CN113912587A - Surface ligand, quantum dot and preparation method thereof, and quantum dot light-emitting diode - Google Patents
Surface ligand, quantum dot and preparation method thereof, and quantum dot light-emitting diode Download PDFInfo
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Abstract
The invention belongs to the technical field of quantum dots, and particularly relates to a surface ligand and application thereof, a quantum dot and a preparation method thereof. The structural general formula of the surface ligand is as follows:
wherein R is1And R2Each independently selected from at least one of quinolinyl, pyrazinyl, and pyridinyl. The surface ligand can make the quantum dots more stable, and the quantum dots can form a film stably and orderly, and the luminous efficiency and the electric charge of the quantum dots are enabledThe transmission is more balanced, and the luminescent performance of the device can be improved when the quantum dot light-emitting diode is used.
Description
Technical Field
The invention belongs to the technical field of quantum dots, and particularly relates to a surface ligand, a quantum dot, a preparation method of the quantum dot and a quantum dot light-emitting diode.
Background
In recent years, with the rapid development of display technologies, quantum dot light emitting diodes (QLEDs) having semiconductor Quantum Dot (QDs) materials as light emitting layers have received much attention. The quantum dot light-emitting diode has the advantages of high color purity, high luminous efficiency, adjustable luminous color, stable device and the like, so that the quantum dot light-emitting diode has wide application prospect in the fields of flat panel display, solid state lighting and the like. Although the performance (including device efficiency and service life) of the existing QLED is greatly improved by the improvement of quantum dot materials and the continuous optimization of the structure of the QLED device, the efficiency of the existing QLED is far from the requirement of industrial production. In the research process of the quantum dot light-emitting diode, the surface ligands of the quantum dots have a large influence on the QLED device, common ligands such as trioctylphosphine, trioctylphosphine oxide, oleic acid, stearic acid, oleylamine, thioglycolic acid, mercaptopropionic acid and the like are used as surface capping agents of the quantum dots, the surface defects of the quantum dots can be reduced to improve the luminous efficiency of the quantum dots, and the ligands and the quantum dots have weak binding capacity and poor stability of a formed quantum dot colloidal solution, so that the performance of the QLED device can be influenced. In addition, in quantum dot thin films, these surface ligand molecules determine the spatial distance between the quantum dots and affect the optoelectronic properties of the film. Generally, the longer the chain of the ligand molecule is, the larger the spatial distance between the quantum dots after the film is formed is, so that the energy transfer between the quantum dots is relatively small, and the quantum dot film (i.e. the quantum dot light-emitting layer) can keep higher fluorescence efficiency; however, for the quantum dot light emitting diode device, the large spatial distance between the quantum dots adversely affects the transport of carriers in the quantum dot light emitting layer, thereby affecting the current and brightness of the device, which are difficult to balance.
Therefore, the prior art is in need of improvement.
Disclosure of Invention
The invention aims to provide a surface ligand, a quantum dot, a preparation method of the quantum dot and a quantum dot light-emitting diode, and aims to solve the technical problem that the existing quantum dot is unstable so as to influence the light-emitting performance.
In order to achieve the purpose, the invention adopts the following technical scheme:
in one aspect, the present invention provides a surface ligand, wherein the surface ligand has the following structural formula:
wherein R is1And R2Each independently selected from at least one of quinolinyl, pyrazinyl, and pyridinyl.
In the surface ligands provided by the present invention, R1And R2The coordination (similar to tridentate chelation) can effectively control the distance between quantum dots, thereby improving the dispersibility of the quantum dots, improving the binding probability of a surface ligand and the quantum dots, enabling the surface ligand to be difficult to fall off, and improving the stability of the quantum dots; in addition, the C ═ O hydrogen bond acceptor on the amide group can form a hydrogen bond with an N-H hydrogen bond donor on an adjacent amide group, and can also form a hydrogen bond with a C-H hydrogen bond donor of a pyridine ring, a pyrazine ring or a quinoline ring, so that a supermolecular structure can be constructed, and the supermolecular structure enables the quantum dot to be more stable; in addition, a 2D layer can be formed through pi-pi stacking effect among pyridine rings, pyrazine rings or quinoline rings, and the formed quantum dot film is highly stable and ordered; and the pyridine ring, the pyrazine ring or the quinoline ring which provides the coordination dots increases the steric hindrance between the quantum dots, so that the energy transfer between the quantum dots is weakened, and the exciton can effectively emit light in the quantum dots in a composite manner. Therefore, the surface ligand is combined on the surface of the quantum dot, so that the quantum dot can be more stable, the quantum dot can be stably and orderly formed into a film, the uniformity of the film layer is improved, the luminous efficiency and the charge transmission of the quantum dot are more balanced, and the luminous performance of a device can be improved when the surface ligand is used for a quantum dot light-emitting diode.
