Quantum dot light emitting diode and preparation method thereof

Abstract

The present invention discloses a quantum dot light-emitting diode and a preparation method thereof. The quantum dot light-emitting diode comprises a first quantum dot layer, and the surface of the first quantum dot layer is bound with cysteine. The present invention uses cysteine ​​to modify the surface of the first quantum dot layer. Since cysteine ​​contains a thio group, the thio group has a strong binding force with the quantum dot and is not easy to fall off from the surface of the quantum dot. Therefore, when the device is lit for a long time, there will be no problem that the quantum dot film changes and affects the performance of the device due to the fall of organic molecules from the surface of the quantum dot, thereby improving the stability and life of the device. In addition, the chain length of cysteine ​​is relatively short, which is conducive to charge transfer, thereby improving the luminous efficiency of the quantum dot light-emitting diode. In addition, the cysteine ​​bound to the surface of the first quantum dot layer can be connected by dehydration condensation to form a stable and uniform connector, so that the film-forming property of the first quantum dot layer is better, its surface is flat, and the quantum dots are neatly arranged.

Description

Technical Field

[0001] The invention relates to the technical field of quantum dot light emitting diodes, and in particular to a quantum dot light emitting diode and a preparation method thereof. Background Art

[0002] Nanomaterials are known as the most promising materials in the 21st century. Quantum dots, as an important branch of the field of nanomaterials, also have important application and research value. Due to their special optical and electrical properties, they can be used in many fields such as light-emitting diodes, solar cells, fluorescent biomarkers, lighting, etc.

[0003] The application of quantum dots is not only related to their material characteristics and size, but also to the ligands on their surface; the ligands on the surface of quantum dots play an important role in determining related applications. Different types of ligands have a direct correlation with the application performance of quantum dots.

[0004] Certain applications of quantum dots involve the preparation of quantum dot films. For quantum dot films, the electrical conductivity of the film has a great influence on the performance of the device. The ligands on the surface of quantum dots mainly affect the charge conductivity of the quantum dot film. The longer organic ligands on the surface of quantum dots have a certain inhibitory effect on the electrical conductivity of the quantum dot film.

[0005] In the research process of quantum dot light-emitting diodes (QLED), the surface ligands of quantum dots have a great influence on QLED devices. Ligands with relatively long carbon chains, such as oleic acid (OA) and oleylamine (OLA), will affect the charge transfer between quantum dots and thus affect the performance of QLED devices. Therefore, corresponding ligand exchange is required during the synthesis and post-processing of quantum dots to meet the needs of QLED devices.

[0006] Therefore, the prior art still needs to be improved and developed. Summary of the invention

[0007] In view of the above-mentioned deficiencies in the prior art, an object of the present invention is to provide a quantum dot light emitting diode and a method for preparing the same, aiming to solve the problem that the existing quantum dot film has poor conductivity and affects the performance of QLED devices.

[0008] The technical solution of the present invention is as follows:

[0009] A first aspect of the present invention provides a quantum dot light emitting diode, which includes a first quantum dot layer, and a surface of the first quantum dot layer is bound to cysteine.

[0010] Optionally, the cysteines bound to the surface of the first quantum dot layer are connected to each other.

[0011] Optionally, the quantum dot light-emitting diode also includes a second quantum dot layer adjacent to the first quantum dot layer, and a ligand is bound to the surface of the second quantum dot layer away from the first quantum dot layer, and the ligand is a ligand obtained by dehydration condensation of cysteine ​​and a long-chain organic carboxylic acid or a long-chain organic amine.

[0012] Optionally, the quantum dot light-emitting diode also includes an anode, a cathode, a hole functional layer and an electron functional layer, the first quantum dot layer and the second quantum dot layer are both arranged between the hole functional layer and the electron functional layer, the first quantum dot layer is arranged close to the side where the hole functional layer is located, the second quantum dot layer is arranged close to the side where the electron functional layer is located, the anode is arranged on the side of the hole functional layer away from the first quantum dot layer, and the cathode is arranged on the side of the electron functional layer away from the second quantum dot layer.

