Quantum dot light-emitting diode and method for preparing the same

Abstract

The present invention discloses a quantum dot light-emitting diode and a preparation method thereof. The quantum dot light-emitting diode includes: a quantum dot layer and an electron transport layer formed on the quantum dot layer, and an ester substance is bonded to the surface of the quantum dot layer on the side close to the electron transport layer. In the present invention, the surface of the quantum dot layer on the side where the electron transport layer is located is replaced with an ester substance. Since the ester substance has good hydrophilicity and the same polarity as that of the electron transport layer, the compatibility between the quantum dot layer and the electron transport layer is effectively improved, the surface contact angle between the quantum dot layer and the electron transport layer is reduced, the voids and defects between the film layers are filled, the occurrence of non-radiative recombination is effectively avoided, the generation of leakage current is reduced, and thus the light-emitting performance of the device is significantly improved.

Description

Technical Field

[0001] The present invention relates to the field of quantum dot light-emitting devices, and particularly to a quantum dot light-emitting diode and a preparation method thereof. Background Art

[0002] Quantum dots (QDs) are usually composed of dozens to millions of atoms, with geometric sizes similar to exciton sizes. They not only partially inherit the characteristics of bulk semiconductors but also exhibit unique optoelectronic properties, specifically including: high color purity, continuously tunable emission spectra with respect to size and composition, narrow full-width at half-maximum, high fluorescence efficiency, long lifetime, excellent monodispersity and photothermal stability, and also excellent solution processability. They have broad application prospects in the fields of display, laser, photovoltaics, biological labeling, etc. Among them, the application of quantum dots in the display field, especially the QD-LCD TVs vigorously promoted by manufacturers such as TCL and Samsung, also indicates that their initial commercialization has begun to take shape.

[0003] With the continuous improvement of quantum dot synthesis technology, the continuous optimization of device structures, and the continuous in-depth theoretical research on the lifetime problem of QLED (quantum dot light-emitting diode) devices, the efficiency and lifetime of the devices have been greatly improved. In particular, the device performance of red and green QLEDs has been able to rival that of existing widely used OLEDs (organic light-emitting diodes), marking a solid step towards the true commercialization of QLEDs. Existing high-performance QLEDs usually use ZnO with high electron mobility as the electron transport layer. On the one hand, this kind of ZnO is usually prepared by a low-temperature solution method and has a polar surface. When depositing the ZnO layer onto the quantum dot light-emitting layer, the contact angle between the polar surface of ZnO and the non-polar surface of the quantum dots is large, the film-forming property of ZnO nanoparticles becomes poor, and electron injection becomes difficult. At the same time, the surface structure of the quantum dots is disordered and the lattice is incomplete, resulting in changes in the electrical properties of the quantum dot layer. On the other hand, due to the fact that the electron mobility in the existing QLED device structure is much higher than the hole mobility, the interfacial charge accumulation between the quantum dots and the electron transport layer is very serious, which not only causes excessive heat generation in the device but also leads to an increase in voltage and a significant decrease in lifetime and efficiency.

[0004] Therefore, the existing technology still needs to be improved and developed. Summary of the Invention

[0005] In view of the above deficiencies of the existing technology, the purpose of the present invention is to provide a quantum dot light-emitting diode and a preparation method thereof, aiming to solve the problem that the large polarity difference between the existing quantum dot layer and the electron transport layer leads to poor film-forming quality of the electron transport layer.

[0006] The technical solution of the present invention is as follows:

[0007] In a first aspect of the present invention, there is provided a quantum dot light-emitting diode, which includes: a quantum dot layer and an electron transport layer formed on the quantum dot layer, and an ester substance is bonded to the surface of the quantum dot layer on the side close to the electron transport layer.

[0008] In the present invention, the surface of the quantum dot layer on the side where the electron transport layer is located is replaced with an ester substance. Since the ester substance has good hydrophilicity and its polarity is the same as that of the electron transport layer, the compatibility between the quantum dot layer and the electron transport layer is effectively improved, the surface contact angle between the quantum dot layer and the electron transport layer is reduced, the voids and defects between the film layers are filled, the occurrence of non-radiative recombination is effectively avoided, the generation of leakage current is reduced, and thus the luminescence performance of the device is significantly improved.

[0009] In a second aspect of the present invention, there is provided a method for preparing a quantum dot light-emitting diode, which includes the steps of:

[0010] providing a quantum dot layer with an ester substance bonded to its surface;

[0011] forming an electron transport layer on the surface of the quantum dot layer where the ester substance is bonded.

[0012] In the present invention, by replacing the surface of the quantum dot layer on the side where the electron transport layer is located with an ester substance, while the remaining regions of the quantum dot layer still remain as non-polar ligands of the quantum dots. Since the ester substance has good hydrophilicity, the surface of the quantum dot layer in contact with the electron transport layer presents a polar state, and the remaining regions of the quantum dot layer still remain in the original non-polar state. In this way, without loss of the original fluorescence efficiency of the quantum dots, the compatibility between the quantum dot layer and the electron transport layer is effectively improved, the surface contact angle between the quantum dot layer and the electron transport layer is reduced, the voids and defects between the film layers are filled, the occurrence of non-radiative recombination is effectively avoided, the generation of leakage current is reduced, and thus the luminescence performance of the device is significantly improved. Description of the Drawings

[0013] Figure 1 It is a schematic structural diagram of a quantum dot light-emitting diode provided in an embodiment of the present invention.