In another aspect, the present invention provides a quantum dot having the above surface ligand bound thereto.
And, a method for preparing quantum dots, comprising the steps of:
providing an initial quantum dot solution, wherein an initial ligand is bound to the surface of an initial quantum dot in the initial quantum dot solution;
and mixing the surface ligand and the initial quantum dot solution to perform ligand exchange reaction to obtain the quantum dot.
The quantum dot and the preparation method provided by the invention are used for obtaining the quantum dot, the surface of the quantum dot is combined with the special surface ligand of the invention, and the surface ligand is used as the surface ligand of the quantum dot, so that the quantum dot can be more stable, the quantum dot can be more stably and orderly formed into a film, the quantum dot light efficiency and the charge transmission are more balanced, and the quantum dot light-emitting diode can be used for improving the light-emitting performance of a device.
Finally, the invention also provides a quantum dot light-emitting diode which comprises an anode, a cathode and a quantum dot light-emitting layer positioned between the anode and the cathode, wherein the quantum dot light-emitting layer is composed of the quantum dot and/or the quantum dot obtained by the preparation method.
The quantum dot light-emitting layer of the quantum dot light-emitting diode provided by the invention is composed of the special quantum dots, the surface ligand combined with the quantum dots can make the quantum dots more stable and orderly form a film, the light efficiency and the charge transmission of the quantum dots are more balanced, and the light-emitting performance of the quantum dot light-emitting diode can be improved.
Drawings
FIG. 1 is a schematic representation of surface ligand-bound quantum dots according to an embodiment of the present invention;
fig. 2 is a schematic flow chart of a method for preparing quantum dots according to an embodiment of the present invention;
fig. 3 is a schematic structural diagram of a quantum dot light emitting diode according to an embodiment of the present invention.
Detailed Description
In order to make the technical problems, technical solutions and advantageous effects to be solved by the present invention more clearly apparent, the present invention is further described in detail below with reference to the following embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.
In one aspect, an embodiment of the present invention provides a surface ligand, where the structural formula of the surface ligand is as follows:
wherein R is1And R2Each independently selected from at least one of quinolinyl, pyrazinyl, and pyridinyl.
In the surface ligands provided in the examples of the present invention, R is1And R2The coordination (similar to tridentate chelation) can effectively control the distance between quantum dots, thereby improving the dispersibility of the quantum dots, improving the binding probability of a surface ligand and the quantum dots, enabling the surface ligand to be difficult to fall off, and improving the stability of the quantum dots; in addition, the C ═ O hydrogen bond acceptor on the amide group can form a hydrogen bond with an N-H hydrogen bond donor on an adjacent amide group, and can also form a hydrogen bond with a C-H hydrogen bond donor of a pyridine ring, a pyrazine ring or a quinoline ring, so that a supermolecular structure can be constructed, and the supermolecular structure enables the quantum dot to be more stable; in addition, a 2D layer can be formed through pi-pi stacking effect among pyridine rings, pyrazine rings or quinoline rings, and the formed quantum dot film is highly stable and ordered; and the pyridine ring, pyrazine ring or quinoline ring providing the coordination dots increases the steric hindrance between the quantum dots, so that the energy transfer between the quantum dots is weakened, and excitons are facilitated to be excitedThe quantum dots can effectively emit light in a composite manner. Therefore, the surface ligand is combined on the surface of the quantum dot, so that the quantum dot can be more stable, the quantum dot can be stably and orderly formed into a film, the uniformity of the film layer is improved, the light efficiency and the charge transmission of the quantum dot are more balanced, and the light-emitting performance of a device can be improved when the surface ligand is used for a quantum dot light-emitting diode.
The amide compound has polydentate coordination and bridging coordination, and the embodiment of the invention can effectively improve the fluorescence efficiency and stability of the quantum dots and control the distance between the quantum dots by selecting the surface ligand, so that the luminous performance of the QLED device can be improved, and the industrialization requirement can be met.
Further, in the structural general formula of the surface ligand, R1And R2The same is true. Due to R1And R2And the two ends of the surface ligand are provided with symmetrical N electron clouds, the probability of combining quantum dots at the two ends of the surface ligand is the same, and the surface ligand is combined with the quantum dots, so that the film forming of the quantum dots is more uniform.
