Quantum dot solution, quantum dot layer formation method, light-emitting element, display device, and quantum dot solution production method

Abstract

A quantum dot solution comprising light-emitting quantum dots, a precursor of a metal sulfide, and a solvent, wherein: the precursor is a metal complex with a thermogravimetric weight loss rate of 60-90% when heated from 50°C to 200°C; the solid after the metal complex is heated to 200°C comprises at least 80 weight% of an inorganic compound; and at least 50 weight% of the inorganic compound is the metal sulfide.

Description

Quantum dot solution, method for forming quantum dot layer, light-emitting element, display device, and method for manufacturing quantum dot solution

[0001] The present disclosure relates to quantum dot solutions and the like.

[0002] Patent Document 1 discloses a method for producing a quantum dot-containing coating using a composition containing quantum dots containing a chalcogenide as a surface material, a base component, and a solvent.

[0003] Japanese Patent Publication No. 2020-101769

[0004] Quantum dot layers modified with organic ligands have the problem of low durability, while quantum dot layers coated with inorganic materials formed by conventional methods have the problem of low carrier injection.

[0005] A quantum dot solution according to one aspect of the present disclosure includes luminescent quantum dots, a metal sulfide precursor, and a solvent, wherein the precursor is a metal complex that experiences a weight loss rate of 60% to 90% when heated from 50°C to 200°C, and the solid obtained after heating the metal complex to 200°C contains 80% by weight or more of an inorganic compound, and 50% by weight or more of the inorganic compound is the metal sulfide.

[0006] By using a quantum dot solution according to one aspect of the present disclosure, it is possible to improve durability while maintaining the carrier injection properties of the quantum dot layer.

[0007] 6B is a flowchart illustrating a method for forming a quantum dot layer according to an embodiment; FIG. 6C is a schematic diagram illustrating a method for forming a quantum dot layer according to an embodiment; FIG. 6D is an example of the structural formula of a ligand and a metal complex; FIG. 6E is a graph illustrating thermogravimetric properties of a metal complex; FIG. 6F is a graph illustrating X-ray diffraction profiles of zinc xanthate before and after heating, and an X-ray diffraction profile of zinc sulfide; FIG. 6G is a graph illustrating the results of Fourier transform infrared spectroscopy (FTIR) measurement of zinc xanthate after heating; FIG. 6H is a graph illustrating the results of Fourier transform infrared spectroscopy (FTIR) measurement of zinc xanthate after heating; FIG. 6I is a graph illustrating the results of Fourier transform infrared spectroscopy (FTIR) measurement of zinc xanthate after heating; FIG. 6J is a graph illustrating the results of Fourier transform infrared spectroscopy (FTIR) measurement of zinc xanthate after heating; FIG. 6J is a graph illustrating an enlarged portion of FIG. 6E. 1 is a graph showing the results of Fourier transform infrared spectroscopy (FTIR) measurement of an organic ligand. 2 is an ultraviolet-visible absorption spectrum (UV-VIS) of a thin film obtained by heating a coating liquid of zinc xanthogenate. 3 is a Tauc plot graph of a thin film obtained by heating a coating liquid of zinc xanthogenate. 4 is a schematic diagram showing another method of forming a quantum dot layer. 5 is a table showing the results of comparing quantum dot layers of a comparative example and quantum dot layers of an embodiment in terms of structure, PLQY (photoluminescence quantum efficiency), and PL lifetime (photoluminescence lifetime). 6 is a graph showing the results of an atmospheric light exposure test of a quantum dot layer of a comparative example and the results of an atmospheric light exposure test of a quantum dot layer of an embodiment. 7 is a graph showing the results of a nitrogen heating test of a quantum dot layer of a comparative example and the results of a nitrogen heating test of a quantum dot layer of an embodiment. 8 is a schematic diagram showing an example of a method for preparing a quantum dot solution. 9 is a graph showing the relationship between PLQY and the molar ratio of potassium xanthogenate to zinc chloride (chloride ion source) in the solution used to prepare the quantum dot solution. FIG. 1 is a schematic diagram showing the state of modification of quantum dots; FIG. 2 is a schematic cross-sectional view showing an example of the configuration of the light-emitting device of the present embodiment; FIG. 3 is a cross-sectional photograph showing an example of the configuration of the light-emitting device of the present embodiment; FIG. 4 is a graph showing the current density-external quantum effect characteristics of the light-emitting device; FIG. 5 is a graph showing the voltage-current density characteristics of the light-emitting device;２ 10 is a graph showing the EL spectrum at 25 mA / m ２ 10 is a graph showing the change in relative luminance and voltage over time when a constant current of 25 mA / m is continuously applied to the light-emitting element of the comparative example. ２ 1 is a graph showing changes in relative luminance and voltage over time when a constant current of 100 kJ / s is continuously applied to the display device according to the embodiment. 2 is a schematic diagram showing a configuration example of a display device according to the embodiment.