[0014] Figure 2 It is a schematic flow diagram of a method for preparing a quantum dot light-emitting diode provided in an embodiment of the present invention.

[0015] Figure 3 It is a schematic diagram before and after the treatment (ligand exchange + esterification reaction) of the quantum dot layer in an embodiment of the present invention. Detailed Embodiments

[0016] The present invention provides a quantum dot light-emitting diode and a preparation method thereof. To make the objectives, technical solutions and effects of the present invention clearer and more definite, the present invention is further described in detail below. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0017] An embodiment of the present invention provides a quantum dot light-emitting diode, which includes: a quantum dot layer and an electron transport layer formed on the quantum dot layer, and an ester substance is bonded to the surface of the quantum dot layer on the side close to the electron transport layer.

[0018] In the existing quantum dot light-emitting diodes, the surface of the entire quantum dot layer is usually coordinated with original ligands. In the quantum dot layer of the quantum dot light-emitting diode of this embodiment, the surface in contact with the electron transport layer is replaced with an ester substance, and the remaining regions of the quantum dot layer still remain as original ligands. According to the principle that like charges repel and opposite charges attract, when the ligand on the surface of the quantum dot layer on the side close to the electron transport layer is replaced with an ester substance, it can, to a certain extent, hinder the injection of electrons with the same charge as the ester group functional group, while the original ligands in some regions of the quantum dot layer can, to a certain extent, accelerate the injection of holes with the opposite charge to the functional group of this kind of ligand, thereby helping to improve the injection balance of electrons and holes, enhancing the recombination efficiency of carriers in the entire quantum dot layer, and improving the device performance.

[0019] In addition, in the existing quantum dot light-emitting diodes, the original ligands coordinated with the quantum dots are usually non-polar ligands, and the entire surface of the quantum dot layer is in a non-polar state, while the surface of the electron transport layer in contact with the quantum dot layer is in a polar state. In this way, due to the large polarity difference between the quantum dot layer and the electron transport layer, the surface contact angle between the quantum dot layer and the electron transport layer is large, there are a large number of defects, electron injection is difficult, non-radiative recombination is serious, and the light-emitting performance of the device is low. To solve this technical problem, the existing method is to first perform ligand exchange in the form of a quantum dot solution, and then deposit the quantum dot solution after ligand exchange to form a quantum dot layer, so as to improve the compatibility between the quantum dot layer and the electron transport layer by this method. Although this method can achieve effective ligand exchange, the ligand exchange occurs on the surface of all quantum dots, which results in a significant reduction in the fluorescence efficiency of the quantum dots themselves.

[0020] In the quantum dot light-emitting diode of this embodiment, by replacing the region of the quantum dot layer close to the electron transport layer with an ester substance, while the remaining regions of the quantum dot layer still remain as non-polar ligands of the quantum dots. Since the ester substance has good hydrophilicity, the surface of the quantum dot layer in contact with the electron transport layer presents a polar state, and the remaining regions of the quantum dot layer still remain in the original non-polar state. In this way, without losing the original fluorescence efficiency of the quantum dots, the compatibility between the quantum dot layer and the electron transport layer is effectively improved, the surface contact angle between the quantum dot layer and the electron transport layer is reduced, the voids and defects between the film layers are filled, the occurrence of non-radiative recombination is effectively avoided, the generation of leakage current is reduced, and thus the luminescence performance of the device is significantly improved.

[0021] In one embodiment, a first ligand (i.e., the original ligand bound to the quantum dots) is bonded to the surface of the quantum dot layer on the side away from the electron transport layer.

[0022] In one embodiment, the first ligand is selected from one or more of organic carboxylic acids having 8 or more carbon atoms, primary amines having 8 or more carbon atoms, secondary or tertiary amines having 4 or more branched-chain carbon atoms, and organic phosphines having 4 or more branched-chain carbon atoms.

[0023] In one embodiment, the first ligand is selected from one or more of organic carboxylic acids having 8 to 20 carbon atoms, primary amines having 8 to 20 carbon atoms, secondary or tertiary amines having 4 to 20 branched-chain carbon atoms, and organic phosphines having 4 to 20 branched-chain carbon atoms.

[0024] As an example, the organic carboxylic acids having 8 to 20 carbon atoms are selected from one or more of octanoic acid, nonanoic acid, decanoic acid, undecanoic acid, dodecanoic acid, tridecanoic acid, tetradecanoic acid, hexadecanoic acid, octadecanoic acid, undecenoic acid, dodecenoic acid, tridecenoic acid, tetradecenoic acid, pentadecenoic acid, hexadecenoic acid, heptadecenoic acid, and octadecenoic acid.

[0025] As an example, the primary amines having 8 to 20 carbon atoms are selected from one or more of octylamine, nonylamine, decylamine, dodecylamine, tetradecylamine, hexadecylamine, and octadecylamine.

[0026] As an example, the secondary or tertiary amines having 4 to 20 branched-chain carbon atoms are selected from one or more of tributylamine, trihexylamine, triheptylamine, trioctylamine, trinonylamine, and tridecylamine.

[0027] As an example, the organic phosphines having 4 to 20 branched-chain carbon atoms are selected from one or more of tributylphosphine, trihexylphosphine, triheptylphosphine, trioctylphosphine, trinonylphosphine, and tridecylphosphine.

