CN114695821A

CN114695821A - Preparation method of quantum dot light-emitting diode

Info

Publication number: CN114695821A
Application number: CN202011637282.8A
Authority: CN
Inventors: 郭煜林; 吴龙佳; 张天朔; 李俊杰; 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 electron transport layer, wherein, electron transport layer includes first electron transport layer, and the preparation method of zinc oxide film that surface hydroxyl amount is less than or equal to 0.4 includes: mixing the zinc salt solution with alkali liquor for reaction to prepare zinc oxide; dissolving the zinc oxide to obtain a zinc oxide colloidal solution; adding an acid solution into the zinc oxide colloidal solution, and adjusting the pH of the zinc oxide colloidal solution to 7-8 to obtain a zinc oxide solution; and forming a zinc oxide solution on the substrate of the prefabricated device, and removing the solvent to prepare the zinc oxide film with the surface hydroxyl content less than or equal to 0.4. The preparation method of the quantum dot light-emitting diode effectively prolongs the service life 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 remarkably improved.

Because the QLED display technology and the organic light-Emitting Diode (OLED) display technology have similarities in the principle of higher light emission, 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 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 the light emitting layer of the device. When the classical physical conclusion of the above-mentioned 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 good 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, in attempting to improve and enhance another key performance of QLED devices, device lifetime, using the above strategy, problems have been encountered: the classical thought and strategy formed in the OLED are not easy to realize effective improvement of the service life of the QLED device so far, and although high QLED device efficiency is obtained through the classical thought and strategy, the service life of the high-efficiency QLED device is obviously inferior to that of a similar device with lower efficiency. Meanwhile, with the gradual development and deepening of research on the QLED device mechanism, the QLED has some special mechanisms different from an OLED device system due to the use of nano materials with special material surfaces, such as quantum dots, ZnO nano particles and the like, and the mechanisms are closely related to the performance of the QLED device, particularly the service life of the device. Therefore, the existing QLED device structure designed based on the OLED device theoretical system can not meet the requirements for improving the performance of the QLED device, particularly the service life of the device; corresponding to the unique device mechanism of the QLED device system, a new and more targeted QLED device structure needs to be developed.

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 the service life of an existing quantum dot light-emitting diode device is not long.

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 less than or equal to 0.4,

the preparation method of the zinc oxide film with the surface hydroxyl content less than or equal to 0.4 comprises the following steps:

mixing the zinc salt solution with alkali liquor for reaction to prepare zinc oxide nano-particles; dissolving the zinc oxide nano particles to obtain a zinc oxide colloidal solution; adding an acid solution into the zinc oxide colloidal solution, and adjusting the pH of the zinc oxide colloidal solution to 7-8 to obtain a zinc oxide solution;

and forming a zinc oxide solution on the prefabricated device substrate of the zinc oxide film with the surface hydroxyl content less than or equal to 0.4, and removing the solvent to prepare the zinc oxide film with the surface hydroxyl content less than or equal to 0.4.

According to the preparation method of the quantum dot light-emitting diode, firstly, a zinc oxide colloidal solution is prepared by a solution method, then acid liquor is added into the zinc oxide colloidal solution, the pH value of the zinc oxide colloidal solution is adjusted to 7-8, the zinc oxide solution is obtained, and zinc oxide with the surface hydroxyl group amount smaller than or equal to 0.4 is obtained. The zinc oxide film with the surface hydroxyl quantity less than or equal to 0.4 is used as the first electron transport layer, the electron is smoothly transported to the quantum dot light-emitting layer, electrons injected into the quantum dot light-emitting layer are increased, the injection rate of the electrons to the quantum dot light-emitting layer is higher than that of holes to the quantum dot light-emitting layer, and the quantum dots in the quantum dot light-emitting layer are negatively charged under the condition. The negatively charged state can be maintained due to the binding effect of the quantum dot core-shell structure and the electrically inert surface ligand, and simultaneously, the coulomb repulsion effect makes the further injection of electrons into the quantum dot light-emitting layer more and more difficult. When the quantum dot light-emitting diode device is continuously lightened to work to a stable state, the state of the quantum dot with negative electricity tends to be stable, namely electrons newly captured and bound by the quantum dot and electrons consumed by radiation transition reach dynamic balance, the injection rate of the electrons to the quantum dot light-emitting layer is much lower than that in the initial stage, and the lower injection rate of the electrons and the lower injection rate of holes just reach carrier injection balance, so that 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 0.4 or less 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 0.4 or less 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 0.4 or less according to the examples of the present application;

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 showing the life 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;

fig. 17 is a graph of the device EQE test results of the quantum dot light emitting diodes provided in example 4 and comparative example 1 of the present application;

fig. 18 is a graph showing the life test results of the quantum dot light emitting diodes provided in example 4 and comparative example 1 of the present application;

fig. 19 is a graph of the device EQE test results of the quantum dot light emitting diodes provided in example 5 and comparative example 1 of the present application;

fig. 20 is a graph showing the life test results of the quantum dot light emitting diodes provided in example 5 and comparative example 1 of the present application;

fig. 21 is a graph showing the results of the device EQE test of the quantum dot light emitting diode provided in example 6 and comparative example 1 of the present application;

fig. 22 is a graph showing the life test results of the quantum dot light emitting diodes provided in example 6 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 quantum dot light emitting diode devices, 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, but due to the fact that the low electron injection rate can only form the instant balance of charge injection at the initial stage of the work of the quantum dot light-emitting diode device, the high device efficiency at the initial moment is achieved. With the continuous operation of the device, before the state of the device reaches dynamic balance, the electron injection rate is continuously reduced, so that the carrier injection balance is broken rapidly, the high device efficiency is reduced and cannot be maintained, and the unbalanced state of the carrier injection may be continuously increased with the continuous operation, so that the service life of the device is correspondingly rapidly reduced.

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, achieves the carrier injection balance in a stable working state, and realizes the continuous maintenance of the device efficiency, thereby effectively improving the working life of the quantum dot light emitting diode device. Specifically, the quantum dot light-emitting diode provided by the application utilizes the zinc oxide film with a small surface hydroxyl group amount as an electron transport layer. In this case, since the injection rate of electrons into the quantum dot light-emitting layer is higher than that of holes, the quantum dots in the light-emitting layer are negatively charged, and the state of the negative charge can be maintained due to the core-shell structure of the quantum dots and the constraint effect of electrically inert surface ligands, and meanwhile, the further injection of electrons into the quantum dot light-emitting layer becomes more and more difficult due to the coulomb repulsion effect. When the quantum dot light-emitting diode device is continuously lightened to a stable state, the state of the quantum dot with negative electricity tends to be stable, namely, electrons newly captured and restrained by the quantum dot and electrons consumed by radiative transition reach dynamic balance, the injection rate of the electrons to the quantum dot light-emitting layer is much lower than that in the initial stage, and the lower injection rate of the electrons and the lower injection rate of holes just reach carrier injection balance, so that the service life of the device is prolonged.

In a first aspect, embodiments of the present application provide a quantum dot light emitting diode, including an anode and a cathode disposed opposite to each other, a quantum dot light emitting layer disposed between the anode and the cathode, an electron transport layer disposed 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 less than or equal to 0.4.

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 0.4 or less, i.e., the electron transport layer is the first electron transport layer. In this case, the electron in the electron transport layer has a small transport resistance to the quantum dot light-emitting layer, which is advantageous for causing the quantum dots in the quantum dot light-emitting layer to be negatively charged. Under the constraint of a quantum dot structure (a quantum dot core-shell structure and an electrically inert surface ligand), the state of a negative dot kept by the quantum dot and the coulomb repulsion effect ensure that the state of the negative charge of the quantum dot is stable when the quantum dot light-emitting diode device is continuously lightened to work to a stable state. At this time, the electrons newly captured and bound by the quantum dots and the electrons consumed by radiation transition reach dynamic balance, the injection rate of the electrons to the quantum dot light-emitting layer is much lower than that in the initial stage, and the lower injection rate of the electrons and the hole injection rate just reach carrier injection balance, so that the service life of the device is prolonged.

