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

Abstract

This light-emitting element (10) comprises: an anode (2) and a cathode (9); and a light-emitting layer (6) positioned between the anode and the cathode, the light-emitting layer (6) having a quantum dot group (QG) that includes first quantum dots (Q1), and an inorganic medium (MX). The light-emitting layer has a first portion (FP) that contains a halogen (7), and a second portion (SP) that is closer to the cathode than the first portion and contains the halogen (7) at a higher concentration than the first portion.

Description

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

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

[0002] US Pat. No. 6,299,499 discloses a light-emitting device comprising an inorganic matrix and quantum dots.

[0003] Japanese Patent No. 6434039

[0004] The light-emitting device containing an inorganic medium as disclosed in Patent Document 1 has a problem of high driving voltage.

[0005] A light-emitting element according to the present disclosure includes an anode, a cathode, and a light-emitting layer located between the anode and the cathode and having a group of quantum dots including a first quantum dot and an inorganic medium, the light-emitting layer having a first portion containing a halogen and a second portion that is closer to the cathode than the first portion and contains a halogen at a higher concentration than the first portion.

[0006] The driving voltage of the light emitting element can be reduced.

[0007] 10 is a cross-sectional view showing a configuration example of a light-emitting element according to the present embodiment; FIG. 11 is a band diagram of the light-emitting element according to the present embodiment; FIG. 12 is a band diagram showing a light-emitting element of a comparative example; FIG. 13 is an example of the composition of a light-emitting layer; FIG. 14 is an enlarged view of a light-emitting layer; FIG. 15 is a graph showing the relationship between the ratio (atomic ratio) of halogen to metal elements and the distance from the HTL; FIG. 16 is a cross-sectional view showing an example of the configuration of a light-emitting element; FIG. 17 is a band diagram of the light-emitting element of FIG. 8; FIG. 18 is a schematic cross-sectional view showing a partial configuration of a light-emitting layer; FIG. 19 is a schematic cross-sectional view showing a partial configuration of a light-emitting layer; FIG. 19 is a flowchart showing a method for manufacturing a light-emitting element; FIG. 19 is a schematic view showing a method for preparing a coating liquid; FIG. 19 is a schematic view showing a configuration example of a display device according to the present embodiment.

[0008] Fig. 1 is a cross-sectional view showing an example of the configuration of a light-emitting element according to this embodiment. Fig. 2 is a band diagram of the light-emitting element according to this embodiment. Fig. 3 is a band diagram showing a light-emitting element of a comparative example. As shown in Fig. 1, the light-emitting element 10 includes, in order from the lower layer side (pixel circuit substrate 1 side), an anode 2, a charge functional layer 5, a light-emitting layer 6, an electron transport layer 8, and a cathode 9. The light-emitting layer 6 emits light by recombination of holes from the anode 2 and electrons from the cathode 9.

[0009] The light-emitting layer 6 is located between the anode 2 and the cathode 9, and has a quantum dot group QG including the first quantum dots Q1 and an inorganic medium MX, and has a first portion FP containing halogen 7, and a second portion SP which is closer to the cathode 9 than the first portion FP and contains halogen 7 at a higher concentration than the first portion FP. The charge functional layer 5 may be a hole transport layer (HTL), or may be a laminate of a hole injection layer (HIL) on the anode side and a hole transport layer (HTL) on the light-emitting layer side.

[0010] The quantum dot group QG may include a second quantum dot Q2, the first portion FP may include a first quantum dot Q1 surrounded by a halogen 7, and the second portion SP may include a second quantum dot Q2 surrounded by a halogen 7. The first and second quantum dots Q1 and Q2 may share a core material and emit light of the same color. Hereinafter, the first and second quantum dots Q1 and Q2 may be collectively referred to as quantum dots Q. The quantum dot Q may have a core-shell structure. Note that in FIG. 2, the core and shell SH levels of the quantum dot Q are individually shown. Note that the term "surroundings" in this disclosure may refer to a region extending outward from the surface of an object (here, the quantum dot Q) over a given width, or may refer to a region from the quantum dot Q having a width approximately within the distance between adjacent quantum dots Q.

[0011] The inorganic medium MX may be an inorganic insulator (e.g., silicon oxide). The quantum dot group QG may be composed of a plurality of quantum dots Q of the same type, with the inorganic medium MX being located between the plurality of quantum dots Q. The inorganic medium MX is not limited to an inorganic insulator and may be a semiconductor such as zinc sulfide.