[0028] In one embodiment, the ester substance is formed by an esterification reaction of a second ligand and a third ligand, and the second ligand and the third ligand are bound to the surface of the quantum dot layer.

[0029] In one embodiment, the carbon chain length of the first ligand is the same as that of the second ligand and the third ligand forming the ester substance, and the molar ratio of the second ligand to the third ligand is 1:1. For example, when the first ligand is undecanoic acid, the second ligand is 11-mercaptoundecanoic acid, the third ligand is 11-mercapto-1-undecanol, and the molar ratio of 11-mercaptoundecanoic acid to 11-mercapto-1-undecanol is 1:1. In this embodiment, the carbon chain lengths of the respective ligands are the same, which can further avoid the problem of a significant reduction in the fluorescence efficiency of the quantum dots caused by conventional ligand replacement.

[0030] In one embodiment, one end of the carbon chain of the second ligand is a mercapto group and the other end is a carboxyl group, and the second ligand is bound to the surface of the quantum dot layer through the mercapto group. The carbon chain of the second ligand can be the same as that of the first ligand.

[0031] In one embodiment, one end of the carbon chain of the third ligand is a mercapto group and the other end is a hydroxyl group, and the third ligand is bound to the surface of the quantum dot layer through the mercapto group. The carbon chain of the third ligand can be the same as that of the first ligand.

[0032] In this embodiment, the quantum dot light-emitting diode is divided into two types: a normal structure and an inverted structure, and each structure of the quantum dot light-emitting diode can have multiple forms. Below, Figure 1 taking the quantum dot light-emitting diode with the normal structure shown as an example, the structure and material selection of the quantum dot light-emitting diode in this embodiment are introduced. As Figure 1 shown, the quantum dot light-emitting diode sequentially includes a substrate, an anode, a hole injection layer, a hole transport layer, a quantum dot layer, an electron transport layer, and a cathode from bottom to top; wherein, an ester substance is bound to the surface of the quantum dot layer on the side where the electron transport layer is located.

[0033] In one embodiment, the substrate can be a rigid substrate or a flexible substrate, and the specific substrate is selected from glass, silicon wafer, polycarbonate, polymethyl methacrylate, polyethylene terephthalate, polyethylene naphthalate, polyamide, polyethersulfone, or a combination thereof, etc.

[0034] In one embodiment, the material of the anode can be selected from nickel, platinum, vanadium, chromium, copper, zinc, gold, or their alloys; the material of the anode can also be selected from one or more of zinc oxide, indium oxide, tin oxide, indium zinc oxide, indium-doped tin oxide, fluorine-doped tin oxide, antimony-doped tin oxide, and aluminum-doped zinc oxide, etc.; the material of the anode can also be a combination of any two or more of the above.

[0035] In one embodiment, the material of the hole injection layer can be selected from materials with good hole injection performance, such as but not limited to poly(3,4-ethylenedioxythiophene)-polystyrene sulfonic acid (PEDOT:PSS), copper phthalocyanine (CuPc), 2,3,5,6-tetrafluoro-7,7',8,8'-tetracyanoquinodimethane (F4-TCNQ), 2,3,6,7,10,11-hexacyano-1,4,5,8,9,12-hexaazatriphenylene (HATCN), doped or undoped transition metal oxides, doped or undoped metal chalcogenides; wherein, the transition metal oxides include but not limited to one or more of MoO3, VO2, WO3, CuO; the metal chalcogenides include but not limited to one or more of MoS2, MoSe2, WS2, WSe2, CuS. In one embodiment, the thickness of the hole injection layer is 10-150 nm.

[0036] In one embodiment, the material of the hole transport layer can be selected from organic materials with good hole transport ability, such as but not limited to 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',4''-tris(carbazol-9-yl)triphenylamine (TCTA), 4,4'-bis(9-carbazolyl)biphenyl (CBP), N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1'-biphenyl-4,4'-diamine (TPD), N,N'-diphenyl-N,N'-(1-naphthyl)-1,1'-biphenyl-4,4'-diamine (NPB), doped graphene, undoped graphene, C60. In one embodiment, the thickness of the hole transport layer is 10-150 nm.

[0037] In one embodiment, the quantum dots can be selected from CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, ZnO, HgS, HgSe, HgTe, CdSeS, CdSeTe, CdSTe, ZnSeS, ZnSeTe, ZnSTe, HgSeS, HgSeTe, HgSTe, CdZnS, CdZnSe, CdZnTe, CdHgS, CdHgSe, CdHgTe, HgZnS, HgZnSe, HgZnTe, CdZnSeS, CdZnSeTe, CdZnSTe, CdHgSeS, CdHgSeTe, CdHgSTe, HgZnSeS, HgZnSeTe, HgZnSTe of Group II-VI; or GaN, GaP, GaAs, GaSb, AlN, AlP, AlAs, AlSb, InN, InP, InAs, InSb, GaNP, GaNAs, GaNSb, GaPAs, GaPSb, AlNP, AlNAs, AlNSb, AlPAs, AlPSb, InNP, InNAs, InNSb, InPAs, InPSb, GaAlNP, GaAlNAs, GaAlNSb, GaAlPAs, GaAlPSb, GaInNP, GaInNAs, GaInNSb, GaInPAs, GaInPSb, InAlNP, InAlNAs, InAlNSb, InAlPAs, InAlPSb of Group III-V; or SnS, SnSe, SnTe, PbS, PbSe, PbTe, SnSeS, SnSeTe, SnSTe, PbSeS, PbSeTe, PbSTe, SnPbS, SnPbSe, SnPbTe, SnPbSSe, SnPbSeTe, SnPbSTe of Group IV-VI; or a combination of any one or more of the above.