[0013] Optionally, the long-chain organic carboxylic acid is selected from one or more of octanoic acid, nonanoic acid, decanoic acid, undecyl acid, dodecyl acid, tridecyl acid, tetradecyl acid, hexadecyl acid, octadecyl acid, undecylenic acid, dodecylenic acid, tridecylenic acid, tetradecylenic acid, pentadecylenic acid, hexadecylenic acid, heptadecenic acid, and octadecylenic acid;

[0014] The long-chain organic amine is selected from one or more of octylamine, nonylamine, decylamine, dodecylamine, tetradecylamine, hexadecylamine, octadecylamine, dodecenylamine, tetradecenylamine, hexadecenylamine and octadecenylamine.

[0015] Optionally, the thickness of the first quantum dot layer is 30-50 nm, and the thickness of the second quantum dot layer is 40-60 nm.

[0016] A second aspect of the present invention provides a method for preparing the quantum dot light-emitting diode of the present invention, comprising the steps of:

[0017] Providing an original first quantum dot layer, the surface of which is bound with a long-chain organic carboxylic acid or a long-chain organic amine;

[0018] The cysteine ​​solution is added to the surface of the original first quantum dot layer to perform the first ligand exchange, thereby obtaining the first quantum dot layer with cysteine ​​bound to the surface.

[0019] Optionally, after obtaining the first quantum dot layer, the method further comprises the following steps:

[0020] Forming an original second quantum dot layer on the surface of the first quantum dot layer, wherein the surface of the original second quantum dot layer is combined with a long-chain organic carboxylic acid or a long-chain organic amine;

[0021] The ligand solution is added to the surface of the original second quantum dot layer for a second ligand exchange to obtain a second quantum dot layer with ligands bound to the surface; wherein the ligands are ligands obtained by dehydration condensation of cysteine ​​and long-chain organic carboxylic acids or long-chain organic amines.

[0022] Optionally, the cysteine ​​solution is prepared by the following method: dissolving cysteine ​​in a solvent, stirring and heating, and controlling the heating temperature between 80-100° C. to obtain a cysteine ​​solution of 5-10 mg / ml.

[0023] Optionally, the ligand is prepared by the following method: a cysteine ​​solution is mixed with a long-chain organic carboxylic acid or a long-chain organic amine, and stirred evenly to allow cysteine ​​to undergo a condensation reaction with the long-chain organic carboxylic acid or the long-chain organic amine to obtain the ligand.

[0024] Beneficial effect: In the present invention, cysteine ​​is used to modify the surface of the first quantum dot layer. Since cysteine ​​contains a thio group (S-), the thio group has a strong binding force with the quantum dot and is not easy to fall off from the surface of the quantum dot. Therefore, when the device (quantum dot light-emitting diode) is lit for a long time, there will be no problem of changes in the quantum dot film caused by the fall of organic molecules from the surface of the quantum dot (such as cracks caused by dense packing) affecting the performance of the device, thereby improving the stability and life of the device. In addition, the chain length of cysteine ​​is relatively short, which is conducive to charge transfer, thereby improving the luminous efficiency of the quantum dot light-emitting diode. In addition, the cysteine ​​bound to the surface of the first quantum dot layer can be connected by dehydration condensation to form a stable and uniform connector, so that the first quantum dot layer has better film-forming properties, its surface is flat, and the quantum dots are neatly arranged. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 A schematic structural diagram of a quantum dot light emitting diode provided in an embodiment of the present invention.

[0026] Figure 2 A schematic flow chart of a method for preparing a quantum dot light-emitting diode provided in an embodiment of the present invention.

[0027] Figure 3 A schematic flow chart of a method for preparing a quantum dot light-emitting diode provided in accordance with another embodiment of the present invention. DETAILED DESCRIPTION

[0028] The present invention provides a quantum dot light emitting diode and a method for preparing the same. To make the purpose, technical solution and effect of the present invention clearer and more specific, 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 intended to limit the present invention.

[0029] An embodiment of the present invention provides a quantum dot light emitting diode, which includes a first quantum dot layer, and cysteine ​​is bound to the surface of the first quantum dot layer.

[0030] In this embodiment, cysteine ​​is used to modify the surface of the first quantum dot layer, which has the following advantages: First, since cysteine ​​contains a thio group (S-), the thio group has a strong binding force with the quantum dot and is not easy to fall off from the surface of the quantum dot. Therefore, when the device is lit for a long time, the quantum dot film will not change due to the fall of organic molecules from the surface of the quantum dot (such as cracks caused by dense stacking) affecting the performance of the device, thereby improving the stability and life of the device. Secondly, the short chain length of cysteine ​​is conducive to charge transfer, thereby improving the luminous efficiency of the quantum dot light-emitting diode.

