A method for preparing high-quality quantum dot ink

Abstract

The present invention discloses a method for preparing high-quality quantum dot ink by single-phase ligand exchange. The method comprises: removing the original long-chain ligand of the quantum dot by means of a thiol ligand having strong binding property with the metal site on the surface of the quantum dot in a non-polar phase, so that the quantum dot is deposited in a single phase from the non-polar phase to obtain a quantum dot solid. The high-quality quantum dot ink is obtained by dissolving the ligand additive in a polar dispersant in advance and then dissolving the quantum dot solid with the dispersant. The present invention expands the ligands that cannot be studied by the existing solid / liquid phase ligand exchange by dissolving the ligand additive in the dispersant, and can accurately design the passivation and doping of the quantum dot surface by regulating the composition and content of the ligand. The invented method can be applied to quantum dot materials such as PbX, HgX, CdX (X=S, Se, Te), and has broad application prospects in the fields of solar cells, infrared detectors, photodiode devices, etc.

Description

Technical Field

[0001] The present invention belongs to the field of compound semiconductor nanomaterial technology preparation, and specifically, relates to a new ligand exchange and surface passivation method for colloidal quantum dots, which achieves precise control of quantum dot surface passivation and doping by changing the composition and content of ligand additives in the dispersant. Background Art

[0002] Colloidal quantum dots (CQDs) have become a hot topic for light-emitting diodes, lasers, photodetectors, and solar cells due to their tunable optical and electrical properties and solution processability. In the past decade, the power conversion efficiency (PCE) of PbS quantum dot solar cells (CQD-SCs) has been rapidly improved, reaching 14.0% through improvements in CQD surface chemistry and device structure. To date, the most efficient CQD-SCs consist of a thin electron transport layer of zinc oxide deposited on indium tin oxide / active layer (CQD solids passivated with halide atoms) / and a thin hole transport layer and gold electrodes.

[0003] Generally, colloidally synthesized quantum dots are capped with fatty acids with long hydrocarbon chains. These long-chain organic molecules help the dispersion of quantum dots in organic solvents to avoid aggregation. However, after being processed into solid films in quantum dots, these organic surface ligands become an insulating layer on individual quantum dots and affect the charge injection of quantum dots. At the same time, the long ligands increase the distance between quantum dots, affecting the carrier transport within the quantum dot layer. A key step to achieve high-performance CQD photovoltaic devices is to exchange these long-chain ligands with short-chain ligands while maintaining good surface passivation. So far, there are two main ligand exchange methods.

[0004] (1) Solid-state ligand exchange: The lead sulfide quantum dot solution is deposited on the substrate and then exchanged with short-chain ligands such as ethylenedithiol (EDT), mercaptopropionic acid (MPA), and iodide to remove the electrically insulating oleic acid. The exchanged long ligands and excess short ligands are then washed away with a rinse solvent.

[0005] (2) Liquid phase ligand exchange: a two-phase ligand exchange process, in which the long-chain ligands of oleylamine coated on the surface of quantum dots are dissolved in the non-polar phase, and the long-chain ligands are exchanged by the short-chain ligands in the polar phase, so that the quantum dots complete the phase transition from non-polar to polar. Ligand treatment can enhance the electrical coupling performance between quantum dots, passivate the surface defects of quantum dots, reduce the defect state density between the band gaps of thin films, inhibit the non-radiative recombination of photogenerated carriers, and regulate the Fermi level of quantum dots; different types of ligands have different passivation mechanisms for quantum dots, and the content of ligands also has different passivation degrees for quantum dots.

[0006] Although the above two methods achieve good surface passivation, they are not efficient in controlling the type and content of ligands. Therefore, in order to further optimize the optoelectronic properties of quantum dots, we need to start with ligand exchange and develop a simple and efficient method to accurately design the ligands on the surface of quantum dots. Summary of the invention

[0007] The technical problem to be solved by the present invention is to provide a method for preparing high-quality quantum dot ink in view of the fact that the existing technology cannot accurately control the type and content of ligands while achieving good surface passivation.