[0012] The band diagram of the light-emitting element 10 ( FIG. 2 ) is compared with the band diagram of a comparative example ( FIG. 3 ) in which the halogen concentration is uniform. It can be seen that the HOMO of the first quantum dot Q1 in the first portion FP is shallower, promoting hole injection into the light-emitting layer 6 and improving carrier balance, thereby reducing the driving voltage of the light-emitting element 10. In the comparative example ( FIG. 3 ), the carrier balance in the light-emitting layer EML is poor, resulting in a higher driving voltage. Furthermore, the second quantum dot Q2, which is easily exposed to excess electrons and is located on the opposite side of the pixel circuit substrate 1 and therefore easily exposed to air and moisture, can be protected by the high concentration of halogen 7.

[0013] Furthermore, by forming a level difference within the light-emitting layer 6, the light-emitting position can be shifted inward (e.g., near the center in the depth direction) from the edge (e.g., the interface B1 on the anode side). This suppresses quenching due to interface defects and enhances the external quantum effect. Even if the charge functional layer 5 (e.g., the hole transport layer) deteriorates faster than the electron transport layer 8, the light-emitting position is less likely to shift (e.g., toward the interface B1 toward the anode side), and the light-emitting position is less likely to move toward the edge. This suppresses quenching due to interface defects and improves reliability. Furthermore, a reduction in the cavity effect due to a change in the light-emitting position (the difference in optical path length between light emitted toward the light-extraction side and light emitted toward the opposite side of the light-extraction side, for example, reflected by the anode 2, deviates from the optimal value, reducing the optical resonance effect) is suppressed, improving reliability. Furthermore, by shifting the light-emitting position within the light-emitting layer 6 inward from the edge, the light-extraction efficiency is also improved. Furthermore, even if the charge functional layer 5 (for example, the hole transport layer) deteriorates faster than the electron transport layer 8, the light emitting position is less likely to shift, and reliability is improved.

[0014] The thickness of the first portion FP may be 20% to 50% of the thickness of the light-emitting layer 6. The first portion FP may include the anode-side interface B1 of the light-emitting layer 6, and the second portion SP may include the cathode-side interface B2 of the light-emitting layer 6. In the light-emitting layer 6, the halogen concentration at the cathode-side interface B2 may be higher than the halogen concentration at the anode-side interface B1, and the halogen concentration at the cathode-side interface B2 may be 1.5 to 10.0 times, 2.0 to 6.0 times, or 3.0 to 5.0 times the halogen concentration at the anode-side interface B1. The "interface" between the anode-side interface B1 and the cathode-side interface B2 can be rephrased as a "boundary" having a predetermined thickness. This predetermined thickness may be 5 nm or less, or may be 10% or less of the thickness of the light-emitting layer 6. X-ray photoelectron spectroscopy (XPS) may be used to measure the halogen concentration.

[0015] 1, the inorganic medium MX may contain halogen 7. In this case, the small content of halogen 7 on the charge functional layer 5 (HTL) side prevents the level of the inorganic medium MX (e.g., silicon oxide) from becoming too deep, enhancing the effect of increasing the probability of hole injection into the first quantum dot Q1. Furthermore, since halogen 7 modifies defects in the inorganic medium MX, quenching of the quantum dot Q is suppressed, improving the reliability of the light-emitting element 10.

[0016] The quantum dots Q (Q1 and Q2) may include at least one of ZnSe, CdSe, ZnTe, CdTe, AlP, GaP, and AlAs. These materials have relatively deep HOMOs (the driving voltage is likely to increase due to the inorganic medium MX and the halogen 7), so the formation of the first and second sites FP and SP is effective.

[0017] A charge functional layer 5 formed of an inorganic oxide may be provided between the anode 2 and the light-emitting layer 6, and the charge functional layer 5 may be a hole transport layer (HTL) in contact with the light-emitting layer 6. The inorganic oxide may be NiO, MgO, MgNiO, LaNiO ３ ,CuO,Cu ２ O, MoO ３ , W.O. ３While inorganic HTLs have the advantage of high reliability, they are prone to exciton deactivation due to defect levels. However, this phenomenon can be suppressed by moving the recombination position (light-emitting position) inward through the formation of the first and second sites FP and SP. As a result, it is possible to enjoy both the high reliability of the inorganic HTL and the high light-emitting efficiency due to suppressed deactivation.

[0018] The first quantum dot Q1 may emit blue light. When the emission wavelength is short, the extraction efficiency decreases significantly due to a change in the emission position, so the formation of the first and second portions FP and SP has a significant advantage (the emission position is less likely to change). One of the anode 2 and the cathode 9 may be located on the upper layer side and be optically transparent, while the other may be located on the lower layer side and be optically reflective. Top emission is more susceptible to optical interference due to the cavity effect (optical resonance effect) than bottom emission, so the formation of the first and second portions FP and SP has a significant advantage (the emission position is less likely to change).

