CN114695825A

CN114695825A - Preparation method of quantum dot light-emitting diode

Info

Publication number: CN114695825A
Application number: CN202011640396.8A
Authority: CN
Inventors: 吴龙佳; 张天朔; 李俊杰; 郭煜林; 童凯
Original assignee: TCL Technology Group Co Ltd
Current assignee: TCL Technology Group Co Ltd
Priority date: 2020-12-31
Filing date: 2020-12-31
Publication date: 2022-07-01

Abstract

The application relates to the technical field of display, and provides a preparation method of a quantum dot light-emitting diode. The application provides a quantum dot light-emitting diode includes an electron transport layer, wherein the electron transport layer includes a first electron transport layer, and a preparation method of a zinc oxide film with the surface hydroxyl amount of more than or equal to 0.6 comprises the following steps: mixing the zinc salt solution with a first alkali liquor at the temperature of 0-70 ℃, and reacting for 30 min-4 h to prepare zinc oxide; dissolving the zinc oxide to obtain a zinc oxide colloidal solution; forming a zinc oxide colloid solution on the substrate of the prefabricated device of the zinc oxide film with the surface hydroxyl content of more than or equal to 0.6 to be prepared, and removing the solvent to prepare the zinc oxide prefabricated film; and depositing a second alkaline solution on the surface of the zinc oxide prefabricated film, and then drying to obtain the zinc oxide film. The preparation method of the quantum dot light-emitting diode effectively improves the external quantum efficiency of the quantum dot light-emitting diode device.

Description

Preparation method of quantum dot light-emitting diode

Technical Field

The application belongs to the technical field of display, and particularly relates to a preparation method of a quantum dot light-emitting diode.

Background

Quantum Dots (QDs) are a class of nanomaterials composed of a small number of atoms, typically with radii smaller than or close to the exciton bohr radius, exhibiting significant quantum confinement effects with unique optical properties. Recently, with the development of display technology, Quantum Dot Light Emitting diodes (QLEDs) using Quantum Dot materials as Light Emitting layers have attracted more and more attention. The quantum dot light-emitting diode has the characteristics of high luminous efficiency, controllable luminous color, high color purity, good device stability, flexible application and the like, and has great application prospect in the fields of display technology, solid-state lighting and the like.

The QLED mainly includes a cathode, an anode, and a quantum dot light emitting layer. In order to improve the performance of the device, the QLED also introduces one or more layers of a hole transport injection layer, a hole transport layer, an electron transport layer and an electron injection layer as functional layers. The zinc oxide is used as an electron transport layer material commonly adopted in the QLED, and has a good energy level matching relationship with the cathode and the quantum dot light-emitting layer, so that the injection barrier of electrons from the cathode to the quantum dot light-emitting layer is obviously reduced, and the deeper valence band energy level of the zinc oxide can play a role in effectively blocking holes. In addition, the zinc oxide material also has excellent electron transport capacity, and the electron mobility is as high as 10^-3cm²V.S. The zinc oxide material becomes the first material of an electron transport layer in a quantum dot light-emitting diode device due to the characteristics, and the stability and the luminous efficiency of the device are obviously improved.

Because the QLED display technology and the organic light-Emitting Diode (OLED) display technology have similarities in light-Emitting principle, the explanation of device physics in the QLED device, the selection and collocation principle of functional layer material energy level, and the like all follow the existing theoretical system in the OLED at present. For example, in order to obtain higher device performance in an OLED device, fine control of carrier injection of holes and electrons on both sides of the device is required to achieve carrier injection balance in a light emitting layer of the device. When the classical physical conclusion of the above OLED device is applied to a QLED device system, considering that the electron mobility of the zinc oxide layer is often higher than the hole mobility of the hole transport layer, in order to achieve a better carrier injection balance in the QLED device, it is necessary to reduce the electron mobility of the zinc oxide layer by means of inserting an electron blocking layer between the quantum dot light emitting layer and the zinc oxide layer. When the method is applied to the QLED device, the performance of the QLED device is improved obviously, particularly the efficiency of the QLED device, and the external quantum efficiency of the QLED device is over 20 percent and is close to the upper limit of a theoretical value.

However, there is a limitation in improving the carrier injection balance by a method of changing the device structure by inserting an electron blocking layer or the like. On one hand, the method is difficult to realize in the actual device preparation, has strict thickness requirements on an electron blocking layer, is difficult to play an effective role when being too thick or too thin, and even reduces the device performance of the QLED, so that the method is difficult to control in the actual operation. In addition, the method of changing the device structure (adding the electron blocking layer) also significantly increases the manufacturing cost of the device, and increases the cost burden in mass production of the future QLED device. Therefore, a more effective and lower-cost method is needed to be found to reduce the electron mobility of the zinc oxide layer, so as to realize the carrier injection balance of the QLED device and improve the external quantum efficiency of the quantum dot light-emitting diode.

Disclosure of Invention

The application aims to provide a preparation method of a quantum dot light-emitting diode, and aims to solve the problem that external quantum efficiency is poor due to unbalanced carrier injection in the existing quantum dot light-emitting diode.

In order to achieve the purpose of the application, the technical scheme adopted by the application is as follows:

the application provides a preparation method of a quantum dot light-emitting diode, the quantum dot light-emitting diode comprises an anode and a cathode which are oppositely arranged, a quantum dot light-emitting layer arranged between the anode and the cathode, and an electron transmission layer arranged between the quantum dot light-emitting layer and the cathode, wherein the electron transmission layer comprises a first electron transmission layer, the first electron transmission layer is a zinc oxide film with the surface hydroxyl group amount more than or equal to 0.6,

the preparation method of the zinc oxide film with the surface hydroxyl content of more than or equal to 0.6 comprises the following steps:

preparing a zinc oxide prefabricated film on a prefabricated device substrate of the zinc oxide thin film with the surface hydroxyl quantity of more than or equal to 0.6;

and depositing a second alkaline solution on the surface of the zinc oxide prefabricated film, and then drying to obtain the zinc oxide film.

According to the preparation method of the quantum dot light-emitting diode, the zinc oxide prefabricated film is subjected to alkali treatment, and the surface of the zinc oxide film can form a liquid film, so that the hydroxyl content of the surface of the zinc oxide prefabricated film can form dynamic balance with the alkali content in the liquid film, and further the hydroxyl content of the surface of the zinc oxide prefabricated film is increased, and the zinc oxide with the surface hydroxyl content of 0.6 or more is obtained. Under the condition, the zinc oxide film with the surface hydroxyl quantity being greater than or equal to 0.6 is used as the first electron transmission layer, the transmission of electrons to the quantum dot light-emitting layer is inhibited, and the electrons injected into the quantum dot light-emitting layer are reduced, so that the injection of hole electrons in the quantum dot light-emitting diode is more balanced, and the service life of the device is prolonged.

Drawings

In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings needed to be used in the embodiments or the prior art descriptions will be briefly introduced below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art that other drawings can be obtained according to the drawings without inventive labor.

Fig. 1 is a schematic structural diagram of an electron transport layer provided in an embodiment of the present application;

FIG. 2 is a schematic structural diagram of another electron transport layer provided in an embodiment of the present application;

fig. 3 is a schematic structural diagram of a light emitting diode provided in an embodiment of the present application;

fig. 4 is a schematic structural diagram of an upright led provided in the embodiment of the present application;

fig. 5 is a schematic structural diagram of an inverted light emitting diode provided in an embodiment of the present application; fig. 6 is a flowchart of a first process for preparing a zinc oxide thin film having a surface hydroxyl group amount of 0.6 or more according to an embodiment of the present disclosure;

fig. 7 is a flowchart of a second process for preparing a zinc oxide thin film having a surface hydroxyl group amount of 0.6 or more according to the examples of the present application;

fig. 8 is a flow chart of a third process for preparing a zinc oxide thin film having a surface hydroxyl group amount of 0.6 or more according to the present invention;

FIG. 9 is a schematic diagram of hydroxyl group content obtained by measuring hydroxyl group oxygen peak area and lattice oxygen peak area by X-ray photoelectron spectroscopy (XPS) and calculating the ratio of the two areas;

FIG. 10 is a graph of EQE-luminance provided by an embodiment of the present application;

FIG. 11 is a schematic diagram of characterizing device lifetime provided by embodiments of the present application;

fig. 12 is a graph of the device EQE test results of the quantum dot light emitting diodes provided in example 1 and comparative example 1 of the present application;

fig. 13 is a graph of the device EQE test results of the quantum dot light emitting diodes provided in example 2 and comparative example 1 of the present application;

fig. 14 is a graph showing the life test results of the quantum dot light emitting diodes provided in example 2 and comparative example 1 of the present application;

fig. 15 is a graph of the device EQE test results of the quantum dot light emitting diodes provided in example 3 and comparative example 1 of the present application;

fig. 16 is a graph showing the life test results of the quantum dot light emitting diodes provided in example 3 and comparative example 1 of the present application.

Detailed Description

In order to make the technical problems, technical solutions and advantageous effects to be solved by the present application more clearly apparent, the present application is further described in detail below with reference to the embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the present application and are not intended to limit the present application.

In the description of the present application, it is to be understood that the terms "first", "second" are used for descriptive purposes only and are not to be construed as indicating or implying relative importance or implying any number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of the present application, "a plurality" means two or more unless specifically limited otherwise.

In a quantum dot light emitting diode device, the electron mobility of the zinc oxide layer is often higher than the hole mobility of the hole transport layer. In order to realize better carrier injection balance in the quantum dot light-emitting diode device, the electron mobility of the zinc oxide layer is reduced by means of inserting an electron barrier layer between the quantum dot light-emitting layer and the zinc oxide layer and the like in the traditional scheme, so that electrons and holes injected into the quantum dot light-emitting layer are balanced. However, there is a limitation in improving the carrier injection balance by a method of changing the device structure by inserting an electron blocking layer or the like. On one hand, the method is difficult to realize in the actual device preparation, because the electron blocking layer has strict thickness requirements, and the electron blocking layer is difficult to play an effective role when being too thick or too thin, and even the device performance of the quantum dot light-emitting diode is reduced, so that the method is difficult to control in the actual operation. On the other hand, the method of changing the device structure (adding the electron blocking layer) also increases the manufacturing cost of the device, and increases the cost burden when the quantum dot light emitting diode device is produced in mass in the future.

In view of this, the present application realizes the regulation of the electron injection rate by regulating the hydroxyl amount on the surface of the zinc oxide film, reduces the electrons injected in the quantum dot light emitting layer, realizes the injection balance of carriers in the quantum dot light emitting diode, and finally obtains the quantum dot light emitting diode device with higher external quantum efficiency. Specifically, the quantum dot light-emitting diode provided by the application utilizes the zinc oxide film with more surface hydroxyl groups as an electron transport layer. In this case, since the injection rate of electrons to the quantum dot light emitting layer is reduced, the injection of holes and electrons is more balanced, and the external quantum efficiency of the device is improved.

The application provides a quantum dot light emitting diode, which comprises an anode and a cathode which are oppositely arranged, a quantum dot light emitting layer arranged between the anode and the cathode, an electron transmission layer arranged between the quantum dot light emitting layer and the cathode,

the electron transport layer comprises a first electron transport layer, and the first electron transport layer is a zinc oxide film with the surface hydroxyl content of more than or equal to 0.6.

In the examples of the present application, the measurement of the amount of hydroxyl groups on the surface of the zinc oxide thin film was performed by detecting the zinc oxide thin film using X-ray photoelectron spectroscopy (XPS). Specifically, in the results of X-ray photoelectron spectroscopy (XPS), the O1s spectrum can be separated into three sub-peaks, namely an OM peak (peak position is 529ev-531 ev) representing the molar concentration of oxygen atoms in the zinc oxide crystal, an OV peak (peak position is 531ev-532ev) representing the molar concentration of oxygen vacancies in the zinc oxide crystal, and an OH peak (peak position is 532ev-534ev) representing the molar concentration of hydroxyl ligands on the surface of the zinc oxide crystal. The area ratio among the sub-peaks represents the ratio of the molar concentrations of different oxygen atoms in the zinc oxide film, and therefore the hydroxyl content on the surface of the zinc oxide film is defined as follows: the area of OH peak area/OM peak, namely the amount of hydroxyl on the surface of the zinc oxide film is as follows: the ratio of the molar concentration of the hydroxyl ligand on the surface of the zinc oxide film to the molar concentration of oxygen atoms in the zinc oxide crystal.

In one possible embodiment, the electron transport layer comprises only one thin film, and the thin film is a zinc oxide thin film having a surface hydroxyl group amount of 0.6 or more, i.e., the electron transport layer is the first electron transport layer. Under the condition, a large number of hydroxyl groups with negative electricity are adsorbed on the surface of the zinc oxide, so that the transmission of electrons in the zinc oxide film can be inhibited and hindered to a certain extent, electrons injected into the quantum dot light-emitting layer can be reduced, the electron injection efficiency of the quantum dot light-emitting diode device in the initial working stage is low, the injection balance of carriers in the quantum dot light-emitting diode device is realized, and the device is in the state of the same balance of the carriers, so that the device has high external quantum efficiency.

When the electron transport layer is the first electron transport layer, the zinc oxide in the first electron transport layer is metal-doped or metal-undoped zinc oxide.

In some embodiments, the first electron transport layer is a zinc oxide thin film that is free of doping metals, i.e., the electron transport layer is made of zinc oxide that does not contain doping metals. In some embodiments, the undoped zinc oxide thin film has a surface hydroxyl number greater than or equal to 0.8; in some embodiments, the undoped zinc oxide thin film has a surface hydroxyl number of greater than or equal to 1.0. It should be noted that the undoped zinc oxide thin film referred to in the examples of the present application is a zinc oxide thin film doped with a metal, and is a zinc oxide thin film formed by a zinc oxide thin film, which is not doped with other metal ions. Namely, the undoped zinc oxide film is a pure zinc oxide film.

In some embodiments, the first electron transport layer is a metal-doped zinc oxide thin film, i.e., the zinc oxide in the electron transport layer is a metal-doped zinc oxide. It should be understood that reference to a doped metal in this application refers to ions of other metals than zinc ions that are ionically doped into the zinc oxide. When the doped zinc oxide obtained by doping the metal element in the zinc oxide is used as an electron transport layer material of a quantum dot light-emitting diode, the quantum dot light-emitting diode device is favorable for obtaining higher device efficiency, but the service life of the device is not ideal and even is inferior to that of the quantum dot light-emitting diode with an undoped pure zinc oxide electron transport layer. This is because while the energy level/oxygen vacancy (electron mobility) of the doped zinc oxide electron transport layer is changed, the doped ions enter the surface of the zinc oxide particles and then preferentially fill the surface defects, which serves the purpose of passivating the defects to a certain extent, and the newly filled doped ion sites coordinate new surface hydroxyl groups, so that the total amount of surface hydroxyl groups is increased.