[0008] In order to solve the above technical problems, the present invention provides a method for preparing high-quality quantum dot ink. A method for preparing quantum dot materials based on homogeneous ligands, first using thiol ligands with stronger binding ability to the quantum dot surface to exchange the original oleylamine ligands on the quantum dot surface, so that the quantum dots are precipitated from a non-polar solvent, and the quantum dots are converted into solid powders by centrifugal drying, and then the ligand additives are dissolved in the quantum dot dispersant, and then the quantum dot solids are dissolved to prepare quantum dot ink. This method can accurately and quantitatively design the components and contents of the ligand additives on the surface of the quantum dots by changing the components and contents of the ligand additives in the dispersant.

[0009] According to the purpose of the present invention, a method for preparing high-quality quantum dot ink is provided, comprising the following steps:

[0010] (1) obtaining a quantum dot solid whose surface ligand is ligand A, and dissolving the quantum dot solid in a non-polar solvent;

[0011] (2) adding n-butylamine to the quantum dot solution obtained in step (1), oscillating the reaction for 1 to 10 minutes, then adding the thiol ligand, oscillating the reaction for 1 to 10 minutes, then adding the antisolvent to the reaction system, and allowing the quantum dots to precipitate from the non-polar solvent after standing;

[0012] (3) removing the supernatant from the quantum dot solution obtained in step (2), adding polar solvent a to wash the quantum dots, centrifuging at 4000-10000 rpm for 1-5 minutes, removing the supernatant, and evacuating for 5-50 minutes to obtain a quantum dot solid, at which point the quantum dot surface has thiol and butylamine ligands;

[0013] (4) dissolving the ligand additive in a polar solvent b, and using the solvent b to dissolve the quantum dot solid obtained in step (3) at a certain concentration to obtain a quantum dot ink with a concentration of 20 to 500 mg / mL;

[0014] The steps (1) to (4) are all carried out under an inert environment.

[0015] Preferably, in step (1), the quantum dots are one of PbX, HgX, CdX (X=S, Se, Te) or CdSe / ZnSe core-shell quantum dots.

[0016] Preferably, in step (1), ligand A is a long-chain ligand containing 8 to 16 carbon atoms, including a mixture of one or more of oleic acid and oleylamine.

[0017] Preferably, in step (1), the non-polar solvent is a mixture of one or more of n-hexane, n-octane, toluene and tetrachloroethylene.

[0018] Preferably, in step (1), the concentration of the quantum dot solid solution is 1 mg / mL to 1000 mg / mL.

[0019] Preferably, in step (2), the thiol ligand is one of mercaptoethanol ME and mercaptopropionic acid MPA.

[0020] Preferably, in step (2), the volume ratio of the mercaptan to the n-butylamine solution is 1 to 100:1.

[0021] Preferably, in step (2), the anti-solvent and the non-polar solvent for dissolving the quantum dots are the same, which is a mixture of one or more of n-octane, n-hexane, toluene or tetrachloroethylene.

[0022] Preferably, in step (2), the standing time is 1 to 15 minutes.

[0023] Preferably, in step (3), the polar solvent a used to clean the quantum dots comprises a mixture of one or more of dimethyl sulfoxide and N,N-dimethylformamide.

[0024] Preferably, in step (4), the additive includes one or a mixture of more of lead iodide, lead bromide, lead chloride, cadmium iodide, cadmium bromide, cadmium chloride, anhydrous cadmium acetate, ammonium iodide, butylammonium iodide, ammonium chloride, zinc iodide, zinc bromide, zinc chloride, silver iodide, cesium iodide, and arsenic sulfide.

[0025] Preferably, in step (4), the polar solvent b used to dissolve the additive comprises a mixture of one or more of dimethyl sulfoxide, N,N-dimethylformamide, and n-butylamine.

[0026] The implementation of the embodiments of the present invention has the following beneficial effects:

[0027] (1) The method for preparing high-quality quantum dot ink described in the present invention has two steps: ligand exchange plus surface passivation and doping. First, in the ligand exchange part, thiol ligands are used to exchange the original long-chain ligands on the surface of quantum dots in a single phase, which shortens the distance between quantum dots and increases the coupling between quantum dots. This exchange method is different from the existing solid-phase and liquid-phase ligand exchange technology. It is a homogeneous phase exchange and the process is simpler. Then there is the surface passivation and doping part. Different from the existing ligand exchange technology, the passivation and doping of the quantum dot surface do not rely on the exchange part, but are achieved by adding ligand additives to the dispersant.