[0019] A hole transport layer 5 may be provided between the anode 2 and the light-emitting layer 6, and the relationship HT-H1≦H1-H2 may be satisfied, where HT is the HOMO of the hole transport layer 5, H1 is the HOMO of the first quantum dot Q1, and H2 is the HOMO of the second quantum dot Q2. In this case, holes are more likely to accumulate at the interface BM between the first and second portions FP and SP than at the interface B1 between the hole transport layer 5 and the first portion FP, and recombination is promoted.

[0020] Hereinafter, the HOMO level refers to the energy level of the highest occupied molecular orbital, and the LUMO level refers to the energy level of the lowest unoccupied molecular orbital. The HOMO level and LUMO level are expressed as negative values ​​with the vacuum level as the reference (0). The absolute value of the difference between the vacuum level and the HOMO level can be rephrased as the ionization potential (absolute value), and the absolute value of the difference between the vacuum level and the LUMO level can be rephrased as the electron affinity (absolute value). Regarding levels such as the HOMO level, "deep" means "the corresponding ionization energy is large or far from the vacuum level," and "shallow" means "the corresponding ionization energy is small or close to the vacuum level." Hereinafter, the HOMO level can also be replaced with the valence band edge (VBM), and the LUMO can also be replaced with the conduction band edge (CBM).

[0021] An electron transport layer 8 may be provided between the cathode 9 and the light-emitting layer 6, and the following relationship may be satisfied: L2-LT≦L1-L2, where LT is the LUMO of the electron transport layer 8, L1 is the LUMO of the first quantum dot Q1, and L2 is the LUMO of the second quantum dot Q2. In this case, electrons are more likely to accumulate at the interface BM between the first and second portions FP and SP than at the interface B2 between the second portion SP and the electron transport layer 8, thereby promoting recombination.

[0022] The relationship 0.2 eV≦H1−H2≦0.8 eV may also be satisfied. Since the energy difference between the hole transport layer 5 and the first quantum dot Q1 is often about 0.2 eV, it is preferable that H1−H2 be 0.2 eV or more. In this case, holes are more likely to accumulate at the interface BM between the first and second portions FP and SP than at the interface B1 between the hole transport layer 5 and the first portion FP. If the barrier is greater than 1.0 eV, the number of holes or electrons that can overcome the barrier will be significantly reduced, so it is preferable that it be 0.8 eV or less.

[0023] The hole transport layer 5 may be provided between the anode 2 and the light-emitting layer 6, and the ratio of halogen to metal elements at the interface B1 between the light-emitting layer 6 and the hole transport layer 5 may be 12% or less. In this case, the energy difference between the hole transport layer 5 and the first quantum dots Q1 becomes small, making it difficult for holes to accumulate at the interface B1 between the hole transport layer 5 and the light-emitting layer 6, which is preferable.

[0024] 4 shows an example of the composition of the light-emitting layer. In FIG. 4, ZnSeTe is used for the core of the quantum dots Q (Q1 and Q2) in the light-emitting layer 6, and silicon oxide (SiO ２ ), and chlorine (Cl) is used as the halogen 7. From Figure 4, it can be seen that the halogen content in the light-emitting layer 6 is higher than when the quantum dots are protected with an organic ligand. In quantum dots with organic ligands, the halogen only coordinates to the surface of the quantum dots, whereas in the light-emitting layer 6 containing the inorganic medium MX, the halogen is also present in the inorganic medium MX, increasing the halogen content. In the case of Figure 4, it can be seen that a 6% increase in Cl / Zn (atomic ratio) deepens the HOMO level of the core by about 0.1 eV. Furthermore, even in the absence of a halogen ligand, the level difference between the HTL and the quantum dots is about 0.2 eV, and if the level difference between the HTL and the quantum dots increases by 0.2 eV to about 0.4 eV, hole injection becomes difficult. Therefore, it is preferable that the Cl / Zn (atomic ratio) at the interface B1 be 12% or less. When the inorganic medium MX is silicon oxide (SiO ２ ), the inorganic medium MX is likely to be porous. Therefore, the amount of halogen taken into the inorganic medium MX increases, and the level of the light-emitting layer 6 becomes deeper, which may result in a worsening of hole injection compared to when the inorganic medium MX is not porous. Therefore, when the inorganic medium MX is made of silicon oxide (SiO ２ In the case of (1), the second portion SP contains a higher halogen concentration than the first portion FP, which significantly improves hole injection.

