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

Abstract

A quantum dot solution containing luminescent quantum dots, a precursor of a metal sulfide, and a solvent, the precursor being a metal complex having a weight loss rate of 60-90% when heated from 50 DEG C to 200 DEG C, and the solid obtained by heating the metal complex to 200 DEG C containing 80 wt% or more of an inorganic compound. 50 wt% or more of the inorganic compound is the metal sulfide.

Description

Technical Field

[0001] The present disclosure relates to a quantum dot solution and the like. Background Art

[0002] In Patent Document 1, a method for manufacturing a quantum dot-containing coating film using a composition containing a quantum dot, a substrate component, and a solvent is disclosed. Among them, the quantum dot contains a chalcogenide as a surface material. Prior Art Documents Patent Documents

[0003] Patent Document 1: Japanese Unexamined Patent Publication "Japanese Patent Application Laid-Open No. 2020-101769" Summary of the Invention Problems to be Solved by the Invention

[0004] There is a problem of low durability in the quantum dot layer modified with an organic ligand. In addition, there is a problem of low carrier injectability in the quantum dot layer covered with an inorganic substance formed by an existing method. Solutions to the Problems

[0005] The quantum dot solution according to one aspect of the present disclosure is a quantum dot solution containing a luminescent quantum dot, a precursor of a metal sulfide, and a solvent. The precursor is a metal complex having a weight reduction rate of 60% to 90% when heated from 50°C to 200°C. The solid obtained by 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. Effects of the Invention

[0006] By using the quantum dot solution according to one aspect of the present disclosure, it is possible to improve the durability while maintaining the carrier injectability of the quantum dot layer. Brief Description of the Drawings

[0007] Figure 1 It is a flowchart showing a method for forming a quantum dot layer according to an embodiment. Figure 2 It is a schematic diagram showing a method for forming a quantum dot layer according to an embodiment. Figure 3 It is an example of the structural formula of a ligand and a metal complex. Figure 4 It is a graph showing the thermogravimetric characteristics of a metal complex. Figure 5 It is a graph showing the X-ray diffraction distribution maps before and after heating of zinc xanthate and the X-ray diffraction distribution map of zinc sulfide. Figure 6AIt is a chart showing the results of Fourier transform infrared spectroscopy (FTIR) measurement after heating zinc xanthate. Figure 6B It is a chart showing the results of Fourier transform infrared spectroscopy (FTIR) measurement after heating zinc xanthate. Figure 6C It is a chart showing the results of Fourier transform infrared spectroscopy (FTIR) measurement after heating zinc xanthate. Figure 6D A figure showing the results of Fourier transform infrared spectroscopy (FTIR) measurement after heating zinc xanthate. Figure 6E It is a chart showing the results of Fourier transform infrared spectroscopy (FTIR) measurement after heating zinc xanthate. Figure 7A It is an enlarged view showing Figure 6E a part of Figure 7B A figure showing the results of Fourier transform infrared spectroscopy (FTIR) measurement of an organic ligand. Figure 8A The ultraviolet-visible absorption spectrum (UV-VIS) of a thin film obtained by heating a coating solution of zinc xanthate. Figure 8B It is a Tauc distribution chart of a thin film obtained by heating a coating solution of zinc xanthate. Figure 9 It is a schematic diagram showing another method for forming a quantum dot layer. Figure 10 It is a table showing the results of comparing the quantum dot layer of a comparative example and the quantum dot layer of an embodiment in terms of structure, PLQY (photoluminescence quantum yield), and PL lifetime (photoluminescence lifetime). Figure 11A A figure showing the results of an atmospheric light exposure test of the quantum dot layer of a comparative example and the results of an atmospheric light exposure test of the quantum dot layer of an embodiment. Figure 11B It is a graph showing the results of a heating test in nitrogen of the quantum dot layer of a comparative example and the results of a heating test in nitrogen of the quantum dot layer of an embodiment. Figure 12 It is a schematic diagram showing an example of a method for preparing a quantum dot solution. Figure 13 A figure showing the relationship between the molar ratio of potassium xanthate to zinc chloride (chloride ion source) in the solution used in the preparation of the quantum dot solution and PLQY. Figure 14 It is a schematic diagram showing the modification state of quantum dots. Figure 15It is a cross-sectional schematic diagram showing a structural example of the light-emitting element of the present embodiment. Figure 16 It is a cross-sectional photograph showing a structural example of the light-emitting element of the present embodiment. Figure 17 It is a graph showing the current density-external quantum efficiency characteristics of the light-emitting element. Figure 18 It is a graph showing the voltage-current density characteristics of the light-emitting element. Figure 19 It is a graph showing the EL spectra when the current density of the embodiment and the comparative example is 10 mA / m 2 in. Figure 20A It is a graph showing the temporal changes in relative luminance and voltage when a constant current of 25 mA / m 2 flows continuously through the light-emitting element of the embodiment. Figure 20B It is a graph showing the temporal changes in relative luminance and voltage when a constant current of 25 mA / m 2 flows continuously through the light-emitting element of the comparative example. Figure 21 It is a schematic diagram showing a structural example of the display device according to the embodiment. Figure 22 It is a cross-sectional view showing a structural example of the display device according to the embodiment. Detailed Embodiment