[0028] (2) The ligands used in the exchange method described in the present invention are mercaptoethanol ME and n-butylamine. The functional group -SH of the short-chain ligand mercaptoethanol ME has a strong binding force with the metal site on the surface of the quantum dot (such as Pb on the surface of PbS / PbSe, or Cd on the surface of CdSe), which can fully exchange the original ligand on the surface of the quantum dot. The -NH2 functional group of n-butylamine can deprotonate the -SH of mercaptoethanol ME. The double electric layer formed by the two ligands provides electrostatic force so that the quantum dots can form a good colloidal ink.

[0029] (3) The method for preparing high-quality quantum dot ink described in the present invention is different from the existing two-phase and solid-phase ligand exchange methods. The passivation and doping of the quantum dot surface are achieved by dissolving the ligand additive in the dispersant and changing the composition and content of the ligand added to the dispersant, thereby accurately designing the ligand on the quantum dot surface.

[0030] (4) The preparation of high-quality quantum dot ink described in the present invention can control the doping of quantum dots under the same exchange conditions and can prepare n-type, p-type and i-type quantum dot materials, which is impossible to achieve with all current technologies. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0032] Figure 1 This is a comparison diagram of the absorption of the PbS quantum dot ink and the quantum dot stock solution after the present invention is applied to PbS quantum dots and adding different concentrations of PbI2 ligand passivation in the dispersant after single-phase exchange;

[0033] Figure 2This is a comparison diagram of the absorption of the PbS quantum dot ink and the quantum dot stock solution after the present invention is applied to PbS quantum dots and adding different types of ligands passivated in the dispersant after single-phase exchange;

[0034] Figure 3 This is a comparison diagram of the absorption of PbS quantum dot ink prepared by the present invention and dual-phase exchange;

[0035] Figure 4 This is a comparison diagram of the absorption of the CdSe / ZnSe quantum dot ink and the quantum dot stock solution applied to CdSe / ZnSe core-shell quantum dots after single-phase exchange by adding different types of ligands passivated in the dispersant. DETAILED DESCRIPTION

[0036] The technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0037] Unless otherwise specified, the raw materials in the examples of the present invention were purchased from commercial sources.

[0038] The PbS quantum dots in the embodiment of the present invention are obtained by the method in Patent No. ZL201611050553.3 “A method for preparing PbS quantum dots”.

[0039] Example 1

[0040] 1. Obtain PbS quantum dots wrapped with oleic acid and oleylamine ligands, dissolve the quantum dots in a non-polar solvent, toluene, and prepare a PbS quantum dot solution with a concentration of 50 mg / mL.

[0041] 2. Take 1 mL of PbS quantum dot solution, add 300 μL of n-butylamine, oscillate for 2 minutes, add 10 μL of mercaptoethanol ME, continue oscillating for 2 minutes, add 5 mL of anti-solvent toluene and mix with the quantum dots, and the quantum dots are precipitated from the non-polar phase.

[0042] 3. Remove the supernatant from the quantum dot solution obtained in step 2, add 2 mL of N,N-dimethylformamide to wash the precipitated quantum dots, centrifuge at 6000 r / min for 2 minutes, remove the supernatant, and dry at room temperature for 30 minutes to obtain PbS quantum dot solid.

[0043] 4. Mix n-butylamine and N,N-dimethylformamide in a volume ratio of 4:1 to obtain a mixed dispersant. Dissolve the lead iodide ligand as an additive in the mixed dispersant to prepare dispersant solutions containing different lead iodide concentrations, including 0 mg / mL, 12 mg / mL and 30 mg / mL. Dissolve the PbS quantum dot solid obtained in step 3 in the mixed dispersant solution to prepare a lead iodide passivated PbS quantum dot ink with a concentration of 320 mg / mL.