[0038] In one embodiment, the electron transport layer can be selected from one or more of ZnO, TiO2, Alq3, SnO, ZrO, AlZnO, ZnSnO, BCP, TAZ, PBD, TPBI, Bphen, CsCO3. In one embodiment, the thickness of the electron transport layer is 5 to 100 nm.

[0039] In one embodiment, the cathode can be selected from metals or their alloys, such as magnesium, calcium, sodium, potassium, titanium, indium, yttrium, lithium, gadolinium, aluminum, silver, tin, lead, cesium, barium, or their alloys; the cathode can also be composed of a multi-layer structural material, such as composed of a first layer and a second layer located on the first layer, the material of the first layer is alkali metal halide, alkaline earth metal halide, alkali metal oxide, or a combination thereof, and the material of the second layer is alkaline earth metal, group 13 metal, or a combination thereof. For example, the cathode is LiF / Al, LiO2 / Al, LiF / Ca, Liq / Al, and BaF₂ / Ca, but not limited thereto.

[0040] An embodiment of the present invention provides a method for preparing a quantum dot light-emitting diode, as Figure 2 shown, including the following steps:

[0041] S10. Provide a quantum dot layer with an ester substance bound to its surface;

[0042] S20. Form an electron transport layer on the surface of the quantum dot layer where the ester substance is bound.

[0043] In the quantum dot light-emitting diode of this embodiment, by replacing the region of the quantum dot layer close to the electron transport layer with an ester substance, while the remaining regions of the quantum dot layer still remain as non-polar ligands of the quantum dots. Since the ester substance has good hydrophilicity, the surface of the quantum dot layer in contact with the electron transport layer presents a polar state, and the remaining regions of the quantum dot layer still remain in the original non-polar state. In this way, without losing the original fluorescence efficiency of the quantum dots, the compatibility between the quantum dot layer and the electron transport layer is effectively improved, the surface contact angle between the quantum dot layer and the electron transport layer is reduced, the voids and defects between the film layers are filled, the occurrence of non-radiative recombination is effectively avoided, the generation of leakage current is reduced, and thus the light-emitting performance of the device is significantly improved.

[0044] In step S10, in one embodiment, the method for preparing a quantum dot layer with an ester substance bound to its surface includes the following steps:

[0045] S11. Provide a quantum dot layer with a first ligand bound to both surfaces (for example, Figure 3 the upper surface and the lower surface in

[0046] S12. Under a vacuum condition of <5×10 -4 Pa, use a second ligand and a third ligand to perform ligand exchange with the first ligand on one of the two surfaces of the quantum dot layer (for example, Figure 3 the upper surface in

[0047] S13. Make the second ligand and the third ligand undergo an esterification reaction to obtain a quantum dot layer with an ester substance bound to its surface.

[0048] In this embodiment, when the second ligand and the third ligand are added to the surface of the quantum dot layer having the first ligand, under the dual induction of the ligand concentration difference and the interaction between the cations on the surface of the quantum dots and the functional groups of the second ligand and the third ligand, the second ligand and the third ligand will directly diffuse along the direction of the hole transport layer (usually the quantum dot layer is located on the surface of the hole transport layer), thereby inducing ligand exchange of some ligands on the surface of the quantum dot layer (the surface to be in contact with the electron transport layer). At the same time, in order to prevent the entire quantum dot film layer from undergoing ligand exchange due to an excessive concentration difference, vacuum treatment is performed above the quantum dot film layer (the vacuum condition is preferably <5×10 -4 Pa). Under the condition of vacuum pumping, an upward pulling force is generated on the ligand to be exchanged, so as to better balance the ligand to be exchanged due to an excessive concentration difference, resulting in ligand exchange occurring in the entire quantum dot film layer. And ligand exchange does not occur in the region in contact with the hole transport layer, and this region remains the original ligand of the quantum dots (i.e., the first ligand). All the surfaces of the quantum dots in the quantum dot layer before ligand exchange are coordinated with the first ligand. After ligand exchange, the surfaces on the side close to the electron transport layer in the quantum dot layer are simultaneously coordinated with the second ligand and the third ligand, while the remaining regions all maintain the original first ligand. By using the above ligand exchange method, the problem of a significant reduction in the fluorescence efficiency of the quantum dots caused by conventional ligand exchange can be effectively avoided without loss of the original fluorescence efficiency of the quantum dots, and ligand exchange occurs in some regions on the surface of the quantum dots.

[0049] Next, the second ligand and the third ligand on the surface of the quantum dot layer after ligand exchange undergo an esterification reaction to form an ester substance. Since the ester substance has good hydrophilicity and its polarity is the same as that of the electron transport layer, this effectively improves the compatibility between the quantum dot layer and the electron transport layer, reduces the surface contact angle between the quantum dot layer and the electron transport layer, fills the voids and defects between the film layers, effectively avoids the occurrence of non-radiative recombination, reduces the generation of leakage current, and thus significantly improves the light-emitting performance of the device. In addition, this embodiment has the advantages of simple, mild, effective, fast method and strong universality, and is very suitable for the future large-scale application of quantum dots.