[0008] FIG. 1 is a flowchart illustrating a method for forming a quantum dot layer according to an embodiment. FIG. 2 is a schematic diagram illustrating the method for forming a quantum dot layer according to an embodiment. As shown in FIGS. 1 and 2 , the method for forming a quantum dot layer according to an embodiment includes a step S10 of preparing a quantum dot solution 10 containing luminescent quantum dots QDs and a metal sulfide precursor J, a step S20 of forming a coating liquid 20 of the quantum dot solution 10 on an underlayer UL, a step S30 of drying the coating liquid 20, and a step S40 of modifying the precursor J of the coating liquid 20 to form an inorganic matrix material MX, which is a metal sulfide A, encapsulating a plurality of quantum dots QDs (filling spaces between the plurality of quantum dots QDs). This results in the formation of a quantum dot layer 30 containing a plurality of quantum dots QDs and the inorganic matrix material MX filling spaces between the plurality of quantum dots QDs.

[0009] In step S40, the precursor J can be decomposed and crystallized by at least one of heating and irradiating the coating liquid 20 with light. In step S40, the coating liquid 20 of the quantum dot solution 10 applied to the underlayer UL may be heated to 100° C. or higher and 600° C. or lower. In step S40, the coating liquid 20 of the quantum dot solution 10 applied to the underlayer UL including an organic layer may be heated to 100° C. or higher and 250° C. or lower.

[0010] In step S40, the coating liquid 20 of the quantum dot solution 10 applied to the underlayer UL may be irradiated with light having a wavelength of 200 nm to 400 nm.In step S40, the coating liquid 20 of the quantum dot solution 10 applied to the underlayer UL may be irradiated with laser light having a wavelength of 400 nm to 2000 nm.

[0011] The quantum dot solution 10 contains luminescent quantum dots QD, a precursor J of metal sulfide A, and a solvent Y. Precursor J is a metal complex that experiences a weight loss rate of 60% to 90% when heated from 50°C to 200°C, and the solid obtained after heating this metal complex to 200°C contains 80% by weight or more of an inorganic compound, and 50% by weight or more of this inorganic compound is metal sulfide A. The weight loss rate is the ratio, expressed as a percentage, of the amount of weight loss due to heating from 50°C to the weight at 50°C.

[0012] By using precursor J, which exhibits a weight loss rate of 60% to 90% when heated from 50°C to 200°C, it is possible to form an inorganic matrix material MX with less residue (e.g., organic matter) in a low-temperature process compared to using a precursor with a weight loss rate of less than 60% when heated from 50°C to 200°C. The low residue in the inorganic matrix material MX allows for improved durability while maintaining the carrier injection properties of the quantum dot layer 30 when applied to a light-emitting device. This also results in reduced driving voltage and improved optical transparency. The low-temperature process reduces thermal damage to the carrier transport layer, even when the underlayer UL is an organic carrier transport layer. The quantum dot layer 30 can also be formed on a substrate with low heat resistance (e.g., a flexible substrate).

[0013] The quantum dot solution 10 contains a ligand L1 for the quantum dots QD, and the ligand L1 for the quantum dots QD and the ligand (ligand) L2 for the metal complex (precursor J) may have the same structure (a relationship represented by the same structural formula). The ligand L1 enhances the dispersibility of the quantum dots QD in the quantum dot solution 10. The same structure of the ligands L1 and L2 allows the denaturation (decomposition and crystallization) of the precursor J to proceed quickly. Furthermore, the quantum dot solution 10 can be easily prepared. The ligand L1 may be coordinated to the quantum dots QD in an amount of 0.1 wt % or more.

[0014] The quantum dots QD may have a core 2 and a shell 3, and the material constituting the shell 3 may be the metal sulfide A. In this case, the ligand L1 is more likely to coordinate to the quantum dots QD, and the dispersibility of the quantum dots QD in the quantum dot solution 10 is improved.

[0015] FIG. 3 shows examples of structural formulas of ligands and metal complexes. The ligand L2 of the metal complex (precursor J) may be a dithiocarboxylic acid. The dithiocarboxylic acid has a structure represented by XC(=S)SH (X is a carbon substituent). The quantum dot QD may be modified with a dithiocarboxylic acid. That is, the ligand L1 of the quantum dot QD may be a dithiocarboxylic acid. The metal complex (precursor J) may contain elemental zinc (Zn). The metal complex (precursor J) may be zinc dithiocarboxylic acid. The metal sulfide A may be zinc sulfide (ZnS). The ligand L2 of the metal complex (precursor J) may be xanthogenic acid. The xanthogenic acid has a structure represented by ROC(=S)SH (R is hydrogen, a hydrocarbon group, etc.). The metal complex (precursor J) may be zinc xanthogenate. The shell 3 of the quantum dot QD may be composed of zinc sulfide (ZnS).

[0016] When the metal sulfide A is zinc sulfide, the precursor J is not limited to zinc xanthate. The precursor J may be a zinc dialkylthiourea, and the ligand L2 of the precursor J (metal complex) may be a dialkylthiourea.