[0008] Figure 1 It is a flowchart showing a method for forming a quantum dot layer according to the embodiment. Figure 2 It is a schematic diagram showing a method for forming a quantum dot layer according to the embodiment. As Figure 1 and Figure 2 shown, the method for forming a quantum dot layer according to the embodiment includes: a step S10 of preparing a quantum dot solution 10 containing luminescent quantum dots QD and a precursor J of a metal sulfide; a step S20 of forming a coating liquid 20 of the quantum dot solution 10 on a base layer UL; a step S30 of drying the coating liquid 20; and a step S40 of forming an inorganic base material MX, which is a metal sulfide A and encapsulates a plurality of quantum dots QD (filled between the plurality of quantum dots QD), by modifying the precursor J of the coating liquid 20. Thus, a quantum dot layer (30) containing a plurality of quantum dots (QD) and an inorganic base material (MX) filled between the plurality of quantum dots (QD) is formed.

[0009] In step S40, by performing at least one of heating and light irradiation on the coating solution 20, the precursor J can be decomposed and crystallized. In step S40, the coating solution 20 of the quantum dot solution 10 coated on the base layer UL can also be heated at 100°C or higher and 600°C or lower. In step S40, the coating solution 20 of the quantum dot solution 10 coated on the base layer UL including an organic layer can also be heated at 100°C or higher and 250°C or lower.

[0010] In step S40, light having a wavelength of 200 nm to 400 nm can also be irradiated on the coating solution 20 of the quantum dot solution 10 coated on the base layer UL. In step S40, laser light having a wavelength of 400 nm to 2000 nm can also be irradiated on the coating solution 20 of the quantum dot solution 10 coated on the base layer UL.

[0011] The quantum dot solution 10 contains luminescent quantum dots QD, a precursor J of a metal sulfide A, and a solvent Y. The precursor J is a metal complex having a weight reduction rate of 60% to 90% when heated from 50°C to 200°C. The solid obtained by heating the 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 the metal sulfide A. The weight reduction rate is a graph showing the ratio of the weight reduction amount from 50°C to 200°C with respect to the weight at 50°C, expressed as a percentage.

[0012] By using the precursor J having a weight reduction rate of 60% to 90% when heated from 50°C to 200°C, compared with the case of using a precursor having a weight reduction rate of less than 60% when heated from 50°C to 200°C, an inorganic base material MX with less residue (e.g., organic matter) can be formed in a low-temperature process. Since there is less residue in the inorganic base material MX, when the quantum dot layer 30 is applied to a light-emitting element, the durability can be improved while maintaining the carrier injection property of the quantum dot layer 30. Also, a reduction in driving voltage and an improvement in light transmittance are achieved. By being a low-temperature process, for example, even when the base layer UL is an organic carrier transport layer, it is difficult to cause thermal damage to the carrier transport layer. In addition, the quantum dot layer 30 can be formed on a substrate with low heat resistance (e.g., a flexible substrate).

[0013] The quantum dot solution 10 contains a ligand L1 of the quantum dots QD, and the ligand L1 of the quantum dots QD and the ligand (ligand) L2 of the metal complex (precursor J) can have the same structure (the relationship shown by the same structural formula). Through the ligand L1, the dispersibility of the quantum dots QD in the quantum dot solution 10 is improved. By the ligand L1 and the ligand L2 having the same structure, the denaturation (decomposition and crystallization) of the precursor J proceeds rapidly. In addition, the preparation of the quantum dot solution 10 becomes easy. The ligand L1 can be coordinated to the quantum dots QD at 0.1% by weight or more.

[0014] The quantum dot QD has a core 2 and a shell 3, and the constituent material of the shell 3 can be a metal sulfide A. In this case, the ligand L1 easily coordinates with the quantum dot QD, and the dispersibility of the quantum dot QD in the quantum dot solution 10 is improved.

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

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

[0017] Figure 4 This is a graph showing the thermogravimetric characteristics of a metal complex. The normalized weight on the vertical axis means: taking the weight at 50°C as the reference value (100%), and expressing the weight at 50°C and above as a percentage relative to the reference value. Here, using a thermogravimetric analysis (TGA) apparatus, while heating four powders (a mixture of zinc xanthate, zinc acetate, and ammonium thiocyanate, zinc alkylthiourea, and zinc thiourea) in nitrogen, the weight change is measured.