[0031] In one embodiment, the cysteines bound to the surface of the first quantum dot layer are connected to each other. The cysteines bound to the surface of the first quantum dot layer can be connected by dehydration condensation (amino and carboxyl groups are connected by dehydration condensation) to form a stable and uniform connector, so that the film-forming property of the first quantum dot layer is better, the surface is flat, and the quantum dots are arranged neatly. And because the cysteines are closely connected, the agglomeration of the quantum dots can be effectively avoided, ensuring that the photoelectric properties of the quantum dots themselves are not affected.

[0032] In one embodiment, the quantum dot light-emitting diode also includes a second quantum dot layer adjacent to the first quantum dot layer, and a ligand is bound to the surface of the second quantum dot layer away from the first quantum dot layer, and the ligand is a ligand obtained by dehydration condensation of cysteine ​​and a long-chain organic carboxylic acid or a long-chain organic amine.

[0033] In one embodiment, the quantum dot light-emitting diode also includes an anode, a cathode, a hole functional layer and an electron functional layer. The first quantum dot layer and the second quantum dot layer are both arranged between the hole functional layer and the electron functional layer. The first quantum dot layer is arranged close to the side where the hole functional layer is located, and the second quantum dot layer is arranged close to the side where the electron functional layer is located. The anode is arranged on the side of the hole functional layer away from the first quantum dot layer, and the cathode is arranged on the side of the electron functional layer away from the second quantum dot layer.

[0034] In this embodiment, considering that the electron injection rate on the electronic functional layer side is much higher than the hole injection rate on the hole functional layer side, a second quantum dot layer with weaker conductivity than the first quantum dot layer is arranged close to the electronic functional layer side, so that the electrons entering the quantum dots from the electronic functional layer will be reduced, thereby reducing the accumulation of charges in the device and improving the luminous efficiency of the device. In other words, this embodiment sets a double-layer quantum dot layer (a first quantum dot layer and a second quantum dot layer), and utilizes the difference in conductivity between the double-layer quantum dot layers to effectively increase the hole transmission rate entering the quantum dots from the hole functional layer side, and reduce the electron transmission rate entering the quantum dots from the electronic functional layer side, thereby balancing the hole-electron pairs injected into the quantum dots, reducing the accumulation of charges in the device, and improving the luminous efficiency of the device.

[0035] In this embodiment, the ligand on the surface of the second quantum dot layer is obtained by dehydration condensation of cysteine ​​and long-chain organic carboxylic acid or long-chain organic amine. Since the ligand has long-chain organic carboxylic acid or long-chain organic amine, the conductivity of the second quantum dot layer is weaker than that of the first quantum dot layer.

[0036] In addition, the surface ligand of the first quantum dot layer is cysteine, and the ligand on the surface of the second quantum dot layer is a ligand obtained by dehydration condensation of cysteine ​​and a long-chain organic carboxylic acid or a long-chain organic amine. This ensures that the first quantum dot layer and the second quantum dot layer have good matching and affinity, avoiding obvious stratification.

[0037] It should be noted that the long-chain organic carboxylic acid refers to an organic carboxylic acid having 8 or more carbon atoms, and the long-chain organic amine refers to an organic amine having 8 or more carbon atoms.

[0038] In one embodiment, the long-chain organic carboxylic acid is an organic carboxylic acid having a carbon number of 8 or more and 20 or less. For example, the organic carboxylic acid having a carbon number of 8 or more and 20 or less is selected from one or more of octanoic acid, nonanoic acid, decanoic acid, undecanoic acid, dodecanoic acid, tridecanoic acid, tetradecanoic acid, hexadecanoic acid, octadecanoic acid, undecylenic acid, dodecylenic acid, tridecylenic acid, tetradecylenic acid, pentadecylenic acid, hexadecylenic acid, heptadecenic acid and octadecylenic acid (i.e., oleic acid).