[0025] The light-emitting element 10 includes a hole transport layer 5 between the anode 2 and the light-emitting layer 6, and an electron transport layer 8 between the cathode 9 and the light-emitting layer 6. The difference between the ratio of halogen to metal elements at the interface B2 between the light-emitting layer 6 and the electron transport layer 8 and the ratio of halogen to metal elements at the interface B1 between the light-emitting layer 6 and the hole transport layer 5 may be 12% to 48%. This is preferable because holes are more likely to accumulate at the interface BM between the first and second portions FP and SP than at the interface B1 between the hole transport layer 5 and the first portion FP. In the light-emitting layer 6 of FIG. 4, it has been found that a 6% increase in Cl / Zn deepens the core level by about 0.1 eV. Therefore, the energy barrier within the light-emitting layer 6 is preferably 0.2 eV or more and 0.8 eV or less. Therefore, the difference between the Cl / Zn (atomic ratio) at the interface B2 and the Cl / Zn (atomic ratio) at the interface B1 may be in the range of 12% to 48%.

[0026] FIG. 5 is an enlarged view of the light-emitting layer. FIG. 6 is a graph showing the relationship between the ratio (atomic ratio) of halogen to metal elements and the distance from the HTL. As shown in FIGS. 5 and 6 , the light-emitting layer 6 may include two positions P1 and P2 spaced apart by d in the thickness direction of the light-emitting layer 6, where d≦5 nm is the difference in the ratio of halogen to metal elements, and DR is 12% to 48%. One of the two positions P1 and P2, P1, may be located at the first portion FP, and the other, P2, may be located at the second portion SP. If the halogen concentration changes sharply in this way, the effect of the light-emitting position being less likely to change due to a difference in level becomes more pronounced.

[0027] Fig. 7 is a cross-sectional view showing an example of the configuration of a light-emitting device. Fig. 7 is a cross-sectional view showing an example of the configuration of a light-emitting device. The anode 2 may be on the lower layer side as in Fig. 1, or the anode 2 may be on the upper layer side (light extraction side) as in Fig. 7.

[0028] FIG. 8 is a cross-sectional view showing an example of the configuration of a light-emitting device. FIG. 9 is a band diagram of the light-emitting device of FIG. 8. As shown in FIG. 8, the light-emitting layer 6 includes a third portion TP located between the first portion FP and the second portion SP. The third portion TP may contain halogen 7 at a higher concentration than the first portion FP and a lower concentration than the second portion SP. The third portion TP may include third quantum dots Q3. The third quantum dots Q3 may be of the same type as the first and second quantum dots (at least the core material is the same). In this way, the HOMO level in the light-emitting layer 6 becomes stepped, as shown in FIG. 9, thereby further improving the hole injection efficiency.

[0029] The quantum dots Q (Q1 to Q3) may be dots (particles) with a maximum width of 100 nm or less. The shape of the quantum dots Q 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, they 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. The quantum dots may be semiconductor single crystals and may have a particle size of 1.0 nm to 50 nm. The quantum dots may have at least one of crystals of 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, crystals of III-V group semiconductor compounds such as GaAs, GaP, InN, InAs, InP, and InSb, and crystals of Group IV semiconductor compounds such as Si and Ge. The quantum dots Q may be, for example, a core-shell type in which the above crystal is used as a core and the core is overcoated with a shell material having a high band gap, or a shell-less type constituted only by the core.

[0030] The inorganic medium MX of the light-emitting layer 6 may be an inorganic matrix material that fills the spaces between the multiple light-emitting quantum dots Q. The inorganic medium MX may also be referred to as a substrate, a base material, or a filler. The inorganic medium MX may be a component of the light-emitting layer 6 that includes the multiple quantum dots Q. The inorganic medium MX may be amorphous. The elemental proportion of carbon in the inorganic medium MX may be 10% or less.

[0031] The inorganic medium MX preferably has a wider band gap than the core material of the quantum dots Q. A semiconductor or an insulator can be used as the inorganic medium MX. The inorganic medium MX may contain at least one of a metal sulfide and a metal oxide. Examples of metal sulfides include zinc sulfide (ZnS), zinc magnesium sulfide (ZnMgS, ZnMgS ２ ), gallium sulfide (GaS, Ga ２ S ３ ), zinc tellurium sulfide (ZnTeS), magnesium sulfide (MgS), zinc gallium sulfide (ZnGa ２ S ４ ), magnesium gallium sulfide (MgGa ２ S ４ Examples of metal oxides include zinc oxide (ZnO), titanium oxide (TiO ２ ), tin oxide (SnO ２ ), tungsten oxide (WO ３ ), zirconium oxide (ZrO ２ ), silicon oxide (SiO ２ ) can be mentioned.

[0032] 10A to 10C are schematic cross-sectional views showing a partial configuration of the light-emitting layer. As shown in FIG. 10A, the inorganic medium MX may be an inorganic matrix material including a continuous film J that extends from one side of adjacent quantum dots Q1 and Q2 to the other and contacts the quantum dots Q1 and Q2. The inorganic medium MX may be primarily composed of an inorganic material and may include at least one of a void VD and an organic material YB. As shown in FIGS. 10B and 10C, in a cross-sectional view, the inorganic medium MX (inorganic matrix material) may fill a region K surrounded by two straight lines (common circumscribing lines) circumscribing the peripheries of the two adjacent quantum dots Q1 and Q2 and the opposing peripheries of these quantum dots Q1 and Q2.