[0018] From Figure 4 it can be seen that as the precursor J of zinc sulfide with a weight reduction rate of 60% to 90% when heated from 50°C to 200°C, zinc xanthate (the normalized weight at 200°C is 35%, and the weight reduction rate is 65%) and zinc dialkylthiourea (the normalized weight at 200°C is 30%, and the weight reduction rate is 70%) are suitable. It can also be seen that if the weight reduction rate when heated from 50°C to 200°C is less than 60%, the mixture of zinc acetate and ammonium thiocyanate (the normalized weight at 200°C is 85%, and the weight reduction rate is 15%) is not suitable as the precursor J.

[0019] Zinc xanthate can be prepared, for example, as follows. A mixed solution of an aqueous zinc chloride solution and an aqueous potassium xanthate solution (molar amount of zinc chloride: molar amount of potassium xanthate = 1:2.2) is stirred for 24 hours, and the precipitate formed after stirring 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 dried in nitrogen to obtain zinc xanthate (precursor J). In addition, a solution prepared by dissolving zinc acetate and ammonium thiocyanate in 2-methoxyethanol as a solvent (molar ratio of zinc acetate: molar ratio of ammonium thiocyanate = 1:1) is stirred for 1 hour, and a mixture of zinc acetate and ammonium thiocyanate is obtained by removing the solvent.

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

[0021] Figure 5 It is a graph showing the X-ray diffraction before and after heating of zinc xanthate and the X-ray diffraction graph of zinc sulfide. In the graph after heating (when zinc xanthate is heated to 150 °C), characteristic peaks appear in zinc sulfide, so it can be known that precursor J is modified (decomposed, crystallized) into zinc sulfide.

[0022] Figures 6A - 6E It is a graph showing the results of Fourier transform infrared spectroscopy (FTIR) measurement after heating zinc xanthate. In addition, the horizontal axis is the wavenumber, the vertical axis is the transmittance, the unit of the wavenumber is [cm -1 , and the unit of the transmittance is [%]. Figure 6A is after heating at 100 °C, Figure 6B is after heating at 125 °C, Figure 6C is after heating at 150 °C, Figure 6D is after heating at 175 °C, Figure 6E is the measurement result after heating at 200 °C. Figure 7A is a graph showing an enlarged part of Figure 6E . Figure 7B is a graph showing the results of Fourier transform infrared spectroscopy (FTIR) measurement of the organic ligand.

[0023] It can be seen from Figure 6A that zinc xanthate as a metal complex has absorption peaks at 1040 cm -1 , 1122 cm -1 and 1217 cm -1 in the absorption spectrum of Fourier transform infrared spectroscopy. Therefore, it can be said that the quantum dot solution has absorption peaks at 1040 cm -1 , 1122 cm -1and 1217 cm -1 a ligand having an absorption peak.

[0024] In addition, the quantum dot solution contains, in the absorption spectrum of Fourier transform infrared spectroscopy, at 1040 cm -1 , 1122 cm -1 and 1217 cm -1 having an absorption peak, and a compound having an absorption peak intensity of less than 20% by heating to 200 °C.

[0025] When zinc xanthate is heated to 125 °C or higher, it exhibits Figure 7B a characteristic absorption spectrum that cannot be seen in the organic ligand shown. That is, it can be seen that if zinc xanthate is heated, the peaks near the wave numbers 1000 - 1300 [cm -1 , 3000 [cm -1 disappear, but a peak appears at the wave number 1650 [cm -1 . As Figure 7A shown, it is considered that this peak originates from the S 2 C=O bond.

[0026] That is, the quantum dot layer (light-emitting layer) obtained by heating the coating liquid of the quantum dot solution may include a plurality of luminescent quantum dots QD, a metal sulfide (for example, an inorganic base material mainly composed of ZnS) encapsulating a plurality of quantum dots QD, and a compound having an S 2 C=O bond. In addition, the quantum dot layer (light-emitting layer) may include a plurality of luminescent quantum dots QD, a metal sulfide (for example, an inorganic base material mainly composed of ZnS) encapsulating a plurality of quantum dots QD, and a compound having a peak at 1650 cm -1 in the absorption spectrum of Fourier transform infrared spectroscopy.

[0027] Figure 8A is the ultraviolet-visible absorption spectrum (UV-VIS) of the film obtained by heating the coating liquid of zinc xanthate. Specifically, zinc xanthate is dissolved in DMF (N,N-dimethylformamide) at a concentration of 0.2 mol / l, the solution is coated on glass at 2000 rpm, heated at each temperature for 30 minutes, and the ultraviolet-visible absorption spectrum of the film formed at each temperature is measured. From Figure 8A it can be seen that the absorption peak of zinc xanthate at 305 nm disappears, and zinc xanthate is decomposed at 150 °C.