Further, the surface ligand is one or more of 2, 6-bis (quinoline-2-carboxamide) -pyridine, 2, 6-bis (pyrazine-2-carboxamide) -pyridine and 2, 6-bis (pyridine-2-carboxamide) -pyridine.
2, 6-bis (quinoline-2-carboxamide) -pyridine:
2, 6-bis (pyrazine-2-carboxamide) -pyridine:
2, 6-bis (pyridine-2-carboxamide) -pyridine:
in the examples of the present invention, 2, 6-bis (quinoline-2-carboxamide) -pyridine is preferable, and in the 2, 6-bis (quinoline-2-carboxamide) -pyridine, R is1And R2Both ends are quinolines, which have larger rings, so that the pi-pi stacking is moreIn this way, a more stable and ordered quantum dot film can be formed.
The surface ligand can be better bonded on the surface of the quantum dot (as shown in figure 1), so that the stability and the luminescence property of the quantum dot can be improved.
Correspondingly, the invention also provides the application of the surface ligand as the surface ligand of the quantum dot.
In particular, a quantum dot having the above surface ligand bound thereto. The surface ligand of the quantum dot is combined with the surface ligand which is special in the embodiment of the invention, and the surface ligand is used as the surface ligand of the quantum dot, so that the quantum dot can be more stable, the quantum dot can be stably and orderly formed into a film, the quantum dot light efficiency and the charge transmission are more balanced, and the quantum dot light-emitting diode can improve the light-emitting performance of the device.
Accordingly, as shown in fig. 2, a method for preparing a quantum dot includes the following steps:
s01: providing an initial quantum dot solution, wherein an initial ligand is bound to the surface of an initial quantum dot in the initial quantum dot solution;
s02: and mixing the surface ligand and the initial quantum dot solution to perform ligand exchange reaction to obtain the quantum dot.
According to the preparation method, the quantum dots are subjected to ligand exchange reaction and combined with the surface ligand special for the embodiment of the invention, so that the combination firmness of the quantum dots and the surface ligand can be improved, the stability of the quantum dot colloidal solution is better, and the luminous performance of a quantum dot device can be improved after printing and film forming.
Specifically, a prepared initial quantum dot solution combined with an initial ligand is mixed with the surface ligand, and after stirring for a predetermined time at a certain temperature (such as 20-300 ℃), a ligand exchange reaction occurs, and the quantum dot with the surface ligand is obtained after cleaning. In one embodiment, the initial ligand is selected from at least one of oleic acid, oleylamine, trioctylphosphine and trioctylphosphine; the solvent in the initial quantum dot solution is at least one selected from toluene, chloroform, chlorobenzene, carbon tetrachloride, n-hexane and octane.
Further, the temperature of the ligand exchange reaction is 20-300 ℃; the time of the ligand exchange reaction is 3-5 h. The ligand exchange effect is better under the condition.
Further, the molar ratio of the initial ligand to the surface ligand is 1: 1-1: 50, and the ligand can be fully exchanged under the condition, particularly 1: 5; further, the initial quantum dot concentration in the initial quantum dot solution is 10-50 mg/mL.
Further, after the ligand exchange reaction, a step of adding a polar solvent for washing is also included.
In one embodiment, a method for preparing quantum dots comprises the steps of:
mixing the prepared initial quantum dot solution with the surface ligand (the molar ratio of the initial ligand to the surface ligand is 1: 1-1: 50), stirring for a predetermined time at a certain temperature (20-300 ℃), performing a ligand exchange reaction, and cleaning to obtain the quantum dot with the surface being the ligand. Wherein the mycoplasma comprises but is not limited to at least one of oleic acid, oleylamine, trioctylphosphine and trioctylphosphine oxide; the quantum dots are binary phase quantum dots, ternary phase quantum dots or quaternary phase quantum dots, wherein the binary phase quantum dots include but are not limited to at least one of CdS, CdSe, CdTe, InP, AgS, PbS, PbSe and HgS; the ternary phase quantum dots include but are not limited to ZnXCd1-XS、CuXIn1-XS、ZnXCd1-XSe、ZnXSe1-XS、ZnXCd1-XTe、PbSeXS1-XAt least one of; the quaternary phase quantum dots comprise ZnXCd1-XS/ZnSe、CuXIn1-XS/ZnS、ZnXCd1-XSe/ZnS、CuInSeS、ZnXCd1-XTe/ZnS、PbSeXS1-XAt least one of/ZnS, wherein 0<X<1。
Finally, an embodiment of the present invention further provides a quantum dot light emitting diode, as shown in fig. 3, including an anode 1, a cathode 2, and a quantum dot light emitting layer 3 located between the anode 1 and the cathode 2, where the quantum dot light emitting layer 3 is composed of the quantum dot according to the embodiment of the present invention and/or the quantum dot obtained by the preparation method according to the embodiment of the present invention.