[0044] As attached Figure 1 As shown, the absorption peak of the PbS quantum dot stock solution is at 934nm. The quantum dots are first subjected to single-phase ligand exchange, and then the obtained quantum dot solid is dissolved in a mixed dispersant containing 0mg / mL lead iodide. Its absorption is red-shifted from 934nm to 965nm, and the half-peak width is significantly widened. This indicates that the quantum dots are partially aggregated and fused after ligand exchange. When the quantum dot solid is dissolved in a mixed dispersant containing 12mg / mL or 30mg / mL lead iodide, its absorption peak is less red-shifted, from 934nm to 945nm, and the half-peak width gradually narrows with the increase of lead iodide ligands in the dispersant. This shows that the lead iodide ligands in the dispersant significantly improve the monodispersity of the quantum dots and reduce the aggregation and fusion of the quantum dots.

[0045] A solar cell was prepared using the 30 mg / mL lead iodide passivated PbS quantum dot ink obtained in step 4, and the device structure was ITO electrode / electron transport layer ZnO / active layer single-phase exchange PbS layer / hole transport layer PbS-EDT / gold electrode Au.

[0046] Example 2

[0047] 1. Obtain PbS quantum dots wrapped with oleic acid and oleylamine ligands, dissolve the quantum dots in a non-polar solvent, toluene, and prepare a PbS quantum dot solution with a concentration of 50 mg / mL.

[0048] 2. Take 1 mL of PbS quantum dot solution, add 300 μL of n-butylamine, oscillate for 2 minutes, add 10 μL of mercaptoethanol ME, continue oscillating for 2 minutes, add 5 mL of anti-solvent toluene and mix with the quantum dots, and the quantum dots are precipitated from the non-polar phase.

[0049] 3. Remove the supernatant from the quantum dot solution obtained in step 2, add 2 mL of N,N-dimethylformamide to wash the precipitated quantum dots, centrifuge at 6000 r / min for 2 minutes, remove the supernatant, and dry at room temperature for 30 minutes to obtain PbS quantum dot solid.

[0050] 4. Mix n-butylamine and N,N-dimethylformamide in a volume ratio of 4:1 to obtain a mixed dispersant. Dissolve different types of ligands as additives in the mixed dispersant to prepare dispersant solutions containing different types of ligands; including lead iodide, cadmium iodide and butylamine iodide, all with a concentration of 0.065M. Dissolve the PbS quantum dot precipitate obtained in step 3 in the dispersant solution containing different types of ligand additives to prepare a PbS quantum dot ink passivated with different types of ligands at a concentration of 320 mg / mL.

[0051] As attached Figure 2 As shown in the figure, after the single-phase exchange of quantum dots, ligands such as lead iodide, cadmium iodide and butylamine iodide are added to the dispersant. The absorption of the quantum dot solution will reduce the degree of red shift compared to when there is no ligand in the dispersant, and the half-peak width will be narrowed after the addition of ligands. At the same molar concentration, the quantum dots have the least red shift and the narrowest half-peak width after the addition of lead iodide ligands.

[0052] Example 3

[0053] 1. Obtain CdSe / ZnSe core-shell quantum dots wrapped with oleic acid ligands, dissolve the quantum dots in a non-polar solvent n-octane, and prepare a CdSe / ZnSe quantum dot solution with a concentration of 20 mg / mL.

[0054] 2. Take 1 mL of CdSe / ZnSe quantum dot solution, add 300 μL of n-butylamine, shake and react for 2 minutes, then add 30 μL of mercaptoethanol ME, continue shaking and react for 2 minutes, add 2 mL of anti-solvent n-octane and mix with the quantum dots, and the quantum dots are precipitated from the non-polar phase.

[0055] 3. Remove the supernatant from the quantum dot solution obtained in step 2, add 2 mL of N,N-dimethylformamide to wash the precipitated quantum dots, centrifuge at 6000 r / min for 2 minutes, remove the supernatant, and dry at room temperature for 10 minutes to obtain CdSe / ZnSe quantum dot solid.