[0039] In one embodiment, the long-chain organic amine is an organic amine having a carbon number greater than or equal to 8 and less than or equal to 20. For example, the organic amine having a carbon number greater than or equal to 8 and less than or equal to 20 is selected from one or more of octylamine, nonylamine, decylamine, dodecylamine, tetradecylamine, hexadecylamine, octadecylamine, dodecenylamine, tetradecenylamine, hexadecenylamine and octadecenylamine (i.e., oleylamine).

[0040] In one embodiment, the thickness of the first quantum dot layer is 30-50 nm. If it is too thin, the upper and lower layers may be conductive, affecting the function of the first quantum dot layer, while if it is too thick, it is not conducive to the transmission of carriers.

[0041] In one embodiment, the thickness of the second quantum dot layer is 40-60 nm. The second quantum dot layer is slightly thicker than the first quantum dot layer to effectively reduce the electron injection rate, but too thick is not conducive to the transmission of carriers.

[0042] In one embodiment, the hole functional layer is a hole injection layer and / or a hole transport layer, and the electron functional layer is an electron injection layer and / or an electron transport layer. When both the hole injection layer and the hole transport layer are present, the hole transport layer is disposed close to the first quantum dot layer. When both the electron injection layer and the electron transport layer are present, the electron transport layer is disposed close to the second quantum dot layer.

[0043] In one embodiment, if Figure 1 As shown, the quantum dot light-emitting diode specifically includes an anode, a hole transport layer, a first quantum dot layer, a second quantum dot layer, an electron transport layer and a cathode which are stacked in sequence.

[0044] It should be noted that the present embodiment does not limit the type of quantum dot light emitting diode, and the quantum dot light emitting diode may be a positive structure or an inverted structure. For example, the quantum dot light emitting diode may also include, but is not limited to, an optional interface modification layer and an optional interface functional layer. When the anode is located on the substrate, the quantum dot light emitting diode is a positive structure; when the cathode is located on the substrate, the quantum dot light emitting diode is an inverted structure.

[0045] The present invention provides a method for preparing a quantum dot light-emitting diode as described above, Figure 2 As shown, the steps include:

[0046] S10, providing an original first quantum dot layer, wherein a long-chain organic carboxylic acid or a long-chain organic amine is bonded to a surface of the original first quantum dot layer;

[0047] S20, adding the cysteine ​​solution to the surface of the original first quantum dot layer to perform a first ligand exchange, thereby obtaining a first quantum dot layer with cysteine ​​bound to the surface.

[0048] In this embodiment, cysteine ​​is used to replace the original ligand (i.e., long-chain organic carboxylic acid or long-chain organic amine) to modify the surface of the first quantum dot layer, which has the following advantages: First, since cysteine ​​contains a thio group (S-), the thio group has a strong binding force with the quantum dot and is not easy to fall off from the surface of the quantum dot. Therefore, when the device is lit for a long time, there will be no problems such as changes in the quantum dot film caused by the fall of organic molecules from the surface of the quantum dot (such as cracks caused by dense packing) that affect the performance of the device, thereby improving the stability and life of the device. Secondly, the chain length of cysteine ​​is short, which is conducive to charge transfer, thereby improving the luminous efficiency of the quantum dot light-emitting diode. In addition, the preparation process of this embodiment is simple, highly repeatable, low cost, green and environmentally friendly, and easy to operate.

[0049] In one embodiment, the cysteines bound to the surface of the first quantum dot layer are connected to each other. The cysteines bound to the surface of the first quantum dot layer can be connected by dehydration condensation (amino and carboxyl groups are connected by dehydration condensation) to form a stable and uniform connector, so that the film-forming property of the first quantum dot layer is better, the surface is flat, and the quantum dots are arranged neatly. And because the cysteines are closely connected, the agglomeration of the quantum dots can be effectively avoided, ensuring that the photoelectric properties of the quantum dots themselves are not affected.

[0050] In this embodiment, since long-chain organic carboxylic acids (such as oleic acid) or long-chain organic amines (such as oleylamine) have good film-forming properties, when forming a film, quantum dots modified with long-chain organic carboxylic acids or long-chain organic amines are first used for film formation, and then the cysteine ​​solution is fully covered on the film surface for ligand exchange, and the exchanged long-chain organic carboxylic acids or long-chain organic amines are removed to obtain a first quantum dot layer with cysteine ​​bound to the surface. The first quantum dot layer prepared in this embodiment has good film-forming properties and excellent electrical conductivity. It should be noted that the thickness of the original first quantum dot layer is relatively thin, which is conducive to exchanging the ligands in the original first quantum dot layer to obtain a first quantum dot layer with higher electrical conductivity.