[0050] As Figure 3As shown, the surfaces of all the quantum dots in the quantum dot layer 1 before ligand exchange are coordinated with the first ligand. After ligand exchange and esterification reaction, the surface ligand on the side close to the electron transport layer in the quantum dot layer 2 is replaced by an ester substance (obtained by the esterification reaction of the second ligand and the third ligand), while the remaining regions retain the original first ligand. According to the principle of like charges repelling and opposite charges attracting, when the surface ligand on the side close to the electron transport layer of the quantum dot layer is replaced by an ester substance, it can, to a certain extent, hinder the injection of electrons with the same charge as the ester group functional group, while the original ligand in some regions of the quantum dot layer can, to a certain extent, accelerate the injection of holes with the opposite charge to the functional group of this ligand, thus helping to improve the injection balance of electrons and holes, enhance the recombination efficiency of carriers in the entire quantum dot layer, and improve the device performance.

[0051] In step S11, in the quantum dot layer with the first ligand bound to the surface, this first ligand is also the original ligand of the quantum dots, and the surface on the side of the original quantum dot layer far from the hole transport layer presents a non-polar state. For the specific type of the first ligand, see the above text and will not be elaborated here. It should be noted that in this step, in the quantum dot layer, the surfaces of all the quantum dots are bound to this first ligand, so the surface of the entire quantum dot layer presents a non-polar state.

[0052] In one embodiment, the quantum dot layer is prepared by the following method: spin-coating the prepared quantum dot solution on the prepared hole transport layer, and then performing thermal annealing treatment to obtain the quantum dot layer. Among them, the film thickness can be controlled by adjusting the concentration of the solution, the spin-coating speed, and the spin-coating time. Among them, the quantum dots are quantum dots with the first ligand bound to the surface. In one embodiment, the thickness of the quantum dot layer can be 20 - 60 nm, such as 30 nm.

[0053] In one embodiment, the concentration of the quantum dot solution is 10 - 30 mg / ml. In one embodiment, the solvent used to disperse the quantum dots is selected from at least one of n-octane, n-hexane, cyclohexane, cyclooctane, etc., but is not limited thereto.

[0054] In one embodiment, step S12 includes: adding the second ligand and the third ligand to one surface of the quantum dot layer, and then placing it under a vacuum of <5×10 -4 Pa, performing spin-coating, the spin-coating speed is 100 - 2000 rpm, the spin-coating time is 10 s - 2 min, and using the second ligand and the third ligand to perform ligand exchange with the first ligand located on one surface of the quantum dot layer; after the ligand exchange is completed, the surface of the quantum dot layer is washed, and finally dried to obtain the quantum dot layer with the second ligand and the third ligand bound to the surface.

[0055] In one embodiment, the ratio of the total volume of the second ligand and the third ligand to the mass of the quantum dots is 0.01 - 10:1, and the second ligand and the third ligand are added to one surface of the quantum dot layer for ligand exchange. In one embodiment, the molar ratio of the second ligand to the third ligand is 1:1.

[0056] In one embodiment, after completion, a small amount of isopropanol is added to wash the surface of the quantum dot layer, and finally it is dried at 50 - 120 °C for 30 min - 4 h to obtain a quantum dot layer with the second ligand and the third ligand bound to the surface. In one embodiment, the ratio of the volume of isopropanol to the total volume of the second ligand and the third ligand is 1 - 20:1.

[0057] In one embodiment, the lengths of the carbon chains of the first ligand, the second ligand, and the third ligand are the same. For example, when the first ligand is undecanoic acid, the second ligand is 11-mercaptoundecanoic acid, and the third ligand is 11-mercapto-1-undecanol, and the molar ratio of 11-mercaptoundecanoic acid to 11-mercapto-1-undecanol is 1:1. Since the lengths of the carbon chains of the second ligand and the third ligand are the same as the length of the carbon chain of the first ligand, it is equivalent to an in-situ ligand exchange occurring on the surface of the quantum dots in contact with the electron transport layer, further ensuring that, on the premise of no loss of the original quantum dot fluorescence, the problem of a significant reduction in the fluorescence efficiency of the quantum dots caused by conventional ligand exchange is effectively avoided.

[0058] In this embodiment, one end of the carbon chain of the second ligand is a mercapto group, and the other end is a carboxyl group. The second ligand is bound to the surface of the quantum dot layer through the mercapto group. The carbon chain of the second ligand can be the same as the carbon chain of the first ligand.

[0059] In this embodiment, one end of the carbon chain of the third ligand is a mercapto group, and the other end is a hydroxyl group. The third ligand is bound to the surface of the quantum dot layer through the mercapto group. The carbon chain of the third ligand can be the same as the carbon chain of the first ligand.

[0060] In this embodiment, the first ligand is usually an organic carboxylic acid, primary amine, secondary amine, tertiary amine, or organic phosphine, etc. Since the binding force between the mercapto groups in the second ligand and the third ligand and the quantum dots is greater than the binding force between carboxyl groups, amino groups, phosphoric acid, etc. and the quantum dots, the coordination ability of the second ligand and the coordination ability of the third ligand are both greater than the coordination ability of the first ligand, which is conducive to promoting ligand exchange.