According to the quantum dot light-emitting diode provided by the embodiment of the invention, the quantum dot light-emitting layer is composed of the specific quantum dots, the surface ligand combined with the quantum dots can enable the quantum dots to be more stable and orderly formed into a film, the light efficiency and the charge transmission of the quantum dots are more balanced, and the light-emitting performance of the quantum dot light-emitting diode can be improved.
In one embodiment, a hole function layer may be further disposed between the anode and the quantum dot light emitting layer, and the hole function layer may be a hole transport layer, or a hole injection layer and a hole transport layer that are stacked, wherein the hole injection layer is adjacent to the anode. Or, an electronic function layer can be further arranged between the cathode and the quantum dot light-emitting layer, and the electronic function layer can be an electron transport layer, or a stacked electron injection layer and an electron transport layer, wherein the electron injection layer is adjacent to the cathode.
Further, the quantum dot light emitting diode may be an upright type quantum dot light emitting diode or an inverted type quantum dot light emitting diode.
Correspondingly, the preparation method of the QLED device comprises the following steps:
step S1 is to process the substrate with the bottom electrode. Wherein the substrate may be a rigid substrate, such as glass, or a flexible substrate, such as PI. And manufacturing a bottom electrode on the substrate, for example, forming an ITO substrate. And cleaning the patterned ITO substrate, and treating the cleaned ITO substrate with ultraviolet-ozone or oxygen plasma before depositing other functional layers so as to further remove organic matters attached to the surface of the ITO and improve the work function of the ITO.
Step S2, depositing a hole injection layer on the processed substrate surface, wherein the thickness of the hole injection layer is 10-100nm, and the hole injection material can be water-soluble PEDOT (PSS), or other materials with good hole injection performance, such as NiO and MoO3、WO3Or V2O5And so, PEDOT: PSS is preferred here as the hole injection layer.
Step S3, placing the substrate in a nitrogen atmosphere, depositing a hole transport layer on the surface of the hole injection layer, wherein the hole transport layer can be Poly (9, 9-dioctylfluorene-CO-N- (4-butylphenyl) diphenylamine) (TFB), Polyvinylcarbazole (PVK), Poly (N, N '-bis (4-butylphenyl) -N, N' -bis (phenyl) benzidine) (Poly-TPD), Poly (9, 9-dioctylfluorene-CO-bis-N, N-phenyl-1, 4-Phenylenediamine) (PFB), 4', 4 ″ -tris (carbazol-9-yl) triphenylamine (TCTA), 4' -bis (9-Carbazole) Biphenyl (CBP), N '-diphenyl-N, N' -bis (3-methylphenyl) -1, one or more of 1 '-biphenyl-4, 4' -diamine (TPD) and N, N '-diphenyl-N, N' - (1-naphthyl) -1,1 '-biphenyl-4, 4' -diamine (NPB), and can also be other high-performance hole transport materials, wherein the thickness of the deposited hole transport layer is 1-100 nm.
Step S4, depositing a quantum dot light emitting layer on the hole transport layer, wherein the material of the quantum dot light emitting layer may be one or more of common red light quantum dots, green light quantum dots, and blue light quantum dots, and the thickness is about 10-100nm, and the quantum dots are binary phase quantum dots, ternary phase quantum dots, or quaternary phase quantum dots, wherein the binary phase quantum dots include but are not limited to at least one of CdS, CdSe, CdTe, InP, AgS, PbS, PbSe, HgS; the ternary phase quantum dots include but are not limited to ZnXCd1-XS、CuXIn1-XS、ZnXCd1-XSe、ZnXSe1-XS、ZnXCd1-XTe、PbSeXS1-XAt least one of; the quaternary phase quantum dots comprise ZnXCd1-XS/ZnSe、CuXIn1-XS/ZnS、ZnXCd1-XSe/ZnS、CuInSeS、ZnXCd1-XTe/ZnS、PbSeXS1-XAt least one of/ZnS; wherein 0<X<1. The surface of the quantum dot is combined with the surface ligand of the embodiment of the invention.