[0017] Figure 4 is a graph showing the thermogravimetric properties of metal complexes. The normalized weight on the vertical axis is the weight at 50°C as a reference value (100%), and the weight at temperatures above 50°C is expressed as a percentage of the reference value. Here, a thermogravimetric analyzer (TGA) was used to measure the weight changes while heating four types of powders (zinc xanthogenate, a mixture of zinc acetate and ammonium thiocyanate, a zinc alkylthiourea, and a zinc thiourea) in nitrogen.

[0018] 4 shows that zinc xanthate (normalized weight: 35%, weight loss rate: 65%) and zinc dialkylthiourea (normalized weight: 30%, weight loss rate: 70%) are suitable as precursor J of zinc sulfide, which shows a weight loss rate of 60% to 90% when heated from 50° C. to 200° C. Furthermore, it is also shown that a mixture of zinc acetate and ammonium thiocyanate (normalized weight: 85%, weight loss rate: 15%), which shows a weight loss rate of less than 60% when heated from 50° C. to 200° C., is not suitable as precursor J.

[0019] Zinc xanthogenate can be prepared, for example, as follows: A mixture of an aqueous zinc chloride solution and an aqueous potassium xanthogenate solution (molar ratio of zinc chloride to potassium xanthate = 1:2.2) is stirred for 24 hours, and the resulting precipitate is filtered. The precipitate obtained by filtration is washed three times with distilled water (to remove residual raw materials and by-product potassium chloride) and then dried under nitrogen to obtain zinc xanthogenate (precursor J). A solution of zinc acetate and ammonium thiocyanate dissolved in 2-methoxyethanol (molar ratio of zinc acetate to ammonium thiocyanate = 1:1) is stirred for one hour, and the solvent is removed to obtain a mixture of zinc acetate and ammonium thiocyanate.

[0020] Furthermore, X-ray diffraction (XRD) of the four powders (zinc xanthate, a mixture of zinc acetate and ammonium thiocyanate, zinc alkylthiourea, and zinc thiourea) was measured after heating them to 200°C in nitrogen. For comparison, XRD of the powders ZnO and ZnS was also measured.

[0021] 5 is a graph showing the X-ray diffraction profiles of zinc xanthate before and after heating, and the X-ray diffraction profile of zinc sulfide. The peak characteristic of zinc sulfide appears in the graph after heating (when zinc xanthate was heated to 150° C.), indicating that precursor J was modified (decomposed and crystallized) to zinc sulfide.

[0022] 6A to 6E are graphs showing the results of Fourier transform infrared spectroscopy (FTIR) measurements after heating zinc xanthate. The horizontal axis represents wave number, and the vertical axis represents transmittance, with the wave number expressed in units of cm. －１ ], and the unit of transmittance is [%]. Figure 6A shows the measurement results after heating at 100°C, Figure 6B shows the measurement results after heating at 125°C, Figure 6C shows the measurement results after heating at 150°C, Figure 6D shows the measurement results after heating at 175°C, and Figure 6E shows the measurement results after heating at 200°C. Figure 7A is a graph showing an enlarged portion of Figure 6E. Figure 7B is a graph showing the results of Fourier transform infrared spectroscopy (FTIR) measurement of the organic ligand.

[0023] As shown in FIG. 6A, zinc xanthate, which is a metal complex, has a peak at 1040 cm in the Fourier transform infrared absorption spectrum. －１ , 1122 cm －１ , and 1217 cm －１ Therefore, the quantum dot solution has an absorption peak at 1040 cm around the quantum dots QD in the Fourier transform infrared absorption spectrum. －１ , 1122 cm －１ , and 1217 cm －１ It can be said that the ligand has an absorption peak at

[0024] In addition, the quantum dot solution exhibited a Fourier transform infrared absorption spectrum of 1040 cm －１ , 1122 cm －１ , and 1217 cm －１ The compound contains a compound that has an absorption peak at 200° C. and whose absorption peak intensity becomes less than 20% when heated to 200° C.

[0025] When zinc xanthate is heated to 125°C or higher, a characteristic absorption spectrum not seen in organic ligands is observed, as shown in Figure 7B. －１ ], 3000 [cm －１ ] disappears, but the peak near wavenumber 1650 [cm －１ As shown in Figure 7A, this peak appears at S ２ It is believed to be derived from a C=O bond.

[0026] That is, the quantum dot layer (light-emitting layer) obtained by heating the coating liquid of the quantum dot solution contains a plurality of light-emitting quantum dots QDs, a metal sulfide (for example, an inorganic matrix material mainly composed of ZnS) containing the plurality of quantum dots QDs, and S ２ The quantum dot layer (light-emitting layer) may include a compound having a C═O bond, a metal sulfide (for example, an inorganic matrix material mainly composed of ZnS) containing the quantum dots QDs, and a compound having a wavelength of 1650 cm in a Fourier transform infrared absorption spectrum. －１and a compound having a peak at .

[0027] Figure 8A shows the ultraviolet-visible absorption spectrum (UV-VIS) of a thin film obtained by heating a coating solution of zinc xanthate. Specifically, zinc xanthate was dissolved in DMF (N,N-dimethylformamide) to a concentration of 0.2 mol / L, and the solution was applied to glass at 2000 rpm and heated at various temperatures for 30 minutes. The ultraviolet-visible absorption spectrum of the thin film formed at each temperature was measured. Figure 8A shows that the absorption peak of zinc xanthate at 305 nm disappeared, indicating that the zinc xanthate was decomposed at 150°C.