In some embodiments, the surface hydroxyl content of the metal-doped zinc oxide film is greater than or equal to 0.8; in some embodiments, the amount of surface hydroxyl groups of the zinc oxide thin film containing the doping metal is greater than or equal to 1.0.

In some embodiments, the dopant metal in the zinc oxide thin film containing the dopant metal is selected from Mg²⁺、Mn²⁺At least one of (1). In this case, the doped metal ion and the zinc ion have the same valence state, but the oxide thereof has metal ions with different conduction band energy levels, and at this time, the conduction band energy level of the zinc oxide electron transport layer can be adjusted by doping such metal ions, so as to optimize the energy level matching between the quantum dot light emitting layer and the electron transport layer in the quantum dot light emitting diode device, and improve the EQE of the device.

In some embodiments, the dopant metal in the doped metal-containing zinc oxide thin film is selected from Al³⁺、Y³⁺、La³⁺、Li⁺、 Gd³⁺、Zr⁴⁺、Ce⁴⁺At least one of (1). Under the condition, the doped metal ions and the zinc ions have different valence states, and the oxygen vacancy (electron mobility) of the zinc oxide electron transport layer can be adjusted by doping the metal ions, so that the carrier injection balance of the QLED device is optimized, and the EQE of the device is improved.

The ionic radius of the doped metal ion is different from that of the zinc ion, and the crystal structures of the oxides of the two ions are different (for example, MgO and MnO are of the NaCl type cubic system, ZrO₂Monoclinic system, etc., and ZnO is wurtzite-type hexagonal system), so that the doping metal ions have a doping limit in the zinc oxide material. When the doping amount exceeds the doping limit, the doping metal ions are precipitated from the surface of the zinc oxide material in the form of a second phase, thereby adversely affecting the performance of the zinc oxide material. The ion radius ratio of the doped metal ions to the zinc ions provided in the examples of the present application is shown in table 1 below.

TABLE 1

The embodiment of the application is based on the selected doping metal ions and Zn²⁺The ion radius difference regulates the doping amount of the doped metal ions, and the closer the ion radius of the doped metal ions is to the ion radius of the zinc ions and the more similar the crystal structures of the oxides of the zinc ions and the doped metal ions are, the higher the doping limit of the doped metal ions in the zinc oxide material is. The following are exemplary: when the doping metal is Mg²⁺In the case of the zinc oxide film containing the doped metal, Mg²⁺The doping molar concentration of the silicon nitride is 0.1 to 35 percent; when the doping metal is Mn²⁺In the zinc oxide film containing the doped metal, Mn is contained²⁺The doping molar concentration of the silicon nitride is 0.1 to 30 percent; when the doping metal is Al³⁺In the case of Al in the metal-doped zinc oxide film³⁺The doping molar concentration of the silicon nitride is 0.1 to 15 percent; when the doping metal is Y³⁺In the zinc oxide film containing the doped metal, Y is³⁺The doping molar concentration of the silicon nitride is 0.1 to 10 percent; when the doped metal is La³⁺In the case of the metal-doped zinc oxide film, La is contained in the film³⁺The doping molar concentration of the silicon nitride is 0.1 to 7 percent; when the doping metal is Li⁺In the case of the zinc oxide film containing the doped metal, Li⁺The doping molar concentration of the silicon nitride is 0.1 to 45 percent; when the doping metal is Gd³⁺In the zinc oxide film containing the doped metal, Gd is contained³⁺The doping molar concentration of the silicon nitride is 0.01-8 percent; when the doping metal is Zr⁴⁺In the case of Zr in the zinc oxide film containing the doped metal⁴⁺The doping molar concentration of the silicon nitride is 0.1 to 45 percent; when the doping metal is Ce⁴⁺In the zinc oxide film containing the doped metal, Ce is contained⁴⁺The doping molar concentration of (A) is 0.1% -10%.

In some embodiments, when the electron transport layer is a first electron transport layer, the thickness of the first electron transport layer, i.e., the electron transport layer, is 10 to 100 nm.

In one possible embodiment, the electron transport layer further comprises a second electron transport layer, and the second electron transport layer is a zinc oxide thin film with a surface hydroxyl group amount of less than or equal to 0.4. Namely, the electron transport layer simultaneously comprises a zinc oxide film with the surface hydroxyl group amount less than or equal to 0.4 and a zinc oxide film with the surface hydroxyl group amount greater than or equal to 0.6, and the two films are stacked along the direction vertical to the quantum dot light-emitting layer or the cathode. Under the condition, when double-layer zinc oxide electron transmission is used, the zinc oxide film with high hydroxyl content on the surface can reduce electrons injected in the quantum dot light emitting layer, so that the electron injection efficiency of the quantum dot light emitting diode device in the initial working stage is low, the injection balance of carriers in the quantum dot light emitting diode device is realized, and the device is in the state of the same balance of the carriers, so that the device has high external quantum efficiency; when the quantum dot light-emitting diode device continuously works to a stable state, due to the existence of the zinc oxide film with low surface hydroxyl quantity, the state of negative charge of the quantum dot light-emitting layer still occurs, dynamic balance is achieved, and finally the electron injection efficiency is in a lower level and forms carrier injection balance with the hole injection efficiency, so that the service life of the obtained quantum dot light-emitting diode device is also improved.

The first electron transport layer and the second electron transport layer are stacked, and the relative positions of the first electron transport layer and the second electron transport layer can be flexibly set. In some embodiments, the second electron transport layer is disposed on a side surface of the first electron transport layer adjacent to the quantum dot light emitting layer. Under the condition, when the zinc oxide colloidal solution with less surface hydroxyl group is deposited on the quantum dot light-emitting layer, a more smooth zinc oxide film can be obtained. In some embodiments, the second electron transport layer is disposed on a side surface of the first electron transport layer adjacent to the quantum dot light emitting layer.

In some embodiments, the zinc oxide in both the first electron transport layer and the second electron transport layer is undoped zinc oxide. Namely, the first electron transport layer and the second electron transport layer are made of zinc oxide, and the zinc oxide does not contain doped metal. In some embodiments, the electron transport layer is composed of a zinc oxide thin film, i.e., a first electron transport layer, having a surface hydroxyl group amount of 0.6 or more and a zinc oxide thin film, i.e., a second electron transport layer, having a surface hydroxyl group amount of 0.4 or less. In some embodiments, the amount of surface hydroxyl groups of the first electron transport layer is greater than or equal to 0.8, the amount of surface hydroxyl groups of the second electron transport layer is less than or equal to 0.25, or even less than or equal to 0.15; in some embodiments, the amount of surface hydroxyl groups of the first electron transport layer is greater than or equal to 1.0 and the amount of surface hydroxyl groups of the second electron transport layer is less than or equal to 0.25, or even less than or equal to 0.15.

In some embodiments, the zinc oxide in at least one of the first electron transport layer and the second electron transport layer is a metal doped zinc oxide.

In some embodiments, the zinc oxide in the zinc oxide thin film having a surface hydroxyl group amount of 0.6 or more is a metal-doped zinc oxide, and the zinc oxide thin film having a surface hydroxyl group amount of 0.4 or less, i.e., the second electron transport layer, is an undoped zinc oxide thin film. In this case, as described above, on the one hand, the low hydroxyl group amount enables the quantum dot light emitting diode device to reach the carrier injection equilibrium state even when the quantum dot light emitting diode device continuously operates to the steady state, thereby obtaining a good device lifetime. On the other hand, the high hydroxyl amount can reduce electrons injected in the quantum dot light emitting layer, so that the injection balance of carriers in the quantum dot light emitting diode device is realized, and finally, the quantum dot light emitting diode device with higher external quantum efficiency is obtained. In addition, metal ions are doped in zinc oxide with the surface hydroxyl amount of more than or equal to 0.6 to realize effective carrier injection regulation, the EQE higher than that of a quantum dot light-emitting diode device with an undoped zinc oxide film as an electron transport layer can be obtained in the initial working period of the device, and the surface hydroxyl amount of the first electron transport layer is more than or equal to 0.6, so that the EQE of the quantum dot light-emitting diode device can be effectively improved through the cooperation of the zinc oxide and the electron transport layer. In the embodiment, the carrier injection balance of the QLED device can be realized by adjusting and controlling the hydroxyl quantity on the surface of the zinc oxide film, the structure of the device does not need to be changed (an electron blocking layer is inserted), the zinc oxide film does not need to be modified by means of doping and the like, and the whole process is simple to operate, low in cost and good in repeatability.

Illustratively, the electron transport layer is composed of a first electron transport layer and an undoped zinc oxide thin film (second electron transport layer) having a surface hydroxyl group amount of 0.4 or less, and the zinc oxide in the first electron transport layer is a metal-doped zinc oxide. In some embodiments, the amount of surface hydroxyl groups of the first electron transport layer is greater than or equal to 0.8, and the amount of surface hydroxyl groups of the second electron transport layer is less than or equal to 0.25, or even less than or equal to 0.15; in some embodiments, the amount of surface hydroxyl groups of the first electron transport layer is greater than or equal to 1.0 and the amount of surface hydroxyl groups of the second electron transport layer is less than or equal to 0.25, or even less than or equal to 0.15.

In some embodiments, the zinc oxide film with the surface hydroxyl group amount of greater than or equal to 0.6, i.e., the first electron transport layer, is an undoped zinc oxide film, and the zinc oxide film with the surface hydroxyl group amount of less than or equal to 0.4, i.e., the zinc oxide in the second electron transport layer, is a metal-doped zinc oxide. In this case, on the one hand, the low hydroxyl group amount enables the quantum dot light emitting diode device to reach a carrier injection equilibrium state when continuously operating to a stable state, thereby obtaining a good device lifetime. On the other hand, the high hydroxyl amount can reduce electrons injected in the quantum dot light emitting layer, so that the injection balance of carriers in the quantum dot light emitting diode device is realized, and the quantum dot light emitting diode device with higher external quantum efficiency is obtained finally. In addition, metal ions are doped in zinc oxide with the surface hydroxyl quantity of less than or equal to 0.4 to realize effective carrier injection regulation, the EQE higher than that of a quantum dot light-emitting diode device with an undoped zinc oxide film as an electron transport layer can be obtained in the initial working period of the device, and the surface hydroxyl quantity of the first electron transport layer is greater than or equal to 0.6, so that the EQE of the quantum dot light-emitting diode device can be effectively improved through the cooperation of the zinc oxide and the electron transport layer.

Illustratively, the electron transport layer is composed of an undoped zinc oxide thin film (first electron transport layer) having a surface hydroxyl group amount of 0.6 or more and a zinc oxide thin film (second electron transport layer) having a surface hydroxyl group amount of 0.4 or less, and the zinc oxide in the second electron transport layer is metal-doped zinc oxide. In some embodiments, the amount of surface hydroxyl groups of the first electron transport layer is greater than or equal to 0.8, the amount of surface hydroxyl groups of the second electron transport layer is less than or equal to 0.25, or even less than or equal to 0.15; in some embodiments, the amount of surface hydroxyl groups of the first electron transport layer is greater than or equal to 1.0 and the amount of surface hydroxyl groups of the second electron transport layer is less than or equal to 0.25, or even less than or equal to 0.15.

In some embodiments, the zinc oxide film having a surface hydroxyl group amount of 0.6 or more, i.e., the zinc oxide in the first electron transport layer, is a metal-doped zinc oxide, and the zinc oxide film having a surface hydroxyl group amount of 0.4 or less, i.e., the zinc oxide in the second electron transport layer, is a metal-doped zinc oxide. In this case, on the one hand, the low hydroxyl group amount enables the quantum dot light emitting diode device to reach a carrier injection equilibrium state even when the quantum dot light emitting diode device continuously works to a steady state, and thus, a good device life is obtained. On the other hand, the high hydroxyl amount can reduce electrons injected in the quantum dot light emitting layer, so that the injection balance of carriers in the quantum dot light emitting diode device is realized, and the quantum dot light emitting diode device with higher external quantum efficiency is finally obtained. In addition, metal ions are doped in zinc oxide with the surface hydroxyl group amount of more than or equal to 0.6 and zinc oxide with the surface hydroxyl group amount of less than or equal to 0.4, effective carrier injection regulation is realized, the EQE higher than that of a quantum dot light-emitting diode device with an undoped zinc oxide film as an electron transport layer can be obtained in the initial working period of the device, and the surface hydroxyl group amount of the first electron transport layer is more than or equal to 0.6, so that the EQE of the quantum dot light-emitting diode device can be remarkably improved through cooperation of the zinc oxide with the zinc oxide.

Illustratively, the electron transport layer is composed of a zinc oxide film (first electron transport layer) with the surface hydroxyl group amount of more than or equal to 0.6 and a zinc oxide film (second electron transport layer) with the surface hydroxyl group amount of less than or equal to 0.4, and the zinc oxide in the first electron transport layer core and the second electron transport layer is metal-doped zinc oxide. In some embodiments, the amount of surface hydroxyl groups of the first electron transport layer is greater than or equal to 0.8, the amount of surface hydroxyl groups of the second electron transport layer is less than or equal to 0.25, or even less than or equal to 0.15; in some embodiments, the amount of surface hydroxyl groups of the first electron transport layer is greater than or equal to 1.0 and the amount of surface hydroxyl groups of the second electron transport layer is less than or equal to 0.25, or even less than or equal to 0.15.

In one implementation of the above embodiment, as shown in fig. 1, the electron transport layer 50 is composed of a first electron transport layer 51 and a second electron transport layer 52, and the second electron transport layer 52 is closer to the quantum dot light emitting layer 40 than the first electron transport layer 51, that is, the first electron transport layer 51 is closer to the cathode 60. Under the condition, when the zinc oxide colloidal solution with less surface hydroxyl is deposited on the quantum dot light-emitting layer, a more smooth zinc oxide film can be obtained.