[0061] In step S13, in one embodiment, the step of subjecting the second ligand and the third ligand to an esterification reaction includes: under the catalysis of an inorganic acid, the second ligand and the third ligand are subjected to an esterification reaction, where the temperature of the esterification reaction is 50 to 180 °C and the time of the esterification reaction is 30 min to 4 h. In this embodiment, the inorganic acid acts as a catalyst to catalyze the reaction between the carboxyl group at one end of the second ligand and the hydroxyl group at one end of the third ligand to form an ester substance.

[0062] In one embodiment, after the esterification reaction, the steps further include: adding a small amount of isopropanol to clean the surface of the quantum dot layer, and finally drying it at 50 - 120 °C for 30 min - 4 h.

[0063] In one embodiment, the inorganic acid can be at least one of dichromic acid, diphosphoric acid, hypoiodous acid, hypochlorous acid, hypobromous acid, hypophosphorous acid, hyposulfurous acid, superoxo acid, iodic acid, telluric acid, fluoroboric acid, fluorosilicic acid, fluoroantimonic acid, fluorophosphoric acid, fluorosulfuric acid, fluoroplatinic acid, fluorooxy acid, vanadic acid, perchloric acid, permanganic acid, ferrate(VI) acid, periodic acid, perbromic acid, perxenic acid, chromic acid, silicic acid, cobaltous acid, peroxymonosulfuric acid, peroxydisulfuric acid, peroxydicarbonic acid, perboric acid, percarbonic acid, peroxy acid, pyrophosphoric acid, pyrosulfuric acid, metabisulfurous acid, tetrathionic acid, dithionous acid, phosphoric acid, phosphomolybdic acid, thiosulfuric acid, sulfuric acid, thiocyanic acid, chloric acid, chloroauric acid, chloroplatinic acid, chlorosulfonic acid, aluminic acid, manganous acid, molybdic acid, nickelous acid, boric acid, metaboric acid, metaphosphoric acid, metaarsenous acid, metaaluminic acid, plumbic acid, cyanic acid, hydroiodic acid, hydrazoic acid, hydrotelluric acid, hydrofluoric acid, hydrosulfuric acid, hydrochloric acid, hydrocyanic acid, hydrobromic acid, arsenic acid, trisilicic acid, tetraboric acid, tetrahydroxyaluminate, tetrahydroxycuprate, titanic acid, carbonic acid, ferrate(III) acid, tungstic acid, nitric acid, selenic acid, stannic acid, zincic acid, bromic acid, xenic acid, tellurous acid, sulfurous acid, phosphorous acid, chlorous acid, hydrochloric acid, chromous acid, plumbous acid, arsenous acid, nitrous acid, selenous acid, stannous acid, uranic acid, orthosilicic acid, orthophosphoric acid, orthosulfuric acid, orthocarbonic acid, etc., but not limited thereto.

[0064] In this embodiment, the obtained quantum dot light-emitting diode can be subjected to a packaging process. The packaging process can be carried out by using a common machine packaging or manual packaging. In one embodiment, in the environment of the packaging process, the oxygen content and the water content are both lower than 0.1 ppm to ensure the stability of the device.

[0065] The present invention will be described in detail below through examples.

[0066] Example 1:

[0067] 1. Ligand exchange is carried out on the CdZnSe / ZnSe / ZnS quantum dot layer using 11-mercaptoundecanoic acid and 11-mercapto-1-undecanol, and then a quantum dot layer is obtained through an esterification reaction. The original ligand of the quantum dots is undecanoic acid.

[0068] (1) First, deposit a CdZnSe / ZnSe / ZnS quantum dot layer on the hole transport layer. The concentration of the quantum dots is 20 mg / ml, the solvent is n-octane, and the volume is 0.8 ml.

[0069] (2) Then, drop 0.4 ml of 11-mercaptoundecanoic acid and 11-mercapto-1-undecanol (the molar ratio between the two is 1:1) onto the surface of the quantum dot layer, and then perform spin coating (at a speed of 1000 rpm for 2 minutes) under a vacuum condition of 6×10 -4 Pa. After completion, add 1.5 ml of isopropanol to rinse the surface of the quantum dot layer.

[0070] (3) Next, drop 0.1 ml of HCl onto the surface of step (2), and then place it at 100 °C for an esterification reaction for 60 minutes to obtain an ester substance bound to the surface of the quantum dot layer. After completion, add a small amount of isopropanol to rinse the surface of the quantum dot layer.

[0071] 2. Preparation of quantum dot light-emitting diodes:

[0072] The quantum dot light-emitting diode of this embodiment includes, from bottom to top in sequence: an anode, a hole injection layer, a hole transport layer, a quantum dot layer, an electron transport layer, and a cathode, which are sequentially deposited on a substrate. Among them, the substrate is a glass substrate; the anode is ITO with a thickness of 110 nm; the hole injection layer is PEDOT:PSS with a thickness of 90 nm; the hole transport layer is TFB with a thickness of 70 nm. The quantum dot layer is the quantum dot layer prepared in step 1 above, with a thickness of 60 nm. The electron transport layer is ZnO with a thickness of 50 nm; the cathode is Al with a thickness of 60 nm.