[0028] Figure 8B is a Tauc plot graph of a thin film obtained by heating a zinc xanthate coating solution. The UV-VIS spectrum of a thin film obtained by heating a zinc xanthate coating solution at 200°C was Tauc plotted, and the band gap was calculated. h is Planck's constant, ν is the frequency of light, and α is absorbance. The band gap of the obtained thin film was 3.65 eV. Since the band gap of commonly known zinc sulfide is 3.6 to 3.7 eV, it can be seen that highly transparent zinc sulfide was formed.

[0029] The quantum dots QDs used in this embodiment may be fine particles having a particle size (outer diameter) of 1.0 nm to 100 nm, and may be spherical or non-spherical in shape. The shape of the quantum dots QDs is not limited to a spherical three-dimensional shape (circular cross-sectional shape) as long as it satisfies the above particle size range. 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 QDs may be composed of a semiconductor material, and may be inorganic semiconductor nanocrystals. The semiconductor material may have a certain band gap and may be a material that generates electroluminescence. The wavelength range of the electroluminescence may be any of the red, green, and blue ranges.

[0030] The quantum dots (QDs) may include at least one of a crystal of a II-VI group semiconductor such as MgS, MgSe, MgTe, CaS, CaSe, CaTe, SrS, SrSe, SrTe, BaS, BaSe, BaTe, ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe, or HgTe, a crystal of a III-V group semiconductor such as GaAs, GaP, InN, InAs, InP, or InSb, or a crystal of a group IV semiconductor such as Si or Ge.

[0031] A quantum dot layer including an inorganic matrix material MX can be used as a light-emitting layer of a light-emitting device. The inorganic matrix material MX, which is primarily composed of an inorganic material (inorganic substance), may be a component of a light-emitting layer including a plurality of quantum dots QDs. The inorganic matrix material MX may be a component composed of an inorganic substance (e.g., an inorganic semiconductor) that contains and holds other substances, and may be referred to as a substrate, a base material, or a filler. The inorganic matrix material MX may be solid at room temperature. The inorganic matrix material MX may encapsulate a plurality of quantum dots QDs. The inorganic matrix material MX may fill regions (spaces) other than the plurality of quantum dots QDs in the light-emitting layer (quantum dot layer), or may fill spaces between the plurality of quantum dots QDs. The inorganic matrix material MX may fill regions (spaces) other than the plurality of quantum dots QDs in the light-emitting layer. The inorganic matrix material MX may partially or completely fill spaces between the plurality of quantum dots QDs. The plurality of quantum dots QDs may be embedded in the inorganic matrix material MX at intervals. The inorganic matrix material MX may refer to a portion of the light-emitting layer excluding the plurality of quantum dots QD. The inorganic matrix material MX may completely or incompletely fill the region (space) other than the quantum dot group in the light-emitting layer. Here, three or more quantum dots QD are collectively referred to as a quantum dot group. The structure of the inorganic matrix material MX only needs to be such that the above-described configuration can be seen in a width of about 100 nm in a cross-sectional observation of the light-emitting layer, and it is not necessary that the above-described configuration be observed throughout the entire light-emitting layer. The inorganic matrix material MX in the quantum dot layer is formed within a 1000 nm width along a plane direction perpendicular to the layer thickness direction. ２The inorganic matrix material MX may be formed as a continuous film having an area of ​​1000 nm or more. A continuous film means a film that is not divided by materials other than the material that constitutes the continuous film in one plane. The continuous film may be an integrated film that is connected without interruption by chemical bonds of the inorganic matrix material MX.

[0032] The outer periphery of the light-emitting layer may be made of an inorganic matrix material MX, and the quantum dots QD may be positioned away from the outer periphery. The outer periphery of the light-emitting layer does not have to be made of the inorganic matrix material MX alone, and some of the quantum dots QD may be exposed from the inorganic matrix material MX.

[0033] The inorganic matrix material MX may be the same material as the shell contained in each of the multiple quantum dots QDs. It is desirable that the inorganic matrix material MX has a wider band gap than the core of the quantum dot QDs. When the shell of the quantum dot QD and the inorganic matrix material MX are indistinguishable, the shell may be considered as part of the inorganic matrix material MX. The inorganic matrix material MX may contain a substance different from the main material (e.g., ZnS) as an additive, impurity, or residue. When an emitting layer containing the inorganic matrix material MX is analyzed, the carbon atom content may be 5 atomic % or less.