Step S5, preparing an electron transport layer, wherein the electron transport layer can be made of materials selected from materials with good electron transport propertyThe material for transferring performance can be, for example but not limited to, n-type ZnO, TiO2、Fe2O3、SnO2、Ta2O3One or more of AlZnO, ZnSnO, InSnO and the like. Preferably, the electron transport layer is made of n-type ZnO, and the preferred thickness of the electron transport layer is 10-60 nm.
And S6, finally, placing the wafer in an evaporation bin, and thermally evaporating a layer of top electrode which can be Al, Ag, Au or Cu and the like through a mask plate, wherein the thickness of the top electrode is 60-120 nm, and the QLED device is obtained.
In addition, the scheme can also be used in an inversion type QLED device and comprises the step of depositing an electron transport layer, a quantum dot light emitting layer, a hole injection layer, a hole transport layer and a top electrode on the bottom electrode.
The invention is described in further detail with reference to a part of the test results, which are described in detail below with reference to specific examples.
Example 1
Preparing quantum dots: firstly, 200mg of CdSe/ZnSe quantum dots (the ligand is oleic acid) and 10ml of n-hexane are added into a three-neck flask, argon is introduced for protection, the mixture is fully stirred for 10min at normal temperature to completely dissolve the quantum dots, 8mmol of 2, 6-bis (quinoline-2-formamide) -pyridine is added into the quantum dot solution, and the mixture is stirred for 4h at normal temperature to fully exchange the oleic acid on the surfaces of the quantum dots. And then adding ethyl acetate and methanol into the exchanged quantum dot mixed solution for centrifugal separation for multiple times to obtain the quantum dot with the surface bound with the 2, 6-bis (quinoline-2-formamide) -pyridine ligand.
The preparation steps of the QLED device are as follows:
firstly, placing a patterned ITO substrate in acetone washing liquor, deionized water and isopropanol in sequence for ultrasonic cleaning, wherein the ultrasonic cleaning of each step lasts for about 15 minutes. After the ultrasonic treatment is finished, placing the ITO in a clean oven for drying for later use; after the ITO substrate is dried, treating the ITO surface for 5 minutes by using ultraviolet-ozone so as to further remove organic matters attached to the ITO surface and improve the work function of the ITO;
then, a hole injection layer PEDOT, PSS, with the thickness of 30nm, is spin-coated on the ITO substrate processed in the previous step, and the substrate is placed on a heating table at 150 ℃ for 30 minutes to remove moisture, and the step needs to be completed in the air;
next, the dried substrate coated with the hole injection layer was placed in a nitrogen atmosphere, a layer of hole transport layer material TFB was spin-coated, the thickness of this layer was 30nm, and the substrate was placed on a heating stage at 150 ℃ and heated for 30 minutes to remove the solvent;
after the chip treated in the previous step is cooled, the red quantum dot (surface-bound 2, 6-bis (quinoline-2-formamide) -pyridine ligand) solution with the concentration of 20mg/ml is spin-coated on the surface of the hole transport layer, and the thickness of the hole transport layer is 20 nm. After the deposition in this step, the wafer was heated on a heating table at 80 ℃ for 10 minutes to remove the residual solvent;
then, a ZnO electron transmission layer is coated in a spinning mode, after deposition is finished, the wafer is placed on a heating table at the temperature of 80 ℃ to be heated for 30 minutes, and the thickness of the wafer is 50 nm;
and finally, placing the sheets with the deposited functional layers in an evaporation bin, and thermally evaporating a layer of 100nm silver as a cathode through a mask plate, thereby completing the preparation of the device.
The test result of the prepared device shows that compared with the device prepared by the crude oil acid ligand quantum dots, the device prepared by the quantum dots with the surface ligand of 2, 6-bis (quinoline-2-formamide) -pyridine has the advantage that the External Quantum Efficiency (EQE) is obviously improved and is improved to 18.3% from the original 12.2%.
Example 2
Preparing quantum dots: firstly, 200mg of CdSe/ZnSe quantum dots (the ligand is oleic acid) and 10ml of n-hexane are added into a three-neck flask, argon is introduced for protection, the mixture is fully stirred for 10min at normal temperature to completely dissolve the quantum dots, then 8mmol of 2, 6-bis (pyrazine-2-formamide) -pyridine is added into the quantum dot solution, and the mixture is stirred for 4h at normal temperature to fully exchange the oleic acid on the surfaces of the quantum dots. And then adding ethyl acetate and methanol into the exchanged quantum dot mixed solution for centrifugal separation for multiple times to obtain the quantum dot with the surface bound with the 2, 6-bis (quinoline-2-formamide) -pyridine ligand.