[0051] It should be noted that the long-chain organic carboxylic acid refers to an organic carboxylic acid having 8 or more carbon atoms, and the long-chain organic amine refers to an organic amine having 8 or more carbon atoms.

[0052] In one embodiment, the long-chain organic carboxylic acid is an organic carboxylic acid having a carbon number of 8 or more and 20 or less. For example, the organic carboxylic acid having a carbon number of 8 or more and 20 or less is selected from one or more of octanoic acid, nonanoic acid, decanoic acid, undecanoic acid, dodecanoic acid, tridecanoic acid, tetradecanoic acid, hexadecanoic acid, octadecanoic acid, undecylenic acid, dodecylenic acid, tridecylenic acid, tetradecylenic acid, pentadecylenic acid, hexadecylenic acid, heptadecenic acid and octadecylenic acid (i.e., oleic acid).

[0053] In one embodiment, the long-chain organic amine is an organic amine having a carbon number greater than or equal to 8 and less than or equal to 20. For example, the organic amine having a carbon number greater than or equal to 8 and less than or equal to 20 is selected from one or more of octylamine, nonylamine, decylamine, dodecylamine, tetradecylamine, hexadecylamine, octadecylamine, dodecenylamine, tetradecenylamine, hexadecenylamine and octadecenylamine (i.e., oleylamine).

[0054] In one embodiment, step S20 specifically includes: uniformly and comprehensively covering the surface of the original first quantum dot layer with a cysteine ​​solution, heating to 80-150°C for the first ligand exchange, the first ligand exchange time is 10-20 minutes, and then using 800-2000rpm to spin off the exchanged long-chain organic carboxylic acid or long-chain organic amine, repeating multiple times to complete the ligand exchange and obtain the first quantum dot layer with cysteine ​​bound to the surface. It should be noted that during the ligand exchange process, the cysteines on the surface of the first quantum dot layer can also be connected by dehydration condensation.

[0055] In one embodiment, the cysteine ​​solution is prepared by the following method: dissolving an appropriate amount of cysteine ​​in a solvent, stirring and heating, and controlling the heating temperature between 80-100° C. to finally obtain a 5-10 mg / ml cysteine ​​solution. In one embodiment, the solvent can be one or more of weakly polar solvents such as n-octane, n-hexane, n-heptane, and chloroform.

[0056] In one embodiment, Figure 3 As shown, after obtaining the first quantum dot layer, the following steps are also included:

[0057] S30, forming an original second quantum dot layer on the surface of the first quantum dot layer, wherein the surface of the original second quantum dot layer is combined with a long-chain organic carboxylic acid or a long-chain organic amine;

[0058] S40, adding the ligand solution to the surface of the original second quantum dot layer for a second ligand exchange to obtain a second quantum dot layer with ligands bound to the surface; wherein the ligand is a ligand obtained by dehydration condensation of cysteine ​​and a long-chain organic carboxylic acid or a long-chain organic amine.

[0059] In this embodiment, since long-chain organic carboxylic acids (such as oleic acid) or long-chain organic amines (such as oleylamine) have good film-forming properties, when forming a film, quantum dots modified with long-chain organic carboxylic acids or long-chain organic amines are first used for film formation, and then the ligand solution is fully covered on the film surface for ligand exchange, and the exchanged long-chain organic carboxylic acids or long-chain organic amines are removed to obtain a second quantum dot layer with ligands bound to the surface. This embodiment adopts this method to prepare a second quantum dot layer with good film-forming properties. It should be noted that the thickness of the original second quantum dot layer is relatively thin, which is conducive to exchanging the ligands in the original second quantum dot layer.

[0060] In step S30, the long-chain organic carboxylic acid or long-chain organic amine on the surface of the original second quantum dot layer is the same as the long-chain organic carboxylic acid or long-chain organic amine on the surface of the original first quantum dot layer. The specific types can be selected as described above and will not be repeated here. The original double-layer quantum dot layer is modified with the same ligand to ensure the matching degree and affinity of the double-layer quantum dot layer, and no obvious stratification phenomenon will occur.