[0028] Figure 8BIt is a Tauc distribution chart of a thin film obtained by heating a coating solution of zinc xanthate. The UV-VIS spectrum of the thin film obtained by heating the coating solution of zinc xanthate at 200 °C is plotted as Tauc, and the band gap is calculated. h is a standard constant, V is the vibration number of light, and α is the absorbance. The band gap of the obtained thin film is 3.65 eV. Since the band gap of commonly known zinc sulfide is 3.6 - 3.7 eV, it can be known that zinc sulfide with high transparency is formed.

[0029] The quantum dot QD used in this embodiment can be fine particles having a particle diameter (outer diameter) of 1.0 [nm] to 100 [nm], and the shape can be spherical or non-spherical. The shape of the quantum dot QD only needs to satisfy the above particle diameter range, and is not limited to a spherical three-dimensional shape (circular cross-sectional shape). For example, it can be a cross-sectional shape of a polygon, a three-dimensional shape of a rod, a three-dimensional shape of a branch, a three-dimensional shape having irregularities on the surface, or a combination of these shapes. The quantum dot QD can be composed of a semiconductor material or can be an inorganic semiconductor nanocrystal. The semiconductor material can have a certain band gap or can be a material that generates electroluminescence. The wavelength region of electroluminescence can be any one of the red region, the green region, and the blue region.

[0030] The quantum dot QD can also contain at least one of crystals of II-VI group semiconductors such as MgS, MgSe, MgTe, CaS, CaSe, CaTe, SrS, SrSe, SrTe, BaS, BaSe, BaTe, ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe, crystals of III-V group semiconductors such as GaAs, GaP, InN, InAs, InP, InSb, and crystals of IV group semiconductors such as Si, Ge.

[0031] The quantum dot layer containing the inorganic substrate material MX can be used as the light-emitting layer of a light-emitting element. The inorganic substrate material MX mainly composed of an inorganic material (inorganic substance) can be a constituent element of a light-emitting layer including a plurality of quantum dots QD. The inorganic substrate material MX can be a component made of an inorganic substance (e.g., inorganic semiconductor), containing other substances and holding them, and can be called a base material, a mother material, or a filling material. The inorganic substrate material MX can be solid at room temperature. The inorganic substrate material MX can enclose a plurality of quantum dots QD. The inorganic substrate material MX can fill the regions (spaces) other than the plurality of quantum dots QD in the light-emitting layer (quantum dot layer), or can fill the spaces between the plurality of quantum dots QD. The inorganic substrate material MX can fill the regions (spaces) other than the plurality of quantum dots QD in the light-emitting layer. The inorganic substrate material MX can be locally or completely filled between the plurality of quantum dots QD. The plurality of quantum dots QD can also be buried in the inorganic substrate material MX at intervals. The inorganic substrate material MX can show a part where the plurality of quantum dots QD are removed in the light-emitting layer. The inorganic substrate material MX can completely or incompletely fill the regions (spaces) 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. In the cross-sectional observation of the light-emitting layer, the structure of the inorganic substrate material MX only needs to be the above structure with a width of about 100 nm, and it is not necessary to observe the above structure in the entire light-emitting layer. The inorganic substrate material MX in the quantum dot layer can also be formed into a continuous film having an area of 1000 nm 2 or more in the plane direction orthogonal to the layer thickness direction. A continuous film means a film that is not interrupted by materials other than the materials constituting the continuous film in a plane. The continuous film can also be an integral film-like structure that is continuously connected through chemical bonding of the inorganic matrix material MX.

[0032] The outer periphery of the light-emitting layer can also be composed of the inorganic substrate material MX, and is configured such that the quantum dots QD are located at positions away from the outer edge. The outer edge of the light-emitting layer does not need to be formed only by the inorganic substrate material MX, and a part of the quantum dots QD can also protrude from the inorganic substrate material MX.

[0033] The inorganic substrate material MX can also be the same material as the shell contained in each of the plurality of quantum dots QD. It is preferable that the band gap of the inorganic substrate material MX is wider than the core of the quantum dot QD. In the case where the shell of the quantum dot QD and the inorganic substrate material MX cannot be distinguished, the shell can also be regarded as a part of the inorganic substrate material MX. The inorganic substrate material MX can also include substances different from the main material (e.g., ZnS) as additives, impurities, or residues. When analyzing the light-emitting layer including the inorganic substrate material MX, the carbon atoms contained can be 5 atomic% or less.