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 of less than or equal to 0.25; in some embodiments, the undoped zinc oxide thin film has a surface hydroxyl amount of less than or equal to 0.15. 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 means that the zinc oxide forming the zinc oxide thin film 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 the doped zinc oxide electron transport layer changes energy level/oxygen vacancy (electron mobility), and at the same time, doped ions enter the surface of zinc oxide particles and then preferentially fill the surface defects, thereby serving the purpose of passivating the defects to some extent, and newly filled doped ion sites coordinate new surface hydroxyl groups, so that the total amount of surface hydroxyl groups increases. Therefore, in the embodiment, the amount of the surface hydroxyl group of the metal-doped zinc oxide film is less than or equal to 0.4, so that effective carrier injection regulation is realized. Specifically, compared with the adjustment of the hydroxyl quantity on the surface of the undoped zinc oxide film, when the hydroxyl quantity on the surface of the doped zinc oxide film is adjusted, on one hand, through the energy level matching optimization or the electron mobility optimization of the doped zinc oxide, the quantum dot light-emitting diode device is already under the excellent carrier injection balance, and the External Quantum Efficiency (EQE) higher than that of the quantum dot light-emitting diode device taking the undoped zinc oxide film as the electron transport layer can be obtained in the initial working stage of the device; on the other hand, 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 easily reach a carrier injection balance state when continuously working to a stable state, and therefore the service life of the device is good. Finally, the quantum dot light emitting diode device can have a longer service life while maintaining a higher EQE.

In some embodiments, the amount of surface hydroxyl groups of the metal-doped zinc oxide film is less than or equal to 0.25; in some embodiments, the amount of surface hydroxyl groups of the metal-doped zinc oxide film is less than or equal to 0.15.

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 doping metal ion and the zinc ion have the same valence, but the oxide thereof has a valence ofAnd metal ions with the same conduction band energy level are doped, so that the conduction band energy level of the zinc oxide electron transport layer can be adjusted, 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 further optimized, and the EQE of the device is improved.

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). 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 quantum dot light emitting diode 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, doping metal ions can be precipitated from the surface of the zinc oxide material in the form of a second phase, and the performance of the zinc oxide material is influenced. The ion radius ratio of the doped metal ions and 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 in general, 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 doping metal, Mg²⁺The doping molar concentration of (A) is 0.1% -35%; 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 a zinc oxide film containing a 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, Gd³⁺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 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 zinc oxide film containing the doped metal, Ce is present⁴⁺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 10-100 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 greater than or equal to 0.6. 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 low-surface hydroxyl zinc oxide film, the state of the quantum dot light-emitting layer with negative electricity still occurs, dynamic balance is easily achieved, and finally, the electron injection efficiency is in a lower level so as to form carrier injection balance with the hole injection efficiency, and 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 cathode. Under the condition, when zinc oxide with less surface hydroxyl or metal-doped zinc oxide colloidal solution is deposited on the quantum dot light-emitting layer, a smooth zinc oxide film is favorably 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 0.4 or less and a second electron transport layer having a surface hydroxyl group amount of 0.6 or more. In some embodiments, the amount of surface hydroxyl groups of the first electron transport layer is less than or equal to 0.25, the amount of surface hydroxyl groups of the second electron transport layer is greater than or equal to 0.8, or even greater than or equal to 1.0; in some embodiments, the amount of surface hydroxyl groups of the first electron transport layer is less than or equal to 0.15, and the amount of surface hydroxyl groups of the second electron transport layer is greater than or equal to 0.8, or even greater than or equal to 1.0.

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 0.4 or less is a metal-doped zinc oxide, and the zinc oxide thin film having a surface hydroxyl group amount of 0.6 or more, 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 makes it easy for the quantum dot light emitting diode device to reach a carrier injection equilibrium state when continuously operating to a 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 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 amount 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 amount of the second 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. The embodiment can realize carrier injection balance of the quantum dot light-emitting diode device by regulating the hydroxyl quantity on the surface of the zinc oxide film, does not need to change the device structure (insert an electron blocking layer), does not need to modify the zinc oxide film by means of doping and the like, and has the advantages of simple operation in the whole process, low cost and good 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 0.6 or more, 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 less than or equal to 0.25, and the amount of surface hydroxyl groups of the second electron transport layer is greater than or equal to 0.8, or even greater than or equal to 1.0; in some embodiments, the amount of surface hydroxyl groups of the first electron transport layer is less than or equal to 0.15 and the amount of surface hydroxyl groups of the second electron transport layer is greater than or equal to 0.8, or even greater than or equal to 1.0.

In some embodiments, the zinc oxide film with the surface hydroxyl group amount less than or equal to 0.4, i.e., the first electron transport layer, is an undoped zinc oxide film, and the zinc oxide film with the surface hydroxyl group amount greater than or equal to 0.6, 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 content makes the quantum dot light emitting diode device easily reach a carrier injection equilibrium state when continuously operating to a stable state, and thus a good device lifetime is obtained. On the other hand, the high hydroxyl amount can reduce electrons injected in the quantum dot light emitting layer, so that injection balance of carriers in the quantum dot light emitting diode device is realized, and the quantum dot light emitting diode device with high external quantum efficiency is finally 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 second 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.

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

In some embodiments, the zinc oxide in the zinc oxide thin film with a surface hydroxyl group amount of less than or equal to 0.4, i.e., the first electron transport layer, is a metal-doped zinc oxide, and the zinc oxide in the zinc oxide thin film with a surface hydroxyl group amount of greater than or equal to 0.6, i.e., the second electron transport layer, is a metal-doped zinc oxide. In this case, on the one hand, the low hydroxyl group content makes the quantum dot light emitting diode device easily reach a carrier injection equilibrium state when continuously working to a steady state, and thus a good device lifetime 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 amount of more than or equal to 0.6 and zinc oxide with the surface hydroxyl amount 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 transmission layer can be obtained in the initial working period of the device, and the surface hydroxyl amount of the second electron transmission 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 the cooperation of the zinc oxide with the electron transmission layer and 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 less than or equal to 0.4 and a zinc oxide film (second electron transport layer) with the surface hydroxyl group amount greater than or equal to 0.6, 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 less than or equal to 0.25, the amount of surface hydroxyl groups of the second electron transport layer is greater than or equal to 0.8, or even greater than or equal to 1.0; in some embodiments, the first electron transport layer has a surface hydroxyl number of less than or equal to 0.15, and the second electron transport layer has a surface hydroxyl number of greater than or equal to 0.8, or even greater than or equal to 1.0.

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 first electron transport layer 51 is closer to the quantum dot light emitting layer 40 than the second electron transport layer 52, that is, the second electron transport layer 52 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 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 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. Under the condition, the External Quantum Efficiency (EQE) of the quantum dot light-emitting diode is optimized by directly optimizing the energy level matching of the doped zinc oxide or optimizing the electron mobility under the condition of better carrier injection balance; the quantum dot light-emitting diode device can easily reach a carrier injection balance state when continuously working to a stable state through the surface hydroxyl amount of the first electron transport layer, and further, the service life of the device is good.

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 side surface of the first electron transport layer 51 close to the cathode 60, that is, the first electron transport layer 51 is close to the quantum dot light emitting layer 40. In the double-layer zinc oxide electron transport layer obtained under the condition, due to the existence of the metal-doped zinc oxide layer, the quantum dot light-emitting diode device is in a state of relatively balanced current carriers in the initial working stage, so that the double-layer zinc oxide electron transport layer has higher 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 can be improved. In addition, because the doped metal ions are partially dispersed on the surface of the zinc oxide, the function of passivating the surface defects of the zinc oxide layer is achieved to a certain extent, so that the doped zinc oxide layer has the function of inhibiting the quenching of the surface defects of the zinc oxide layer on excitons, and the zinc oxide film with low hydroxyl content is matched, so that the finally formed double-layer zinc oxide film structure has excellent device service life (under the dual functions of inhibiting the quenching of the surface defects on the excitons and ensuring that the zinc oxide with low hydroxyl content achieves carrier injection balance during continuous work).

