as the metal oxide contained in the matrix material MX. The acceptor level depends on metal defects and excess oxygen, which are hole generation sources. The smaller the difference between the VBM and the acceptor level of a metal oxide, the higher the hole density at room temperature, the higher the electrical conductivity, and the smaller the increase in hole density with increasing temperature. The larger the difference between the VBM and the acceptor level of a metal oxide, the lower the hole density at room temperature, the lower the electrical conductivity, and the higher the temperature range in which the hole density increases with increasing temperature. For metal oxides with a large difference between the VBM and the acceptor level, the temperature range in which the hole density increases with increasing temperature is higher than the operating temperature range of the light-emitting element 10. The operating temperature range of the light-emitting element 10 is typically 0°C to 100°C. The amount of heat generated as Joule heat can be calculated from the driving current and luminous efficiency of the light-emitting element 10. When the light emitting element 10 is used in a normal display panel, the driving current of the light emitting element 10 is 10 [mA / cm ２], and the light-emitting element 10 can reach approximately 100 [°C] due to its own heat generation. In applications where the light-emitting element 10 is driven with a high current, such as outdoor and virtual reality (VR) display panels, the upper limit of the operating temperature range of the light-emitting element 10 is expected to be higher.

[0037] Therefore, to improve IQE, the B constant, which indicates the thermistor characteristics of the light-emitting layer EL, is preferably 1300 to 6000, and the activation energy of the light-emitting layer EL is preferably 80 meV to 300 meV. In other words, the temperature coefficient of change of the carrier density in the light-emitting layer EL is preferably 1.3 percent / °C to 7.0 percent / °C.

[0038] (Electron Blocking) FIG. 3 is a schematic diagram showing an example of the band structure of the light-emitting layer EL shown in FIG. 1. In the example shown in FIG. 3, the light-emitting element 10 includes a hole transport layer HTL and an electron transport layer ETL, but does not include an electron blocking layer. Ｆ " indicates the Fermi level of the light-emitting layer EL. The top of each rectangle indicates the bottom of the conduction band (CBM), and the bottom of each rectangle indicates the top of the valence band (VBM). As shown in FIG. 1, the metal compound body MY is in contact with only a portion of the surface of the quantum dot QD, does not surround the quantum dot QD, and is not in contact with most of the surface of the quantum dot QD. In this case, the metal compound body MY does not substantially affect the band structure of the light-emitting layer EL, and is therefore omitted from FIG. 3.

[0039] As shown in FIG. 3 , the matrix material MX has p-type conductivity and can block electrons injected from the electron transport layer ETL. If electrons reach the hole transport layer HTL without being blocked, parasitic luminescent or non-luminescent recombination occurs in the hole transport layer HTL (or a layer closer to the anode), resulting in a decrease in IQE. According to the configuration of the present disclosure, the matrix material MX blocks electrons, thereby improving the IQE of the light-emitting device 10. To improve the probability that the matrix material MX blocks electrons, it is preferable that the CBM of the quantum dots QDs is deeper than the CBM of the matrix material MX. For example, the matrix material MX may contain the same metal oxide as the hole transport layer THL. Furthermore, the temperature rise of the light-emitting device 10 is reduced due to reduced conversion to thermal energy.

[0040] 4 is a cross-sectional view showing an example of a method for manufacturing a light-emitting device according to an embodiment of the present disclosure. As shown in FIG. 4, the method for manufacturing a light-emitting device according to the present disclosure is a method for manufacturing a light-emitting device 10 including an anode E1, a cathode E2, and an emitting layer EL located between the anode E1 and the cathode E2, and includes a step of applying a solution L1 containing a precursor NX of a matrix material MX having p-type conductivity and containing a metal oxide and a plurality of quantum dots QD to an underlayer UL (step S10), and a step of heating the applied solution L1 (step S20).

[0041] The precursor NX may contain a metal organic compound. For example, when the matrix material MX, which is a metal oxide, is nickel oxide (NiO), the precursor NX may contain nickel nitrate and nickel acetate. By using a sol-gel method or a metal organic deposition (MOD) method, the precursor NX is decomposed and oxidized by heating in step S20 to obtain the matrix material MX. The heating temperature in step S20 is preferably within a temperature range that does not damage the quantum dots QD, and is preferably, for example, 200°C or less.

[0042] The solution L1 may also include a solvent L2 and a precursor NY of the metal compound MY. The plurality of quantum dots QD may include a first quantum dot Q1 and a second quantum dot Q2.