[0034] Figure 9 is a schematic diagram showing another method for forming the quantum dot layer. InFigure 9 In the formation method shown, after performing the step of forming a coating film 25 including an organic ligand, quantum dots QD, and a first solvent SA (non-polar solvent), and the step of coating a second solution containing a metal xanthate (precursor J) and a second solvent SB (polar solvent) on the coating film 25, by performing at least one of heating and light irradiation on the thin film 35 composed of the coating film 25 and the second solution, an inorganic base material MX that encapsulates a plurality of quantum dots QD (filled between the plurality of quantum dots QD) and is a metal sulfide (e.g., ZnS) is formed. Thus, a quantum dot layer 30 including a plurality of quantum dots QD and an inorganic base material MX filled between the plurality of quantum dots QD is formed. In the thin film 35, the ligand of the quantum dots QD is replaced from the organic ligand L0 to xanthic acid (ligand L2 of the metal xanthate). At this time, the free organic ligand L0 is almost completely removed by rinsing by dropping a prescribed organic solvent onto the thin film 35 or the quantum dot layer before and after the treatment (including at least one of heating and light irradiation) of the thin film 35.

[0035] Figure 10 It is a table showing the results of comparing the quantum dot layers of the comparative example and the quantum dot layers of the embodiment in terms of structure, PLQY (photoluminescence quantum efficiency), and PL lifetime. The quantum dot layer of the embodiment was obtained by coating a solution containing quantum dots QD modified with xanthic acid and zinc xanthate (precursor J) on glass and heating at 185 °C. From Figure 10 It can be seen that the quantum dot layer of the embodiment in which the inorganic base material MX encapsulates the quantum dots QD (PLQY is 53%, PL lifetime is 32 [ns]) is superior in PLQY and PL lifetime respectively compared to the quantum dot layer of the comparative example in which an organic ligand is disposed around the quantum dots (PLQY is 50%, PL lifetime is 24 [ns]).

[0036] Figure 11A It is a graph showing the results of the light exposure test in the air of the quantum dot layer of the comparative example and the results of the light exposure test in the air of the quantum dot layer of the embodiment. The quantum dot layer of the comparative example has a structure in which an organic ligand is disposed around the quantum dots. The light is yellow. From Figure 11A It can be seen that the quantum dot layer of the embodiment in which the inorganic base material (ZnS) is formed is less likely to have its PL lifetime reduced after light exposure in the air (the air exposure resistance is improved) compared to the comparative example.

[0037] Figure 11B It is a graph showing the results of the heating test in nitrogen of the quantum dot layer of the comparative example and the results of the heating test in nitrogen of the quantum dot layer of the embodiment. The quantum dot layer of the comparative example has a structure in which an organic ligand is disposed around the quantum dots. From Figure 11BIt can be seen that in the quantum dot layer of the embodiment, compared with the comparative example, the PL lifetime after heating is not easily reduced (the heat resistance is improved).

[0038] Figure 12 Schematic diagram showing an example of a method for preparing a quantum dot solution. A solution (1 mg / mL) obtained by dispersing red light-emitting quantum dots (core: InP, shell: ZnS) modified with a non-polar organic ligand in a non-polar solvent (first solvent: e.g., hexane), a solution (0.1 mol / L) obtained by dissolving zinc ethylxanthate (xanthate source) in a polar solvent (second solvent: e.g., NMF: N-methylformamide), and a solution (0.2 mol / L) obtained by dissolving zinc chloride (halogen source) in a polar solvent (second solvent) are mixed, and the mixture is vigorously stirred for 24 hours in a state where the upper hexane layer and the lower NMF layer are separated. Here, the halogen source can be, for example, a halogen compound, and the halogen compound can be a metal halide or an ammonium halide. In addition, the xanthate source and the halogen source may contain the same metal element. Thereby, the quantum dots QD move to the lower layer and are modified with ethylxanthic acid and chloride ions (halogens). After that, the upper hexane layer is removed, and a medium-polarity solvent (third solvent: e.g., ethyl acetate) is added to the lower layer to precipitate the lower layer. At this time, since zinc chloride and zinc ethylxanthate do not precipitate, the quantum dots modified with xanthic acid and chloride ions precipitate. By dispersing a predetermined amount of the precipitate (including quantum dots modified with xanthic acid and chloride 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 can be adjusted to a desired concentration.

[0039] The xanthic acid may have an alkyl chain having 1 to 5 carbon atoms. The xanthic acid may also have an ether chain having 1 to 20 carbon atoms. In these cases, the solvent may also be a polar solvent containing 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. The xanthic acid may have an alkyl chain having 6 or more carbon atoms, and in this case, the solvent may be a non-polar solvent.

[0040] As the xanthate source used in the preparation of the quantum dot solution, a metal xanthate can be used, for example, zinc ethylxanthate or potassium ethylxanthate can be used.

[0041] As Figure 12 shown, the quantum dot solution may contain halogens, and the quantum dots QD may also be modified with dithiocarboxylic acid (e.g., xanthic acid) and halogens.