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 the zinc oxide thin film or the metal-doped zinc oxide thin film (second electron transport layer) having a surface hydroxyl group amount of 0.6 or more, the film thickness is not preferably too thick because of its low electron mobility. Illustratively, the zinc oxide film or metal-doped zinc oxide film having a surface hydroxyl 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 thickness of the zinc oxide thin film or the metal-doped zinc oxide thin film (first electron transport layer) having a surface hydroxyl group amount of 0.4 or less is 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 amount less than or equal to 0.4, namely a first electron transport layer, a second electron transport layer and a third electron transport layer. Wherein, the second electron transmission layer is a zinc oxide film or a metal-doped zinc oxide film with the surface hydroxyl content of more than or equal to 0.6.

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 transporting layer 50 includes a zinc oxide thin film (first electron transporting layer 51) having a surface hydroxyl group amount of 0.4 or less, a zinc oxide thin film (second electron transporting layer 52) having a surface hydroxyl group amount of 0.6 or more, and a zinc oxide thin film (third electron transporting layer 53) having a surface hydroxyl group amount of 0.4 or less, wherein the third electron transporting layer 53 is provided on a side surface of the second electron transporting layer 52 facing away from the first electron transporting layer 51. Under the condition, the electron mobility is further enhanced by the two zinc oxide films with low hydroxyl content, so that the quantum dot light-emitting diode device can easily 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 in the quantum dot light-emitting layer, realize the injection balance of carriers at the initial working stage of the quantum dot light-emitting diode device, and finally obtain the quantum dot light-emitting diode device with higher external quantum efficiency. The finally obtained quantum dot light-emitting diode device has higher EQE in the initial working period, and meanwhile, the service life of the finally obtained quantum dot light-emitting diode device is improved. In addition, when zinc oxide with less surface hydroxyl group or doped zinc oxide colloidal solution is deposited on the quantum dot light-emitting layer, a smooth zinc oxide film is obtained. Therefore, the obtained quantum dot light-emitting diode device has good EQE and long service life.

In some embodiments, as shown in fig. 2, the electron transport layer 50 includes a zinc oxide thin film (first electron transport layer 51) having a surface hydroxyl group amount of 0.4 or less, a metal-doped zinc oxide thin film (second electron transport layer 52), and a zinc oxide thin film (third electron transport layer 53) having a surface hydroxyl group amount of 0.4 or less, wherein the third electron transport layer 53 is disposed on a side surface of the second electron transport layer 52 facing away from the first electron transport layer 51. Under the condition, on one hand, the two layers of low hydroxyl content enable the quantum dot light emitting diode device to easily reach a carrier injection balance state when the quantum dot light emitting diode device continuously works to a stable state, and therefore good service life of the device is obtained. On the other hand, the zinc oxide of the second electron transport layer contains doped metal ions, so that effective carrier injection regulation and control can be realized, and the EQE higher than that of a quantum dot light-emitting diode device using an undoped zinc oxide film as an electron transport layer can be obtained in the initial working period of the device, so that the finally obtained quantum dot light-emitting diode has higher EQE and the service life of the device.

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 transporting layer includes a zinc oxide thin film (first electron transporting layer) having a surface hydroxyl group amount of 0.4 or less, a zinc oxide thin film (second electron transporting layer) having a surface hydroxyl group amount of 0.6 or more, and a zinc oxide thin film (third electron transporting layer) having a surface hydroxyl group amount of 0.6 or more, and the third electron transporting layer is provided on a side surface of the first electron transporting layer facing away from the second electron transporting layer. In this case, this example newly adds a zinc oxide thin film having a surface hydroxyl group amount of 0.6 or more, as compared with an electron transporting layer including a first electron transporting layer having a surface hydroxyl group amount of 0.4 or less and a second electron transporting layer having a surface hydroxyl group amount of 0.6 or more. According to the quantum dot light-emitting diode obtained by the method, the two layers of high-hydroxyl zinc oxide films formed by the second electron transmission layer and the third electron transmission layer endow the quantum dot light-emitting diode with an excellent carrier balance state, so that high EQE is obtained, and meanwhile, the first electron transmission layer endows the device with a better service life, so that the finally obtained quantum dot light-emitting diode has higher EQE and service life of the device.

In some embodiments, the electron transport layer includes a zinc oxide film (first electron transport layer) having a surface hydroxyl group amount of 0.4 or less, a metal-doped zinc oxide film (second electron transport layer), and a zinc oxide film (third electron transport layer) having a surface hydroxyl group amount of 0.6 or more, and the third electron transport layer is disposed on a side surface of the first electron transport layer facing away from the second 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 easily reach a carrier injection balance state when continuously working to a stable state, and further, the service life of the device is good; the zinc oxide film with high hydroxyl content can reduce electrons injected into the quantum dot light-emitting layer, so that injection balance of carriers in the initial working stage of 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. 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; and 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 easily reach a carrier injection balance state when continuously working to a stable state. Therefore, the quantum dot light-emitting diode can obtain better service life of the device, and higher EQE can be kept in the initial working period.

In some embodiments, the electron transport layer includes a zinc oxide film with a surface hydroxyl group amount of less than or equal to 0.4, i.e., a first electron transport layer, a metal-doped zinc oxide film (a second electron transport layer), and a zinc oxide film with a surface hydroxyl group amount of greater than or equal to 0.6, i.e., 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. In this case, the additional addition of the third electron transport layer, i.e., the high hydroxyl zinc oxide thin film, further increases the EQE in the initial state of the quantum dot device, compared to an electron transport layer including only the first electron transport layer and the second electron transport layer. The finally obtained quantum dot light-emitting diode can achieve high EQE in the initial working period and has good service life of a final device.

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 (first electron transport layer) having a surface hydroxyl group amount of 0.4 or less, a zinc oxide thin film (second electron transport layer) having a surface hydroxyl group amount of 0.6 or more, and a metal-doped zinc oxide thin film (third electron transport layer), and the third electron transport layer is disposed on a side surface of the first electron transport layer facing away from the second 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 easily reach a carrier injection balance state when continuously working to a stable state, and further, the service life of the device is good; the zinc oxide film with high hydroxyl content can reduce electrons injected in the quantum dot light-emitting layer, realize the injection balance of carriers in the working initial stage 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 a 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 easily 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 smooth zinc oxide film is obtained.

In some embodiments, the electron transport layer includes a zinc oxide thin film having a surface hydroxyl group amount of 0.4 or less, i.e., a first electron transport layer, a metal-doped zinc oxide thin film (a second electron transport layer), 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 first electron transport layer facing away from the second electron transport layer. Under the condition, the zinc oxide film with low hydroxyl group content further enhances the electron mobility, so that the quantum dot light-emitting diode device can easily reach a carrier injection balance state when continuously working to a stable state, and further a good device life is obtained; a metal-doped zinc oxide film. Therefore, the obtained quantum dot light-emitting diode device has good EQE and long service life.

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 second electron transport layer is a zinc oxide film or a metal-doped zinc oxide film with the surface hydroxyl content of 0.6 or more, so the thickness of the second electron transport layer is 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 transport layer are kept. 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, 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 therefore, for the sake of brevity, no further description is given here.

In some embodiments, the doping metal in the metal-doped zinc oxide thin film is selected from Mg²⁺、Mn²⁺At least one ofOne kind of the medicine. 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⁺When 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⁺When the metal is doped with Al in the zinc oxide film³⁺The doping molar concentration of the silicon nitride is 0.1 to 15 percent; 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 (A) is 0.1% -7%; when the doping metal is Li⁺In the process, Li in the metal-doped zinc oxide film⁺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 to 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 comprises 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 comprises 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 contain cadmium or do not contain cadmium. 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 and continuous excitation spectrum distribution, high emission spectrum stability 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 easy to be broken down by electrons, and the injection performance of current carriers is not easy to ensure; when the thickness of the electron transport layer is greater than 100nm, electron injection is easily inhibited, 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 less than or equal to 0.4 is used as the first electron transport layer, the electron is smoothly transported to the quantum dot light-emitting layer, and the electricity injected into the quantum dot light-emitting layer is increased, so that the injection rate of the electron to the quantum dot light-emitting layer is higher than that of the hole to the quantum dot light-emitting layer, and the situation easily causes the quantum dot in the quantum dot light-emitting layer to be negatively charged. The negatively charged state can be maintained due to the binding effect of the quantum dot core-shell structure and the electrically inert surface ligand, and simultaneously, the coulomb repulsion effect makes the further injection of electrons into the quantum dot light-emitting layer more and more difficult. When the quantum dot light-emitting diode device is continuously lightened to a stable state, the state of the quantum dot with negative electricity tends to be stable, namely, electrons newly captured and bound by the quantum dot and electrons consumed by radiation transition reach dynamic balance, the injection rate of the electrons to the quantum dot light-emitting layer is lower than that in the initial stage, and the lower electron injection rate and the lower hole injection rate are unified to achieve carrier injection balance, so that the service life of the device is prolonged. That is, although in the initial working period of the quantum dot light emitting diode device, the quantum dot light emitting diode device is in a state of unbalanced carrier injection due to the high electron injection rate, which affects the device performance; however, when the quantum dot light emitting diode device is continuously lighted to a stable state, the reduced electron injection rate and the hole injection rate are easy to form carrier injection balance, so that the efficiency of the device is continuously maintained, and the service life of the quantum dot light emitting diode device is prolonged.