[0061] In step S40, in one embodiment, the ligand is prepared by the following method: a cysteine ​​solution is mixed with a long-chain organic carboxylic acid or a long-chain organic amine, and stirred evenly, so that a condensation reaction occurs between cysteine ​​and the long-chain organic carboxylic acid or the long-chain organic amine, and the reaction time is 1-2 hours to obtain a ligand. Wherein, 10 ml of a cysteine ​​solution (at a concentration of 5-10 mg / ml) is mixed with 3-8 ml of a long-chain organic carboxylic acid or a long-chain organic amine. Wherein, while stirring, appropriate heating can also be performed, and the heating temperature is controlled between 120-150°C, which is conducive to promoting the condensation reaction between cysteine ​​and the long-chain organic carboxylic acid or the long-chain organic amine. The specific types of long-chain organic carboxylic acids or long-chain organic amines are described above and will not be repeated here.

[0062] In one embodiment, step S40 specifically includes: evenly and comprehensively covering the surface of the original second quantum dot layer with the ligand solution, heating to 80-150°C for a second ligand exchange, the second ligand exchange time is 10-20 minutes, and then the exchanged long-chain organic carboxylic acid or long-chain organic amine is rotated away at a speed of 800-2000 rpm, and the process is repeated multiple times to complete the ligand exchange and obtain a second quantum dot layer with ligands bound to the surface.

[0063] In one embodiment, the first quantum dot layer is located on the surface of the hole functional layer, and after obtaining the second quantum dot layer with ligands bound to the surface, the step further includes: forming an electron functional layer on the surface of the second quantum dot layer.

[0064] That is to say, the quantum dot light-emitting diode also includes a hole functional layer and an electron functional layer, the first quantum dot layer and the second quantum dot layer are both arranged between the hole functional layer and the electron functional layer, the first quantum dot layer is arranged close to the side where the hole functional layer is located, and the second quantum dot layer is arranged close to the side where the electron functional layer is located.

[0065] In one embodiment, the hole functional layer is a hole injection layer and / or a hole transport layer, and the electron functional layer is an electron injection layer and / or an electron transport layer. When both the hole injection layer and the hole transport layer are present, the hole transport layer is prepared close to the first quantum dot layer. When both the electron injection layer and the electron transport layer are present, the electron transport layer is prepared close to the second quantum dot layer.

[0066] In one embodiment, the quantum dot light emitting diode specifically includes an anode, a hole transport layer, a first quantum dot layer, a second quantum dot layer, an electron transport layer and a cathode which are stacked in sequence.

[0067] It should be noted that the present embodiment does not limit the type and preparation method of the quantum dot light emitting diode, and the quantum dot light emitting diode can be a positive structure or an inverted structure. For example, the quantum dot light emitting diode can also include, but is not limited to, an optional interface modification layer and an optional interface functional layer. When the anode is located on the substrate, the quantum dot light emitting diode is a positive structure; when the cathode is located on the substrate, the quantum dot light emitting diode is an inverted structure.

[0068] The present invention will be further described below by means of specific examples.

[0069] 1. Preparation of quantum dots with oleic acid ligands on the surface

[0070] 1) Preparation of cadmium oleate Cd(OA)2 and zinc oleate Zn(OA)2 precursors:

[0071] Take 0.8mmol of cadmium oxide (CdO), 6mmol of zinc acetate {Zn(Ac)2}, 8mL of oleic acid (OA), and 15mL of octadecene (ODE) and add them into a three-necked flask. First, exhaust the air at room temperature for 10min, then heat to 170℃ and exhaust the air for 60min, and then maintain the temperature at 170℃.