[0034] 9 is a schematic diagram illustrating another method for forming a quantum dot layer. The method shown in FIG. 9 includes the steps of forming a coating film 25 containing an organic ligand, quantum dots QDs, and a first solvent SA (nonpolar solvent), and then applying a second solution containing a metal xanthate (precursor J) and a second solvent SB (polar solvent) onto the coating film 25. The coating film 25 and the second solution are then heated and / or irradiated with light to form a thin film 35 comprising the coating film 25 and the second solution. This forms an inorganic matrix material MX, which is a metal sulfide (e.g., ZnS) that encapsulates a plurality of quantum dots QDs (filling the spaces between the quantum dots QDs). This results in a quantum dot layer 30 containing a plurality of quantum dots QDs and the inorganic matrix material MX filling the spaces between the quantum dots QDs. In the thin film 35, the ligand of the quantum dots QDs is exchanged from the organic ligand L0 to xanthate (metal xanthate ligand L2). The released organic ligands L0 can be almost completely removed by dropping a predetermined organic solvent onto the thin film 35 or the quantum dot layer and rinsing it off before or after the treatment of the thin film 35 (which may include at least one of heating and light irradiation).

[0035] 10 is a table showing the results of comparing the structure, PLQY (photoluminescence quantum efficiency), and PL lifetime (photoluminescence lifetime) of the quantum dot layer of the comparative example and the quantum dot layer of the embodiment. The quantum dot layer of the embodiment was obtained by applying a solution containing quantum dots QDs modified with xanthogenic acid and zinc xanthogenate (precursor J) to glass and heating it at 185°C. From FIG. 10, it can be seen that the quantum dot layer of the embodiment in which the quantum dots QDs are encapsulated in an inorganic matrix material MX (PLQY 53%, PL lifetime 32 [ns]) is superior in both PLQY and PL lifetime to the quantum dot layer of the comparative example in which organic ligands are arranged around the quantum dots (PLQY 50%, PL lifetime 24 [ns]).

[0036] 11A is a graph showing the results of atmospheric light exposure tests of a quantum dot layer of a comparative example and a quantum dot layer of an embodiment. The quantum dot layer of the comparative example has an organic ligand disposed around the quantum dots. The light is yellow light. FIG. 11A shows that the quantum dot layer of the embodiment, which forms an inorganic matrix material (ZnS), is less susceptible to deterioration in PL lifetime after light exposure in the atmosphere (has improved atmospheric exposure resistance) than the comparative example.

[0037] 11B is a graph showing the results of a heating test in nitrogen for a quantum dot layer of a comparative example and a quantum dot layer of an embodiment. The quantum dot layer of the comparative example has a configuration in which organic ligands are arranged around the quantum dots. From FIG. 11B, it can be seen that the quantum dot layer of the embodiment is less likely to decrease in PL lifetime after heating (has improved heat resistance) than the comparative example.

[0038] FIG. 12 is a schematic diagram illustrating an example of a method for preparing a quantum dot solution. A solution (1 mg / mL) of red-emitting quantum dots (core: InP, shell: ZnS) modified with nonpolar organic ligands dispersed in a nonpolar solvent (first solvent: e.g., hexane) was mixed with a solution (0.1 mol / L) of zinc ethylxanthate (xanthogenic acid source) dissolved in a polar solvent (second solvent: e.g., N-methylformamide (NMF)), and a solution (0.2 mol / L) of zinc chloride (halogen source) dissolved in a polar solvent (second solvent). The mixture was vigorously stirred for 24 hours while the upper hexane layer and the lower NMF layer were separated. The halogen source is, for example, a halogen compound, which may be a metal halide or ammonium halide. The xanthogenic acid source and halogen source may contain the same metal element. As a result, the quantum dots (QDs) migrated to the lower layer and were modified with ethylxanthogenic acid and chlorine ions (halogen). The upper hexane layer was then removed, and a medium-polarity solvent (third solvent: e.g., ethyl acetate) was added to the lower layer to precipitate it. While zinc chloride and zinc ethylxanthate did not precipitate, the quantum dots modified with xanthogenic acid and chlorine ions precipitated. By dispersing a predetermined amount of the precipitate (including the quantum dots modified with xanthogenic acid and chlorine ions) and a predetermined amount of zinc ethylxanthate (ZnS precursor) in a polar solvent (fourth solvent: e.g., DMF), the quantum dots and zinc ethylxanthate could be adjusted to the desired concentration.

[0039] The xanthogenic acid may have an alkyl chain having 1 to 5 carbon atoms. The xanthogenic acid may have an ether chain having 1 to 20 carbon atoms. In these cases, the solvent may be a polar solvent including at least one of formamide-based solvents, acetamide-based solvents, ester-based solvents, ketone-based solvents, sulfoxide solvents, ether-based solvents, thioether-based solvents, and nitrile-based solvents. The xanthogenic acid may have an alkyl chain having 6 or more carbon atoms, in which case the solvent may be a non-polar solvent.

[0040] The xanthogen acid source used in preparing the quantum dot solution may be a metal xanthogenate, such as zinc ethylxanthogenate or potassium ethylxanthogenate.

[0041] As shown in FIG. 12, the quantum dot solution may include a halogen, and the quantum dots QDs may be modified with a dithiocarboxylic acid (e.g., xanthic acid) and a halogen.