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 with a surface hydroxyl group amount less than or equal to 0.4,

as shown in fig. 6, the method for preparing a zinc oxide thin film having a surface hydroxyl group amount of 0.4 or less includes:

s11, mixing the zinc salt solution with alkali liquor for reaction, adding a precipitator into the mixed solution after the reaction is finished, and collecting precipitates; cleaning the precipitate twice or more by using 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 less than or equal to 0.4 to be prepared, and removing the solvent to prepare the zinc oxide film with the surface hydroxyl content less than or equal to 0.4.

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 less than or equal to 0.4. In the preparation process of preparing the zinc oxide colloidal solution by using the solution method, the obtained precipitate is cleaned twice or more by using a reaction solvent to obtain the zinc oxide with the surface hydroxyl content less than or equal to 0.4. The zinc oxide film with the surface hydroxyl quantity less than or equal to 0.4 is used as the first electron transmission layer, the transmission of electrons to the quantum dot light-emitting layer becomes smooth, electrons injected into the quantum dot light-emitting layer are increased, the injection rate of the electrons to the quantum dot light-emitting layer is higher than that of holes to the quantum dot light-emitting layer, and the quantum dots in the quantum dot light-emitting layer are negatively charged under the condition. This negatively charged state can be maintained due to the quantum dot core-shell structure and the binding of electrically inert surface ligands, while the coulomb repulsion effect makes further injection of electrons into the quantum dot light-emitting layer more difficult. When the quantum dot light-emitting diode device is continuously lightened to a stable state, the state of the quantum dot with negative electricity tends to be stable, namely, electrons newly captured and bound by the quantum dot and electrons consumed by radiation transition reach dynamic balance, the injection rate of the electrons to the quantum dot light-emitting layer is much lower than that in the initial stage, and the lower electron injection rate and the lower hole injection rate at the moment easily reach carrier injection balance, so that the service life of the device is obviously prolonged.

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 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 one of the salts which can react with the alkaline solution to generate the hydroxide of zinc, including but not limited to zinc acetate, zinc nitrate, zinc sulfate and zinc chloride. The solvent is selected from solvents having good solubility for zinc salt and the generated zinc oxide nanoparticles, including but not limited to water, organic alcohol, organic ether, sulfone and other solvents with high polarity. 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 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 present application, the alkali solution is a solution formed by an alkali capable of reacting with a zinc salt to generate a hydroxide of zinc, and specifically, the 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 lye reacts with both the zinc ions and the hydroxide ions of the dopant metal ions. In the embodiment of the application, alkali liquor is obtained by dissolving or diluting alkali with a solvent. On one hand, solid alkali such as sodium hydroxide can be dissolved by a solvent to form liquid alkali liquor, and then the liquid alkali liquor is added into a reaction system, so that the dispersion uniformity of the alkali liquor in the reaction system is facilitated; on the other hand, the concentration of alkali in the alkali liquor can be adjusted to be 0.1-2mol/L through dissolution or dilution, 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 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 lye is selected from K_b＞10^-1Of (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 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 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, the solvent used to dissolve or dilute the base to form the lye 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 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 more 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 some embodiments, the zinc salt solution and the alkali solution are mixed and treated at the temperature of 0-70 ℃ and react for 30 min-4 h to prepare the zinc oxide nanoparticles. In some embodiments, the zinc salt solution is mixed with the alkali liquor 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 alkali liquor; adjusting the temperature of the zinc salt solution to 0-70 ℃, and adding 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 nanoparticles can be remarkably slowed down, the reaction can be realized only by special equipment, the reaction difficulty is increased, and the zinc oxide nanoparticles cannot be generated even under certain conditions, but 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 easy to agglomerate, 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 alkali liquor 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, so that the film formation of the zinc oxide colloid solution is facilitated. In some embodiments, under the condition that the temperature is 0-30 ℃, a zinc salt solution and an alkali liquor are mixed for treatment, so that a qualified zinc oxide colloidal solution is easily generated; in some embodiments, the colloidal zinc oxide solution can be formed at a temperature of 30-70 ℃, and the quality of the colloidal zinc oxide solution is generally inferior to that of the colloidal zinc oxide solution formed at a temperature of 0-30 ℃, and the reaction time is generally reduced. In some embodiments, in the step of mixing the zinc salt solution with the alkali solution, the zinc salt solution is mixed with the alkali solution according to a molar ratio of hydroxide ions to zinc ions of 1.5: 1-2.5: 1, so as to ensure the formation of zinc oxide nanoparticles and reduce the generation of reaction byproducts. When the molar ratio of hydroxide ions to zinc ions is less than 1.5:1, excessive zinc salt causes that a large amount of zinc salt can not generate zinc oxide nano particles; and when the molar ratio of hydroxide ions to zinc ions is more than 2.5:1, the alkali liquor is excessive, and the excessive hydroxide ions and the zinc hydroxide intermediate form a stable complex 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 alkali liquor, the addition amount of the zinc salt solution and the alkali liquor is: the ratio of the molar amount of hydroxide ions provided by the 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 alkali solution, reacting for 30min to 4h at a reaction temperature of 0 to 70 ℃ to ensure the formation of the 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, the crystalline state of the sample is incomplete, the crystal structure is poor, and if the zinc oxide is 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 is high after the zinc oxide colloidal solution is formed into a film, thus affecting the electron transmission performance. In some embodiments, the zinc salt solution is mixed with the alkali solution and then reacted for 1-2 hours at the reaction temperature.

In some embodiments, the zinc salt solution is mixed with alkali liquor at the temperature of 0-70 ℃, reacted for 30 min-4 h, and stirred to promote the uniformity of the reaction and the particle uniformity of the obtained zinc oxide nanoparticles, so as to obtain zinc oxide nanoparticles with uniform size.

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 quantity, 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, and the zinc oxide film can play a certain role in inhibiting and hindering the transmission of electrons in the zinc oxide layer, so that 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 washed for a plurality of times, the residual hydroxyl amount on the surfaces of the zinc oxide nanoparticles is relatively small; 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 more by adopting the reaction solvent, so that the surface hydroxyl amount of the precipitate is less than or equal to 0.4.

In one possible embodiment, if the base in the lye is K_b＞10^-1The number of washing treatments is 3 or more. In this case, since K_b＞10^-1The ionization coefficient of the alkali (2) is larger, so that the hydroxyl group amount on the surface of the finally synthesized zinc oxide colloid is larger, and the surface is easy to reduce the hydroxyl group amount by cleaning for more than or equal to 3 times.

In one possible embodiment, if the base in the lye is K_b＜10^-1The number of washing treatments is 2 or more. When the reaction base is K_b＜10^-1The alkali has a small ionization coefficient, so that the hydroxyl content of the surface of the finally synthesized zinc oxide colloid is small, and the surface of the finally synthesized zinc oxide colloid is small, so that the surface of the finally synthesized zinc oxide colloid can be cleaned for more than or equal to 2 times.

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 alkali liquor is at least one selected from potassium hydroxide, sodium hydroxide and lithium hydroxide, and the collected precipitate is washed by using a reaction solvent for 3-5 times, so that zinc oxide nanoparticles with the surface hydroxyl content of less than or equal to 0.4 can be obtained; in some embodiments, the alkali in the alkaline solution is at least one selected from TMAH, ammonia water, ethanolamine, and ethylenediamine, and the collected precipitate is washed with the reaction solvent 2-4 times, so as to obtain zinc oxide nanoparticles with surface hydroxyl content less than or equal to 0.4.