[0072] 2) Preparation of selenium (Se) precursor:

[0073] Weigh 4 mmol of Se and add it to 4 ml of trioctylphosphine oxide (TOP), heat to 170 °C for 30 min, then cool to 140 °C;

[0074] 3) Preparation of sulfur (S) precursor:

[0075] Weigh 4 mmol of S and add it to 6 mL of trioctylphosphine oxide (TOP), heat to 170 °C for 30 min, then cool to 140 °C;

[0076] 4) After the temperature of the mixed solution prepared in step 1) is raised to 300°C, 2 mL of selenium (Se) precursor is quickly injected into the mixed solution for reaction for 10 minutes, and then 2 mL of sulfur (S) precursor is extracted and injected into the reacted mixed solution for reaction for 30 minutes. Finally, after the reaction solution temperature is cooled to room temperature, it is cleaned by centrifugation to obtain oil-soluble CdSe / ZnS red quantum dots and annealed. The ligand on its surface is oleic acid (OA). The prepared quantum dots with oleic acid ligands on the surface are placed in an oven for drying. Weigh an appropriate amount of quantum dot powder and dissolve it in n-hexane to prepare a 20 mg / ml quantum dot solution for standby use.

[0077] 2. Preparation of cysteine ​​hexane solution

[0078] Dissolve an appropriate amount of cysteine ​​in n-octane, heat while stirring, and control the heating temperature between 80-100 degrees Celsius to finally prepare an 8 mg / ml cysteine ​​n-hexane solution.

[0079] 3. Preparation of ligand solution

[0080] Take 10 ml of the cysteine ​​n-hexane solution prepared above, add 5 ml of oleic acid thereto, heat while stirring, control the heating temperature between 120-150 degrees Celsius, and react for 2 hours to finally obtain the desired ligand solution.

[0081] 4. The preparation process of quantum dot light-emitting diodes is as follows:

[0082] 1) First, clean the ITO glass: wipe the surface of the ITO glass with a cotton swab dipped in a small amount of soapy water to remove impurities visible to the naked eye, then ultrasonically clean it with deionized water, acetone, ethanol, and isopropanol for 15 minutes in sequence, and then blow it dry with nitrogen for later use;

[0083] 2) Then prepare the hole transport layer: prepare a layer of TFB solution on the cleaned ITO glass surface by spin coating, and then anneal at 100 degrees Celsius for 20 minutes for standby use;

[0084] 3) Then prepare the first quantum dot layer: take 50 microliters of the prepared quantum dot solution with a pipette, drop it on the ITO glass with a TFB layer on the surface, and spin coat it. The spin coating speed is 3000 rpm, and the time is controlled at 50 seconds. Then, drop the cysteine ​​n-hexane solution evenly and comprehensively on the membrane surface, and heat it to 140 degrees Celsius for ligand exchange for 15 minutes. Then, use a speed of 1500 rpm to spin off the exchanged oleic acid ligand, repeat three times, so that the ligand exchange is complete, and finally cure at 80 degrees Celsius for 30 minutes to dry completely to obtain the first quantum dot layer;

[0085] 4) Then prepare the second quantum dot layer: take 50 microliters of the prepared quantum dot solution with a pipette, drop it on the ITO glass with the first quantum dot layer on the surface, and spin coat it. The spin coating speed is 3000 rpm, and the time is controlled at 50 seconds. Then, the ligand solution is evenly and comprehensively dropped on the film surface, and heated to 140 degrees Celsius for ligand exchange for 15 minutes. Then, the exchanged oleic acid ligand is removed by spinning at a speed of 1500 rpm. Repeat three times to complete the ligand exchange. Finally, cure at 80 degrees Celsius for 30 minutes to completely dry to obtain the second quantum dot layer.

[0086] 5) Then, the electron transport layer was prepared: 40 μl of a 20 mg / ml standard zinc oxide ethanol solution was taken with a pipette and slowly dripped onto the second quantum dot layer, and then spin-coated at a speed of 5000 rpm for 20 seconds, followed by annealing at a temperature of 120 degrees Celsius for 15 minutes;

[0087] 6) Finally, prepare the cathode: transfer the substrate with the prepared electron transport layer to a vacuum coating machine, evaporate a layer of metal aluminum on the electron transport layer, and complete the preparation of the quantum dot light-emitting diode.