[0042] Figure 13 Graph showing the relationship between the molar ratio of potassium ethylxanthate to zinc chloride (chloride ion source) in the solution used in the preparation of the quantum dot solution and the PLQY. Figure 14It is a schematic diagram showing the modification state of quantum dots. From Figure 13 it can be seen that as the molar ratio of potassium xanthate increases from 0% to 50%, the xanthate-modified quantum dots QDx change to xanthate- and halogen-modified quantum dots QDw, and then to halogen-modified quantum dots QDh. Among the PLQYs, the xanthate- and halogen-modified quantum dots QDw (compared with QDx and QDh) are higher.

[0043] In addition, it is known that xanthate coordinates on the surface of quantum dots QD to increase the PL intensity. Also, it is known that xanthate decomposes approximately by heating and does not have an adverse effect on photoluminescence or electroluminescence. It is also known that by coordinating halogen (chloride ion) together with xanthate in quantum dots QD ( Figure 14 QDw), the dispersibility in a polar solvent is improved.

[0044] Figure 15 It is a cross-sectional schematic diagram showing a structural example of the light-emitting element of the present embodiment. Figure 16 It is a cross-sectional photograph showing a structural example of the light-emitting element of the present embodiment. As Figure 15 , 16 shown, the light-emitting element 5 sequentially 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).

[0045] The light-emitting element 5 can be formed, for example, as follows: namely, a process of dissolving zinc ethylxanthate in a DMF solvent to prepare a solution with a concentration of 0.04 mol / l (12 mg / ml), and using this solution to prepare a quantum dot solution containing 15 mg / ml of quantum dots QD modified with ethylxanthic acid and halogen (chloride ions) (a DMF solution containing 15 mg / ml of quantum dots modified with chloride ions and xanthic acid and 0.04 mol / l (12 mg / ml) of zinc xanthate); a process of forming a first electrode D1 using ITO (indium tin oxide); a process of coating and forming a hole injection layer 28 on the first electrode D1 using NiO nanoparticles (which may include a process of coating 15 mg / ml of NiO nanoparticles on ITO as an anode in nitrogen at 2000 rpm); a process of coating and forming a hole transport layer 29 on the hole injection layer 28 using, for example, P-TPD (which may include a process of coating P-TPD dissolved in chlorobenzene at a concentration of 8 mg / ml at 1500 rpm); a process of coating the quantum dot solution prepared in the above process on the hole transport layer 29 by spin coating (for example, at a rotation speed of 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 process of coating and forming an electron transport layer 31 using ZnMgO (which may include a step of coating ZnMgO nanoparticles); a process of forming a second electrode D2 using Ag (which may include a process of silver evaporation). In addition, the quantum dot solution may contain 5 to 100 mg / ml of quantum dots QD and 1 to 100 mg / ml of metal complexes (for example, zinc ethylxanthate). In addition, as a comparative example, a light-emitting element having a light-emitting layer was fabricated, and the light-emitting layer was obtained by heating a coating solution of an octane solution containing 15 mg / ml of quantum dots modified with an organic ligand at 90 °C. In the embodiment and the comparative example, except for the light-emitting layer, they are common, and commercially available red-emitting InP / ZnS was used as the quantum dots.

[0046] Figure 17 It is a graph showing the current density-external quantum efficiency characteristics of the light-emitting element. It can be seen that the maximum value of the external quantum efficiency (EQE) is 6.5% in the embodiment and 6.4% in the comparative example. Figure 18 It is a graph showing the voltage (applied voltage)-current density characteristics of the light-emitting element. The current density at 6V application is 29 m / m 2 in the embodiment and 12 mA / m 2 in the comparative example. According to the embodiment, since it is easy to inject carriers into the light-emitting layer, the voltage required for the desired light emission luminance (current density) can be reduced.

[0047] Figure 19 It is a graph showing the current density of 10 mA / m in the embodiment and the comparative example 2Graph of the EL spectrum at that time. In an embodiment, in order to suppress the Förster energy transfer between quantum dots, the peak wavelength is shifted to the short-wavelength side compared with the comparative example, and has the same emission peak as in the solution.

[0048] Figure 20A It shows the relative luminance and the change over time of the voltage when a constant current of 25 mA / m is continuously supplied to the light-emitting element of the embodiment. 2 Graph of the change over time. Figure 20B It shows the relative luminance and the change over time of the voltage when a constant current of 25 mA / m is continuously supplied to the light-emitting element of the comparative example. 2 Graph of the change over time. 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, showing a significant improvement compared with the comparative example. In addition, the voltage increase of the embodiment is very slow compared with the comparative example.