In some embodiments, the electron transport layer includes a first electron transport layer and a second electron transport layer, the second electron transport layer is disposed on a side surface of the first electron transport layer close to the cathode or the quantum dot light emitting layer, and the second electron transport layer is a metal-doped zinc oxide thin film. In this case, the External Quantum Efficiency (EQE) of the quantum dot light emitting diode is optimized by energy level matching optimization or electron mobility optimization of the doped zinc oxide while increasing the amount of hydroxyl groups on the surface of the zinc oxide.

Illustratively, as shown in fig. 1, the electron transport layer 50 includes a first electron transport layer 51 and a second electron transport layer 52, the second electron transport layer 52 is a metal-doped zinc oxide thin film, and the second electron transport layer 52 is disposed on a surface of the first electron transport layer 51 near the quantum dot light emitting layer 40, that is, the first electron transport layer 51 is near the cathode 60. Under the condition, when the metal-doped zinc oxide colloidal solution is deposited on the quantum dot light-emitting layer, a more smooth zinc oxide film can be obtained. In the above embodiment, when the electron transport layer is a double-layered zinc oxide film, the thickness of each layer of zinc oxide film is 10 to 100 nm. Under the condition, the zinc oxide film has proper thickness and is not easy to be broken down by electrons, thereby being beneficial to keeping the injection performance, the film forming quality and the surface smoothness of the electron transmission layer. Considering that the zinc oxide thin film or the metal-doped zinc oxide thin film having a surface hydroxyl group amount of 0.6 or more is not too thick because of its low electron mobility. Illustratively, a zinc oxide film or a metal-doped zinc oxide film having a surface hydroxyl group amount of 0.6 or more has a thickness of 10 to 30 nm. The zinc oxide thin film having a surface hydroxyl group amount of 0.4 or less can be appropriately thick because of its high electron mobility. Illustratively, the zinc oxide film or metal-doped zinc oxide film having a surface hydroxyl number of 0.4 or less has a thickness of 20 to 60 nm.

In some of the above embodiments, the electron transport layer comprises n thin film stacked units composed of the first electron transport layer and the second electron transport layer, where n is greater than or equal to 2. The electron transmission layer adopts a laminated mode, so that the energy level matching is better, and the service life of the device is prolonged to a greater extent. In some embodiments, n is an integer greater than or equal to 2 and less than or equal to 9.

In one possible embodiment, the electron transport layer further comprises a third electron transport layer. Namely, the electron transport layer comprises a zinc oxide film with the surface hydroxyl content of more than or equal to 0.6, namely a first electron transport layer, a second electron transport layer and a third electron transport layer. The second electron transport layer is a zinc oxide film with the surface hydroxyl group amount less than or equal to 0.4, a zinc oxide film with the surface hydroxyl group amount greater than or equal to 0.6 or a metal-doped zinc oxide film.

In one embodiment, the third electron transport layer is a zinc oxide thin film having a surface hydroxyl amount of 0.6 or more.

In some embodiments, the electron transport layer includes a zinc oxide thin film (i.e., a first electron transport layer) having a surface hydroxyl group amount of 0.6 or more, a zinc oxide thin film (i.e., a second electron transport layer) having a surface hydroxyl group amount of 0.4 or less, and a zinc oxide thin film (i.e., a third electron transport layer) having a surface hydroxyl group amount of 0.6 or more, wherein the third electron transport layer is disposed on a side surface of the second electron transport layer facing away from the first electron transport layer. Under the condition, the quantum dot light-emitting diode device can reach a carrier injection balance state when continuously working to a stable state by the zinc oxide film with low hydroxyl content, so that the service life of the device is prolonged; the two zinc oxide thin films with high hydroxyl content can reduce electrons injected into the quantum dot light-emitting layer, realize the injection balance of carriers in the quantum dot light-emitting diode device, and finally obtain the quantum dot light-emitting diode device with higher external quantum efficiency.

In one embodiment, the third electron transport layer is a zinc oxide thin film having a surface hydroxyl amount of 0.4 or less.

In some embodiments, as shown in fig. 2, the electron transport layer 50 includes a zinc oxide thin film having a surface hydroxyl group amount of 0.6 or more (i.e., a first electron transport layer 51), a zinc oxide thin film having a surface hydroxyl group amount of 0.4 or less (i.e., a second electron transport layer 52), and a zinc oxide thin film having a surface hydroxyl group amount of 0.4 or less (i.e., a third electron transport layer 53), and the third electron transport layer 53 is disposed on a side surface of the first electron transport layer 51 facing away from the second electron transport layer 52. Under the condition, the double-layer zinc oxide film with low hydroxyl content enables the quantum dot light-emitting diode device to reach a carrier injection balance state when continuously working to a stable state, so that the service life of the device is prolonged; the zinc oxide film with high hydroxyl content can reduce electrons injected in the quantum dot light-emitting layer, realize the injection balance of current carriers in the quantum dot light-emitting diode device, and finally obtain the quantum dot light-emitting diode device with higher external quantum efficiency. In addition, when zinc oxide with less surface hydroxyl or doped zinc oxide colloidal solution is deposited on the quantum dot light-emitting layer, a more smooth zinc oxide film can be obtained. Therefore, the obtained quantum dot light-emitting diode device has good EQE and device service life.

In some embodiments, the electron transport layer includes a zinc oxide thin film having a surface hydroxyl group amount of 0.6 or more (i.e., a first electron transport layer), a metal-doped zinc oxide thin film (a second electron transport layer), and a zinc oxide thin film having a surface hydroxyl group amount of 0.4 or less (i.e., a third electron transport layer), and the third electron transport layer is disposed between the second electron transport layer and the first electron transport layer. In this case, on the one hand, the low hydroxyl group amount enables the quantum dot light emitting diode device to reach a carrier injection equilibrium state even when the quantum dot light emitting diode device continuously operates to a steady state, and thus, a good device life is obtained. On the other hand, the high hydroxyl amount can reduce electrons injected in the quantum dot light emitting layer, so that the injection balance of carriers in the quantum dot light emitting diode device is realized, and the quantum dot light emitting diode device with higher external quantum efficiency is finally obtained. In addition, metal ions are doped in the zinc oxide of the second electron transport layer to achieve effective carrier injection regulation, and the EQE higher than that of a quantum dot light emitting diode device with an undoped zinc oxide film as an electron transport layer can be obtained in the initial working period of the device, so that the EQE of the quantum dot light emitting diode device can be improved more remarkably.

In some embodiments, the third electron transport layer is a metal-doped zinc oxide thin film.

In some embodiments, the electron transport layer includes a zinc oxide thin film (i.e., a first electron transport layer) having a surface hydroxyl group amount of 0.6 or more, a zinc oxide thin film (i.e., a second electron transport layer) having a surface hydroxyl group amount of 0.4 or less, and a metal-doped zinc oxide thin film (a third electron transport layer), and the third electron transport layer is disposed on a side surface of the second electron transport layer facing away from the first electron transport layer. Under the condition, the zinc oxide film with low hydroxyl content further enhances the electron mobility, so that the quantum dot light-emitting diode device can reach a carrier injection balance state when continuously working to a stable state, and further a good device service life is obtained; the zinc oxide film with high hydroxyl content can reduce electrons injected into the quantum dot light-emitting layer, realize the injection balance of carriers in the initial working period of the quantum dot light-emitting diode device, and finally obtain the quantum dot light-emitting diode device with higher external quantum efficiency. Meanwhile, through energy level matching optimization or electron mobility optimization of doped zinc oxide, the quantum dot light-emitting diode device is already under the condition of better carrier injection balance, and the External Quantum Efficiency (EQE) higher than that of the quantum dot light-emitting diode device taking an undoped zinc oxide film as an electron transmission layer can be obtained in the initial working stage of the device; due to the fact that the amount of hydroxyl on the surface of the doped zinc oxide film is low, the quantum dot light-emitting diode device can reach a carrier injection balance state when continuously working to a stable state. Therefore, the quantum dot light emitting diode can obtain better device life and keep higher EQE in the initial working period. In some embodiments, a third electron transport layer is disposed adjacent to the quantum dot light emitting layer. When the doped zinc oxide colloidal solution is deposited on the quantum dot light-emitting layer, a more smooth zinc oxide film can be obtained.

It is to be understood that in the embodiment where the electron transport layer contains the third electron transport layer, the zinc oxide in the zinc oxide thin film having a surface hydroxyl group amount of 0.4 or less may be undoped zinc oxide or metal-doped zinc oxide; similarly, the zinc oxide in the zinc oxide thin film with the surface hydroxyl group amount of more than or equal to 0.6 can be undoped zinc oxide or metal doped zinc oxide.

In embodiments where the electron transport layer comprises a third electron transport layer, in some embodiments, the electron transport layer has a thickness of 10nm to 100 nm. In some embodiments, the zinc oxide thin film having a surface hydroxyl group amount of 0.4 or less has a thickness of 20 to 60 nm. In some embodiments, the zinc oxide thin film having a surface hydroxyl group amount of 0.6 or more has a thickness of 10 to 30 nm. In some embodiments, the metal-doped zinc oxide thin film has a thickness of 10-30 nm. The thickness of each layer is within the range, so that the thickness of the zinc oxide film is proper under the condition, the zinc oxide film is not easy to be broken down by electrons, and the injection performance, the film forming quality and the surface smoothness of the electron transmission layer are favorably maintained. In particular, a zinc oxide thin film or a metal-doped zinc oxide thin film having a surface hydroxyl group amount of 0.6 or more has a relatively thin film thickness due to its low electron mobility; the zinc oxide thin film having a surface hydroxyl group amount of 0.4 or less is relatively thick because of its high electron mobility.

In the embodiment where the electron transport layer includes the second electron transport layer and the second electron transport layer is a metal-doped zinc oxide film, and the electron transport layer includes the second electron transport layer and the third electron transport layer and the second electron transport layer and/or the third electron transport layer is a metal-doped zinc oxide film, in some embodiments, the type of the doped metal in the metal-doped zinc oxide film, the influence of the doped metal, and the doping content of the doped metal are as described above (in the case where the electron transport layer is the first electron transport layer), and are not described herein again for the sake of brevity.

In some embodiments, the doping metal in the metal-doped zinc oxide thin film is selected from Mg²⁺、Mn²⁺At least one of (a). In some embodiments, the doping metal in the metal-doped zinc oxide thin film is selected from Al³⁺、Y³⁺、La³⁺、Li⁺、Gd³⁺、Zr⁴⁺、 Ce⁴⁺At least one of (1).

In some embodiments, when the doping metal is Mg²⁺Then, the metal is doped with Mg in the zinc oxide film²⁺The doping molar concentration of the silicon nitride is 0.1 to 35 percent; when the doping metal is Mn²⁺In the case of metal doping with Mn in the zinc oxide film²⁺The doping molar concentration of the silicon nitride is 0.1 to 30 percent; when the doping metal is Al³⁺In the case of metal doping with Al in the zinc oxide film³⁺The doping molar concentration of (A) is 0.1% -15%; when the doping metal is Y³⁺When Y is in the metal-doped zinc oxide film³⁺The doping molar concentration of the silicon nitride is 0.1 to 10 percent; when the doped metal is La³⁺Then, the metal is doped with La in the zinc oxide film³⁺The doping molar concentration of the silicon nitride is 0.1 to 7 percent; when the doping metal is Li⁺In the case of metal-doped zinc oxide thin film, Li⁺The doping molar concentration of the silicon nitride is 0.1 to 45 percent; when the doping metal is Gd³⁺When the metal is doped with Gd in the zinc oxide film³⁺The doping molar concentration of the silicon nitride is 0.01-8 percent; when the doping metal is Zr⁴⁺In the process, Zr in the metal doped zinc oxide film⁴⁺The doping molar concentration of the silicon nitride is 0.1 to 45 percent; when the doping metal is Ce⁴⁺In the process, Ce is doped in the zinc oxide film⁴⁺The doping molar concentration of the silicon nitride is 0.1-10%.

In some embodiments, as shown in fig. 3, a quantum dot light emitting diode includes an anode 10 and a cathode 60 disposed opposite each other, and a quantum dot light emitting layer 40 disposed between the cathode 60 and the anode 10, and an electron transport layer 50 disposed between the cathode 60 and the quantum dot light emitting layer 40.

In some embodiments, the light emitting diode further comprises a hole functional layer disposed between the anode 10 and the quantum dot light emitting layer 40. Wherein the hole function layer comprises at least one of a hole transport layer, a hole injection layer and an electron blocking layer. In some embodiments, the quantum dot light emitting diode further includes an electron injection layer disposed between the cathode 60 and the electron transport layer 50.

In the above embodiments, the light emitting diode may further include a substrate on which the anode 10 or the cathode 60 is disposed.

The light emitting diode provided by the embodiment of the application is divided into a positive light emitting diode and an inverted light emitting diode.

In one embodiment, the positive quantum dot light emitting diode includes an anode 10 and a cathode 60 disposed opposite to each other, a quantum dot light emitting layer 40 disposed between the anode 10 and the cathode 60, and an electron transport layer 50 disposed between the cathode 60 and the quantum dot light emitting layer 40, and the anode 10 is disposed on a substrate. In some embodiments, a hole transport layer 30 is disposed between the anode 10 and the quantum dot light emitting layer 40, and further, a hole injection layer 20 is disposed between the anode 10 and the hole transport layer; and/or an electron injection layer is disposed between the cathode 60 and the electron transport layer 50. In some embodiments of an upright led, as shown in fig. 4, the qd-led comprises a substrate 100, an anode 10 disposed on the surface of the substrate 100, a hole injection layer 20 disposed on the surface of the anode 10, a hole transport layer disposed on the surface of the hole injection layer 20, a qd light emitting layer 40 disposed on the surface of the hole transport layer, an electron transport layer 50 disposed on the surface of the qd light emitting layer 40, and a cathode 60 disposed on the surface of the electron transport layer 50.

In one embodiment, the inverted quantum dot light emitting diode includes a stacked structure including an anode 10 and a cathode 60 disposed opposite to each other, a quantum dot light emitting layer 40 disposed between the anode 10 and the cathode 60, and an electron transport layer 50 disposed between the cathode 60 and the quantum dot light emitting layer 40, and the cathode 60 is disposed on a substrate. In some embodiments, a hole transport layer 30 is disposed between the anode 10 and the quantum dot light emitting layer 40, and further, a hole injection layer 20 is disposed between the anode 10 and the hole transport layer; and/or an electron injection layer is disposed between the cathode 60 and the electron transport layer 50. In some embodiments of the inverted led, as shown in fig. 5, the quantum dot led includes a substrate 100, a cathode 60 disposed on a surface of the substrate 100, an electron transport layer 50 disposed on a surface of the cathode 60, a light emitting layer 40 disposed on a surface of the electron transport layer 50, a hole transport layer disposed on a surface of the quantum dot light emitting layer 40, a hole injection layer 20 disposed on a surface of the hole transport layer, and an anode 10 disposed on a surface of the hole injection layer 20.