[0056] 4. Disperse cadmium chloride, cadmium acetate, cadmium iodide, zinc iodide and zinc chloride in N,N-dimethylformamide solution at the same molar concentration of 0.05M to obtain a mixed dispersant containing different types of additives. Dissolve the CdSe / ZnSe quantum dot solid obtained in step 3 in the dispersant solution containing different types of additives and without additives obtained above to prepare a CdSe / ZnSe quantum dot ink passivated with different types of additives at a concentration of 20 mg / mL.

[0057] As attached Figure 4 As shown, after single-phase exchange of CdSe / ZnSe quantum dots, various ligands were added to the dispersant, and the absorption of the quantum dot solution was almost the same as that before exchange.

[0058] The photoluminescence quantum yield (PLQY) of the original solution and the CdSe / ZnSe core-shell quantum dots passivated with different types of additives after exchange were tested, and the results are listed in Table 1.

[0059] Table 1 shows the PLQY parameters of CdSe / ZnSe core-shell quantum dots passivated with different additives and stock solution in Example 3

[0060] Additives PLQY(%) Original solution 77.5 No Additives 56.6 Cadmium chloride 73.8 Cadmium acetate 74.5 Cadmium iodide 72.0 Zinc iodide 77.2 Zinc chloride 67.8

[0061] Combined with the data in Table 1, it can be seen that in Example 3, the PLQY of CdSe / ZnSe quantum dots after exchange decreased from 77.5% to 56.6%. By dissolving additives such as cadmium chloride, cadmium acetate, cadmium iodide, zinc iodide and zinc chloride into N,N-dimethylformamide dispersant, the passivation of quantum dots can be achieved. This method can compare the passivation effects of various different additives. Test data show that when zinc iodide is used as an additive, the passivation effect on quantum dots is the best. The PLQY of quantum dots after exchange can be increased from 56.6% to 77.2%, which is almost consistent with the 77.5% PLQY of the original solution. And for CdSe / ZnSe quantum dots, metal halides such as zinc iodide, zinc chloride and cadmium iodide cannot be studied by existing ligand exchange methods.

[0062] Comparative Example 1

[0063] 1. Obtain PbS quantum dots wrapped with oleic acid and oleylamine ligands, dissolve the quantum dots in a non-polar solvent n-octane, and prepare a PbS quantum dot solution with a concentration of 10 mg / mL.

[0064] 2. Prepare a lead halide ligand precursor solution, mix 0.1M lead iodide, 0.02M lead bromide, 0.04M ammonium acetate and 5mL N,N-dimethylformamide to obtain a lead halide ligand precursor solution. Mix the PbS quantum dot solution obtained in step 1 with the lead halide ligand precursor solution in a volume ratio of 1:1, shake vigorously for 2 minutes, and then stand for 2 minutes to separate the layers; take the lower layer solution and n-octane in a volume ratio of 1:1, shake vigorously for 1 minute to wash, and then stand for 1 minute to separate the layers; repeat the washing and standing steps twice to obtain a lower layer solution; centrifuge the obtained lower layer solution and toluene at a volume ratio of 1:1 at a speed of 9000r / min for 5 minutes; dry the precipitate obtained by centrifugation at room temperature for 40 minutes to obtain a PbS-PbX2 quantum dot precipitate.

[0065] 3. Mix n-butylamine and N,N-dimethylformamide in a volume ratio of 4:1 to obtain a mixed dispersant. Dissolve the PbS-PbX2 quantum dot precipitate obtained in step 2 in the mixed dispersant solution at a concentration of 320 mg / mL to obtain PbS-PbX2 ink; compare the absorption of the PbS quantum dot ink prepared in Example 1 with the absorption of the quantum dot stock solution and the absorption of the ink containing 30 mg / mL lead iodide ligand in the dispersant prepared in Example 1. Figure 3 As shown, the quantum dots are passivated by single-phase ligand exchange and then adding lead iodide ligands to the dispersant; the absorption of the prepared ink is narrower than that of the two-phase exchange, and the peak position red shift is smaller.

[0066] 4. Use the PbS-PbX2 quantum dot ink obtained in step 3 to prepare a solar cell, and the device structure is ITO electrode / electron transport layer ZnO / active layer PbS-PbX2 layer / hole transport layer PbS-EDT / gold electrode Au.