[0073] Comparative Example 1:

[0074] The main difference from Example 1 is that the quantum dot layer does not undergo the ligand exchange process (i.e., step (2)) and the esterification reaction process (i.e., step (3)) in step 1 above.

[0075] Example 2:

[0076] 1. For Cd x Zn 1-x Se / Cd y Zn 1-y The Se / ZnSe / CdZnS (0 < x < 1, 0 < y < 1, and x < y) quantum dot layer undergoes ligand exchange with 11-mercaptoundecanoic acid and 11-mercapto-1-undecanol, and then a quantum dot layer is obtained through an esterification reaction. The original ligand of the quantum dots is undecanoic acid.

[0077] (1) First, deposit Cd x Zn 1-xSe / Cd y Zn 1-y A Se / ZnSe / CdZnS (0 < x < 1, 0 < y < 1, and x < y) quantum dot layer. The quantum dot concentration is 20 mg / ml, the solvent is n-octane, and the volume is 0.8 ml.

[0078] (2) Then, 0.4 ml of 11-mercaptoundecanoic acid and 11-mercapto-1-undecanol (the molar ratio between the two is 1:1) are dropped onto the surface of the quantum dot layer, and then spin coating is carried out under a vacuum condition of 6×10 -4 Pa (the speed is 1000 rpm and the time is 2 min). After completion, 1.5 ml of isopropanol is added to rinse the surface of the quantum dot layer.

[0079] (3) Next, 0.1 ml of HCl is dropped onto the surface of step (2), and then it is dried at 100 °C for 60 min to obtain an ester substance bound to the surface of the quantum dot layer. After completion, a small amount of isopropanol is added to rinse the surface of the quantum dot layer.

[0080] 2. Preparation of quantum dot light-emitting diodes:

[0081] The quantum dot light-emitting diode of this embodiment includes, from bottom to top in sequence: an anode, a hole injection layer, a hole transport layer, a quantum dot layer, an electron transport layer, and a cathode, which are sequentially deposited on a substrate. Among them, the substrate is a glass substrate; the anode is ITO with a thickness of 110 nm; the hole injection layer is PEDOT:PSS with a thickness of 90 nm; the hole transport layer is TFB with a thickness of 70 nm. The quantum dot layer is the quantum dot layer prepared in step 1 above with a thickness of 60 nm. The electron transport layer is ZnO with a thickness of 50 nm; the cathode is Al with a thickness of 60 nm.

[0082] Comparative Example 2:

[0083] The main difference from Example 2 is that the quantum dot layer does not go through the ligand exchange process (i.e., step (2)) and the esterification reaction process (i.e., step (3)) in step 1 above.

[0084] Example 3:

[0085] 1. For Cd x Zn 1-x S / Cd y Zn 1-y The S / ZnS (0 < x < 1, 0 < y < 1, and x < y) quantum dot layer is subjected to ligand exchange using 11-mercaptoundecanoic acid and 11-mercapto-1-undecanol, and then a quantum dot layer is obtained through an esterification reaction. The original ligand of the quantum dot is undecanoic acid.

[0086] (1) First, Cd is deposited on the hole transport layerx Zn 1-x S / Cd y Zn 1-y An S / ZnS (0 < x < 1, 0 < y < 1, and x < y) quantum dot layer. The quantum dot concentration is 20 mg / ml, the solvent is n-octane, and the volume is 0.8 ml.

[0087] (2) Then, 0.4 ml of 11-mercaptoundecanoic acid and 11-mercapto-1-undecanol (the molar ratio between the two is 1:1) are dropped onto the surface of the quantum dot layer, and then spin coating is carried out under a vacuum condition of 6×10 -4 Pa (the speed is 1000 rpm and the time is 2 min). After completion, 1.5 ml of isopropanol is added to rinse the surface of the quantum dot layer.

[0088] (3) Next, 0.1 ml of HCl is dropped onto the surface of step (2), and then it is dried at 100 °C for 60 min to obtain an ester substance bound to the surface of the quantum dot layer. After completion, a small amount of isopropanol is added to rinse the surface of the quantum dot layer.

[0089] 2. Preparation of quantum dot light-emitting diodes:

[0090] The quantum dot light-emitting diode of this embodiment includes, from bottom to top in sequence: an anode, a hole injection layer, a hole transport layer, a quantum dot layer, an electron transport layer, and a cathode, which are sequentially deposited on a substrate. Among them, the substrate is a glass substrate; the anode is ITO with a thickness of 110 nm; the hole injection layer is PEDOT:PSS with a thickness of 90 nm; the hole transport layer is TFB with a thickness of 70 nm. The quantum dot layer is the quantum dot layer prepared in step 1 above, with a thickness of 60 nm. The electron transport layer is ZnO with a thickness of 50 nm; the cathode is Al with a thickness of 60 nm.

[0091] Comparative Example 3:

[0092] The main difference from Example 3 is that the quantum dot layer does not undergo the ligand exchange process (i.e., step (2)) and the esterification reaction process (i.e., step (3)) in step 1 above.