[0043] For example, a method for manufacturing a light-emitting device according to the present disclosure may include a step of forming an anode E1 (Step S110), optionally a step of forming a first charge functional layer F1 on the anode E1 (Step S120), and a step of forming an emitting layer EL on the underlayer UL using the anode E1 or the first charge functional layer F1 as the underlayer UL (Step S130), where Step S130 may include Steps S10 and S20.

[0044] Next, for example, the manufacturing method of the light-emitting element according to the present disclosure may optionally include a step of forming a second charge functional layer F2 on the light-emitting layer EL (step S140), and may include a step of forming a cathode E2 on the light-emitting layer EL or the second charge functional layer F2 (step S150).

[0045] Example 1 Fig. 5 is a cross-sectional view showing an example of the configuration of a light-emitting device according to Example 1 of the present disclosure. As shown in Fig. 5, a light-emitting device 10 according to Example 1 of the present disclosure was fabricated. The light-emitting device 10 according to Example 1 included an anode E1, a hole injection layer HIL, a hole transport layer HTL, an emitting layer EL, an electron transport layer ETL, and a cathode E2, in this order. The anode E1 was a 100 nm thick film containing indium tin oxide, the hole injection layer HIL was a 50 nm thick film containing poly(3,4-ethylenedioxythiophene) / poly(4-styrenesulfonic acid) (PEDOT:PSS), the hole transport layer HTL was a 60 nm thick film containing poly(2,7-(9,9-di-n-octylfluorene)-(1,4-phenylene-((4-sec-butylphenyl)imino)-1,4-phenylene (TFB), the electron transport layer ETL was a 50 nm thick film containing zinc oxide (ZnO), and the cathode E2 was a 100 nm thick film containing aluminum (Al).

[0046] The light-emitting layer EL included a plurality of quantum dots QD, a matrix material MX, and a metal compound MY. The quantum dots QD were red-emitting core-shell quantum dots, and the shell material was zinc sulfide (ZnS). The matrix material MX was oxygen-rich nickel oxide (NiO) and had p-type conductivity. The metal compound MY was zinc sulfide (ZnS).

[0047] The anode E1 was located on a glass substrate G1 having a thickness of 1 mm, and the cathode E2 was covered with a cover glass G2 having a thickness of 0.5 mm via an air layer having a thickness of 0.5 mm.

[0048] (Comparative Example 1) A light-emitting device according to Comparative Example 1 of the present disclosure was fabricated. The light-emitting device according to Comparative Example 1 was fabricated in the same manner as the light-emitting device 10 according to Example 1, except for the light-emitting layer. The light-emitting layer according to Comparative Example 1 had a plurality of quantum dots and a matrix material positioned between the plurality of quantum dots. The plurality of quantum dots according to Comparative Example 1 were the same as the plurality of quantum dots QD according to Example 1. The matrix material according to Comparative Example 1 was zinc sulfide (ZnS) and had n-type conductivity.

[0049] 6 is a graph showing the external quantum efficiency (EQE) versus current density for the light-emitting device according to Example 1 of the present disclosure, when the temperature is kept constant. Fig. 6 is a semi-logarithmic graph with the horizontal axis representing logarithms. The temperature of the light-emitting device 10 according to Example 1 was kept constant (27°C) by forced cooling.

[0050] As shown in FIG. 6, the current is approximately 0.1 [mA / cm ２], the EQE increases as the current density increases. This is because the current injected into the quantum dots becomes dominant over the leakage current of the entire light-emitting device. As the current density increases further, the EQE peaks and then begins to decrease. This is a phenomenon of current-induced luminous efficiency decline, commonly referred to as roll-off. One cause of the roll-off is an excess of electrons in the light-emitting layer EL. This excess electrons is caused by the carrier mobility of the electron transport layer being one order of magnitude greater than that of the hole transport layer. Furthermore, in some conventional light-emitting devices, parasitic electroluminescence from the hole transport layer is sometimes observed during operation, which is thought to be due to some of the electrons injected into the light-emitting layer reaching the hole transport layer without recombining.