[0088] In summary, in this embodiment, cysteine ​​is used to modify the surface of the first quantum dot layer. Since cysteine ​​contains a thio group (S-), the thio group has a strong binding force with the quantum dot and is not easy to fall off from the surface of the quantum dot, so it can play a better passivation effect on the surface of the first quantum dot layer. In addition, the chain length of cysteine ​​is short, which is conducive to charge transfer, thereby improving the luminous efficiency of the quantum dot light-emitting diode. Secondly, the cysteine ​​bound to the surface of the first quantum dot layer can be connected by dehydration condensation to form a stable and uniform connector, so that the film-forming property of the first quantum dot layer is better, its surface is flat, and the quantum dots are neatly arranged. In addition, this embodiment also sets a second quantum dot layer with a weaker conductivity than the first quantum dot layer close to the electronic functional layer, so that the electrons entering the quantum dot from the electronic functional layer will be reduced, thereby reducing the accumulation of charges in the device and improving the luminous efficiency of the device.

[0089] It should be understood that the application of the present invention is not limited to the above examples. For ordinary technicians in this field, improvements or changes can be made based on the above description. All these improvements and changes should fall within the scope of protection of the claims attached to the present invention.

Claims

1. A quantum dot light emitting diode, characterized in that: It includes an anode, a cathode, a hole functional layer, an electron functional layer, and a first quantum dot layer and a second quantum dot layer stacked between the hole functional layer and the electron functional layer, wherein the first quantum dot layer is arranged close to the side where the hole functional layer is located, the second quantum dot layer is arranged close to the side where the electron functional layer is located, the anode is arranged on the side where the hole functional layer is located away from the first quantum dot layer, and the cathode is arranged on the side where the electron functional layer is located away from the second quantum dot layer; The surface of the first quantum dot layer is bound to cysteine; the surface of the second quantum dot layer away from the first quantum dot layer is bound to a ligand, which is a ligand obtained by dehydration condensation of cysteine ​​and a long-chain organic carboxylic acid or a long-chain organic amine.

2. The quantum dot light emitting diode according to claim 1, characterized in that: The cysteines bound to the surface of the first quantum dot layer are connected to each other.

3. The quantum dot light emitting diode according to claim 1, characterized in that: The long-chain organic carboxylic acid is selected from one or more of octanoic acid, nonanoic acid, decanoic acid, undecyl acid, dodecyl acid, tridecyl acid, tetradecyl acid, hexadecyl acid, octadecyl acid, undecylenic acid, dodecylenic acid, tridecylenic acid, tetradecylenic acid, pentadecylenic acid, hexadecylenic acid, heptadecenic acid, and octadecylenic acid, The long-chain organic amine is selected from one or more of octylamine, nonylamine, decylamine, dodecylamine, tetradecylamine, hexadecylamine, octadecylamine, dodecenylamine, tetradecenylamine, hexadecenylamine and octadecenylamine.

4. The quantum dot light emitting diode according to claim 1, characterized in that: The thickness of the first quantum dot layer is 30-50 nm, and the thickness of the second quantum dot layer is 40-60 nm.

5. A method for preparing a quantum dot light-emitting diode according to any one of claims 1 to 4, characterized in that: Includes steps: Providing an original first quantum dot layer, the surface of which is bound with a long-chain organic carboxylic acid or a long-chain organic amine; Adding a cysteine ​​solution to the surface of the original first quantum dot layer to perform a first ligand exchange, thereby obtaining a first quantum dot layer with cysteine ​​bound to the surface; Forming an original second quantum dot layer on the surface of the first quantum dot layer, wherein the surface of the original second quantum dot layer is combined with a long-chain organic carboxylic acid or a long-chain organic amine; Adding the ligand solution to the surface of the original second quantum dot layer to perform a second ligand exchange, thereby obtaining a second quantum dot layer with ligands bound to the surface; The ligand is a ligand obtained by dehydration condensation of cysteine ​​and long-chain organic carboxylic acid or long-chain organic amine.

6. The method for preparing a quantum dot light-emitting diode according to claim 5, characterized in that: The cysteine ​​solution is prepared by the following method: dissolving cysteine ​​in a solvent, stirring and heating, and controlling the heating temperature between 80-100° C. to obtain a cysteine ​​solution of 5-10 mg / ml.

7. The method for preparing a quantum dot light-emitting diode according to claim 6, characterized in that: The ligand is prepared by the following method: a cysteine ​​solution is mixed with a long-chain organic carboxylic acid or a long-chain organic amine, and the mixture is stirred evenly to allow a condensation reaction between the cysteine ​​and the long-chain organic carboxylic acid or the long-chain organic amine to obtain the ligand.

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