[0093] Table 1. EQE (%) of the devices prepared in Comparative Example and Examples 1 - 3

[0094]

[0095] The performance of the quantum dot layers and quantum dot light-emitting diodes prepared in the comparative example and Examples 1 - 3 was tested as follows:

[0096] External quantum dot efficiency:

[0097] The ratio of the number of electron-hole pairs injected into the quantum dots to the number of emitted photons, in %, is an important parameter for measuring the quality of an electroluminescent device and can be obtained by measuring with an EQE optical test instrument. The specific calculation formula is as follows:

[0098]

[0099] In the formula, ηe is the optical output coupling efficiency, ηr is the ratio of the number of recombined carriers to the number of injected carriers, χ is the ratio of the number of excitons generating photons to the total number of excitons, K R is the radiation process rate, and K NR is the non-radiation process rate.

[0100] It should be understood that the application of the present invention is not limited to the above examples. For those of ordinary skill in the art, improvements or transformations can be made according to the above description, and all such improvements and transformations should fall within the protection scope of the appended claims of the present invention.

Claims

1. A quantum dot light emitting diode, characterized in that, It includes a quantum dot layer and an electron transport layer formed on the quantum dot layer. A first ligand is bonded to the surface of the quantum dot layer on the side away from the electron transport layer; the first ligand is the original ligand of the quantum dot, and the first ligand is non-polar. An ester substance is bonded to the surface of the quantum dot layer on the side close to the electron transport layer; the ester substance has the same polarity as the electron transport layer. The ester substance is formed by ligand exchange of the first ligand on the surface of the quantum dot layer on the side close to the electron transport layer with a second ligand and a third ligand under vacuum conditions, and then an esterification reaction. The carbon chain length of the first ligand is the same as that of the second ligand and the third ligand forming the ester substance.

2. The quantum dot light-emitting diode according to claim 1, wherein, The molar ratio of the second ligand to the third ligand is 1:

1.

3. The quantum dot light emitting diode according to claim 1, wherein The first ligand is selected from one or more of organic carboxylic acids with carbon atoms greater than or equal to 8, primary amines with carbon atoms greater than or equal to 8, secondary amines or tertiary amines with branched-chain carbon atoms greater than or equal to 4, and organic phosphines with branched-chain carbon atoms greater than or equal to 4.

4. The quantum dot light-emitting diode according to claim 3, wherein, The first ligand is selected from one or more of organic carboxylic acids with carbon atoms greater than or equal to 8 and less than or equal to 20, primary amines with carbon atoms greater than or equal to 8 and less than or equal to 20, secondary amines or tertiary amines with branched-chain carbon atoms greater than or equal to 4 and less than or equal to 20, and organic phosphines with branched-chain carbon atoms greater than or equal to 4 and less than or equal to 20, etc.

5. The quantum dot light-emitting diode according to claim 4, wherein, The organic carboxylic acids with carbon atoms greater than or equal to 8 and less than or equal to 20 are selected from one or more of octanoic acid, nonanoic acid, decanoic acid, undecanoic acid, dodecanoic acid, tridecanoic acid, tetradecanoic acid, hexadecanoic acid, octadecanoic acid, undecenoic acid, dodecenoic acid, tridecenoic acid, tetradecenoic acid, pentadecenoic acid, hexadecenoic acid, heptadecenoic acid, and octadecenoic acid, etc. The primary amines with carbon atoms greater than or equal to 8 and less than or equal to 20 are selected from one or more of octylamine, nonylamine, decylamine, dodecylamine, tetradecylamine, hexadecylamine, and octadecylamine, etc. The secondary amines or tertiary amines with branched-chain carbon atoms greater than or equal to 4 and less than or equal to 20 are selected from one or more of tributylamine, trihexylamine, triheptylamine, trioctylamine, trinonylamine, and tridecylamine, etc. The organic phosphines with branched-chain carbon atoms greater than or equal to 4 and less than or equal to 20 are selected from one or more of tributylphosphine, trihexylphosphine, triheptylphosphine, trioctylphosphine, trinonylphosphine, and tridecylphosphine, etc.

6. The quantum dot light-emitting diode according to claim 1, wherein, One end of the carbon chain of the second ligand is a mercapto group, and the other end is a carboxyl group. The second ligand is bonded to the surface of the quantum dot layer through the mercapto group. One end of the carbon chain of the third ligand is a mercapto group, and the other end is a hydroxyl group. The third ligand is bonded to the surface of the quantum dot layer through the mercapto group.

7. The quantum dot light emitting diode according to claim 6, wherein, When the first ligand is undecanoic acid, the second ligand is 11-mercapto-undecanoic acid, and the third ligand is 11-mercapto-1-undecanol.

8. The preparation method of the quantum dot light-emitting diode according to any one of claims 1-7, characterized in that, It includes the following steps: Provide a quantum dot layer with an ester substance bonded to its surface. Under vacuum conditions, perform ligand exchange of the first ligand on the surface of the quantum dot layer on the side close to the electron transport layer with a second ligand and a third ligand. Esterify the second ligand and the third ligand to obtain a quantum dot layer with an ester substance bound to the surface on the side close to the electron transport layer.

9. The method for preparing a quantum dot light-emitting diode according to claim 8, wherein The ratio of the total volume of the second ligand and the third ligand to the mass of the quantum dots is 0.01 - 10:

1.

10. The method for preparing a quantum dot light-emitting diode according to claim 8, wherein The steps for carrying out the esterification reaction include: under the catalysis of an inorganic acid, esterify the second ligand and the third ligand, wherein the temperature of the esterification reaction is 50 - 180 °C, and the time of the esterification reaction is 30 min - 4 h.

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