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 less than or equal to 0.4 is a metal-doped zinc oxide film, and correspondingly, the zinc oxide film with the surface hydroxyl group amount less than or equal to 0.4 is a metal-doped zinc oxide, and in this case, the zinc salt solution further contains doped metal ions. In this embodiment, the doping metal ions are selected as in the above metal-doped zinc oxide thin film.

In some embodiments, the dopant metal ion is selected from Mg²⁺、Mn²⁺At least one of (1). 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, metal ions having different valence states from zinc ions are doped, and oxygen vacancy (electron mobility) of the zinc oxide electron transport layer can be adjusted by doping such metal ions, thereby optimizing the quantum dotsThe carrier injection of the light emitting diode device is balanced, 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 and controls 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 two 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 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 doping metal ion is Li⁺In solution of zinc salt, Li⁺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⁺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 doped metal ions, and in the step of mixing the zinc salt solution and the alkali liquor, the addition amount of the zinc salt solution and the alkali liquor satisfies the following condition: 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 this case, the zinc salt solution is mixed with an 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: when 1, the content of metal ions is excessive, and metal salt is not easy to generate metal-doped zinc oxide nano particles; and when the molar ratio of hydroxide ions to zinc ions is greater than 1.25:1, the alkali liquor is excessive, and the excessive hydroxyl ions and the zinc hydroxide intermediate form a stable complex, so that the zinc oxide nano-particles are not easy to generate by polycondensation. In some embodiments, in the step of mixing the zinc salt solution and the alkali solution, the zinc salt solution and the 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 less than or equal to 0.4 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 less than or equal to 0.4.

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 less than or equal to 0.4 is obtained.

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 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 0.4 or less 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 less than or equal to 0.4 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 greater than or equal to 0.6, 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 first 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 smooth 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 the electron transport layer comprises the third electron transport layer can be referred to as the case where the zinc oxide thin film with the surface hydroxyl group amount less than or equal to 0.4, the second electron transport layer and the third electron transport layer are prepared by the above method.

In the above examples, the zinc oxide thin film having a surface hydroxyl group amount of 0.6 or more may be formed by a zinc oxide colloidal solution having a surface hydroxyl group amount of 0.6 or more.

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 an 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 example, the zinc salt and the solvent type of the zinc salt solution, the content of the zinc salt solution, the type and doping content of the dopant ion, the type and addition amount of the 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 example. 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 alkali liquor, the addition amount of the alkali meets the following requirements: 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 0.4 or less,

as shown in fig. 7, the method for preparing a zinc oxide thin film having a surface hydroxyl group amount of 0.4 or less includes:

s21, mixing and reacting a zinc salt solution with alkali liquor to prepare zinc oxide nano-particles; dissolving zinc oxide nano particles to obtain a zinc oxide colloidal solution; adding an acid solution into the zinc oxide colloidal solution, and adjusting the pH value of the zinc oxide colloidal solution to 7-8 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 less than or equal to 0.4 to be prepared, and removing the solvent to prepare the zinc oxide film with the surface hydroxyl content less than or equal to 0.4.

According to the preparation method of the quantum dot light-emitting diode, the zinc oxide colloidal solution is prepared by a solution method, then the acid solution is added into the zinc oxide colloidal solution, the pH value of the zinc oxide colloidal solution is adjusted to 7-8, the zinc oxide solution is obtained, and the zinc oxide with the surface hydroxyl content less than or equal to 0.4 is obtained. The zinc oxide film with the surface hydroxyl quantity less than or equal to 0.4 is used as the first electron transport layer, the electron is smoothly transported to the quantum dot light-emitting layer, electrons injected into the quantum dot light-emitting layer are increased, the injection rate of the electrons to the quantum dot light-emitting layer is higher than that of holes to the quantum dot light-emitting layer, and the situation can cause the quantum dots in the quantum dot light-emitting layer to be negatively charged. The negatively charged state can be maintained due to the binding effect of the quantum dot core-shell structure and the electrically inert surface ligand, and simultaneously, the coulomb repulsion effect makes the further injection of electrons into the quantum dot light-emitting layer more and more difficult. When the quantum dot light-emitting diode device is continuously lightened to a stable state, the state of the quantum dot with negative electricity tends to be stable, namely electrons newly captured and bound by the quantum dot and electrons consumed by radiation transition reach dynamic balance, the injection rate of the electrons to the quantum dot light-emitting layer is much lower than that in the initial stage, and the lower injection rate of the electrons and the hole injection rate just reach carrier injection balance, so that 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 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 basis and type of the zinc salt solution, the zinc salt in the zinc salt solution, and the formation manner of the zinc salt solution, the selection basis and type of the alkali in the alkali solution, and the formation manner of the alkali solution are as described in the above first embodiment, and are not repeated herein for saving space. The reaction conditions and time for mixing and reacting the zinc salt solution and the alkali solution, the content ratio of the zinc salt solution and the alkali solution, and the preferred situations, etc. as described in the above first embodiment, are not described herein again for saving space.

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 method and conditions for centrifuging the mixed system subjected to the precipitation treatment, collecting the precipitate, are described in the first embodiment above.

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

In the embodiment of the application, acid liquor is added into the zinc oxide colloidal solution, and the pH value of the zinc oxide colloidal solution is adjusted to 7-8. 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 acid solution breaks the balance. Specifically, after the acid solution is added, the amount of hydroxyl in an ionized state in the zinc oxide colloidal solution is reduced, so that the amount of hydroxyl ligands on the surface of the zinc oxide is correspondingly reduced. At the same time, the amount of acid added in the solution cannot be too large (the pH value cannot be too small), otherwise, the amount of hydroxyl ligand on the surface of zinc oxide is too small, so that the ligand protection on the surface of zinc oxide is lost, and zinc oxide particles are seriously agglomerated and even precipitated. Therefore, the pH value of the zinc oxide colloidal solution is adjusted to 7-8 by adding acid liquor in the embodiment of the application. In some embodiments, the pH of the zinc oxide colloidal solution is adjusted to 7.2-7.8 by adding an acid solution, so that on the basis that the amount of hydroxyl groups on the surface of the obtained zinc oxide is less than or equal to 0.4, certain hydroxyl ligands are held on the surface of the zinc oxide nanoparticles, and good dispersibility is obtained. In some embodiments, the pH of the zinc oxide colloidal solution is adjusted to between 7.3 and 7.6 by adding an acid solution.

In some embodiments, the acid in the acid solution is selected from at least one of inorganic strong acids such as hydrochloric acid, sulfuric acid, nitric acid, hydrofluoric acid, and the like, or at least one of organic carboxylic acids such as formic acid, acetic acid, propionic acid, oxalic acid, acrylic acid, and the like. In the embodiment of the application, the acid solution is a solution formed by dissolving an inorganic acid or a solution formed by dissolving or diluting an organic acid. The acid is dissolved or diluted to adjust the concentration of the acid liquor, so that the reaction rate is controlled, and the adjustment of the hydroxyl on the surface of the zinc oxide nano-particle can be fully carried out. Wherein the solvent used for dissolving or diluting the acid to form the acid solution is capable of dissolving or being miscible with the acid, and further the solvent has the same polarity as the zinc oxide nanoparticles. In some embodiments, the solvent used to dissolve or dilute the acid to form the acid 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 acid to form the acid 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 one possible embodiment, the zinc oxide film with the surface hydroxyl group amount less than or equal to 0.4 is a metal-doped zinc oxide film, and correspondingly, the zinc oxide film with the surface hydroxyl group amount less than or equal to 0.4 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 dopant metal ions is described above in the first embodiment of the metal doped zinc oxide film.

In some embodiments, the zinc salt solution contains zinc ions and doped metal ions, and in the step of mixing the zinc salt solution and the alkali liquor, the addition amount of the zinc salt solution and the alkali liquor satisfies the following condition: 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 this case, the zinc salt solution is mixed with an 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 alkali liquor is obviously excessive, and the excessive hydroxide 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 alkali solution, the zinc salt solution and the 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 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 of 0.4 or less is to be prepared, and the solvent may be removed to obtain a zinc oxide thin film with a surface hydroxyl group amount of 0.4 or less, according to the type of the prepared quantum dot light emitting diode device.