[0033] One of the anode 2 and the cathode 9 may be formed of a light-transmitting material. Examples of the light-transmitting material include transparent conductive materials. Examples of the transparent conductive material include indium tin oxide (ITO), indium zinc oxide (IZO), tin oxide (SnO2), and fluorine-doped tin oxide (FTO). The other of the anode 2 and the cathode 9 may be formed of a light-reflecting material. Examples of the light-reflecting material include metal materials. Examples of the metal material include aluminum (Al), silver (Ag), copper (Cu), and gold (Au).

[0034] Materials for the charge functional layer 5 (HIL, HTL) include metal oxides such as nickel oxide (NiO), as well as poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4'-(N-(4-sec-butylphenyl))diphenylamine)] (abbreviated as "TFB"), poly[N,N'-bis(4-butylphenyl)-N,N'-bis(phenyl)-benzidine] (abbreviated as "Poly-TPD"), polyvinylcarbazole (abbreviated as "PVK"), and poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (abbreviated as "PTAA"). These materials may be used alone or in combination of two or more. An inorganic material such as a metal oxide may be used for the HIL in contact with the anode 2, and an organic material may be used for the HTL in contact with the light-emitting layer 6. Nanoparticles of metal oxides such as nickel oxide may be used in the HIL (hole injection layer) or HTL (hole transport layer).

[0035] The electron transport layer 8 may be made of zinc oxide (ZnO), magnesium zinc oxide (MgZnO), titanium oxide (TiO ２ In addition to metal oxides such as zinc oxide, compounds or complexes containing one or more nitrogen-containing heterocycles such as an oxadiazole ring, a triazole ring, a triazine ring, a quinoline ring, a phenanthroline ring, a pyrimidine ring, a pyridine ring, an imidazole ring, or a carbazole ring may be used. Nanoparticles of a metal oxide such as zinc oxide may be used in the ETL (electron transport layer).

[0036] Example 1 In Example 1, a first coating liquid containing first quantum dots Q1 is prepared using the following first and second solutions, and a second coating liquid containing second quantum dots Q2 is prepared using the following first and third solutions. First solution: A liquid in which a group of blue-emitting quantum dots (QG) is dispersed in a non-polar solvent such as hexane at a concentration of 1 mg / mL. Second solution: A liquid containing the following (A) and (B). Third solution: A liquid containing the following (A) and (C). (A) A xanthogenate source such as zinc ethylxanthate (a precursor of zinc sulfide, an inorganic medium) is dissolved in a polar solvent such as N-methylformamide (abbreviated as "NMF") at a concentration of 0.1 mol / L.

[0037] (B) A halogen source such as zinc chloride dissolved in a polar solvent such as NMF at a concentration of 0.05 mol / L.

[0038] (C) A halogen source such as zinc chloride dissolved in a polar solvent such as NMF at a concentration of 0.2 mol / L.

[0039] The halogen source may be, for example, another halogen compound, and the halogen compound may be a metal halide or an ammonium halide. The xanthic acid source and the halogen source may contain the same metal element.

[0040] In the first embodiment, chlorine is used as the halogen element, but other halogen elements (fluorine F, bromine Br, iodine I) may also be used.

[0041] Equal amounts of the first and second solutions were mixed, and the mixture was stirred vigorously for 24 hours in a container with a stir bar, with the upper layer of the first solution and the lower layer of the second solution separated. As a result, the blue quantum dots were transferred from the layer of the first solution to the layer of the second solution, and ethylxanthogenate and chloride ions were coordinated to each quantum dot. This completes the substitution of the ligands coordinated to the blue quantum dots.

[0042] The first solution is removed from the mixture of the first and second solutions by removing the hexane along with the organic ligands, and a medium-polarity solvent such as ethyl acetate is added to the remaining second solution. This precipitates the blue quantum dots and the ethylxanthate and chloride ions that coordinate with these quantum dots. Meanwhile, the zinc chloride and zinc ethylxanthate do not precipitate and remain in the NMF / ethyl acetate mixture.

[0043] Next, the precipitate and zinc ethylxanthogenate are dispersed in a polar solvent such as a mixed solution of N,N-dimethylformamide (DMF) and THT (tetrahydrothiophene) to obtain a first coating liquid (containing first quantum dots Q1).

[0044] Similarly, the first solution and the third solution are used to obtain a second coating liquid (containing second quantum dots Q2).

[0045] 11 is a flowchart showing a method for manufacturing a light-emitting element. In step S10, an anode 2, which is a reflective electrode, is formed on a substrate 1. For example, a 20 nm thick ITO film, a 100 nm thick silver film, and a 50 nm thick ITO film are laminated in this order using a sputtering method, a vapor deposition method, or the like to form the anode 2.