In the above embodiments, the substrate 100 may be a rigid substrate or a flexible substrate, and specifically, glass, a silicon wafer, polycarbonate, polymethyl methacrylate, polyethylene terephthalate, polyethylene naphthalate, polyamide, polyether sulfone, or a combination of at least two of the above materials, or a laminated structure of at least two of the above materials may be selected.

In some embodiments, the hole injection layer 20 can be selected from poly (ethylene dioxythiophene), at least one of polystyrene sulfonate (PEDOT: PSS), HTL-1, HTL-2, and other hole injection materials with high injection performance.

The structure of PEDOT: PSS is as follows:

the structure of HTL-1 is as follows:

the structure of HTL-2 is as follows:

in some embodiments, the material of the hole transport layer 30 may be selected from conventional hole transport materials. Exemplary materials of the hole transport layer 30 include at least one of 4,4'-N, N' -dicarbazolyl-biphenyl (CBP), poly [ (9,9 '-dioctylfluorene-2, 7-diyl) -co- (4,4' - (N- (4-sec-butylphenyl) diphenylamine)) ] (TFB), poly (4-butylphenyl-diphenylamine) (poly-TPD), 4',4' -tris (N-carbazolyl) -triphenylamine (TCTA), poly (N-vinylcarbazole) (PVK), and derivatives thereof, but the material of the hole transport layer 30 may be other hole transport materials having high injection properties.

The quantum dots in the quantum dot light-emitting layer 40 are one of red, green and blue, and may also be yellow quantum dots. The quantum dots can be cadmium-containing or cadmium-free. In some embodiments, the quantum dots in the quantum dot light emitting layer 40 may be single core quantum dots or core-shell structure quantum dots, and the core and shell compounds of the quantum dots may each be independently selected from at least one of CdS, CdSe, CdTe, ZnO, ZnS, ZnSe, ZnTe, GaAs, GaP, GaSb, HgS, HgSe, HgTe, InAs, InP, InSb, AlAs, AlP, CuInS, CuInSe, and various core-shell structure or alloy structure quantum dots. The quantum dot light-emitting layer formed by the method has the characteristics of wide excitation spectrum, continuous distribution, high stability of emission spectrum and the like.

In the embodiment of the present application, the material and thickness of the electron transport layer 50 are as described above, and are not described herein again. In the embodiment of the application, the thickness of the electron transport layer is 10-100 nm. When the thickness of the electron transmission layer is less than 10nm, the film layer is easily broken down by electrons, and the injection performance of carriers is not easily ensured; when the thickness of the electron transport layer is greater than 100nm, electron injection is blocked, and charge injection balance of the device is affected.

The bottom electrode (the anode 10 bonded on the substrate 100 or the cathode 60 bonded on the substrate) may be made of a common bottom electrode material, and in some embodiments, the material of the bottom electrode includes at least one of zinc oxide, indium oxide, tin oxide, Indium Tin Oxide (ITO), Indium Zinc Oxide (IZO), and fluorine-doped tin oxide.

In some embodiments, the top electrode (anode 10 or cathode 60 remote from substrate 100) is a transparent oxide, a thin metal, or a combination of both. In some embodiments, the transparent oxide may be ITO, IZO, AZO; in some embodiments, the thin metal may be Ag, Al, Au, Mg, Ca, Yb, Ba, or alloys thereof; in some embodiments, the top electrode may also be O/M/O, where M is Ag, Al, Au, Mg, Ca, Yb, Ba, or alloys thereof, and O is an oxide including, but not limited to, ITO, IZO, AZO.

According to the quantum dot light-emitting diode provided by the embodiment of the application, the zinc oxide film with the surface hydroxyl amount being greater than or equal to 0.6 is used as the first electron transmission layer, the transmission of electrons in the electron transmission layer is inhibited, and the transmission of electrons in the quantum dot light-emitting diode is reduced, so that the electrons injected into the quantum dot light-emitting layer are reduced, the injection balance of carriers in the quantum dot light-emitting diode is realized, and the quantum dot light-emitting diode with higher external quantum efficiency is finally obtained. The quantum dot light-emitting diode provided by the application can realize the carrier injection balance of a quantum dot light-emitting diode device only by regulating the hydroxyl quantity on the surface of the zinc oxide film, does not need to change the structure of the device (insert an electronic barrier layer), does not need to modify the zinc oxide film by means of doping and the like, and has the advantages of simple operation of the whole process, low cost and good repeatability.

The quantum dot light-emitting diode provided by the embodiment of the application can be prepared by various methods. The present application provides three examples of methods for fabricating the above-described quantum dot light emitting diode.

In a first embodiment, the present application provides a method for preparing a quantum dot light emitting diode, the quantum dot light emitting diode includes an anode and a cathode oppositely disposed, a quantum dot light emitting layer disposed between the anode and the cathode, and an electron transport layer disposed between the quantum dot light emitting layer and the cathode, wherein the electron transport layer includes a first electron transport layer, and the first electron transport layer is a zinc oxide thin film having a surface hydroxyl group amount of 0.6 or more,

as shown in fig. 6, the method for preparing a zinc oxide thin film with a surface hydroxyl group amount of 0.6 or more comprises the following steps:

s11, mixing the zinc salt solution with a first alkali liquor for reaction, adding a precipitator into the mixed solution after the reaction is finished, and collecting precipitate; washing the precipitate twice or less with a reaction solvent, and dissolving the obtained white precipitate to obtain a zinc oxide colloidal solution;

s12, forming a zinc oxide colloidal solution on a prefabricated device substrate of the zinc oxide film with the surface hydroxyl content of more than or equal to 0.6 to be prepared, and removing the solvent to prepare the zinc oxide film with the surface hydroxyl content of more than or equal to 0.6.

According to the preparation method of the quantum dot light-emitting diode provided by the embodiment of the application, a solution method is utilized to prepare a zinc oxide colloid solution which is used as a film forming solution of a zinc oxide film with the surface hydroxyl quantity being more than or equal to 0.6. In the preparation process of preparing the zinc oxide colloidal solution by using a solution method, the obtained precipitate is cleaned twice or less by using a reaction solvent to obtain the zinc oxide with the surface hydroxyl content of more than or equal to 0.6. The zinc oxide film with the surface hydroxyl amount being greater than or equal to 0.6 is used as a first electron transmission layer, the transmission of electrons to the quantum dot light-emitting layer is inhibited, and the electrons injected into the quantum dot light-emitting layer are reduced, so that the electrons and holes in the quantum dot light-emitting diode are more balanced, and the external quantum efficiency of the device is improved.

In the embodiment of the present application, the composition of the quantum dot light emitting diode, especially the case of the electron transport layer, as described above in the first aspect, is not repeated herein for saving space.

The step S11 is to prepare the colloidal zinc oxide solution by a solution method, which may be one of an alcoholysis method, a hydrolysis method, and the like. The basic flow of the solution method for preparing the zinc oxide comprises the following steps: mixing the zinc salt solution with the first alkali liquor, and reacting to generate a hydroxide intermediate such as zinc hydroxide; the hydroxide intermediate undergoes polycondensation to gradually generate the zinc oxide nano-particles.

In the embodiment of the application, the zinc salt solution is formed by dissolving zinc salt in a solvent. Wherein the zinc salt is selected from salts capable of reacting with the first alkali solution to generate hydroxide of zinc, including but not limited to one of zinc acetate, zinc nitrate, zinc sulfate and zinc chloride. The solvent is selected from solvents with better solubility to zinc salt and the generated zinc oxide nanoparticles, and includes but is not limited to solvents with higher polarity such as water, organic alcohol, organic ether, sulfone and the like. In some embodiments, the solvent is selected from at least one of water, organic alcohols, organic ethers, sulfones. The solvent has better solubility to zinc salt, is relatively stable as a reaction medium in an alkaline environment, and is not easy to introduce side reaction; but also has solubility to zinc oxide nano-particles which are final products with polarity. The solvent can ionize the reaction base, and can promote the reaction between the base and the zinc salt by acting as a solvent for dissolving the zinc salt and a solvent for diluting or dissolving the reaction base. Illustratively, the solvent may be selected from at least one of water, methanol, ethanol, propanol, butanol, ethylene glycol monomethyl ether, and dimethyl sulfoxide (DMSO).

In the embodiment of the application, the first alkali solution is a solution formed by alkali capable of reacting with zinc salt to generate hydroxide of zinc, and specifically, the first alkali solution provides hydroxide ions for reacting with zinc ions in the reaction system. It will be appreciated that when the zinc salt contains dopant metal ions, the first basic solution reacts with both zinc ions and the hydroxide ions of the dopant metal ions. In the embodiment of the present application, a solvent is used to dissolve or dilute alkali to obtain a first alkali solution. On one hand, solid alkali such as sodium hydroxide can be dissolved by a solvent to form liquid first alkali liquor, and then the liquid first alkali liquor is added into a reaction system, so that the dispersion uniformity of the first alkali liquor in the reaction system is facilitated; on the other hand, the concentration of alkali in the first alkali liquor can be adjusted to be 0.1-2mol/L by dissolving or diluting, so that the phenomenon that the concentration of the added alkali is too high, the reaction rate is too high, the obtained zinc oxide nano-particles are not uniform in size, and the zinc oxide nano-particles are agglomerated when the zinc oxide particles are too large is avoided.

Wherein, the alkali in the first alkali liquor can be selected from inorganic alkali and organic alkali; a strong base may be selected, and a weak base may be selected. In one possible embodiment, the base in the first lye is selected from K_b＞10^-1Base of (2), exemplary, K_b＞10^-1The alkali is at least one selected from potassium hydroxide, sodium hydroxide and lithium hydroxide. In one possible embodiment, the base in the first lye is selected from K_b＜10^-1Base of (2), exemplary, K_b＜10^-1The alkali is at least one selected from TMAH, ammonia water, ethanolamine and ethylenediamine. The solvent used for dissolving or diluting the alkali to form the first alkali liquor can dissolve the alkali or be mixed and dissolved with the alkali, and the polarity of the solvent is the same as that of the zinc oxide nano-particles. In some embodiments, for dissolving orThe solvent for diluting the alkali to form the first alkali liquor can be the same as the solvent in the zinc salt solution or can be different from the solvent in the zinc salt solution. In some embodiments, the solvent used to dissolve or dilute the base to form the first lye is selected to be the same solvent as the zinc salt solution, which is more advantageous to obtain a stable reaction system. Wherein, the same solvent includes but is not limited to water, organic alcohol, organic ether, sulfone and other solvents with higher polarity. In some embodiments, the solvent is selected from at least one of water, organic alcohols, organic ethers, sulfones. Illustratively, the solvent may be selected from at least one of water, methanol, ethanol, propanol, butanol, ethylene glycol monomethyl ether, and dimethyl sulfoxide (DMSO).

In some embodiments, the zinc salt solution and the first alkali solution are mixed and treated at a temperature of 0-70 ℃ and reacted for 30 min-4 h to prepare the zinc oxide nanoparticles. In some embodiments, the zinc salt solution is mixed with the first lye in the following way: dissolving zinc salt at room temperature (5-40 ℃) to obtain a zinc salt solution, and dissolving or diluting alkali at room temperature to obtain a first alkali liquor; adjusting the temperature of the zinc salt solution to 0-70 ℃, and adding the first alkali liquor. In this case, the added alkali reacts with zinc salt in the zinc salt solution to produce zinc oxide nanoparticles, and good particle dispersibility can be obtained. When the reaction temperature is lower than 0 ℃, the generation of zinc oxide nano particles can be obviously slowed down, the reaction can be realized only by special equipment, the reaction difficulty is increased, the zinc oxide nano particles are not easily generated even under some conditions, and only hydroxide intermediates can be obtained; when the reaction temperature is higher than 70 ℃, the reaction activity is too high, the generated zinc oxide nanoparticles are seriously agglomerated, and a colloidal solution with good dispersibility is not easy to obtain, so that the later-stage film forming of the zinc oxide colloidal solution is influenced. In some embodiments, the reaction temperature of the zinc salt solution and the first alkali solution is between room temperature and 50 ℃, in this case, the formation of zinc oxide nanoparticles is facilitated, and the obtained zinc oxide ions have better particle dispersibility, which is beneficial to the film formation of the zinc oxide colloid solution. In some embodiments, the zinc salt solution and the first alkali solution are mixed at the temperature of 0-30 ℃ to easily generate a qualified zinc oxide colloidal solution; in some embodiments, the zinc oxide colloidal solution can be generated under the condition of the temperature of 30-70 ℃, the quality of the obtained zinc oxide colloidal solution is inferior to that of the zinc oxide colloidal solution generated under the condition of 0-30 ℃, and the reaction time is reduced. In some embodiments, in the step of mixing the zinc salt solution with the first alkali liquor, the zinc salt solution is mixed with the first alkali liquor according to a molar ratio of hydroxide ions to zinc ions of 1.5: 1-2.5: 1, so as to ensure formation of zinc oxide nanoparticles and reduce generation of reaction byproducts. When the molar ratio of hydroxide ions to zinc ions is less than 1.5:1, the zinc salt is obviously excessive, so that a large amount of zinc salt is not easy to generate zinc oxide nano particles; and when the molar ratio of hydroxide ions to zinc ions is more than 2.5:1, the first alkali liquor is obviously excessive, and the excessive hydroxyl ions and the zinc hydroxide intermediate form a stable complex compound which is not easy to generate the zinc oxide nano-particles by polycondensation. In some embodiments, in the step of mixing the zinc salt solution with the first alkali liquor, the addition amount of the zinc salt solution and the first alkali liquor satisfies the following condition: the ratio of the molar amount of hydroxide ions provided by the first alkali solution to the molar amount of zinc ions provided by the zinc salt is 1.7:1 to 1.9: 1.

In some embodiments, after mixing the zinc salt solution with the first alkali solution, reacting at a reaction temperature of 0-70 ℃ for 30 min-4 h to ensure the formation of zinc oxide nanoparticles and control the particle size of the nanoparticles. When the reaction time is less than 30min, cluster seeds of zinc oxide are obtained by the reaction with too low reaction time, and the crystalline state of the sample is incomplete and the crystal structure is poor, so that if the cluster seeds are used as an electron transport layer material, the conductivity of the electron transport layer is poor; when the reaction time exceeds 4 hours, the excessively long particle growth time leads to excessively large and uneven particle size of the generated nanoparticles, and the surface roughness of the zinc oxide colloidal solution after film formation is high, thus affecting the electron transmission performance. In some embodiments, the zinc salt solution is mixed with the first alkali solution and then reacted for 1-2 hours at the reaction temperature.