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 0.4 or less,

as shown in fig. 8, the method for preparing a zinc oxide thin film having a surface hydroxyl group amount of 0.4 or less includes:

s31, preparing a zinc oxide prefabricated film on a prefabricated device substrate of the zinc oxide film of which the surface hydroxyl quantity is less than or equal to 0.4;

s32, depositing acid liquor 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 acid treatment to obtain the zinc oxide with the surface hydroxyl content of less than or equal to 0.4. In this case, when a zinc oxide film having a surface hydroxyl group amount of 0.4 or less is used as the first electron transport layer, electron transport to the quantum dot light-emitting layer becomes smooth, and electrons injected into the quantum dot light-emitting layer increase, so that the rate of injection of electrons into the quantum dot light-emitting layer is higher than the rate of injection of holes into the quantum dot light-emitting layer, which causes the quantum dots in the quantum dot light-emitting layer to be negatively charged. The negatively charged state can be maintained due to the core-shell structure of the quantum dot and the constraint action of an electrically inert surface ligand, and meanwhile, the further injection of electrons into a quantum dot light-emitting layer becomes more and more difficult due to the coulomb repulsion effect. When the quantum dot light-emitting diode device is continuously lightened to a stable state, the state of the quantum dot with negative electricity tends to be stable, namely, electrons newly captured and restrained by the quantum dot and electrons consumed by radiative transition reach dynamic balance, the injection rate of the electrons to the quantum dot light-emitting layer is much lower than that in the initial stage, and the lower injection rate of the electrons and the lower injection rate of holes just reach carrier injection balance, so that 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 alkali liquor 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 the substrate of the prefabricated device of the zinc oxide film with the surface hydroxyl content less than or equal to 0.4 to be prepared, and removing the solvent to prepare the zinc oxide prefabricated film.

Wherein, zinc salt solution and alkali liquor are mixed and reacted to prepare zinc oxide nano particles; the step of dissolving the zinc oxide nanoparticles to obtain the zinc oxide colloidal solution is referred to as step S11 in the first embodiment, and is not repeated herein for the sake of brevity.

In one possible embodiment, the zinc oxide in the zinc oxide thin film with the surface hydroxyl group amount 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 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 choice of doping metal ions and the doping content, such as the choice of doping metal in the metal-doped zinc oxide film above, are used.

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 group amount less than or equal to 0.4 to be prepared, and the solvent is removed to prepare the zinc oxide thin film with the surface hydroxyl group amount less than or equal to 0.4.

In some embodiments, the above-described zinc oxide colloidal solution is formed on a pre-fabricated device substrate, and referring to the step of S12 "forming a zinc oxide colloidal solution on a pre-fabricated device substrate on which a zinc oxide thin film having a surface hydroxyl group amount of 0.4 or less is to be prepared, and removing the solvent to obtain a zinc oxide thin film having a surface hydroxyl group amount of 0.4 or less".

In the above step S32, the amount of hydroxyl groups on the surface of the zinc oxide pre-formed film is changed by depositing an acid solution on the zinc oxide pre-formed film. Specifically, after the acid solution is deposited, a liquid film is formed on the surface of the zinc oxide prefabricated film, so that hydroxyl on the surface of the zinc oxide prefabricated film reacts with ionized hydrogen ions in the liquid film, and the amount of the hydroxyl on the surface of the zinc oxide prefabricated film is reduced.

In some embodiments, the acid in the acid solution includes, but is not limited to, at least one of inorganic strong acids such as hydrochloric acid, sulfuric acid, nitric acid, hydrofluoric acid, and the like, or at least one of organic carboxylic acids such as formic acid, acetic acid, propionic acid, oxalic acid, acrylic acid, and the like. In the embodiment of the present application, the acid solution is a solution formed by an inorganic acid, or a solution formed by diluting or dissolving an organic acid, or may be directly an organic carboxylic acid. The acid solution concentration is adjusted by dissolving or diluting the acid, so that the reaction rate is controlled, and the adjustment of the hydroxyl on the surface of the zinc oxide nanoparticles can be fully performed. Wherein, the solvent used for dissolving or diluting the acid to form the acid solution can dissolve the acid or be mixed and dissolved with the acid, and in addition, the polarity of the solvent is the same as that of the zinc oxide nano-particles. In some embodiments, the solvent used to dissolve or dilute the acid to form the acid 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 acid to form the acid 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 the embodiment of the present application, the concentration and the addition amount of the acid solution need to be controlled. This is because: when the concentration and the addition amount of the acid are too large, the amount of the hydroxyl ligand on the surface of the zinc oxide is too small, so that the ligand protection on the surface of the zinc oxide is lost, zinc oxide particles are seriously agglomerated, and the quality of a zinc oxide film is influenced; when the concentration and the addition amount of the acid are too small, the effect of reducing the hydroxyl amount on the surface of the zinc oxide is not easily achieved. In some embodiments, the concentration of the acid solution is 0.05-0.5mmol/L, so as to obtain a proper concentration for regulating the surface hydroxyl group amount of the zinc oxide prefabricated film. In some embodiments, the acid solution is deposited in an amount sufficient to satisfy the following weight ratio: each 5mg of zinc oxide pre-film is treated with 50. mu.L to 1000. mu.L of an acid solution. The excessive concentration of the acid solution and the excessive acid addition amount can lead the amount of hydroxyl ligands on the surfaces of the zinc oxide nano particles to be too small, lead the surfaces of the zinc oxide to lose the ligand protection, lead the zinc oxide particles to be seriously agglomerated and influence the quality of the zinc oxide film; when the concentration of the acid liquor and the addition amount of the acid are too small, the effect of reducing the amount of hydroxyl on the surface of the zinc oxide is not easily achieved. It will be appreciated that the acid liquor concentration can be flexibly adjusted depending on the different types of acids selected.

The inorganic acid is generally strong acid, and the ionization capacity of hydrogen ions is strong, so that the surface hydroxyl content of the zinc oxide can be adjusted by only a small amount of low-concentration inorganic acid. Organic acids are generally weak acids, and hydrogen ions have weak ionization capacity, so that a large amount of organic acids with relatively high concentration are needed to effectively adjust the surface hydroxyl amount of the zinc oxide.

In some embodiments, the acid in the acid solution is an inorganic acid and the concentration of the acid solution is 0.05 to 0.1 mmol/L. Illustratively, the inorganic acid is at least one selected from hydrochloric acid, sulfuric acid, nitric acid, and hydrofluoric acid. In this case, the deposition amount of the acid solution and the weight of the lower zinc oxide prefabricated film satisfy: each 5mg of zinc oxide pre-film was treated with 50. mu.L to 200. mu.L of an acid solution.

In some embodiments, the base in the acid solution is an organic carboxylic acid, and the concentration of the acid solution formed is 0.2 to 0.4 mmol/L. Illustratively, the organic carboxylic acid is at least one selected from formic acid, acetic acid, propionic acid, oxalic acid, and acrylic acid. In this case, the deposition amount of the acid solution and the weight of the lower zinc oxide prefabricated film satisfy: each 5mg of the zinc oxide pre-film is treated with 100. mu.L to 500. mu.L of an acid solution.

In the embodiment of the present application, the method for depositing the acid 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.

After acid liquor is deposited on the surface of the zinc oxide prefabricated film, drying treatment is carried out, and ionized hydrogen ions in the acid liquor and hydroxyl on the surface of the zinc oxide are fully reacted 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 acid solution sufficiently react with the hydroxyl groups on the surface of the zinc oxide to reduce 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 acid liquor can be quickly dried, the zinc oxide prefabricated film can be quickly changed into a solid film, so that ionized hydrogen ions in the acid 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, so that the preparation of the next layer, particularly the evaporation quality of the electrode, is influenced. In some embodiments, the drying process is carried out at a temperature of 10 ℃ to 50 ℃ for a time of 30 minutes to 2 hours. By changing the hydroxyl group amount on the surface of the zinc oxide by the method, an auxiliary layer formed by a very small amount of acid may be remained on the surface of the finally obtained film.