[0046] In step S20, a hole injection layer (HIL, which is the layer below the charge functional layer 5) is formed on the anode 2. First, a nickel oxide nanoparticle dispersion (for example, a solution in which nickel oxide nanoparticles are dispersed in a solvent in which water and 2-methoxyethanol are mixed in equal volumes) is applied by spin coating in the atmosphere, and then baked at 200°C. This process may be repeated 1 to 5 times. A SAM layer (self-assembled monolayer) may be formed on the hole injection layer (HIL). For example, a 0.01 M ethanol solution of [2-(3,6-dimethyl-9H-carbazol-9-yl)ethyl]phosphonic Acid (Me-2PACz), [2-(3,6-Dibromo-9H-carbazol-9-yl)ethyl]phosphonic Acid (Br-2PACz), or the like is applied to a nickel oxide layer by spin coating in a nitrogen atmosphere, and then the solvent is evaporated by baking to form a SAM layer.

[0047] In step S30, a hole transport layer (HTL, which is the upper layer of the charge functional layer 5) is formed on the hole injection layer. For example, the hole transport layer (HTL) is formed by applying a solution obtained by dispersing poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4'-(N-(4-sec-butylphenyl))diphenylamine)] (abbreviated as "TFB"), poly[N,N'-bis(4-butylphenyl)-N,N'-bis(phenyl)-benzidine] (abbreviated as "Poly-TPD"), polyvinylcarbazole (abbreviated as "PVK"), poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (abbreviated as "PTAA"), or the like, in a chlorobenzene solvent by spin coating under a nitrogen atmosphere.

[0048] In step S40, the light-emitting layer 6 is formed on the hole transport layer. For example, after a first coating liquid containing the first quantum dots Q1 is applied and baked at 150°C (forming the first portion FP), a second coating liquid containing the second quantum dots Q2 is applied and baked at 150°C (forming the second portion SP), thereby forming the light-emitting layer 6 (FIG. 1).

[0049] In step S50, the electron transport layer 8 is formed on the light-emitting layer 6. For example, a solution in which MgZnO nanoparticles are dispersed in a solvent (such as ethanol) is applied by spin coating under a nitrogen atmosphere to form the electron transport layer 8. The Mg:Zn (atomic ratio) in the MgZnO nanoparticles can be, for example, 0:100, 5:95, 10:90, 15:85, or 20:80.

[0050] The electron transport layer 8 may be a non-doped ZnO nanoparticle film, or may be a ZnO nanoparticle film doped with at least one of Li, Al, Ti, Ga, and Zr in addition to Mg. ２ film or ZrO ２ A film may be formed.

[0051] In step S60, a cathode 9, which is a translucent electrode, is formed on the substrate 1. For example, the cathode 9 is made of ITO formed to a thickness of about 100 nm by sputtering, or silver (Ag) or gold (Au) formed to a thickness of about 20 nm by vapor deposition.

[0052] In addition, a third coating liquid containing third quantum dots Q3 may be prepared using the first solution (a liquid in which blue quantum dots are dispersed in a non-polar solvent such as hexane at a concentration of 1 mg / mL) and a fourth solution containing the above (A) and the following (D).

[0053] (D) A liquid in which a halogen source such as zinc chloride is dissolved in a polar solvent such as NMF at a concentration of 0.1 mol / L. In this case, after a first coating liquid containing first quantum dots Q1 is applied and baked at 150°C (forming the first portion FP), a third coating liquid containing third quantum dots Q3 is applied and baked at 150°C (forming the third portion TP), and then a second coating liquid containing second quantum dots Q2 is applied and baked at 150°C (forming the second portion SP), thereby forming the light-emitting layer 6 (Figure 8).

[0054] Alternatively, the light-emitting layer 6 (containing the inorganic material MX of zinc sulfide) may be formed on the inorganic hole-injection layer 3 without performing step S30. In this case, the inorganic hole-injection layer 3 and the light-emitting layer 6 are in contact with each other (form an interface).

[0055] Example 2 A first coating liquid containing first quantum dots Q1 is prepared using the following first and second solutions, and a second coating liquid containing second quantum dots Q2 is prepared using the following first and third solutions: First solution: A solution in which a group of blue-emitting quantum dots (QG) is dispersed in a nonpolar solvent such as octane at a concentration of 2 mg / mL, to which MPS ((3-mercaptopropyl)trimethoxysilane), a precursor of an inorganic medium (silicon oxide), is added to a concentration of 0.4 mol / L, and further, TMOS (tetramethyl orthosilicate), a precursor of an inorganic medium (silicon oxide), is added to a concentration of 0.04 mol / L. Second solution: A solution in which a halogen source such as zinc chloride is dissolved in a polar solvent such as DMF at a concentration of 0.1 mol / L. Third solution: A solution in which a halogen source such as zinc chloride is dissolved in a polar solvent such as DMF at a concentration of 0.4 mol / L. The halogen source may be, for example, another halogen compound, and the halogen compound may be a metal halide or an ammonium halide.