The preparation steps of the QLED device are as follows:
firstly, placing a patterned ITO substrate in acetone washing liquor, deionized water and isopropanol in sequence for ultrasonic cleaning, wherein the ultrasonic cleaning of each step lasts for about 15 minutes. After the ultrasonic treatment is finished, placing the ITO in a clean oven for drying for later use; after the ITO substrate is dried, treating the ITO surface for 5 minutes by using ultraviolet-ozone so as to further remove organic matters attached to the ITO surface and improve the work function of the ITO;
then, a hole injection layer PEDOT, PSS, with the thickness of 30nm, is spin-coated on the ITO substrate processed in the previous step, and the substrate is placed on a heating table at 150 ℃ for 30 minutes to remove moisture, and the step needs to be completed in the air;
next, the dried substrate coated with the hole injection layer was placed in a nitrogen atmosphere, a layer of hole transport layer material TFB was spin-coated, the thickness of this layer was 30nm, and the substrate was placed on a heating stage at 150 ℃ and heated for 30 minutes to remove the solvent;
after the wafer processed in the previous step is cooled, the red quantum dot (surface-bound 2, 6-bis (pyrazine-2-formamide) -pyridine ligand) solution with the concentration of 20mg/ml is spin-coated on the surface of the hole transport layer, and the thickness of the hole transport layer is 20 nm. After the deposition in this step, the wafer was heated on a heating table at 80 ℃ for 10 minutes to remove the residual solvent;
then, a ZnO electron transmission layer is coated in a spinning mode, after deposition is finished, the wafer is placed on a heating table at the temperature of 80 ℃ to be heated for 30 minutes, and the thickness of the wafer is 50 nm;
and finally, placing the sheets with the deposited functional layers in an evaporation bin, and thermally evaporating a layer of 100nm silver as a cathode through a mask plate, thereby completing the preparation of the device.
The test result of the prepared device shows that compared with the device prepared by the crude oil acid ligand quantum dots, the device prepared by the quantum dots with the surface ligand of 2, 6-bis (pyrazine-2-formamide) -pyridine has the advantage that the External Quantum Efficiency (EQE) is obviously improved from 12.2% to 16.5%.
Example 3
Preparing quantum dots: firstly, 200mg of CdSe/ZnSe quantum dots (the ligand is oleic acid) and 10ml of n-hexane are added into a three-neck flask, argon is introduced for protection, the mixture is fully stirred for 10min at normal temperature to completely dissolve the quantum dots, 8mmol of 2, 6-bis (pyridine-2-formamide) -pyridine is added into the quantum dot solution, and the mixture is stirred for 4h at normal temperature to fully exchange the oleic acid on the surfaces of the quantum dots. And then adding ethyl acetate and methanol into the exchanged quantum dot mixed solution for centrifugal separation for multiple times to obtain the quantum dot with the surface bound with the 2, 6-bis (quinoline-2-formamide) -pyridine ligand.
The preparation steps of the QLED device are as follows:
firstly, placing a patterned ITO substrate in acetone washing liquor, deionized water and isopropanol in sequence for ultrasonic cleaning, wherein the ultrasonic cleaning of each step lasts for about 15 minutes. After the ultrasonic treatment is finished, placing the ITO in a clean oven for drying for later use; after the ITO substrate is dried, treating the ITO surface for 5 minutes by using ultraviolet-ozone so as to further remove organic matters attached to the ITO surface and improve the work function of the ITO;
then, a hole injection layer PEDOT, PSS, with the thickness of 30nm, is spin-coated on the ITO substrate processed in the previous step, and the substrate is placed on a heating table at 150 ℃ for 30 minutes to remove moisture, and the step needs to be completed in the air;
next, the dried substrate coated with the hole injection layer was placed in a nitrogen atmosphere, a layer of hole transport layer material TFB was spin-coated, the thickness of this layer was 30nm, and the substrate was placed on a heating stage at 150 ℃ and heated for 30 minutes to remove the solvent;
after the chip treated in the previous step is cooled, the red quantum dot (surface-bound 2, 6-bis (pyridine-2-formamide) -pyridine ligand) solution with the concentration of 20mg/ml is spin-coated on the surface of the hole transport layer, and the thickness of the hole transport layer is 20 nm. After the deposition in this step, the wafer was heated on a heating table at 80 ℃ for 10 minutes to remove the residual solvent;
then, a ZnO electron transmission layer is coated in a spinning mode, after deposition is finished, the wafer is placed on a heating table at the temperature of 80 ℃ to be heated for 30 minutes, and the thickness of the wafer is 50 nm;
and finally, placing the sheets with the deposited functional layers in an evaporation bin, and thermally evaporating a layer of 100nm silver as a cathode through a mask plate, thereby completing the preparation of the device.