In some embodiments, under the condition that the temperature is 0-70 ℃, the zinc salt solution is mixed with the first alkali liquor, the reaction is carried out for 30 min-4 h, and the reaction is carried out under the stirring condition, so that the uniformity of the reaction and the uniformity of the obtained zinc oxide nanoparticles are promoted, and the zinc oxide nanoparticles with uniform size are obtained.

In the present example, after the reaction was completed, a precipitant was added to the mixed solution after the reaction was completed, and the precipitate was collected. The precipitant selects a solvent having a polarity opposite to that of the final product zinc oxide nanoparticles, thereby precipitating the zinc oxide nanoparticles by reducing their solubility. In some embodiments, the precipitating agent is selected to be a less polar solvent, which is opposite in polarity to the zinc oxide nanoparticles and facilitates precipitation of the zinc oxide nanoparticles. Exemplary precipitating agents include, but are not limited to, ethyl acetate, acetone, n-hexane, n-heptane, and the remaining low polarity long chain alkanes, and the like.

In some embodiments, 2 to 6 times of the volume of the precipitant is added to the mixed solution after the reaction is finished (i.e., the volume ratio of the precipitant to the mixed solution is 2:1 to 6:1), and white precipitate is generated in the mixed solution. In this case, it is ensured that the solubility of the zinc oxide particles is not impaired by an excessive amount of the precipitant on the premise that the zinc oxide nanoparticles are sufficiently precipitated. In some embodiments, the volume ratio of the precipitant to the mixed solution is 3:1 to 5: 1.

In the examples of the present application, the mixed system subjected to the precipitation treatment was centrifuged, and the precipitate was collected. The embodiment of the application adopts the reaction solvent to carry out cleaning treatment on the collected precipitate so as to remove reactants which do not participate in the reaction. The obtained zinc oxide nano-particles are cleaned by adopting a reaction solvent, and raw materials such as redundant zinc salt, alkali and the like for preparing the zinc oxide nano-particles can be removed, so that the purity of the zinc oxide nano-particles is improved. It should be noted that the reaction solvent is as described above. In some embodiments, the reaction solvent is selected from at least one of water, organic alcohols, organic ethers, sulfones. Illustratively, the reaction solvent is selected from at least one of water, methanol, ethanol, propanol, butanol, ethylene glycol monomethyl ether, DMSO.

In the embodiment of the application, zinc salt and alkali are adopted to react to form the zinc oxide nano-particles, and in the polar zinc oxide solution, a large number of ionized hydroxyl groups are adsorbed on the surface of the zinc oxide colloid due to the characteristics of the zinc oxide colloid. The hydroxyl groups are negatively charged and are adsorbed on the surfaces of the zinc oxide nano particles in a large amount, so that the surfaces of the zinc oxide nano particles are also negatively charged. Under the action of electrostatic coulomb repulsion among the zinc oxide nano-particles, the zinc oxide nano-particles can be dispersed in a polar solution, and have better solution stability and dispersibility. After the zinc oxide colloidal solution is deposited into a zinc oxide film, a large number of hydroxyl groups still coat the surface of zinc oxide particles after the film is solidified and formed. When the zinc oxide film is used as an electron transmission layer in a quantum dot light-emitting diode structure, a large number of hydroxyl groups with negative charges are adsorbed on the surface of zinc oxide, so that the transmission of electrons in the zinc oxide layer can be inhibited and hindered to a certain extent, and the injection condition of electrons in the quantum dot light-emitting diode device can be directly influenced by the amount of the hydroxyl groups on the surface of the zinc oxide film. When the amount of the surface hydroxyl is large, the transmission of electrons in the quantum dot light-emitting diode device can be inhibited, and the electrons injected into the quantum dot light-emitting layer can be reduced; when the amount of the surface hydroxyl is less, the transmission of electrons in the quantum dot light-emitting diode device is smooth, and the electrons injected into the quantum dot light-emitting layer are increased. Therefore, the surface hydroxyl amount of the obtained zinc oxide nanoparticles is adjusted by controlling the cleaning times in the examples of the present application.

Specifically, when the zinc oxide nanoparticles are cleaned for a plurality of times, the residual hydroxyl quantity on the surfaces of the zinc oxide nanoparticles is relatively low; when the zinc oxide nano-particles are washed for a plurality of times, the residual hydroxyl quantity on the surfaces of the zinc oxide nano-particles is correspondingly less. In the embodiment of the application, the precipitate is cleaned twice or less by using the reaction solvent, so that the surface hydroxyl content is more than or equal to 0.6.

In one possible embodiment, if the base in the first lye is K_b＞10^-1The number of washing treatments is 2 or less. In this case, since K_b＞10^-1The ionization coefficient of the alkali is larger, so that the finally synthesized zinc oxide colloid tableThe hydroxyl content of the surface is large, and the state of high hydroxyl content on the surface of the zinc oxide can be maintained after the cleaning times are less than or equal to 2 times.

In one possible embodiment, if the base in the first lye is K_b＜10^-1The number of washing treatments is 1 or less. When the reaction base is K_b＜10^-1The ionization coefficient of the alkali is smaller, so that the hydroxyl quantity of the surface of the finally synthesized zinc oxide colloid is smaller, and the surface can be cleaned for less than or equal to 1 time.

Wherein, different K_bThe base can be selected as described above. Exemplary, K_b＞10^-1The alkali (b) includes but is not limited to inorganic strong alkali such as potassium hydroxide, sodium hydroxide, lithium hydroxide and the like; k is_b＜10^-1The base (C) includes, but is not limited to, organic weak bases such as TMAH, ammonia, ethanolamine, ethylenediamine, etc.

In some embodiments, the alkali in the first alkali solution is at least one selected from potassium hydroxide, sodium hydroxide and lithium hydroxide, and the collected precipitate is washed with the reaction solvent for 1 time, so that zinc oxide nanoparticles with surface hydroxyl group amount of 0.6 or more can be obtained; in some embodiments, the alkali in the first alkali solution is at least one selected from TMAH, ammonia, ethanolamine, and ethylenediamine, and the collected precipitate is washed with the reaction solvent for 1 time, so as to obtain zinc oxide nanoparticles with surface hydroxyl amount greater than or equal to 0.6.

And (4) obtaining a white precipitate after cleaning treatment, and dissolving the obtained white precipitate to obtain a zinc oxide colloidal solution.

In one possible embodiment, the zinc oxide film with the surface hydroxyl group amount of 0.6 or more is a metal-doped zinc oxide film, and correspondingly, the zinc oxide with the surface hydroxyl group amount of 0.6 or more is a metal-doped zinc oxide, and in this case, the zinc salt solution further contains doped metal ions. In this example, the selection of the doping metal ions is as described above for the doping metal in the metal-doped zinc oxide film.

In some embodiments, the gold is dopedThe metal ion is selected from Mg²⁺、Mn²⁺At least one of (a). In this case, the doped metal ions and the zinc ions have the same valence state, but the oxides of the doped metal ions have metal ions with different conduction band energy levels, and at this time, the conduction band energy level of the zinc oxide electron transport layer can be adjusted by doping the metal ions, so that the energy level matching between the quantum dot light emitting layer and the electron transport layer in the quantum dot light emitting diode device is optimized, and the EQE of the device is improved.

In some embodiments, the dopant metal ion is selected from Al³⁺、Y³⁺、La³⁺、Li⁺、Gd³⁺、Zr⁴⁺、Ce⁴⁺At least one of (1). In this case, the doped metal ions and the zinc ions have different valence states, and the oxygen vacancy (electron mobility) of the zinc oxide electron transport layer can be adjusted by doping the metal ions, so that the carrier injection balance of the QLED device is optimized, and the EQE of the device is improved.

The embodiment of the application is based on the selected doping metal ions and Zn²⁺The ion radius difference regulates the doping amount of the doped metal ions, and the closer the ion radius of the doped metal ions is to the ion radius of the zinc ions and the more similar the crystal structures of the oxides of the zinc ions and the doped metal ions are, the higher the doping limit of the doped metal ions in the zinc oxide material is. The following are exemplary: when the doped metal ion is Mg²⁺In zinc salt solution, Mg²⁺The molar content accounts for 0.1 to 35 percent of the total molar weight of the metal ions; when the doped metal ion is Mn²⁺In zinc salt solution, Mn²⁺The molar content accounts for 0.1 to 30 percent of the total molar weight of the metal ions; when the doped metal ion is Al³⁺In the zinc salt solution, Al³⁺The molar content accounts for 0.1 to 15 percent of the total molar weight of the metal ions; when the doped metal ion is Y³⁺In zinc salt solution, Y³⁺The molar content accounts for 0.1 to 10 percent of the total molar weight of the metal ions; when the doped metal ion is La³⁺In zinc salt solution, La³⁺The molar content accounts for 0.1 to 7 percent of the total molar weight of the metal ions; when the doped metal ion is Li⁺In solution of zinc salt, Li⁺Molar content of0.1 to 45 percent of the total molar weight of metal ions; when the doped metal ion is Gd³⁺Gd in the zinc salt solution³⁺The molar content accounts for 0.01 to 8 percent of the total molar weight of the metal ions; when the doped metal ion is Zr⁴⁺In zinc salt solution, Zr⁴⁺The molar content accounts for 0.1 to 45 percent of the total molar weight of the metal ions; when the doped metal ion is Ce⁴⁺In solution of zinc salt, Ce⁴⁺The molar content accounts for 0.1 to 10 percent of the total molar weight of the metal ions.

In some embodiments, the zinc salt solution contains zinc ions and doping metal ions, and in the step of mixing the zinc salt solution and the first alkali solution, the addition amount of the zinc salt solution and the first alkali solution is satisfied: the ratio of the product of the molar weight and the valence number of the metal ion to the molar weight of the hydroxide ion is 0.75: 1-1.25: 1. In this case, the zinc salt solution is mixed with the primary alkali solution to ensure the formation of metal-doped zinc oxide nanoparticles and to reduce the generation of reaction by-products. When the molar ratio of hydroxide ions to metal ions is less than 0.75: 1, the metal salt is not easy to generate metal-doped zinc oxide nano particles due to excessive metal ion content; and when the molar ratio of hydroxide ions to zinc ions is more than 1.25:1, the first alkali liquor is obviously excessive, and the excessive hydroxide ions and the zinc hydroxide intermediate form a stable complex which is not easy to generate zinc oxide nano-particles by polycondensation. In some embodiments, in the step of mixing the zinc salt solution with the first alkali solution, the zinc salt solution and the first alkali solution are added in an amount such that the ratio of the product of the molar amount of the metal ions and the valence number to the molar amount of the hydroxide ions is 0.85:1 to 0.95: 1.

In step S12, the zinc oxide colloidal solution may be formed on a pre-fabricated device substrate on which a zinc oxide thin film with a surface hydroxyl group amount greater than or equal to 0.6 is to be formed, according to the type of the manufactured quantum dot light emitting diode device, and the solvent is removed to obtain the zinc oxide thin film with a surface hydroxyl group amount greater than or equal to 0.6.

In some embodiments, the zinc oxide colloidal solution is formed on a substrate of a prefabricated device by one of the methods including, but not limited to, spin coating, doctor blading, printing, spraying, roll coating, electrodeposition, and the like. After the zinc oxide colloid solution is formed on a prefabricated device substrate, the solvent is removed through annealing treatment, and the zinc oxide film with the surface hydroxyl content of more than or equal to 0.6 is obtained.

In one possible embodiment, the quantum dot light emitting diode is an orthoscopic quantum dot light emitting diode, and the pre-fabricated device substrate includes an anode substrate, and a quantum dot light emitting layer bonded to the anode substrate. In some embodiments, the pre-fabricated device substrate further comprises a hole-functional layer disposed between the anode substrate and the quantum dot light-emitting layer. Wherein the hole function layer comprises at least one of a hole transport layer, a hole injection layer and an electron blocking layer.

In one possible embodiment, the quantum dot light emitting diode is an inverted quantum dot light emitting diode and the pre-fabricated device substrate is a cathode substrate. In some embodiments, the pre-fabricated device substrate further includes an electron injection layer bonded to the cathode surface of the cathode substrate.

In some embodiments, a zinc oxide thin film having a surface hydroxyl number of 0.6 or more may be used alone as an electron transport layer.

In some embodiments, the electron transport layer comprises two zinc oxide thin films or n thin film stacked units consisting of two zinc oxide thin films, the two zinc oxide thin films are respectively named as a first electron transport layer and a second electron transport layer, and n is greater than or equal to 2. In some embodiments, n is an integer greater than or equal to 2 and less than or equal to 9. Wherein, at least the first electron transmission layer is the zinc oxide film with the surface hydroxyl group amount more than or equal to 0.6 prepared by the method, and the situation of the second electron transmission layer can refer to the situation in the second electron transmission layer of the quantum dot light-emitting diode device.

In some embodiments, the second electron transport layer is a zinc oxide film having a surface hydroxyl amount of less than or equal to 0.4, or the second electron transport layer is a metal-doped zinc oxide film. The first electron transport layer may be disposed at a side adjacent to the quantum dot light emitting layer, or at a side adjacent to the cathode. Preferably, the second electron transport layer is arranged on one side close to the quantum dot light emitting layer or the metal-doped zinc oxide film is arranged on one side close to the quantum dot light emitting layer, so that a more flat zinc oxide film can be obtained.

In some embodiments, the electron transport layer comprises three zinc oxide films, which are designated as a first electron transport layer, a second electron transport layer, and a third electron transport layer, respectively. In the above quantum dot light emitting diode device, the case where at least the first electron transport layer is the zinc oxide thin film with the surface hydroxyl amount greater than or equal to 0.6, the second electron transport layer and the third electron transport layer prepared by the above method can be referred to the case where the electron transport layer in the above quantum dot light emitting diode device includes the third electron transport layer.

In the above examples, the zinc oxide thin film having a surface hydroxyl group amount of 0.4 or less may be formed by a zinc oxide colloidal solution having a surface hydroxyl group amount of 0.4 or less.