[0042] Figure 13 is a graph showing the relationship between the molar ratio of potassium ethylxanthate to zinc chloride (chloride ion source) in the solution used to prepare the quantum dot solution and PLQY. Figure 14 is a schematic diagram showing the modification state of the quantum dots. Figure 13 shows that as the molar ratio of potassium xanthate increases from 0% to 50%, the quantum dots change from xanthate-modified QDx to xanthate- and halogen-modified QDw, and then to halogen-modified QDh, and that the PLQY of the xanthate- and halogen-modified QDw is higher (than that of QDx and QDh).

[0043] We also found that xanthogenic acid coordinates to the surface of quantum dots (QDs) to enhance PL intensity. Furthermore, we found that xanthogenic acid is almost completely decomposed by heating, without adversely affecting photoluminescence or electroluminescence. We also found that coordinating halogens (chloride ions) with xanthogenic acid to quantum dots (QDs) (QDw in Figure 14) improves their dispersibility in polar solvents.

[0044] Fig. 15 is a cross-sectional schematic diagram showing a configuration example of the light-emitting element of this embodiment. Fig. 16 is a cross-sectional photograph showing a configuration example of the light-emitting element of this embodiment. As shown in Figs. 15 and 16, the light-emitting element 5 includes a first electrode D1 (anode), a hole injection layer 28, a hole transport layer 29, a light-emitting layer 30, an electron transport layer 31, and a second electrode D2 (cathode) in this order.

[0045] The light-emitting element 5 can be formed, for example, as follows: dissolving zinc ethylxanthate in a DMF solvent to prepare a 0.04 mol / L (12 mg / mL) solution, and using this solution to prepare a quantum dot solution containing 15 mg / mL of quantum dots QDs modified with ethylxanthate and halogen (chloride ions) (a DMF solution containing 15 mg / mL of quantum dots modified with chloride ions and xanthate and 0.04 mol / L (12 mg / mL) of zinc xanthate); forming a first electrode D1 using ITO (indium tin oxide); and coating and forming a hole injection layer 28 on the first electrode D1 using NiO nanoparticles (15 mg / mL of NiO nanoparticles are coated on the ITO anode in nitrogen). at 2000 rpm), a step of forming a hole transport layer 29 on the hole injection layer 28 using, for example, P-TPD (which may include a step of applying P-TPD dissolved in chlorobenzene at a concentration of 8 mg / ml at 1500 rpm), a step of applying the quantum dot solution prepared in the above step onto the hole transport layer 29 by spin coating (e.g., at 2000 rpm) and then heating the coating solution at 150°C for 30 minutes to form a light-emitting layer 30 (quantum dot layer), a step of applying an electron transport layer 31 using ZnMgO (which may include a step of applying ZnMgO nanoparticles), and a step of forming a second electrode D2 using Ag (which may include a silver vapor deposition step). The quantum dot solution may contain quantum dots QD at a concentration of 5 to 100 mg / ml and a metal complex (e.g., zinc ethylxanthate) at a concentration of 1 to 100 mg / ml. As a comparative example, a light-emitting device was fabricated that had a light-emitting layer obtained by heating a coating liquid of an octane solution containing 15 mg / ml of quantum dots modified with organic ligands at 90° C. The embodiment and the comparative example were the same except for the light-emitting layer, and commercially available red-emitting InP / ZnS was used as the quantum dots.

[0046] Fig. 17 is a graph showing the current density vs. external quantum efficiency characteristics of the light-emitting device. It can be seen that the maximum external quantum efficiency (EQE) is 6.5% in the embodiment and 6.4% in the comparative example. Fig. 18 is a graph showing the voltage (applied voltage) vs. current density characteristics of the light-emitting device. The current density when 6 V is applied is 29 mA / m in the embodiment.２ , 12mA / m in the comparative example ２ According to the embodiment, carrier injection into the light-emitting layer becomes easier, so that the voltage required for a desired light emission luminance (current density) can be reduced.

[0047] FIG. 19 shows the current density of 10 mA / m ２ 10 is a graph showing the EL spectrum in the embodiment, in which the sulfide medium suppresses Förster energy transfer between quantum dots, so that the peak wavelength is shifted to the shorter wavelength side than in the comparative example, and the same emission peak as in the solution is maintained.

[0048] FIG. 20A shows the current consumption of 25 mA / m ２ 20B is a graph showing the change in relative luminance and voltage over time when a constant current of 25 mA / m is continuously applied to the light-emitting element of the comparative example. ２ 1 is a graph showing the change in relative luminance and voltage over time when a constant current of 1000 kJ / s is continuously applied. When the luminance half-life is calculated by linearly approximating the luminance decay curve, the luminance half-life of the comparative example is 52 hours. The luminance half-life of the embodiment is 620 hours, which is a significant improvement over the comparative example. Furthermore, the voltage increase of the embodiment is much slower than that of the comparative example.

[0049] FIG. 21 is a schematic diagram showing a configuration example of a display device according to an embodiment. FIG. 22 is a cross-sectional view showing a configuration example of a display device according to an embodiment. As shown in FIG. 21 , a display device 50 includes a display unit DA including a plurality of subpixels SP, a first driver X1 and a second driver X2 that drive the plurality of subpixels SP, and a display controller DC that controls the first driver X1 and the second driver X2. Each subpixel SP includes a light-emitting element 5 and a pixel circuit PC connected to the light-emitting element 5. The pixel circuit PC may be connected to a scanning signal line GL, a data signal line DL, and an emission control line EL. The scanning signal line GL and the emission control line EL may be connected to a first driver X1, and the data signal line DL may be connected to a second driver X2.