[0049] Figure 21 It is a schematic diagram showing a structural example of the display device according to the embodiment. Figure 22 It is a cross-sectional view showing a structural example of the display device according to the embodiment. As Figure 21 shown, the display device 50 includes: a display unit DA including a plurality of sub-pixels SP, a first driver X1 and a second driver X2 for driving the plurality of sub-pixels SP, and a display control unit DC for controlling the first driver X1 and the second driver X2. The sub-pixel SP includes a light-emitting element 5 and a pixel circuit PC connected to the light-emitting element 5. The pixel circuit PC may also be connected to a scan signal line GL, a data signal line DL, and a light emission control line EL. The scan signal line GL and the light emission control line EL may also be connected to the first driver X1, and the data signal line DL is connected to the second driver X2.

[0050] The display device 50 may also 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, for example, a plurality of pixel circuits PC in which an inorganic matrix is disposed. The pixel circuit PC may include a pixel capacitor to which a gradation signal is written, a transistor for controlling the current value of the light-emitting element 5 according to the gradation signal, a transistor connected to the scan signal line GL and the data signal line DL, and a transistor connected to the light emission control line EL.

[0051] As Figure 22As shown, the display device 50 includes a pixel circuit substrate 13 and a light-emitting element layer 14. The light-emitting element layer 14 includes, in order from the pixel circuit substrate 13 side, a first electrode D1, an edge covering 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 has a hole injection function and a hole transport function, and the second functional layer SK has 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 and a silicon oxide film, and prevents foreign substances (such as water and oxygen) from entering the light-emitting element layer 14.

[0052] As the material of the first functional layer FK, organic materials such as poly[(9,9-dioctylfluorene-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-tetra(4-methoxyphenyl)-9H-carbazole-3,6-diamine] (V886), 7,7'-bis[1,4]benzoxazine[2,3,4-kl]benzoxazine (HN-D1), etc., and inorganic materials such as NiO nanoparticles can be used.

[0053] As the material of the second functional layer SK, organic materials such as (2,2'',2''-(1,3,5-benzenetricarbonyl)-tris(1-phenyl-1-H-benzoimidazole) (TPBi), 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD), nanoparticles of organometallic complexes, etc., and inorganic materials such as nanoparticles of n-type oxide semiconductors can be used. As the organometallic complex, for example, tris(8-hydroxyquinoline) aluminum complex (Alq3) etc. can be cited. As the n-type oxide semiconductor, for example, metal oxides such as ZnO and ZnMgO can be cited.

[0054] In Figure 22 the Figure 2 etc., the quantum dot layer 30 shown is used as the light-emitting layer, but it is not limited thereto. The quantum dot layer 30 shown in Figure 2 etc. can also be used as a wavelength conversion layer or a light sensor layer. In addition, a power generation element having a quantum dot layer 30 between a pair of electrodes can also be formed. For example, holes and electrons can be generated in the quantum dots QD by light incident on the quantum dot layer 30, and each hole and electron can be transported to the electrodes to generate an electromotive force.

[0055] The above-described embodiments are for illustrative and explanatory purposes, not for limiting purposes. Based on these illustrations and explanations, those skilled in the art can clearly perform various deformation methods. Description of Reference Numerals

[0056] 2 Core 3 Housing 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 Substrate Material UL Base Layer

Claims

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

2. The quantum dot solution according to claim 1, Characterized in that, It includes a ligand of the quantum dots, The ligand of the quantum dots has the same structure as the ligand of the metal complex.

3. The quantum dot solution according to claim 2, Characterized in that, The ligand of the quantum dots coordinates with the quantum dots by 0.1% by weight or more.

4. The quantum dot solution according to any one of claims 1 to 3, Characterized in that, The ligand of the metal complex is dithiocarboxylic acid.

5. The quantum dot solution according to any one of claims 1 to 3, Characterized in that, The ligand of the metal complex is dialkylthiourea.

6. The quantum dot solution according to claim 4, Characterized in that, The ligand of the metal complex is xanthic acid.

7. The quantum dot solution according to any one of claims 1 to 6, Characterized in that, The metal complex includes zinc element.

8. The quantum dot solution according to any one of claims 1 to 6, Characterized in that, It contains halogen.

9. The quantum dot solution according to claim 4, Characterized in that, The quantum dots are modified with dithiocarboxylic acid.

10. The quantum dot solution according to claim 9, Characterized in that, The quantum dots are modified with dithiocarboxylic acid and halogen.

11. The quantum dot solution according to claim 6, Characterized in that, The xanthic acid has an alkyl chain with 1 to 5 carbon atoms.

12. The quantum dot solution according to claim 6, Characterized in that, The xanthic acid has an ether chain with 1 to 20 carbon atoms.