[0056] FIG. 12 is a schematic diagram showing a method for preparing a coating solution. As shown in FIG. 12, equal amounts of the first solution and the second solution are mixed and vigorously stirred for 24 hours in a container with a stir bar. As a result, for example, quantum dots (QG) are transferred from the layer of the first solution to the interface between the first solution and the second solution. When centrifuged at 4000 rpm for 5 minutes, the quantum dots (QG) present at the interface are precipitated to the bottom of the container. After removing all the solvent, the mixture is dispersed in a nonpolar solvent such as toluene to obtain a first coating solution (containing first quantum dots Q1) of the desired concentration.

[0057] Similarly, the first solution and the third solution are used to obtain a second coating liquid (containing second quantum dots Q2).

[0058] In this case, a first coating liquid containing first quantum dots Q1 is applied and baked at 100°C (forming the first portion FP), and then a second coating liquid containing second quantum dots Q2 is applied and baked at 100°C (forming the second portion SP), thereby forming the light-emitting layer 6 (Figure 1).

[0059] In addition, a fourth solution (a liquid in which a halogen source such as zinc chloride is dissolved in a polar solvent such as DMF at a concentration of 0.2 mol / L) may be prepared, and a third coating liquid containing the third quantum dots Q3 may be produced using the first solution and the fourth solution.

[0060] In this case, after applying a first coating liquid containing the first quantum dots Q1 and baking it at 100°C (forming the first portion FP), a third coating liquid containing the third quantum dots Q3 is applied and baked at 100°C (forming the third portion TP), and then a second coating liquid containing the second quantum dots Q2 is applied and baked at 100°C (forming the second portion SP), thereby forming the light-emitting layer 6 (Figure 8).

[0061] Alternatively, the light-emitting layer 6 (including the inorganic material MX of silicon oxide) may be formed on the inorganic hole injection layer (charge functional layer 5) without performing step S30. In this case, the inorganic hole injection layer and the light-emitting layer 6 are in contact with each other (an interface is formed).

[0062] 13 is a schematic diagram showing an example of the configuration of a display device according to this embodiment. The display device 20 includes a display unit DA, a first driver circuit X1 (e.g., a data signal line drive circuit) and a second driver circuit X2 (e.g., a scanning signal line drive circuit and a light-emitting control line drive circuit) that drive the display unit DA, and a control circuit CL that controls the first driver circuit X1 and the second driver circuit X2. The display unit DA may include a substrate (pixel circuit substrate 1) and a light-emitting element layer. The light-emitting element layer may include a light-emitting element 10R (10) that emits red light, a light-emitting element 10G (10) that emits green light, and a light-emitting element 10B (10) that emits blue light, and the light-emitting elements 10R, 10G, and 10B may each be connected to a pixel circuit PC formed in the pixel circuit layer.

[0063] [Notes] The above-described embodiments are for the purpose of illustration and description, and are not intended to be limiting. Based on these examples and descriptions, it will be apparent to those skilled in the art that many variations are possible.

[0064] REFERENCE SIGNS LIST 1 Substrate (pixel circuit substrate) 2 Anode 5 Charge functional layer 6 Light-emitting layer 7 Halogen 8 Electron transport layer 9 Cathode 10 Light-emitting element 20 Display device FP First portion SP Second portion 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 located between the anode and the cathode and having a group of quantum dots including a first quantum dot and an inorganic medium, wherein the light-emitting layer has a first portion containing a halogen and a second portion that is closer to the cathode than the first portion and contains a higher concentration of halogen than the first portion.

2. The light-emitting element according to claim 1, wherein the first portion includes an anode-side interface of the light-emitting layer, and the second portion includes a cathode-side interface of the light-emitting layer.

3. The light-emitting device according to claim 1 or 2, wherein the inorganic medium contains a halogen.

4. A light-emitting element according to any one of claims 1 to 3, wherein the quantum dot group includes a second quantum dot, the first portion includes the first quantum dot surrounded by a halogen, and the second portion includes the second quantum dot surrounded by a halogen.

5. The light-emitting device according to claim 2, wherein the halogen concentration at the cathode-side interface of the light-emitting layer is 1.5 times or more the halogen concentration at the anode-side interface of the light-emitting layer.

6. The light-emitting device according to any one of claims 1 to 5, wherein the first quantum dots include at least one of ZnSe, CdSe, ZnTe, CdTe, AlP, GaP, and AlAs.