In the above embodiment, the metal-doped zinc oxide thin film can be prepared by the following method:

mixing a zinc salt solution containing doped metal ions with a first alkali liquor at the temperature of 0-70 ℃, and reacting for 30 min-4 h; adding a precipitator into the mixed solution after the reaction is finished, and collecting precipitates; washing the precipitate by using a reaction solvent, and dissolving the obtained white precipitate to obtain a metal-doped zinc oxide colloidal solution; and forming the doped metal zinc oxide colloidal solution on a substrate of the metal doped zinc oxide film to be prepared to prepare the metal doped zinc oxide film. In this embodiment, the types of zinc salt and solvent of the zinc salt solution and the content of the zinc salt solution, the type and doping content of the doping ion, the type and addition amount of the first alkali solution, the reaction temperature and reaction time, and the selection and addition amount of the precipitant are all performed with reference to step S11 described above in this embodiment of the present application. In the method, the zinc salt solution containing doped metal ions can be obtained by dissolving zinc salt and selected metal salt in a certain proportion in a solvent at room temperature. In the step of mixing the zinc salt solution containing the doped metal ions with the first alkali liquor, the addition amount of alkali is as follows: the ratio of the product of the molar amount of metal ions and the valence number to the molar amount of hydroxide ions is 0.75: 1-1.25: 1.

In a second embodiment, the present application provides a method for preparing a quantum dot light emitting diode, the quantum dot light emitting diode includes an anode and a cathode oppositely disposed, a quantum dot light emitting layer disposed between the anode and the cathode, and an electron transport layer disposed between the quantum dot light emitting layer and the cathode, wherein the electron transport layer includes a first electron transport layer, and the first electron transport layer is a zinc oxide thin film having a surface hydroxyl group amount of 0.6 or more,

as shown in fig. 7, the method for preparing a zinc oxide thin film with a surface hydroxyl content of 0.6 or more comprises the following steps:

s21, reacting a zinc salt solution with a first alkali liquor to prepare zinc oxide nano-particles; dissolving zinc oxide nano particles to obtain a zinc oxide colloidal solution; adding a second alkali liquor into the zinc oxide colloidal solution, and adjusting the pH value of the zinc oxide colloidal solution to be greater than or equal to 8 to obtain a zinc oxide solution;

s22, forming a zinc oxide colloidal solution on a prefabricated device substrate of the zinc oxide film with the surface hydroxyl content of more than or equal to 0.6 to be prepared, and removing the solvent to prepare the zinc oxide film with the surface hydroxyl content of more than or equal to 0.6.

According to the preparation method of the quantum dot light-emitting diode provided by the embodiment of the application, the zinc oxide colloidal solution is prepared by a solution method, then the second alkali liquor is added into the zinc oxide colloidal solution, the pH value of the zinc oxide colloidal solution is adjusted to be greater than or equal to 8, the zinc oxide solution is obtained, and the zinc oxide with the surface hydroxyl group amount greater than or equal to 0.6 is obtained. The zinc oxide film with the surface hydroxyl amount being more than or equal to 0.6 is used as a first electron transmission layer, the transmission of electrons to the quantum dot light-emitting layer is inhibited, and the electrons injected into the quantum dot light-emitting layer are reduced, so that the injection of hole electrons in the quantum dot light-emitting diode is more balanced, and the service life of the device is prolonged.

The step S21 is to prepare the colloidal zinc oxide solution by a solution method, which may be one of an alcoholysis method, a hydrolysis method, and the like. The basic flow of the solution method for preparing the zinc oxide comprises the following steps: mixing the zinc salt solution with the first alkali liquor, and reacting to generate a hydroxide intermediate such as zinc hydroxide; the hydroxide intermediate undergoes polycondensation to gradually generate the zinc oxide nano-particles.

In the embodiment of the present application, the selection criteria and types of the zinc salt solution, the zinc salt in the zinc salt solution, and the formation manner of the zinc salt solution, the selection criteria and types of the alkali in the first alkali solution, and the formation manner of the first alkali solution are not described herein again for the sake of brevity. The reaction conditions and time for mixing the zinc salt solution and the first alkali solution, the content ratio of the zinc salt solution and the first alkali solution, and the like, as described in the above first embodiment, are not described herein again for saving reasons.

In some embodiments, after the reaction is completed, a precipitant is added to the mixed solution after the reaction is completed, and the precipitate is collected. The selection of the precipitating agent is as described above for the first embodiment.

In the examples of the present application, the mixed system subjected to the precipitation treatment was centrifuged, and the precipitate was collected. See the first embodiment above for methods and conditions for centrifugation.

And dissolving the precipitate after the cleaning treatment to obtain a zinc oxide colloidal solution.

In the embodiment of the application, the pH of the zinc oxide colloidal solution is adjusted to be more than or equal to 8 by adding the second alkali liquor into the zinc oxide colloidal solution. The hydroxyl ligand on the surface of the zinc oxide and the hydroxyl in an ionization state in the colloidal solution of the zinc oxide form a dynamic balance, and the addition of the second alkali liquor can break the balance. Specifically, after the second alkali solution is added, the amount of hydroxyl in an ionized state in the zinc oxide colloidal solution is increased, so that the amount of hydroxyl ligands on the surface of the zinc oxide is correspondingly increased. At the same time, the addition amount of the alkali in the second alkali liquor cannot be too large (the pH value cannot be too large), otherwise, the zinc oxide particles can react to form zinc hydroxide, and the concentration of the zinc oxide colloidal solution is reduced. Therefore, in some embodiments, the pH of the zinc oxide colloidal solution is adjusted to be between 9 and 12 by adding the second alkali liquor, so that the zinc oxide nanoparticles have higher yield (concentration) on the basis that the surface hydroxyl group amount of the obtained zinc oxide is greater than or equal to 0.6. In some embodiments, the pH of the zinc oxide colloidal solution is adjusted to be between 9 and 10 by adding a second alkali liquor.

In the embodiment of the application, the alkali in the second alkali liquor can be inorganic alkali or organic alkali; a strong base may be selected, and a weak base may be selected. In some embodiments, the second alkaline solution is selected from the group consisting of potassium hydroxide, sodium hydroxide, lithium hydroxide, TMAH, ammonia, ethanolamine, and ethylenediamine. In the embodiment of the application, the second alkali solution is a solution formed by dissolving an inorganic base or a solution formed by dissolving or diluting an organic base. Through dissolving or diluting alkali, adjust the second alkali lye concentration to control reaction rate, thereby make the adjustment of zinc oxide nano-particle surface hydroxyl can fully go on. Wherein the solvent used to dissolve or dilute the acid to form the second lye is capable of dissolving or being miscible with the base and is otherwise the same polarity as the zinc oxide nanoparticles. In some embodiments, the solvent used to dissolve or dilute the base to form the second basic solution may be the same as the solvent in the zinc salt solution or may be different from the solvent in the zinc salt solution. In some embodiments, the solvent used to dissolve or dilute the base to form the second alkaline solution includes, but is not limited to, water, organic alcohols, organic ethers, sulfones, and the like, which are relatively polar solvents. In some embodiments, the solvent is selected from at least one of water, organic alcohols, organic ethers, sulfones. Illustratively, the solvent may be selected from at least one of water, methanol, ethanol, propanol, butanol, ethylene glycol monomethyl ether, and dimethyl sulfoxide (DMSO).

In step S22, the zinc oxide solution may be formed on a pre-fabricated device substrate on which a zinc oxide thin film with a surface hydroxyl group amount greater than or equal to 0.6 is to be formed, and the solvent may be removed to form a zinc oxide thin film with a surface hydroxyl group amount greater than or equal to 0.6, according to the type of the quantum dot light emitting diode device to be formed.

The embodiment of step S22 refers to the first embodiment described above.

In a third embodiment, the present application provides a method for preparing a quantum dot light emitting diode, the quantum dot light emitting diode includes an anode and a cathode oppositely disposed, a quantum dot light emitting layer disposed between the anode and the cathode, and an electron transport layer disposed between the quantum dot light emitting layer and the cathode, wherein the electron transport layer includes a first electron transport layer, and the first electron transport layer is a zinc oxide thin film having a surface hydroxyl group amount of 0.6 or more,

as shown in fig. 8, the method for preparing a zinc oxide thin film having a surface hydroxyl group amount of 0.6 or more includes:

s31, preparing a zinc oxide prefabricated film on a prefabricated device substrate of the zinc oxide film of which the surface hydroxyl content is greater than or equal to 0.6;

and S32, depositing a second alkaline solution on the surface of the zinc oxide prefabricated film, and then drying to obtain the zinc oxide film.

According to the preparation method of the quantum dot light-emitting diode provided by the embodiment of the application, the zinc oxide prefabricated film is subjected to alkali treatment, and a liquid film can be formed on the surface of the zinc oxide prefabricated film, so that the hydroxyl content on the surface of the zinc oxide prefabricated film and the alkali content in the liquid film form a dynamic balance, and further the hydroxyl content on the surface of the zinc oxide prefabricated film is increased, so that the zinc oxide with the surface hydroxyl content of more than or equal to 0.6 is obtained. Under the condition, the zinc oxide film with the surface hydroxyl quantity being more than or equal to 0.6 is used as the first electron transmission layer, the transmission of electrons to the quantum dot luminous layer is inhibited, and the electrons injected into the quantum dot luminous layer are reduced, so that the injection of hole electrons in the quantum dot luminous diode is more balanced, and the service life of the device is prolonged.

In the step S31, the zinc oxide pre-formed film may be prepared in various ways, for example, by a solution method or a sol-gel method.

In some embodiments, the zinc oxide pre-formed film is prepared by a solution process comprising: mixing a zinc salt solution and a first alkali solution for reaction to prepare zinc oxide nano-particles; dissolving zinc oxide nano particles to obtain a zinc oxide colloidal solution; and forming a zinc oxide colloid solution on a prefabricated device substrate of the zinc oxide film with the surface hydroxyl content of more than or equal to 0.6 to be prepared, and removing the solvent to prepare the zinc oxide prefabricated film.

The step of mixing the zinc salt solution with the first alkali solution for reaction to prepare the zinc oxide nanoparticles is referred to as step S11 in the first embodiment, and is not described herein again for brevity.

In one possible embodiment, the zinc oxide in the zinc oxide thin film with the surface hydroxyl group amount of less than or equal to 0.4 is metal-doped zinc oxide, and correspondingly, the zinc oxide in the zinc oxide thin film with the surface hydroxyl group amount of less than or equal to 0.4 is metal-doped zinc oxide, and in this case, the zinc salt solution further contains doped metal ions. In this example, the selection of the dopant metal ions and the dopant content are as described above for the dopant metal in the metal-doped zinc oxide film.

In the embodiment of the application, according to the type of the prepared quantum dot light-emitting diode device, the zinc oxide colloid solution is formed on the prefabricated device substrate of the zinc oxide thin film with the surface hydroxyl content of more than or equal to 0.6 to be prepared, and the solvent is removed to prepare the zinc oxide thin film with the surface hydroxyl content of more than or equal to 0.6.

In some embodiments, the above-mentioned zinc oxide colloidal solution is formed on a pre-fabricated device substrate, and the step of forming the zinc oxide colloidal solution on the pre-fabricated device substrate on which the zinc oxide thin film having the surface hydroxyl group amount of 0.6 or more is to be prepared, and removing the solvent to obtain the zinc oxide thin film having the surface hydroxyl group amount of 0.6 or more is referred to as the above-mentioned step S12 ".

In the above step S32, the amount of hydroxyl groups on the surface of the zinc oxide pre-formed film is changed by depositing the second alkali solution on the zinc oxide pre-formed film. Specifically, after the second alkali solution is deposited, a liquid film is formed on the surface of the zinc oxide prefabricated film, so that the hydroxyl on the surface of the zinc oxide prefabricated film and the alkali content in the liquid film form a dynamic balance, and further the hydroxyl content on the surface of the zinc oxide prefabricated film is increased.

In the embodiment of the application, the alkali in the second alkali liquor can be inorganic alkali or organic alkali; a strong base may be selected, and a weak base may be selected. In some embodiments, the second alkaline solution is selected from at least one of potassium hydroxide, sodium hydroxide, lithium hydroxide, TMAH, ammonia, ethanolamine, and ethylenediamine. In the embodiment of the application, the second alkali solution is a solution formed by dissolving an inorganic base or a solution formed by dissolving or diluting an organic base. Through dissolving or diluting alkali, adjust the second alkali lye concentration to control reaction rate, thereby make the adjustment of zinc oxide nano-particle surface hydroxyl can fully go on. Wherein the solvent used to dissolve or dilute the acid to form the second lye is capable of dissolving or being miscible with the base and is otherwise the same polarity as the zinc oxide nanoparticles. In some embodiments, the solvent used to dissolve or dilute the base to form the second basic solution may be the same as the solvent in the zinc salt solution or may be different from the solvent in the zinc salt solution. In some embodiments, the solvent used to dissolve or dilute the base to form the second lye includes, but is not limited to, water, organic alcohols, organic ethers, sulfones, and the like, which are relatively polar solvents. In some embodiments, the solvent is selected from at least one of water, organic alcohols, organic ethers, sulfones. Illustratively, the solvent may be selected from at least one of water, methanol, ethanol, propanol, butanol, ethylene glycol monomethyl ether, and dimethyl sulfoxide (DMSO).

In the examples of the present application, it is necessary to control the concentration and the addition amount of the alkali solution. This is because: when the concentration and the addition amount of the alkali are too large, a large amount of zinc hydroxide impurities are generated on the surface of the zinc oxide prefabricated film, and the quality of the zinc oxide film is influenced; when the concentration and the addition amount of the alkali are too small, the effect of increasing the surface hydroxyl amount of the zinc oxide is not easily achieved. In some embodiments, the concentration of the second alkali solution is 0.05-0.5mmol/L, so as to obtain a proper concentration to regulate the surface hydroxyl group amount of the zinc oxide prefabricated film. In some embodiments, the deposition amount of the second alkali solution and the weight of the lower zinc oxide preformed film satisfy: each 5mg of zinc oxide pre-formed film is treated with 50. mu.L to 1000. mu.L of the second alkali solution. The concentration of the second alkali liquor and the addition amount of the alkali are too large, so that a large amount of zinc hydroxide impurities are generated on the surface of the zinc oxide prefabricated film, and the quality of the zinc oxide film is influenced; when the concentration of the second alkali liquor and the addition amount of the alkali are too small, the effect of increasing the surface hydroxyl amount of the zinc oxide is not easily achieved. It will be appreciated that the concentration of the secondary alkali fluid can be flexibly adjusted depending on the type of alkali selected.