In some embodiments, the second electron transport layer is a zinc oxide film having a surface hydroxyl amount of greater than or equal to 0.6, or the second electron transport layer is a metal-doped zinc oxide film. The first electron transport layer may be disposed on a side adjacent to the quantum dot light emitting layer, or on a side adjacent to the cathode. Preferably, the first electron transport layer is arranged at one side adjacent to the quantum dot light emitting layer or the metal-doped zinc oxide film is arranged at one side adjacent to the quantum dot light emitting layer, so that a 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 an 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 with 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 dopant ion, the type and addition amount of the alkali solution, the reaction temperature and reaction time, the selection and addition amount of the precipitant, and the type and content of the dopant 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 doped metal ion with the 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 doping 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 the metal is doped with Y in the 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 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, prior to 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 environment of the packaging treatment 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, heat 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: diluting the prepared zinc oxide solution to 30mg/mL, spin-coating the solution on a pretreated glass sheet, and spin-coating the solution to form a film. Wherein, the hydroxyl amount 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; outputting a certain voltage to the device in the cassette to make the device emit light and record the current, and collecting and analyzing the light source via the silicon photodiodeThe spectral data can be used for calculating the G (lambda) human eye photopic vision function and the S (lambda) normalized electroluminescence spectrum while obtaining the color coordinates, so that the 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: NVO-QLED-LT-128 of new view

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 a unit of_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 80 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) Washing the precipitate with methanol as reaction solvent for 3 times, and dissolving the white precipitate to obtain zinc oxide colloidal solution with surface hydroxyl amount of 0.3.

And step two, forming a zinc oxide colloid solution on the quantum dot light-emitting layer, and removing the solvent to prepare the zinc oxide film with the surface hydroxyl amount less than or equal to 0.4.

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 0.3.

Comparative example 1

The difference from example 1 is that: common zinc oxide nano-particles are adopted as an electron transport layer material. 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 0.7.

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 cathodeWherein the anode is ITO (55nm), the hole injection layer is PEDOT (50nm), the hole transport layer is TFB (30nm), and the quantum dot luminescent layer is red quantum dot Cd_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 chloride in dimethyl sulfoxide at room temperature to prepare a zinc salt solution with the concentration of 0.8mol/L, and dissolving ammonia water in butanol at room temperature to obtain an alkali solution with the concentration of 1.2mol/L, wherein the molar ratio of hydroxide ions to zinc ions is 1.5: 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 1.5:1, and continuously stirring the mixed solution under the condition that the reaction temperature is kept at 40 ℃ for reacting for 60 min; (C) adding a mixed solution after the reaction is finished into the mixed solution in a volume ratio of 5:1, a white precipitate was generated in the mixed solution. Washing the precipitate with methanol as reaction solvent for 2 times, and dissolving the obtained white precipitate to obtain 0.6mol/L first zinc oxide colloidal solution;

(2) dissolving zinc chloride in dimethyl sulfoxide at room temperature to prepare a zinc salt solution with the concentration of 0.8mol/L, and dissolving potassium hydroxide in ethanol at room temperature to obtain an alkali liquor with the concentration of 1.2 mol/L; (B) adjusting the temperature of the zinc salt solution to 45 ℃ according to the molar ratio of hydroxide ions to zinc ions of 1.5:1, dropping alkali liquor into the zinc salt solution, and continuously stirring/reacting the mixed solution for 60min under the condition that the reaction temperature is kept at 45 ℃; (C) adding a mixed solution after the reaction is finished into the mixed solution in a volume ratio of 5:1, white precipitate was generated in the mixed solution. (D) Cleaning the precipitate with ethanol for 2 times, and dissolving the obtained white precipitate to obtain a second zinc oxide colloidal solution with the concentration of 0.6 mol/L;

(3) forming a first zinc oxide colloidal solution on the quantum dot light-emitting layer, removing the solvent to prepare a first zinc oxide film with the surface hydroxyl amount of 0.3, wherein the thickness of the film is 60 nm; 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 0.7, wherein the thickness of the film is 20 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 0.3, and the hydroxyl group content of the second electron transport layer is determined to be 0.7.

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;

(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, generating white precipitate 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 first zinc oxide colloidal solution with the hydroxyl content of 0.25.

(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 3:1, a white precipitate was generated in the mixed solution.

(D) Dissolving the obtained white precipitate, adding 0.1mol/L sodium hydroxide into the zinc oxide colloidal solution, and adjusting the pH of the solution to 8 to obtain a second zinc oxide colloidal solution with the hydroxyl content of 0.85.

(3) Forming a second zinc oxide colloidal solution on the quantum dot light-emitting layer, and removing the solvent to obtain a second zinc oxide film with the surface hydroxyl amount of 0.85; and forming a first zinc oxide colloidal solution on the second zinc oxide film, and removing the solvent to obtain the first zinc oxide film with the surface hydroxyl group content of 0.25. The thickness of the first zinc oxide layer is 60nm, and the thickness of the second zinc oxide layer is 30 nm.

Hydroxyl groups in the prepared 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 0.25, and the hydroxyl group content of the second electron transport layer is determined to be 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.

Example 4

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 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 butanol at room temperature to prepare a zinc salt solution with the concentration of 0.5mol/L, dissolving TMAH in butanol at room temperature to obtain an alkali solution with the concentration of 1mol/L, wherein the molar ratio of hydroxide ions to zinc ions is 2: 1;

(B) adjusting the temperature of the zinc salt solution to 50 ℃, dropwise adding alkali liquor into the zinc salt solution according to the molar ratio of hydroxide ions to zinc ions of 2:1, and continuously stirring the mixed solution under the condition that the reaction temperature is kept at 50 ℃ for reacting for 70 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. Washing the precipitate with reaction solvent butanol for 2 times, and dissolving the obtained white precipitate to obtain a first zinc oxide colloidal solution with a concentration of 0.5 mol/L;

(2) dissolving magnesium acetate and zinc acetate in butanol at room temperature to prepare a mixed salt solution with the concentration of 0.5mol/L, wherein the molar ratio of magnesium ions is 5%, and dissolving potassium hydroxide in ethanol at room temperature to obtain an alkali liquor with the concentration of 1 mol/L;

adjusting the temperature of the zinc salt solution to 40 ℃, and mixing the zinc salt solution and the zinc salt solution according to the molar ratio of hydroxide ions to zinc ions of 2:1, dropping alkali liquor into the mixed salt solution, and continuously stirring/reacting the mixed solution for 90min under the condition that the reaction temperature is kept at 40 ℃; (B) adding a mixed solution after the reaction is finished into the mixed solution in a volume ratio of 5:1, generating white precipitate in the mixed solution; (C) washing the precipitate with butanol for 2 times, and dissolving the obtained white precipitate to obtain a second 5% magnesium-doped zinc oxide colloidal solution with a concentration of 0.5 mol/L;

(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; 0.1mmol/L hydrochloric acid is deposited on the surface of the zinc oxide prefabricated film, and the deposition amount of the acid solution and the weight of the lower zinc oxide prefabricated film meet the following conditions: treating every 5mg of zinc oxide prefabricated film with 80 μ L of acid solution, reacting at 70 deg.C for 60min, and removing solvent to obtain a first zinc oxide film with surface hydroxyl content of 0.3; depositing a second 5% magnesium-doped zinc oxide colloidal solution on the first zinc oxide film, and removing the solvent to prepare a second 5% magnesium-doped zinc oxide film with the surface hydroxyl amount of 0.5;

the thickness of the first zinc oxide film is 60nm, and the thickness of the second 5% magnesium-doped zinc oxide film is 30 nm.

And detecting hydroxyl groups in the prepared first electron transport layer and the second electron transport layer by using X-ray photoelectron spectroscopy (XPS), and determining that the hydroxyl group content of the first electron transport layer is 0.3 and the hydroxyl group content of the second electron transport layer is 0.5.

The device EQE test results of the quantum dot light emitting diodes provided in example 4 and comparative example 1 are shown in fig. 17, and the lifetime test results are shown in fig. 18.