7. The light-emitting device according to any one of claims 1 to 5, further comprising a charge functional layer formed of an inorganic oxide and disposed between the anode and the light-emitting layer, the charge functional layer being in contact with the light-emitting layer.

8. The light-emitting element according to any one of claims 1 to 7, wherein the first quantum dots emit blue light.

9. The light-emitting device according to any one of claims 1 to 8, wherein one of the anode and the cathode is located on the upper layer side and is light-transmitting, and the other is located on the lower layer side and is light-reflective.

10. The light-emitting device according to claim 4, further comprising a hole transport layer between the anode and the light-emitting layer, wherein the HOMO of the hole transport layer is HT, the HOMO of the first quantum dot is H1, and the HOMO of the second quantum dot is H2, and the relationship HT-H1≦H1-H2 holds.

11. The light-emitting device according to claim 4, further comprising an electron transport layer between the cathode and the light-emitting layer, wherein the LUMO of the electron transport layer is LT, the LUMO of the first quantum dot is L1, and the LUMO of the second quantum dot is L2, and the following relationship holds: L2-LT≦L1-L2.

12. The light-emitting device according to claim 10, wherein 0.2 eV≦H1−H2≦0.8 eV holds true.

13. The light-emitting device according to any one of claims 1 to 12, further comprising a hole transport layer between the anode and the light-emitting layer, wherein the ratio of halogen to metal elements at the interface between the light-emitting layer and the hole transport layer is 12% or less.

14. The light-emitting element according to any one of claims 1 to 13, further comprising a hole transport layer between the anode and the light-emitting layer, and an electron transport layer between the cathode and the light-emitting layer, wherein the difference between the ratio of halogen to metal elements at the interface between the light-emitting layer and the electron transport layer and the ratio of halogen to metal elements at the interface between the light-emitting layer and the hole transport layer is 12% to 48%.

15. A light-emitting element according to any one of claims 1 to 14, wherein the light-emitting layer includes two positions that are separated from each other by 5 nm or less in the thickness direction of the light-emitting layer and have a difference in the ratio of halogen to metal elements of 12% to 48%.

16. The light-emitting element of claim 15, wherein one of said two locations is in said first region and the other is in said second region.

17. The light-emitting element according to any one of claims 1 to 16, wherein the light-emitting layer includes a third portion located between the first portion and the second portion, and the third portion contains a halogen at a higher concentration than the first portion and a lower concentration than the second portion.

18. The light-emitting device according to any one of claims 1 to 17, wherein the inorganic medium is an inorganic insulator.

19. The light-emitting device according to claim 1, wherein the inorganic medium includes a continuous film that extends from one of two adjacent quantum dots in the quantum dot group to the other and contacts the two quantum dots, and the inorganic medium includes silicon oxide.

20. The light-emitting device according to claim 19, wherein the quantum dot group includes a second quantum dot, the first portion includes the first quantum dot surrounded by a halogen, the second portion includes the second quantum dot surrounded by a halogen, and a hole transport layer is provided between the anode and the light-emitting layer, and wherein, assuming that a HOMO of the hole transport layer is HT, a HOMO of the first quantum dot is H1, and a HOMO of the second quantum dot is H2, HT-H1≦H1-H2 holds, and 0.2 eV≦H1-H2≦0.8 eV holds.

21. The light-emitting device according to claim 19, further comprising a hole transport layer between the anode and the light-emitting layer, wherein the ratio of halogen to metal elements at the interface between the light-emitting layer and the hole transport layer is 12% or less.

22. The light-emitting element according to claim 19, further comprising a hole transport layer between the anode and the light-emitting layer, and an electron transport layer between the cathode and the light-emitting layer, wherein the difference between the ratio of halogen to metal elements at the interface between the light-emitting layer and the electron transport layer and the ratio of halogen to metal elements at the interface between the light-emitting layer and the hole transport layer is 12% to 48%.

23. The light-emitting element according to claim 19, wherein the light-emitting layer includes two positions separated from each other by 5 nm or less in the thickness direction of the light-emitting layer, and the difference in the ratio of halogen to metal elements is 12% to 48%.

24. A light-emitting device according to any one of claims 1 to 23, wherein the quantum dot group is made up of a plurality of quantum dots of the same type, and the inorganic medium is located in the gaps between the quantum dot group.

25. A display device comprising a light-emitting element according to any one of claims 1 to 24.

26. A method for manufacturing a light-emitting device, comprising: forming an anode; removing a solvent from a first coating liquid containing first quantum dots and an inorganic medium precursor and containing a halogen; removing a solvent from a second coating liquid containing second quantum dots and an inorganic medium precursor and containing a halogen at a higher concentration than the first coating liquid; and forming a cathode.

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