13. The quantum dot solution according to claim 11 or 12, Characterized in that, The solvent is a polar solvent containing at least one of formamide solvents, acetamide solvents, ester solvents, ketone solvents, sulfoxide solvents, ether solvents, thioether solvents, and nitrile solvents.

14. The quantum dot solution according to claim 6, Characterized in that, The xanthic acid has an alkyl chain with 6 or more carbon atoms.

15. The quantum dot solution according to claim 14, Characterized in that, The solvent is a non-polar solvent.

16. The quantum dot solution according to any one of claims 1 to 15, Characterized in that, The quantum dots have a shell composed of the metal sulfide.

17. The quantum dot solution according to any one of claims 1 to 16, Characterized in that, In the absorption spectrum of the metal complex in Fourier transform infrared spectroscopy, there are absorption peaks at 1040 cm -1 , 1122 cm -1 and 1217 cm -1 .

18. The quantum dot solution according to any one of claims 1 to 17, Characterized in that, There is a ligand around the quantum dots, and the ligand has absorption peaks in the absorption spectrum of Fourier transform infrared spectroscopy at 1040 cm -1 , 1122 cm -1 and 1217 cm -1 .

19. The quantum dot solution according to any one of claims 1 to 16, Characterized in that, In the absorption spectrum of Fourier transform infrared spectroscopy, there are absorption peaks at 1040 cm -1 , 1122 cm -1 and 1217 cm -1 , and it contains a compound whose absorption peak intensity is less than 20% when heated to 200 °C.

20. The quantum dot solution according to any one of claims 1 to 19, Characterized in that, containing the quantum dots at a concentration of 5 to 100 mg / ml and containing the metal complex at a concentration of 1 to 100 mg / ml.

21. A method for forming a quantum dot layer, characterized in that, comprising: a step of coating the quantum dot solution according to any one of claims 1 to 20 on a base layer; and a step of heating the coated quantum dot solution at a temperature of 100 °C or higher and 600 °C or lower.

22. A method for forming a quantum dot layer, characterized in that, comprising: a step of coating the quantum dot solution according to any one of claims 1 to 20 on a base layer containing an organic layer; and a step of heating the coated quantum dot solution at a temperature of 100 °C or higher and 250 °C or lower.

23. A method for forming a quantum dot layer, characterized in that, comprising: a step of coating the quantum dot solution according to any one of claims 1 to 20 on a base layer; and a step of irradiating the coated quantum dot solution with light having a wavelength of 200 nm to 400 nm.

24. A method for forming a quantum dot layer, characterized in that, comprising: a step of coating the quantum dot solution according to any one of claims 1 to 20 on a base layer; and a step of irradiating the coated quantum dot solution with a laser having a wavelength of 400 nm to 2000 nm.

25. A method for forming a quantum dot layer, characterized in that, comprising: a step of forming a coating film containing an organic ligand, quantum dots, and a first solvent; and a step of coating a second solution containing a metal xanthate and a second solvent on the coating film.

26. A method for manufacturing a light-emitting element, characterized in that, comprising: a step of forming an anode; a step of forming a quantum dot layer as a light-emitting layer by using the method for forming a quantum dot layer according to any one of claims 21 to 25; and a step of forming a cathode.

27. A light-emitting element, characterized in that, comprising: an anode and a cathode; and a light-emitting layer located between the anode and the cathode, wherein the light-emitting layer includes: a plurality of luminescent quantum dots; a metal sulfide containing the plurality of quantum dots; and Compound having S 2 with a C=O bond.

28. A light-emitting element, characterized in that, comprising: an anode and a cathode; and a light-emitting layer located between the anode and the cathode, wherein the light-emitting layer includes: a plurality of luminescent quantum dots; a metal sulfide encapsulating the plurality of quantum dots; and A compound having a peak at 1650 cm in the absorption spectrum of Fourier transform infrared spectroscopy -1 is provided.

29. The light-emitting element according to claim 27 or 28, characterized in that: the metal sulfide is zinc sulfide.

30. A display device, characterized in that, comprising the light-emitting element according to any one of claims 27 to 29.

31. A method for manufacturing a quantum dot solution, characterized in that, comprising: a step of mixing and stirring a first solution and a second solution, the first solution containing an organic ligand, quantum dots, and a first solvent, and the second solution containing a metal xanthate, a halogen compound, and a second solvent.

32. The method for manufacturing a quantum dot solution according to claim 31, characterized in that, the halogen compound is a metal halide or an ammonium halide, 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. The method for manufacturing a quantum dot solution according to claim 32, wherein, the precipitate contains quantum dots modified with xanthic acid and a halogen.

34. The method for manufacturing a quantum dot solution according to claim 32, wherein, the metal xanthate and the metal halide contain the same metal element.

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

36. The method for manufacturing a quantum dot solution according to 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.

Images