Example 5

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

(1) dissolving zinc sulfate in butanol at room temperature to prepare a zinc salt solution with the concentration of 1mol/L, and dissolving sodium hydroxide in ethanol at room temperature to obtain an alkali solution with the concentration of 1.5mol/L, wherein the molar ratio of hydroxide ions to zinc ions is 1.5: 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 1.5:1, and continuously stirring the mixed solution at the reaction temperature of 60 ℃ for reacting for 60 min;

(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. Washing the precipitate with ethanol as a reaction solvent for 2 times, and dissolving the obtained white precipitate to obtain a first zinc oxide colloidal solution with the concentration of 0.75 mol/L;

(2) dissolving yttrium sulfate and zinc sulfate in butanol at room temperature to prepare a mixed salt solution with the concentration of 1mol/L, wherein the molar ratio of yttrium ions is 10%, and dissolving potassium hydroxide in ethanol at room temperature to obtain an alkali liquor with the concentration of 2 mol/L; adjusting the temperature of the zinc salt solution to 50 ℃ according to the molar ratio of hydroxide ions to zinc ions of 2:1, dropping alkali liquor into the mixed salt solution, and continuously stirring/reacting the mixed solution for 90min under the condition that the reaction temperature is kept at 50 ℃; adding a mixed solution after the reaction is finished into the mixed solution in a volume ratio of 4: 1, generating white precipitate in the mixed solution; cleaning the precipitate with ethanol for 2 times, and dissolving the obtained white precipitate to obtain a second 10% yttrium-doped zinc oxide colloidal solution with the concentration of 0.75 mol/L;

(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; 0.075mmol/L nitric acid is deposited on the surface of the zinc oxide prefabricated film, and the deposition amount of the acid solution and the weight of the lower zinc oxide prefabricated film meet the following conditions: treating every 5mg of zinc oxide prefabricated film with 100 microliter of acid solution, reacting for 90min at 80 ℃, and removing the solvent to obtain a first zinc oxide film with the surface hydroxyl content of 0.35; depositing a second 10% yttrium-doped zinc oxide solution on the first zinc oxide film, and removing the solvent to prepare a second 10% yttrium-doped zinc oxide film with the surface hydroxyl content of 0.75;

the thickness of the first zinc oxide film is 70nm, and the thickness of the second 10% yttrium-doped zinc oxide film is 15 nm.

Hydroxyl groups in the zinc oxide colloidal solution or the zinc oxide solution for preparing the 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 is determined to be 0.35, the hydroxyl group content of the second electron transport layer is determined to be 0.35, and the hydroxyl group content of the third electron transport layer is determined to be 0.75. The device EQE test results of the quantum dot light emitting diodes provided in example 5 and comparative example 1 are shown in fig. 19, and the lifetime test results are shown in fig. 20.

Example 6

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

(C) adding a mixed solution after the reaction is finished into the mixed solution in a volume ratio of 4: 1, white precipitate was generated in the mixed solution. Washing the precipitate with methanol as reaction solvent for 3 times, and dissolving the obtained white precipitate to obtain a first zinc oxide colloidal solution with surface hydroxyl amount of 0.15;

(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 1.9: 1;

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

(D) Dissolving the obtained white precipitate, adding TMAH with the molar concentration of 0.3mol/L into the zinc oxide colloidal solution, and adjusting the pH of the solution to be 8 to obtain a second zinc oxide colloidal solution with the hydroxyl content of 0.70;

(3) 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 1.9: 1;

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

(D) Dissolving the obtained white precipitate, adding 0.1mol/L sulfuric acid into the zinc oxide colloidal solution, and adjusting the pH of the solution to 7.5 to obtain a third zinc oxide colloidal solution with the hydroxyl content of 0.35;

(4) forming a first zinc oxide colloidal solution on the quantum dot light-emitting layer, removing the solvent, and preparing a first zinc oxide film with the surface hydroxyl amount of 0.15; 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 quantity of 0.70; 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 amount of 0.35. The thickness of the first zinc oxide layer is 60nm, the thickness of the second zinc oxide layer is 30nm, and the thickness of the third zinc oxide layer is 60 nm.

Hydroxyl groups in the zinc oxide colloidal solution or the zinc oxide solution for preparing the 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 is determined to be 0.15, the hydroxyl group content of the second electron transport layer is determined to be 0.70, and the hydroxyl group content of the third electron transport layer is determined to be 0.35.

The device EQE test results of the quantum dot light emitting diodes provided in example 6 and comparative example 1 are shown in fig. 21, and the lifetime test results are shown in fig. 22.

TABLE 2

It should be appreciated that testing the lifetime of a quantum dot light emitting diode device is different from characterizing the efficiency of a quantum dot light emitting diode device, which is typically short in time, and thus characterizes the onset transient state of operation of the quantum dot light emitting diode 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 exemplary of the present application and should not be taken as limiting the present application, as any modification, equivalent replacement, or improvement made within the spirit and scope of the present application should be included in the present application.

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 less than or equal to 0.4,

mixing the zinc salt solution with alkali liquor for reaction to prepare zinc oxide nano-particles; dissolving the zinc oxide nano particles to obtain a zinc oxide colloidal solution; adding an acid solution into the zinc oxide colloidal solution, and adjusting the pH value of the zinc oxide colloidal solution to 7-8 to obtain a zinc oxide solution;

and forming a zinc oxide solution on the substrate of the prefabricated device of the zinc oxide film with the surface hydroxyl amount less than or equal to 0.4, and removing the solvent to prepare the zinc oxide film with the surface hydroxyl amount less than or equal to 0.4.

2. The method for preparing the quantum dot light-emitting diode of claim 1, wherein in the step of adding an acid solution to the zinc oxide colloidal solution and adjusting the pH of the zinc oxide colloidal solution to 7-8, an acid solution is added to the zinc oxide colloidal solution so that the pH of the obtained mixed solution is 7.2-7.8.

3. The method for preparing a quantum dot light-emitting diode according to claim 2, wherein in the step of adding an acid solution to the zinc oxide colloidal solution and adjusting the pH of the zinc oxide colloidal solution to 7 to 8, the acid solution is added to the zinc oxide colloidal solution so that the pH of the obtained mixed solution is 7.3 to 7.6.

4. The method of claim 1, wherein the acid in the acid solution is at least one selected from the group consisting of hydrochloric acid, sulfuric acid, nitric acid, hydrofluoric acid, formic acid, acetic acid, propionic acid, oxalic acid, and propylene.

5. The method of any one of claims 1 to 4, wherein the alkali solution is at least one alkali solution selected from potassium hydroxide, sodium hydroxide, lithium hydroxide, TMAH, ammonia, ethanolamine, and ethylenediamine.

6. The method of any one of claims 1 to 4, wherein the solvent in the zinc salt solution and the solvent in the alkali solution are independently selected from at least one of water, organic alcohol, organic ether and sulfone.

7. The method according to any one of claims 1 to 4, wherein the solvent in the acid solution is at least one selected from water, methanol, ethanol, propanol, butanol, ethylene glycol monomethyl ether, and DMSO.

8. The method for preparing a quantum dot light-emitting diode according to any one of claims 1 to 4, wherein the zinc oxide film having a surface hydroxyl group amount of 0.4 or less is a metal-doped zinc oxide film, and the zinc salt solution further contains a doped metal ion.

9. The method of claim 8, wherein the dopant metal ion is selected from the group consisting of Mg²⁺、Mn²⁺At least one of; or

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

10. The method of claim 9, wherein the doping concentration of the doped metal ions is as follows:

when the doped metal ion is Mg²⁺When Mg is contained in the zinc salt solution²⁺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 Mn is contained in the zinc salt solution²⁺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 Al is contained in the zinc salt solution³⁺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 the zinc salt solution, Y is³⁺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⁺While in the 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 the zinc salt solution, Li⁺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 Gd is contained 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 the zinc salt solution, Zr⁴⁺The molar content of the metal ions accounts for 0.1 percent of the total molar weight of the metal ions～45％；

When the doped metal ion is Ce⁴⁺When in the zinc salt solution, Ce is in⁴⁺The mol content accounts for 0.1-10% of the total molar weight of the metal ions.

11. The method for preparing a quantum dot light-emitting diode according to claim 8, wherein in the step of mixing the zinc salt solution with an alkali solution, the addition amounts of the zinc salt solution and the alkali solution are 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.