The test result of the prepared device shows that compared with the device prepared by the crude oil acid ligand quantum dot, the device prepared by the quantum dot with the surface ligand of 2, 6-bis (pyridine-2-formamide) -pyridine has the advantage that the External Quantum Efficiency (EQE) is obviously improved from 12.2 percent to 13.3 percent.
Example 4
Preparing quantum dots: firstly, 200mg of CdSe/ZnSe/ZnS quantum dots (the ligand is oleic acid) and 10ml of n-hexane are added into a three-neck flask, argon is introduced for protection, the mixture is fully stirred for 10min at normal temperature to completely dissolve the quantum dots, 5mmol of 2, 6-bis (quinoline-2-formamide) -pyridine is added into the quantum dot solution, and the mixture is stirred for 4h at normal temperature to fully exchange the oleic acid on the surfaces of the quantum dots. And then adding ethyl acetate and methanol into the exchanged quantum dot mixed solution for centrifugal separation for multiple times to obtain the quantum dot with the surface bound with the 2, 6-bis (quinoline-2-formamide) -pyridine ligand.
The preparation steps of the QLED device are as follows:
firstly, placing a patterned ITO substrate in acetone washing liquor, deionized water and isopropanol in sequence for ultrasonic cleaning, wherein the ultrasonic cleaning of each step lasts for about 15 minutes. After the ultrasonic treatment is finished, placing the ITO in a clean oven for drying for later use; after the ITO substrate is dried, treating the ITO surface for 5 minutes by using ultraviolet-ozone to further remove organic matters attached to the ITO surface and improve the work function of the ITO;
then, a hole injection layer PEDOT, PSS, with the thickness of 30nm, is spin-coated on the ITO substrate processed in the previous step, and the substrate is placed on a heating table at 150 ℃ for 30 minutes to remove moisture, and the step needs to be completed in the air;
next, the dried substrate coated with the hole injection layer was placed in a nitrogen atmosphere, a layer of hole transport layer material TFB was spin-coated, the thickness of this layer was 30nm, and the substrate was placed on a heating stage at 150 ℃ and heated for 30 minutes to remove the solvent;
after the chip treated in the previous step is cooled, the solution of the green quantum dots (surface-bound 2, 6-bis (quinoline-2-carboxamide) -pyridine ligand) with the concentration of 15mg/ml is spin-coated on the surface of the hole transport layer, and the thickness of the hole transport layer is 20 nm. After the deposition in this step, the wafer was heated on a heating table at 80 ℃ for 10 minutes to remove the residual solvent;
then, a ZnO electron transmission layer is coated in a spinning mode, after deposition is finished, the wafer is placed on a heating table at the temperature of 80 ℃ to be heated for 30 minutes, and the thickness of the wafer is 50 nm;
and finally, placing the sheets with the deposited functional layers in an evaporation bin, and thermally evaporating a layer of 100nm silver as a cathode through a mask plate, thereby completing the preparation of the device.
The test result of the prepared device shows that compared with the device prepared by the crude oil acid ligand quantum dot, the device prepared by the quantum dot with the surface ligand of 2, 6-bis (quinoline-2-formamide) -pyridine has obviously improved external quantum efficiency which is improved to 14.6 percent from the original 9.5 percent.
Example 5
Preparing quantum dots: firstly, 200mg of CdZnSe/ZnSe/ZnS quantum dots (the ligand is oleic acid) and 10ml of n-hexane are added into a three-neck flask and are protected by argon, the mixture is fully stirred for 10min at normal temperature to completely dissolve the quantum dots, 2mmol of 2, 6-bis (quinoline-2-formamide) -pyridine is added into the quantum dot solution, and the mixture is stirred for 4h at normal temperature to fully exchange the oleic acid on the surfaces of the quantum dots. And then adding ethyl acetate and methanol into the exchanged quantum dot mixed solution for centrifugal separation for multiple times to obtain the quantum dot with the surface bound with the 2, 6-bis (quinoline-2-formamide) -pyridine ligand.