[0051] 7 is a graph showing the external quantum efficiency (EQE) versus current density as the temperature increases for a light-emitting device according to Example 1 of the present disclosure. FIG. 8 is a graph showing the external quantum efficiency (EQE) versus current density as the temperature increases for a light-emitting device according to Comparative Example 1 of the present disclosure. FIGS. 7 and 8 are semi-logarithmic graphs in which the horizontal axis is logarithmic. The dashed line indicates a constant temperature, and the solid line indicates an increase in temperature. The light-emitting device 10 according to Example 1 and the light-emitting device according to Comparative Example 1 were placed at room temperature and driven while allowing natural cooling, so that the temperatures of each increased according to the current density.

[0052] The current value of the light-emitting region to be measured divided by its area is the current density [mA / cm ２ The temperature of the light-emitting element can be derived from a numerical simulation. The amount of heat generated by the light-emitting element can be calculated based on the voltage and current applied to the light-emitting element and the external quantum efficiency of the light-emitting element, and the temperature rise of the light-emitting element can be calculated based on the thermal conductivity of each layer of the light-emitting element and the environment in which the light-emitting element 10 is placed.

[0053] As shown in Fig. 7, in the light-emitting device 10 according to Example 1, the EQE decreased when the current density was below a certain threshold, but on the other hand, the EQE increased when the current density was above a certain threshold. In contrast, as shown in Fig. 8, in the light-emitting device according to Comparative Example 1, the EQE decreased at all current densities. ２], the temperature of the light-emitting element 10 according to Example 1 was 42°C, and the temperature of the light-emitting element according to Comparative Example 1 was 47°C. Therefore, it is presumed that the light-emitting element 10 according to Example 1 achieved the above-mentioned effects, specifically, the effect of improving the carrier balance in a self-aligned manner due to a temperature rise, the effect of improving IQE, and the effect of reducing the temperature rise of the light-emitting element 10.

[0054] 9 is a diagram showing the temperature versus current density of the light-emitting element according to Example 1 of the present disclosure. The temperature of the light-emitting element 10 according to Example 1 was simulated according to the current density when the light-emitting element 10 was placed at room temperature and driven while allowing it to cool naturally. The input power [W / cm ２ ] (= 5V × current density) is consumed for light emission at an EQE of 20%, and the remaining 80% is converted into thermal energy. The transient characteristics of the temperature change of each layer due to the conduction and outflow of heat generated in the region consisting of the emitting layer EL and the hole transport layer HTL were calculated, and the steady-state temperature at which the generated heat and outflow are balanced was calculated. The heat outflow from each end to the outside was calculated taking into account both radiation and convection, and assuming that there was no convection in the air layer between the cathode E2 and the cover glass G2.

[0055] As shown in Figure 9, even with an EQE of 20%, the temperature inside the element can easily increase depending on the driving current density. In particular, in applications requiring high brightness, a high current density is required, which causes the element temperature to rise significantly above room temperature, so the suppression of the element temperature increase in this application is particularly effective. [Embodiment 2]

[0056] 10 is a cross-sectional view showing an example of the configuration of a light-emitting device according to an embodiment of the present disclosure. As shown in FIG. 10 , in the light-emitting layer EL of the light-emitting device 10 according to the present disclosure, the metal compound body MY has a shell-like shape around the first quantum dot Q1 and may continuously surround the first quantum dot Q1 (surround it without any gaps). In this configuration, the first quantum dot Q1 is not in contact with the matrix material MX. It should be noted that if the metal compound body MY is in contact with the entire surface of the quantum dot QD as shown in FIG. 10 , the metal compound body MY may affect the band structure of the light-emitting layer EL.

[0057] [Embodiment 3] Figure 11 is a cross-sectional view showing an example of the configuration of a light-emitting device according to an embodiment of the present disclosure. As shown in Figure 11, in the light-emitting layer EL of the light-emitting device 10 according to the present disclosure, the matrix material MX is formed in a single layer or multiple layers, with the first quantum dots Q1 and the second quantum dots Q2 located on both sides of one layer of the matrix material MX, and the metal compound bodies MY may be in an inorganic medium (layered) state. According to this configuration, the metal compound bodies MY are formed in multiple layers, with the first quantum dots Q1 located within one layer of the metal compound bodies MY and the second quantum dots Q2 located within another layer of the metal compound bodies MY. In this light-emitting layer EL, the volume occupancy of the metal compound bodies MY may be greater than the volume occupancy of the matrix material MX. It should be noted that when the metal compound bodies MY are in contact with most or all of the surfaces of the quantum dots QDs as shown in Figure 11, the metal compound bodies MY may affect the band structure of the light-emitting layer EL.