The inorganic base is generally strong base, and the ionization capacity of hydroxide ions is strong, so that the amount of hydroxyl groups on the surface of the zinc oxide can be adjusted by only a small amount of low-concentration inorganic base. The organic base is generally weak base, and the ionization capacity of hydroxide ions is weak, so that a large amount of organic base with relatively high concentration is needed to effectively adjust the surface hydroxyl amount of the zinc oxide.

In some embodiments, the base in the second alkali solution is an inorganic base, and the concentration of the second alkali solution is 0.05-0.1 mmol/L. Illustratively, the inorganic base is at least one selected from the group consisting of potassium hydroxide, sodium hydroxide, and lithium hydroxide. In this case, the deposition amount of the second alkali solution and the weight of the lower zinc oxide pre-film satisfy: each 5mg of the zinc oxide pre-film was treated with 50. mu.L to 400. mu.L of the second alkali solution.

In some embodiments, the base in the second alkali solution is an organic base, and the concentration of the second alkali solution formed is 0.2-0.4 mmol/L. Illustratively, the organic carboxylic acid is at least one selected from TMAH, ammonia, ethanolamine, and ethylenediamine. In this case, the deposition amount of the second alkali solution and the weight of the lower zinc oxide prefabricated film satisfy: the treatment is carried out using 500. mu.L to 1000. mu.L of the second alkali solution per 5mg of the zinc oxide pre-formed film.

In the embodiment of the present application, the method for depositing the second alkaline solution on the surface of the zinc oxide pre-film may adopt a solution processing method, including but not limited to one of spin coating, doctor blading, printing, spraying, roll coating, electrodeposition, and the like.

And (3) depositing a second alkali liquor on the surface of the zinc oxide prefabricated film, and then carrying out drying treatment, wherein ionized hydrogen ions in the second alkali liquor are fully reacted with hydroxyl on the surface of the zinc oxide through the drying treatment. In some embodiments, the temperature of the drying process is 10 ℃ to 100 ℃ and the drying time is 10 minutes to 2 hours. In this case, the ionized hydrogen ions in the second alkali fluid react with the hydroxyl groups on the surface of the zinc oxide sufficiently to increase the amount of hydroxyl groups on the surface of the zinc oxide. If the drying temperature is too high or the drying treatment time is too long, the second alkali liquor can be quickly dried, the zinc oxide prefabricated film can be quickly changed into a solid film, and further, ionized hydrogen ions in the second alkali liquor and hydroxyl on the surface of the zinc oxide are not easy to fully react, and the hydroxyl content on the surface of the zinc oxide is not easy to fully reduce; when the drying temperature is too low or the drying time is too short, the zinc oxide prefabricated film is difficult to be sufficiently dried, and the preparation of the next layer, especially the evaporation quality of the electrode, is influenced. In some embodiments, the temperature of the drying process is 10 ℃ to 50 ℃ and the drying time is 30 minutes to 2 hours. By changing the hydroxyl content of the surface of the zinc oxide by the method, the surface of the finally obtained film can remain an auxiliary layer formed by a very small amount of alkali.

In one possible embodiment, the quantum dot light emitting diode is an orthoscopic quantum dot light emitting diode, and the prefabricated device substrate includes an anode substrate and a quantum dot light emitting layer bonded to the anode substrate. In some embodiments, the pre-fabricated device substrate further comprises a hole-functional layer disposed between the anode substrate and the quantum dot light-emitting layer. Wherein the hole function layer comprises at least one of a hole transport layer, a hole injection layer and an electron blocking layer.

In some embodiments, the electron transport layer comprises two zinc oxide thin films or n thin film stacked units consisting of two zinc oxide thin films, the two zinc oxide thin films are respectively named as a first electron transport layer and a second electron transport layer, and n is greater than or equal to 2. In some embodiments, n is an integer greater than or equal to 2 and less than or equal to 9. Wherein, at least the first electron transport layer is the zinc oxide film with the surface hydroxyl amount of more than or equal to 0.6 prepared by the method, and the condition of the second electron transport layer can refer to the condition in the second electron transport layer of the quantum dot light-emitting diode device.

In some embodiments, the second electron transport layer is a zinc oxide film having a surface hydroxyl amount of less than or equal to 0.4, or the second electron transport layer is a metal-doped zinc oxide film. The first electron transport layer may be disposed at a side adjacent to the quantum dot light emitting layer, or at a side adjacent to the cathode. Preferably, the zinc oxide film or the metal-doped zinc oxide film with the surface hydroxyl group amount of less than or equal to 0.4 is arranged at one side adjacent to the quantum dot light-emitting layer, so that a more flat zinc oxide film can be obtained.

In the above examples, the metal-doped zinc oxide thin film can be produced by the method of the first embodiment. In some embodiments, the preparation of the metal-doped zinc oxide thin film comprises:

mixing a zinc salt solution containing doped metal ions with a first alkali liquor at the temperature of 0-70 ℃, and reacting for 30 min-4 h; adding a precipitator into the mixed solution after the reaction is finished, and collecting precipitates; washing the precipitate by using a reaction solvent, and dissolving the obtained white precipitate to obtain a metal-doped zinc oxide colloidal solution; and forming the doped metal zinc oxide colloidal solution on a substrate of the metal doped zinc oxide film to be prepared to prepare the metal doped zinc oxide film. In this embodiment, the types of zinc salt and solvent of the zinc salt solution, the content of the zinc salt solution, the type and doping content of the doping ion, the type and addition amount of the first alkali solution, the reaction temperature and reaction time, the selection and addition amount of the precipitant, and the type and content of the doping metal ion are all performed with reference to step S11 described above in this embodiment of the present application. In some embodiments, in the step of mixing the zinc salt solution containing the doping metal ion with the first alkali solution, the addition amount of the alkali is such that: the ratio of the product of the molar amount of metal ions and the valence number to the molar amount of hydroxide ions is 0.75: 1-1.25: 1.

In some embodiments, the dopant metal ions in the resulting metal-doped zinc oxide film are selected from Mg²⁺、Mn²⁺At least one of; in some embodiments, the dopant metal ions in the resulting metal-doped zinc oxide film are selected from Al³⁺、Y³ ⁺、 La³⁺、Li⁺、Gd³⁺、Zr⁴⁺、Ce⁴⁺At least one of (1).

In some embodiments, the doping concentration of the doped metal ions in the prepared metal-doped zinc oxide film is as follows: when the doped metal ion is Mg²⁺When the metal is doped with Mg in the zinc oxide film²⁺The molar content accounts for 0.1 to 35 percent of the total molar weight of the metal ions; when the doped metal ion is Mn²⁺When the metal is doped with Mn in the zinc oxide film²⁺The molar content accounts for 0.1 to 30 percent of the total molar weight of the metal ions; when the doped metal ion is Al³⁺When the metal is doped with Al in the zinc oxide film³⁺The molar content accounts for 0.1 to 15 percent of the total molar weight of the metal ions; when the doped metal ion is Y³⁺When in use, Y in the metal-doped zinc oxide film³⁺The molar content accounts for 0.1 to 10 percent of the total molar weight of the metal ions; when the doped metal ion is La³⁺When in use, La in the metal-doped zinc oxide film³⁺The molar content accounts for 0.1 to 7 percent of the total molar weight of the metal ions; when the doped metal ion is Li⁺When the metal is doped with Li in the zinc oxide film⁺The molar content accounts for 0.1 to 45 percent of the total molar weight of the metal ions; when the doped metal ion is Gd³⁺When the metal is doped with Gd in the zinc oxide film³⁺The molar content of the metal ions is the total molar amount of the metal ions0.01 to 8 percent; when the doped metal ion is Zr⁴⁺When the metal is doped with Zr in the zinc oxide film⁴⁺The molar content accounts for 0.1 to 45 percent of the total molar weight of the metal ions; when the doped metal ion is Ce⁴⁺When the metal is doped with Ce in the zinc oxide film⁴⁺The molar content accounts for 0.1 to 10 percent of the total molar weight of the metal ions.

It should be understood that, in the three embodiments described above, when the device is an upright quantum dot light emitting diode, after the preparation of the electron transport layer, the method further includes depositing a cathode on the electron transport layer to obtain the quantum dot light emitting diode. In some embodiments, before evaporating the cathode, preparing an electron injection layer on the electron transport layer is further included. When the device is an inverted quantum dot light-emitting diode, after the electron transmission layer is prepared, two light-emitting layers are prepared on the electron transmission layer, and an anode is vapor-plated on the quantum dot light-emitting layer to obtain the quantum dot light-emitting diode. In some embodiments, before the step of evaporating the anode, the step of preparing a hole function layer on the quantum dot light-emitting layer is further included.

In the embodiments of the present application, the method for forming the hole function layer (including at least one of the hole injection layer, the hole transport layer, and the electron blocking layer) and the quantum dot light emitting layer preferably employs a solution processing method, including but not limited to one of spin coating, doctor blading, printing, spraying, roll coating, electrodeposition, and the like.

In the three embodiments of the present application, the prepared quantum dot light emitting diode is subjected to encapsulation treatment, and the encapsulation treatment may be performed by a common machine or by manual encapsulation. Preferably, the oxygen content and the water content are both lower than 0.1ppm in the packaging treatment environment to ensure the stability of the device. The curing resin adopted for packaging is acrylic resin, acrylic resin or epoxy resin; the resin curing employs UV irradiation, heating, or a combination of both.

In some embodiments, after the prepared quantum dot light emitting diode is packaged according to the performance requirements of the quantum dot light emitting diode device, one or more treatments including ultraviolet irradiation, heating, positive and negative pressure, external electric field, and external magnetic field are further performed on the obtained quantum dot light emitting diode to improve one or more aspects of the performance of the quantum dot light emitting diode device, wherein the atmosphere during the application process may be air or inert atmosphere.

The following description will be given with reference to specific examples.

First, three detection methods used in the embodiments of the present application are introduced:

(1) an X-ray photoelectron spectroscopy (XPS) is a surface analysis method that uses X-rays with a certain energy to irradiate a sample, so that electrons or valence electrons in the inner layer of atoms or molecules are excited and emitted, the electrons excited by photons are called photoelectrons, and the energy and quantity of the photoelectrons can be measured, thereby obtaining the composition of an object to be measured. The technology can effectively distinguish the existence of three chemical oxygen states of zinc oxide material, namely lattice oxygen connected with metal atoms, oxygen defect formed in crystal growth and hydroxyl oxygen. When surface hydroxyl group test is performed by using X-ray photoelectron spectroscopy (XPS), the equipment model is as follows: semer fema, sample preparation method: and diluting the prepared zinc oxide solution to 30mg/mL, spin-coating the zinc oxide solution on a pretreated glass sheet, and spin-coating the zinc oxide solution to form a film. Wherein, the hydroxyl content calculation method comprises the following steps: the ratio of the hydroxyl oxygen peak area to the lattice oxygen peak area is the hydroxyl content ratio:

as shown in fig. 9.

(2) JVL (Current Density-Voltage-Brightness) equipment external quantum efficiency test method

The equipment model is as follows: keithley 2400/6485

The external quantum efficiency parameters mainly include six parameters: voltage, current, brightness, external quantum dot efficiency, power efficiency, and luminescence spectrum; the device is output with a certain voltage in the cassette to make the device conduct and emit light, record the current in time, collect the light source through the silicon photodiode, analyze the spectral data, calculate the G (lambda) human eye vision function and S (lambda) normalized electroluminescence while obtaining the color coordinateOptical spectrum, so current efficiency eta_AIs calculated by

Wherein L is the brightness read by the silicon photodiode, JD is the device current density, which is the ratio of the device area (a) to the current (I) flowing through the device

The calculation method of the external quantum efficiency eta EQE comprises the following steps

Where q is the basic charge, h is the Planckian constant, and c is the speed of light in vacuum.

As shown in fig. 10, the EQE peak of the EQE-luminance curve is the external quantum efficiency of the device.

(3) QLED life test system

The model is as follows: new FOV NVO-QLED-LT-128

The working principle is as follows:

the 128-channel QLED life test system controls a digital IO card of NI (American national instruments) to realize the chip selection of the channel number and the output of digital signals through the PCI bus communication of a central processing computer, the corresponding digital signals are converted into analog signals through a D/A chip to finish current output (I), and data acquisition is realized through a data acquisition card. The acquisition of the brightness converts the optical signal into an electrical signal through a sensor, and the electrical signal is used for simulating the brightness change (L).

The test method comprises the following steps:

QLED Life test method (constant current method)

(A) Three to four different constant current densities (e.g., 100mA cm 2, 50mA cm 2, 20mA cm 2, 10mA cm 2) were selected and tested for initial brightness under the corresponding conditions.

(B) Constant current was maintained and changes in brightness and device voltage over time were recorded.

(C) The time for the device to decay to T95, T80, T75, T50 under different constant currents was recorded.

(D) The acceleration factor is calculated by curve fitting.

(E) The lifetimes of the devices 1000nit T95, T80, T75, T50 were extrapolated by empirical equations, as shown in diagram 11.

The calculation method comprises the following steps: t is_T95@1000nits＝(L_MAX/1000)^A*T₉₅

Wherein: l is_MAXMaximum brightness

A-acceleration factor

T₉₅The time elapsed for the maximum brightness of the device to decay to 95%.

Example 1

A quantum dot light-emitting diode comprises an anode substrate and a cathode which are oppositely arranged, a quantum dot light-emitting layer arranged between the anode and the cathode, a hole transport layer arranged between the anode and the quantum dot light-emitting layer, a hole injection layer arranged between the anode and the hole transport layer, and an electron transport layer arranged between the quantum dot light-emitting layer and the cathode, wherein the anode is ITO (55nm), the hole injection layer is PEDOT: PSS (50nm), the hole transport layer is TFB (30nm), and the quantum dot light-emitting layer is red quantum dot Cd_xZn_1-xSe/ZnSe (40nm), the electron transport layer is ZnO material (50nm) prepared by the following method, and the cathode is Ag electrode (100 nm).