[0050] The display device 50 may include a pixel circuit substrate 13 including a substrate 11 and a pixel circuit layer 12, a light emitting element layer 14, and a sealing layer 15. The substrate 11 may be a glass substrate, a resin substrate, or the like. The substrate 11 may be flexible. The pixel circuit layer 12 includes a plurality of pixel circuits PC arranged, for example, in an inorganic matrix. The pixel circuit PC may include a pixel capacitor to which a grayscale signal is written, a transistor that controls the current value of the light emitting element 5 in accordance with the grayscale signal, a transistor connected to a scanning signal line GL and a data signal line DL, and a transistor connected to a light emitting control line EL.

[0051] As shown in FIG. 22 , the display device 50 includes a pixel circuit substrate 13 and a light-emitting element layer 14. The light-emitting element layer 14 may include, in order from the pixel circuit substrate 13 side, a first electrode D1, an edge cover film JF covering the edge of the first electrode D1, a first functional layer FK, a light-emitting layer (quantum dot layer) 30, a second functional layer SK, and a second electrode D2. The first functional layer FK may have a hole injection function and a hole transport function, and the second functional layer SK may have an electron transport function. The light-emitting element layer 14 may include a light-emitting element 5R including a light-emitting layer 30R that emits red light, a light-emitting element 5G including a light-emitting layer 30G that emits green light, and a light-emitting element 5B including a light-emitting layer 30B that emits blue light. The sealing layer 15 includes an inorganic insulating film such as a silicon nitride film or a silicon oxide film, and prevents foreign substances (water, oxygen, etc.) from entering the light-emitting element layer 14.

[0052] Examples of materials that can be used for the first functional layer FK include organic materials such as poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4'-(N-4-sec-butylphenyl))diphenylamine)] (TFB), poly(4-butyltriphenylamine) (p-TPD), poly(9-vinylcarbazole) (PVK), [9,9'-[1,2-phenylenebis(methylene)]bis[N3,N3,N6,N6-tetrakis(4-methoxyphenyl)-9H-carbazole-3,6-diamine] (V886), and 7,7'-bi[1,4]benzoxazino[2,3,4-kl]phenoxazine (HN-D1), and inorganic materials such as NiO nanoparticles.

[0053] As the material for the second functional layer SK, organic materials such as (2,2',2''-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi), bathocuproine (BCP), and nanoparticles of organometallic complexes, and inorganic materials such as nanoparticles of n-type oxide semiconductors can be used. Examples of organometallic complexes include tris(8-quinolinol)aluminum complex (Alq3). Examples of n-type oxide semiconductors include metal oxides such as ZnO and ZnMgO.

[0054] In Fig. 22, the quantum dot layer 30 shown in Fig. 2 and other figures is used as a light-emitting layer, but is not limited to this. The quantum dot layer 30 shown in Fig. 2 and other figures can also be used as a wavelength conversion layer or a photosensor layer. A power generating element can also be configured with the quantum dot layer 30 between a pair of electrodes. For example, holes and electrons may be generated in the quantum dots QDs from light incident on the quantum dot layer 30, and then transported to the electrodes to generate an electromotive force.

[0055] The above-described embodiments are intended to be illustrative and explanatory, and not limiting, and many variations will be apparent to those skilled in the art based on these examples and descriptions.

[0056] 2 Core 3 Shell 5 Light-emitting element 10 Quantum dot solution 20 Coating liquid 28 Hole injection layer 29 Hole transport layer 30 Light-emitting layer (quantum dot layer) 31 Electron transport layer J Precursor (metal complex) Y Solvent QD Quantum dot L1 Ligand of quantum dot L2 Ligand of metal complex MX Inorganic matrix material UL Underlayer

Claims

1. 1. A quantum dot solution comprising luminescent quantum dots, a metal sulfide precursor, and a solvent, The precursor is a metal complex that has a weight loss rate of 60% to 90% when heated from 50°C to 200°C, and after heating the metal complex to 200°C, the solid contains 80% by weight or more of an inorganic compound, and 50% by weight or more of the inorganic compound is the metal sulfide. A quantum dot solution.

2. The quantum dot solution described in claim 1, wherein the quantum dot and the metal complex each have a ligand of the same structure.

3. The quantum dot solution according to claim 2 , wherein the ligand of the quantum dot is coordinated to the quantum dot in an amount of 0.1% by weight or more.

4. The metal complex has a ligand, 4. The quantum dot solution of claim 1 or 3, wherein the ligand is a dithiocarboxylic acid.

5. The metal complex has a ligand, 4. The quantum dot solution of claim 1 or 3, wherein the ligand is a dialkylthiourea.

6. 5. The quantum dot solution of claim 4, wherein the dithiocarboxylic acid is xanthic acid.