[0067] Test Case

[0068] Using a solar simulator, the PbS quantum dot solar cells prepared in Example 1 and Comparative Example 1 were subjected to current-voltage scanning under the illumination intensity of AM1.5G. The photoelectric conversion efficiency results are listed in Table 2, where the test area of ​​the solar cell is 0.0414 cm 2 .

[0069] Table 2 is a table showing the photoelectric conversion efficiency of PbS solar cells prepared in Example 1 and Comparative Example 1

[0070]

[0071] As shown in Table 2, the PbS solar cell device prepared with a lead iodide additive concentration of 30 mg / mL in Example 1 has significantly improved open circuit voltage, short circuit current and photoelectric conversion efficiency compared with the device prepared in Comparative Example 1.

[0072] What is disclosed above is only a preferred embodiment of the present invention, and it certainly cannot be used to limit the scope of rights of the present invention. Ordinary technicians in this field can understand that all or part of the processes of the above embodiment and equivalent changes made according to the claims of the present invention still fall within the scope of the invention.

Claims

1. A method for preparing high-quality quantum dot ink, characterized in that The following steps are involved: (1) obtaining a quantum dot solid whose surface ligand is ligand A, and dissolving the quantum dot solid in a non-polar solvent; (2) adding n-butylamine to the quantum dot solution obtained in step (1), shaking the reaction for 1 to 10 minutes, and then adding the thiol ligand, shaking the reaction for 1 to 10 minutes, and then adding the antisolvent to the reaction system, and after standing, the quantum dots are precipitated from the non-polar solvent; (3) removing the supernatant from the quantum dot solution obtained in step (2), adding polar solvent a to wash the quantum dots, centrifuging at 4000-10000 rpm for 1-10 minutes, removing the supernatant, and vacuuming for 5 minutes-12 hours to obtain a quantum dot solid, at which point the quantum dot surface has thiol and n-butylamine ligands; (4) dissolving the ligand additive in a polar solvent b, and using the solvent b to dissolve the quantum dot solid obtained in step (3) at a certain concentration to obtain a quantum dot ink with a concentration of 20 to 500 mg / mL; The steps (1) to (4) are all carried out under an inert environment; In the step (2), the thiol ligand is one of mercaptoethanol ME and mercaptopropionic acid MPA; the volume ratio of the thiol to the n-butylamine solution is 1-100:1; In step (1), the quantum dots are one of PbX, HgX, CdX or CdSe / ZnSe core-shell quantum dots; wherein X = S, Se, Te; In step (1), ligand A is a long-chain ligand containing 8 to 16 carbon atoms, including one or more mixtures of oleic acid and oleylamine.

2. The method for preparing high-quality quantum dot ink according to claim 1, characterized in that: In step (1), the non-polar solvent is a mixture of one or more of n-hexane, n-octane, toluene and tetrachloroethylene.

3. The method for preparing high-quality quantum dot ink according to claim 1, characterized in that: In step (1), the solubility of the quantum dot solid is 1 mg / mL to 1000 mg / mL.

4. The method for preparing high-quality quantum dot ink according to claim 1, characterized in that: In step (2), the anti-solvent and the non-polar solvent for dissolving the quantum dots are the same, which is a mixture of one or more of n-octane, n-hexane, toluene or tetrachloroethylene; and the standing time is 1 to 15 minutes.

5. The method for preparing high-quality quantum dot ink according to claim 1, characterized in that: In step (3), the polar solvent a used to clean the quantum dots includes a mixture of one or more of dimethyl sulfoxide and N,N-dimethylformamide.

6. The method for preparing high-quality quantum dot ink according to claim 1, characterized in that: In step (4), the additive includes one or a mixture of more of lead iodide, lead bromide, lead chloride, cadmium iodide, cadmium bromide, cadmium chloride, anhydrous cadmium acetate, ammonium iodide, butylammonium iodide, ammonium chloride, zinc iodide, zinc bromide, zinc chloride, silver iodide, cesium iodide, and arsenic sulfide.

7. The method for preparing high-quality quantum dot ink according to claim 1, characterized in that: In step (4), the polar solvent b used to dissolve the additive includes one or more mixtures of dimethyl sulfoxide, N,N-dimethylformamide, and n-butylamine.

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