[0058] 12 is a schematic diagram illustrating an example of a band structure of an emitting layer of a light-emitting element according to an embodiment of the present disclosure. As shown in FIG. 12, in the emitting layer EL of the light-emitting element 10 according to the present disclosure, the volume occupancy of the matrix material MX may be large in a portion close to the anode E1 and small in a portion close to the cathode E2.

[0059] 13 to 15 are cross-sectional views showing an example of the configuration of a light-emitting device in which the present embodiment 4 is combined with the above-described embodiments 1 to 3. As shown in Fig. 13 to 15, the present embodiment 4 can be combined with the above-described embodiments 1 to 3.

[0060] [Embodiment 5] Fig. 16 is a cross-sectional view showing an example of the configuration of a display device according to an embodiment of the present disclosure. As shown in Fig. 16, a display device 20 according to the present disclosure includes one or more light-emitting elements 10 according to the present disclosure. For example, the display device 20 may include a display unit DA in which a plurality of pixels PX are provided, and a frame unit NA in which a drive circuit DC for driving the display unit DP is provided, and each pixel PX may include a light-emitting element 10 and a pixel circuit PC.

[0061] 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.

[0062] 10 Light-emitting element 20 Display device E1 Anode E2 Cathode EL Light-emitting layer MX Matrix material MY Metal compound body Q1 First quantum dot Q2 Second quantum dot

Claims

1. A light-emitting device comprising an anode and a cathode, and a light-emitting layer positioned between the anode and the cathode, wherein the light-emitting layer has a first quantum dot and a second quantum dot, and a matrix material positioned between the first quantum dot and the second quantum dot, and the matrix material is a p-type conductive metal oxide.

2. The light-emitting device according to claim 1, wherein the composition of the metal oxide is oxygen-rich with respect to the stoichiometric ratio.

3. The light-emitting device according to claim 1 or 2, wherein the light-emitting layer includes a metal compound body in contact with the matrix material.

4. The light-emitting device according to any one of claims 1 to 3, wherein the matrix material has an increasing hole density with a temperature rise from 20°C to 100°C.

5. The light-emitting device according to any one of claims 1 to 4, wherein the temperature change rate of the carrier density of the light-emitting layer is from 1.3 [% / °C] to 7.0 [% / °C].

6. The light-emitting device according to any one of claims 1 to 5, wherein the B constant indicating the thermistor characteristics of the light-emitting layer is from 1300 to 6000.

7. The light-emitting device according to any one of claims 1 to 6, wherein the activation energy of the light-emitting layer is from 80 [meV] to 300 [meV].

8. The light-emitting device according to claim 3, wherein the metal compound body is in contact with the first quantum dot.

9. The light-emitting device according to claim 3, wherein the metal compound body is positioned at a part around the first quantum dot.

10. The light-emitting device according to claim 3, wherein the metal compound body has a larger band gap than the first quantum dot.

11. The light-emitting device according to claim 3, wherein the metal compound body includes a metal sulfide.

12. The light-emitting device according to claim 3, wherein in the light-emitting layer, the volume occupancy rate of the metal compound body is smaller than that of the matrix material.

13. The light-emitting device according to any one of claims 1 to 12, wherein the first and second quantum dots are in contact with the matrix material.

14. The light-emitting device according to claim 3, wherein the metal compound body surrounds the first quantum dots without any gaps.

15. The light-emitting device according to claim 3, wherein in the light-emitting layer, the matrix material is formed in a single layer or multiple layers, and the first quantum dots and the second quantum dots are positioned on both sides of one layer of the matrix material.

16. The light-emitting device according to claim 15, wherein in the light-emitting layer, the volume occupancy rate of the metal compound body is larger than the volume occupancy rate of the matrix material.

17. The light-emitting device according to any one of claims 1 to 16, wherein the metal oxide is nickel oxide.

18. The light-emitting device according to claim 11, wherein the metal sulfide is zinc sulfide.

19. The density of nickel in the matrix material is 5×10 ２２ [ / cm ３ , and the light-emitting device according to claim 17.

20. A display device including the light-emitting device according to any one of claims 1 to 19.

21. A method for manufacturing a light-emitting device including an anode and a cathode, and a light-emitting layer positioned between the anode and the cathode, the method including: applying a solution including a precursor of a matrix material that is a p-type conductive metal oxide and a plurality of quantum dots to an underlying layer; and heating the applied solution.

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