The preparation steps of the QLED device are as follows:
firstly, placing a patterned ITO substrate in acetone, washing liquor, deionized water and isopropanol in sequence for ultrasonic cleaning, wherein each step of ultrasonic cleaning lasts for about 15 minutes. After the ultrasonic treatment is finished, placing the ITO in a clean oven for drying for later use; after the ITO substrate is dried, treating the ITO surface for 5 minutes by using ultraviolet-ozone to further remove organic matters attached to the ITO surface and improve the work function of the ITO;
then, a hole injection layer PEDOT, PSS, with the thickness of 30nm, is spin-coated on the ITO substrate processed in the previous step, and the substrate is placed on a heating table at 150 ℃ for 30 minutes to remove moisture, and the step needs to be completed in the air;
next, the dried substrate coated with the hole injection layer was placed in a nitrogen atmosphere, a layer of hole transport layer material TFB was spin-coated, the thickness of this layer was 30nm, and the substrate was placed on a heating stage at 150 ℃ and heated for 30 minutes to remove the solvent;
after the wafer treated in the previous step is cooled, the blue quantum dot (surface-bound 2, 6-bis (quinoline-2-formamide) -pyridine ligand) solution with the concentration of 15mg/ml is spin-coated on the surface of the hole transport layer, and the thickness of the hole transport layer is 20 nm. After the deposition in this step, the wafer was heated on a heating table at 80 ℃ for 10 minutes to remove the residual solvent;
then, a ZnO electron transmission layer is coated in a spinning mode, after deposition is finished, the wafer is placed on a heating table at the temperature of 80 ℃ to be heated for 30 minutes, and the thickness of the wafer is 50 nm;
and finally, placing the sheets with the deposited functional layers in an evaporation bin, and thermally evaporating a layer of 100nm silver as a cathode through a mask plate, thereby completing the preparation of the device.
The test result of the prepared device shows that compared with the device prepared by the crude oil acid ligand quantum dots, the device prepared by the quantum dots with the surface ligand of 2, 6-bis (quinoline-2-formamide) -pyridine has the advantage that the External Quantum Efficiency (EQE) is obviously improved from the original 7.1% to 9.6%.
The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents and improvements made within the spirit and principle of the present invention are intended to be included within the scope of the present invention.
Claims (10)
1. A surface ligand, characterized in that the structural formula of the surface ligand is as follows:
wherein R is1And R2Each independently selected from at least one of quinolinyl, pyrazinyl, and pyridinyl.
2. The surface ligand of claim 1, wherein the surface ligand isIn the general structural formula (II), R1And R2The same is true.
3. The surface ligand of claim 2, wherein the surface ligand is one or more of 2, 6-bis (quinoline-2-carboxamide) -pyridine, 2, 6-bis (pyrazine-2-carboxamide) -pyridine, and 2, 6-bis (pyridine-2-carboxamide) -pyridine.
4. A quantum dot incorporating a surface ligand according to any one of claims 1 to 3.
5. A preparation method of quantum dots is characterized by comprising the following steps:
providing an initial quantum dot solution, wherein an initial ligand is bound to the surface of an initial quantum dot in the initial quantum dot solution;
mixing the surface ligand of any one of claims 1-3 with the initial quantum dot solution to perform a ligand exchange reaction to obtain the quantum dot.
6. The method of claim 5, wherein the temperature of the ligand exchange reaction is 20 to 300 ℃; and/or the presence of a gas in the gas,
the time of the ligand exchange reaction is 3-5 h.
7. The method of claim 5, wherein the molar ratio of the initial ligand to the surface ligand is from 1:1 to 1: 50; and/or the presence of a gas in the gas,
the initial quantum dot concentration in the initial quantum dot solution is 10-50 mg/mL.
8. The method of claim 5, wherein the initial ligand is selected from at least one of oleic acid, oleylamine, trioctylphosphine and trioctylphosphine; and/or the presence of a gas in the gas,
the solvent in the initial quantum dot solution is at least one selected from toluene, chloroform, chlorobenzene, carbon tetrachloride, n-hexane and octane.
9. The method according to any one of claims 5 to 8, further comprising a step of washing with a polar solvent after the ligand exchange reaction.
10. A quantum dot light-emitting diode comprising an anode, a cathode and a quantum dot light-emitting layer disposed between the anode and the cathode, wherein the quantum dot light-emitting layer is composed of the quantum dot of claim 4 and/or the quantum dot obtained by the preparation method of any one of claims 5 to 9.
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