7. The quantum dot solution according to any one of claims 1 to 3, wherein the metal complex contains a zinc element.

8. The quantum dot solution according to any one of claims 1 to 3, comprising a halogen.

9. The quantum dot solution of claim 2, wherein the ligand of the same structure of the quantum dot and the metal complex is a dithiocarboxylic acid.

10. 5. The quantum dot solution of claim 4, wherein the quantum dots are modified with a dithiocarboxylic acid and a halogen.

11. The quantum dot solution according to claim 6, wherein the xanthogenic acid has an alkyl chain having 1 to 5 carbon atoms.

12. The quantum dot solution according to claim 6, wherein the xanthogenic acid has an ether chain having 1 to 20 carbon atoms.

13. 12. The quantum dot solution of claim 11, wherein the solvent is a polar solvent including at least one of a formamide-based solvent, an acetamide-based solvent, an ester-based solvent, a ketone-based solvent, a sulfoxide solvent, an ether-based solvent, a thioether-based solvent, and a nitrile-based solvent.

14. The quantum dot solution of claim 6 , wherein the xanthogenic acid has an alkyl chain having six or more carbon atoms.

15. 15. The quantum dot solution of claim 14, wherein the solvent is a non-polar solvent.

16. The quantum dot solution according to any one of claims 1 to 3 and 9, wherein the quantum dots have a shell made of the metal sulfide.

17. The metal complex has a Fourier transform infrared absorption spectrum of 1040 cm －１ , 1122 cm －１ , and 1217 cm －１ The quantum dot solution according to any one of claims 1 to 3 and 9, having an absorption peak at

18. In the Fourier transform infrared absorption spectrum, the quantum dot is surrounded by a region having a wavelength of 1040 cm －１ , 1122 cm －１ , and 1217 cm －１ The quantum dot solution according to any one of claims 1 to 3 and 9, comprising a ligand having an absorption peak at

19. In the Fourier transform infrared absorption spectrum, －１ , 1122 cm －１ , and 1217 cm －１ The quantum dot solution according to any one of claims 1 to 3 and 9, comprising a compound having an absorption peak at 200°C and having an absorption peak intensity of less than 20% when heated to 200°C.

20. The quantum dot solution according to any one of claims 1 to 3 and 9, comprising the quantum dots at a concentration of 5 to 100 mg / ml and the metal complex at a concentration of 1 to 100 mg / ml.

21. A step of applying a quantum dot solution according to any one of claims 1 to 3 and 9 to an underlayer; and heating the applied quantum dot solution at 100° C. or more and 600° C. or less.

22. A step of applying the quantum dot solution according to any one of claims 1 to 3 and 9 to an underlayer including an organic layer; and heating the applied quantum dot solution at 100° C. or more and 250° C. or less.

23. A step of applying a quantum dot solution according to any one of claims 1 to 3 and 9 to an underlayer; and irradiating the applied quantum dot solution with light having a wavelength of 200 nm to 400 nm.

24. A step of applying a quantum dot solution according to any one of claims 1 to 3 and 9 to an underlayer; and irradiating the applied quantum dot solution with laser light having a wavelength of 400 nm to 2000 nm.

25. forming a coating comprising organic ligands and quantum dots and a first solvent; and applying a second solution containing a metal xanthate and a second solvent onto the coating film.

26. forming an anode; forming a quantum dot layer as a light-emitting layer by using the method for forming a quantum dot layer according to claim 25; and forming a cathode.

27. an anode and a cathode, and a light-emitting layer located between the anode and the cathode; The light-emitting layer includes a plurality of light-emitting quantum dots, a metal sulfide containing the plurality of quantum dots, and S ２ and a compound having a C═O bond.

28. an anode and a cathode, and a light-emitting layer located between the anode and the cathode; The light-emitting layer includes a plurality of luminescent quantum dots, a metal sulfide containing the plurality of quantum dots, and a Fourier transform infrared absorption spectrum having a wavelength of 1650 cm －１ and a compound having a peak at .

29. 29. The light-emitting device according to claim 27 or 28, wherein the metal sulfide is zinc sulfide.

30. A display device comprising the light-emitting element according to claim 27 or 28.

31. A method for producing a quantum dot solution, comprising a step of mixing and stirring a first solution containing an organic ligand, quantum dots, and a first solvent with a second solution containing a metal xanthate, a halide compound, and a second solvent.

32. The halogen compound is a metal halide or an ammonium halide, 32. The method for producing a quantum dot solution according to claim 31, further comprising a step of dispersing a precipitate obtained by adding a third solvent after stirring the first solution and the second solution, and a metal xanthate in a fourth solvent.

33. 33. The method of claim 32, wherein the precipitate comprises quantum dots modified with xanthic acid and a halogen.

34. 33. The method of claim 32, wherein the metal xanthate and the metal halide comprise the same metal element.

35. The method for producing a quantum dot solution according to any one of claims 32 to 34, wherein the metal xanthate contains a zinc element.

36. 33. The method of claim 32, wherein the first solvent is a non-polar solvent, the second solvent is a polar solvent, the third solvent is a medium polar solvent, and the fourth solvent is a polar solvent.

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