The preparation method of the quantum dot light-emitting diode comprises the following steps:

preparing a hole injection layer, a hole transport layer and a quantum dot light emitting layer on an anode substrate in sequence;

preparing an electron transport layer on the quantum dot light emitting layer;

evaporating or sputtering a top electrode on the zinc oxide electron transport layer or the doped zinc oxide electron transport layer to obtain the quantum dot light-emitting diode,

the preparation method of the electron transport layer comprises the following steps:

the first step,

(A) Dissolving zinc acetate in dimethyl sulfoxide at room temperature to prepare a zinc salt solution with the concentration of 0.6mol/L, and dissolving sodium hydroxide in methanol at room temperature to obtain an alkali solution with the concentration of 0.96mol/L, wherein the molar ratio of hydroxide ions to zinc ions is 1.6: 1;

(B) adjusting the temperature of the zinc salt solution to 40 ℃, dropwise adding alkali liquor into the zinc salt solution according to the molar ratio of hydroxide ions to zinc ions of 1.6:1, and continuously stirring the mixed solution under the condition that the reaction temperature is kept at 40 ℃ for reacting for 90 min;

(C) adding a mixed solution after the reaction is finished into the mixed solution in a volume ratio of 4.5: 1, a white precipitate was formed in the mixed solution.

(D) Dissolving the obtained white precipitate, adding 0.2mol/L potassium hydroxide solution into the zinc oxide colloidal solution, and adjusting the pH of the solution to 8 to obtain zinc oxide colloidal solution with hydroxyl content of 0.85.

And step two, forming a zinc oxide colloidal solution on the quantum dot light-emitting layer, and removing the solvent to prepare a zinc oxide film with the surface hydroxyl quantity of 0.85, namely an electron transmission layer, wherein the thickness of the zinc oxide film is 50 nm.

Hydroxyl groups in zinc oxide for forming the electron transport layer were detected by X-ray photoelectron spectroscopy (XPS), and the hydroxyl group content of the electron transport layer was measured to be 0.85.

Comparative example 1

The difference from example 1 is that: common zinc oxide nano-particles sold in the market are adopted as the material of the electron transport layer. Hydroxyl groups in zinc oxide for forming the electron transport layer were detected by X-ray photoelectron spectroscopy (XPS), and the hydroxyl group content of the electron transport layer was measured to be 0.5.

The device lifetime test results of the quantum dot light emitting diodes provided in example 1 and comparative example 1 are shown in fig. 12.

Example 2

A quantum dot light-emitting diode comprises an anode substrate and a cathode which are oppositely arranged, a quantum dot light-emitting layer arranged between the anode and the cathode, a hole transport layer arranged between the anode and the quantum dot light-emitting layer, a hole injection layer arranged between the anode and the hole transport layer, and an electron transport layer arranged between the quantum dot light-emitting layer and the cathode, wherein the anode is ITO (55nm), and the hole is a hole injection layerPSS (50nm) as a hole injection layer, TFB (30nm) as a hole transport layer, and Cd as a red quantum dot light-emitting layer_xZn_1-xSe/ZnSe (40nm), the electron transport layer is ZnO material prepared by the following method, and the cathode is Ag electrode (100 nm).

preparing an electron transport layer on the quantum dot light emitting layer;

(1) dissolving zinc acetate in dimethyl sulfoxide at room temperature to prepare a zinc salt solution with the concentration of 0.5mol/L, and dissolving potassium hydroxide in methanol at room temperature to obtain an alkali solution with the concentration of 0.85mol/L, wherein the molar ratio of hydroxide ions to zinc ions is 1.7: 1;

(B) adjusting the temperature of the zinc salt solution to 60 ℃, dropwise adding alkali liquor into the zinc salt solution according to the molar ratio of hydroxide ions to zinc ions of 1.7:1, continuously stirring the mixed solution under the condition that the reaction temperature is kept at 60 ℃, and reacting for 90 min;

(C) adding a mixed solution after the reaction is finished into the mixed solution in a volume ratio of 3:1, a white precipitate was generated in the mixed solution.

(D) Dissolving the obtained white precipitate to obtain a first zinc oxide colloidal solution with the concentration of 0.6 mol/L;

(2) dissolving zinc acetate in dimethyl sulfoxide at room temperature to prepare a zinc salt solution with the concentration of 0.5mol/L, and dissolving potassium hydroxide in methanol at room temperature to obtain an alkali solution with the concentration of 0.85mol/L, wherein the molar ratio of hydroxide ions to zinc ions is 1.7: 1;

(D) Dissolving the obtained white precipitate, adding 0.05mol/L hydrochloric acid into the zinc oxide colloidal solution, and adjusting the pH of the solution to 7.2 to obtain a second zinc oxide colloidal solution with the hydroxyl content of 0.25;

(3) forming a first zinc oxide colloidal solution on the quantum dot light-emitting layer, and removing the solvent to prepare a zinc oxide prefabricated film; and (2) depositing alkali liquor on the surface of the zinc oxide prefabricated film, wherein the alkali liquor is 0.05mol/L potassium hydroxide solution, and the deposition amount of the alkali liquor and the weight of the lower-layer zinc oxide prefabricated film meet the following requirements: treating every 5mg of zinc oxide prefabricated film with 20 μ L of alkali liquor, heating at 80 deg.C for 30min, removing solvent to obtain a first zinc oxide film with surface hydroxyl amount of 0.85, i.e. a first electron transport layer; and forming a second zinc oxide colloidal solution on the first zinc oxide film, and removing the solvent to obtain a second zinc oxide film with the surface hydroxyl content of 0.25, namely a second electron transport layer. The thickness of the first zinc oxide layer is 60nm, and the thickness of the second zinc oxide layer is 40 nm.

Hydroxyl groups in the zinc oxide for preparing the first electron transport layer and the second electron transport layer are detected by X-ray photoelectron spectroscopy (XPS), and the hydroxyl group content of the first electron transport layer is determined to be 0.85 and the hydroxyl group content of the second electron transport layer is determined to be 0.25.

The device EQE test results of the quantum dot light emitting diodes provided in example 2 and comparative example 1 are shown in fig. 13, and the lifetime test results are shown in fig. 14.

Example 3

A quantum dot light-emitting diode comprises an anode substrate and a cathode which are oppositely arranged, a quantum dot light-emitting layer arranged between the anode and the cathode, a hole transport layer arranged between the anode and the quantum dot light-emitting layer, a hole injection layer arranged between the anode and the hole transport layer, and an electron transport layer arranged between the quantum dot light-emitting layer and the cathode, wherein the anode is ITO (55nm), the hole injection layer is PEDOT: PSS (50nm), the hole transport layer is TFB (30nm), the quantum dot light-emitting layer is red quantum dot CdxZn1-xSe/ZnSe (40nm), the electron transport layer is a ZnO material prepared by the following method, and the cathode is an Ag electrode (100 nm).

preparing an electron transport layer on the quantum dot light emitting layer;

(1) dissolving zinc acetate in dimethyl sulfoxide at room temperature to prepare a zinc salt solution with the concentration of 0.5mol/L, dissolving sodium hydroxide in methanol at room temperature to obtain an alkali solution with the concentration of 0.85mol/L, wherein the molar ratio of hydroxide ions to zinc ions is 1.7: 1;

(C) adding a mixed solution after the reaction is finished into the mixed solution in a volume ratio of 3:1, a white precipitate was generated in the mixed solution. Washing the precipitate with methanol as reaction solvent for 1 time, and dissolving the obtained white precipitate to obtain the first zinc oxide colloidal solution with surface hydroxyl amount of 0.88.

(2) Dissolving zinc acetate in dimethyl sulfoxide at room temperature to prepare a zinc salt solution with the concentration of 0.5mol/L, dissolving sodium hydroxide in methanol at room temperature to obtain an alkali solution with the concentration of 0.85mol/L, wherein the molar ratio of hydroxide ions to zinc ions is 1.7: 1;

(C) adding a mixed solution after the reaction is finished into the mixed solution in a volume ratio of 4: 1, a white precipitate was generated in the mixed solution.

(D) Dissolving the obtained white precipitate, adding 0.05mol/L hydrochloric acid into the zinc oxide colloidal solution, and adjusting the pH of the solution to 7.2 to obtain a second zinc oxide colloidal solution with the hydroxyl content of 0.22.

(3) Dissolving zinc acetate in dimethyl sulfoxide at room temperature to prepare a zinc salt solution with the concentration of 0.5mol/L, and dissolving sodium hydroxide in methanol at room temperature to obtain an alkali solution with the concentration of 0.85mol/L, wherein the molar ratio of hydroxide ions to zinc ions is 1.7: 1;

(D) Dissolving the obtained white precipitate, adding 0.05mol/L potassium hydroxide solution into the zinc oxide colloidal solution, and adjusting the pH of the solution to 8.2 to obtain a second zinc oxide colloidal solution with the hydroxyl content of 0.85.

(4) Forming a first zinc oxide colloidal solution on the quantum dot light-emitting layer, and removing the solvent to obtain a first zinc oxide film with the surface hydroxyl amount of 0.88; forming a second zinc oxide colloidal solution on the first zinc oxide film, and removing the solvent to prepare a second zinc oxide film with the surface hydroxyl content of 0.22; and forming a third zinc oxide colloidal solution on the second zinc oxide film, and removing the solvent to obtain a third zinc oxide film with the surface hydroxyl quantity of 0.85. The thickness of the first zinc oxide layer is 30nm, the thickness of the second zinc oxide layer is 60nm, and the thickness of the third zinc oxide layer is 30 nm. .

Hydroxyl groups in the prepared first electron transport layer, the second electron transport layer and the third electron transport layer are detected by X-ray photoelectron spectroscopy (XPS), and the hydroxyl group content of the first electron transport layer, the hydroxyl group content of the second electron transport layer and the hydroxyl group content of the third electron transport layer are respectively 0.88, 0.22 and 0.85.

The device EQE test results of the quantum dot light emitting diodes provided in example 3 and comparative example 1 are shown in fig. 15, and the lifetime test results are shown in fig. 16.

The quantum dot light emitting diodes provided in the above 3 examples and comparative examples were subjected to performance tests, and the test results are shown in table 2 below:

TABLE 2

It should be understood that the lifetime of a quantum dot light emitting diode device is tested differently than the efficiency of a quantum dot light emitting diode device, which is typically tested for a short period of time, and therefore characterizes the operational initial transient state of the QLED device; the device life is characterized by the capability of keeping the efficiency of the device after the device continuously works and enters a stable state, namely the condition of carrier injection balance in the device after the device enters the stable working state.

The above description is only a preferred embodiment of the present application and should not be taken as limiting the present application, and any modifications, equivalents, improvements, etc. made within the spirit and scope of the present application should be included.

Claims

1. A preparation method of a quantum dot light-emitting diode is characterized in that the quantum dot light-emitting diode comprises an anode and a cathode which are oppositely arranged, a quantum dot light-emitting layer arranged between the anode and the cathode, and an electron transmission layer arranged between the quantum dot light-emitting layer and the cathode, wherein the electron transmission layer comprises a first electron transmission layer, the first electron transmission layer is a zinc oxide film with the surface hydroxyl group amount of more than or equal to 0.6,

2. The method of claim 1, wherein the second alkaline solution is at least one alkaline solution selected from potassium hydroxide, sodium hydroxide, lithium hydroxide, TMAH, ammonia, ethanolamine, and ethylenediamine.

3. The method of claim 2, wherein the concentration of the second alkali solution is 0.05-0.5 mmol/L.

4. The method of claim 3, wherein the base in the second alkali solution is an inorganic base, and the concentration of the second alkali solution is 0.05 to 0.1 mmol/L.

5. The method for preparing the quantum dot light-emitting diode of claim 4, wherein in the step of depositing the second alkali solution on the surface of the zinc oxide prefabricated film, the addition amount of the second alkali solution satisfies the following conditions: each 5mg of zinc oxide pre-formed film is treated with 50. mu.L to 400. mu.L of the second alkali solution.

6. The method of claim 3, wherein the base in the second alkali solution is an organic base, and the concentration of the second alkali solution is 0.2-0.4 mmol/L.

7. The method for preparing the quantum dot light-emitting diode of claim 6, wherein in the step of depositing the second alkali solution on the surface of the zinc oxide prefabricated film, the addition amount of the second alkali solution satisfies the following conditions: each 5mg of the zinc oxide pre-film is treated with 500. mu.L to 1000. mu.L of the second alkali solution.

8. The method of any one of claims 1 to 7, wherein the drying is performed at a temperature of 10 ℃ to 100 ℃ for a time of 10 minutes to 2 hours.

9. The method for preparing a quantum dot light-emitting diode according to any one of claims 1 to 7, wherein the zinc oxide film having a surface hydroxyl group amount of 0.6 or more is a metal-doped zinc oxide film.

10. The method of claim 9, wherein the metal ions in the metal-doped zinc oxide film are selected from Mg²⁺、Mn²⁺At least one of; or

The doped metal ions in the metal-doped zinc oxide film are selected from Al³⁺、Y³⁺、La³⁺、Li⁺、Gd³⁺、Zr⁴⁺、Ce⁴⁺At least one of (1).

11. The method of claim 10, wherein the doping concentration of the doped metal ions is as follows:

when the doped metal ion is Mg²⁺When the metal is doped with Mg in the zinc oxide film²⁺The molar content accounts for 0.1 to 35 percent of the total molar weight of the metal ions;

when the doped metal ion is Mn²⁺When the metal is doped with Mn in the zinc oxide film²⁺The molar content accounts for 0.1 to 30 percent of the total molar weight of the metal ions;

when the doped metal ion is Al³⁺When the metal is doped with Al in the zinc oxide film³⁺Molar content of metal0.1 to 15 percent of the total molar amount of ions;

when the doped metal ion is Y³⁺When in use, Y in the metal-doped zinc oxide film³⁺The molar content accounts for 0.1 to 10 percent of the total molar weight of the metal ions;

when the doped metal ion is La³⁺When in use, La in the metal-doped zinc oxide film³⁺The molar content accounts for 0.1 to 7 percent of the total molar weight of the metal ions;

when the doped metal ion is Li⁺When the metal is doped with Li in the zinc oxide film⁺The molar content accounts for 0.1 to 45 percent of the total molar weight of the metal ions;

when the doped metal ion is Gd³⁺When the metal is doped with Gd in the zinc oxide film³⁺The molar content accounts for 0.01 to 8 percent of the total molar weight of the metal ions;

when the doped metal ion is Zr⁴⁺When the metal is doped with Zr in the zinc oxide film⁴⁺The molar content accounts for 0.1 to 45 percent of the total molar weight of the metal ions;

when the doped metal ion is Ce⁴⁺When in use, Ce in the metal-doped zinc oxide film⁴⁺The molar content accounts for 0.1 to 10 percent of the total molar weight of the metal ions.