Electron transport material, electron transport layer and application thereof

Abstract

This application relates to an electron transport layer material for quantum dot light-emitting diodes (QLEDs), an electron transport layer, and its applications. The electron transport layer material comprises n-type metal oxide nanoparticles and photosensitive ligands modified on their surface. The photosensitive ligands include anchoring groups and photosensitive groups. The electron transport layer is formed by depositing the material into a film and then subjecting it to photo-irradiation and / or heat treatment to form a cross-linked network structure, wherein the n-type metal oxide nanoparticles are dispersed within the cross-linked network structure. This application also provides a QLED device incorporating this electron transport layer and its fabrication method. By anchoring and passivating surface defects through the photosensitive ligands, and by reducing electron mobility and enhancing film stability through the cross-linked network, this electron transport layer can improve carrier injection balance in QLED devices, suppress efficiency roll-off, and improve device lifetime.

Description

Technical Field

[0001] This application relates to the field of display technology, and in particular to an electronic transport material, an electronic transport layer, and their applications. Background Technology

[0002] Quantum dot light-emitting diodes (QLEDs) have attracted widespread attention in the field of next-generation display technology due to their high luminous efficiency, high color purity, wide color gamut, and flexible fabrication capabilities. In recent years, device performance has made continuous progress, with the external quantum efficiency of red, green, and blue QLEDs all exceeding 20%, meeting the basic requirements for commercial applications. QLEDs have application potential in smart terminals, AR / VR, large-size ultra-high-definition displays, and high-end lighting, and the global quantum dot display market is currently showing a growth trend. However, the operating life of QLED devices remains one of the main bottlenecks restricting their commercial application.

[0003] Charge accumulation and efficiency roll-off caused by carrier injection imbalance are key factors affecting device lifetime. At high current densities, efficiency roll-off is related to various factors, including Auger recombination, electric field-induced quenching, Joule heating, and electron leakage to the hole transport layer, with electron leakage being identified as a major factor. The root cause of charge accumulation lies in the difference in carrier mobility among the functional layers of QLED devices: ZnO nanoparticles, commonly used in the electron transport layer, have high electron mobility, while organic polymer materials (such as TFB and PVK), commonly used in the hole transport layer, have relatively low hole mobility. This difference leads to electron injection typically exceeding hole injection in the device, resulting in carrier injection imbalance.

[0004] Several technologies have attempted to address this issue, such as doping ZnO with Mg to reduce electron mobility, or inserting an ultrathin insulating layer between the quantum dot and the electron transport layer to block excess electrons. However, inserting an ultrathin insulating layer between the light-emitting layer and the electron transport layer requires high process control, which is not conducive to industrial mass production. Therefore, effectively reducing excess electron injection and suppressing charge accumulation remain technical challenges for improving the efficiency and lifetime of QLEDs. Summary of the Invention

[0005] This application aims to solve the problems of excessive electron injection, charge accumulation, and efficiency roll-off caused by excessively high carrier mobility in the electron transport layer of quantum dot light-emitting diodes. After film deposition, a cross-linked network structure is formed through photo- or thermal treatment, which reduces electron mobility and passivates surface defects, thereby improving balanced carrier injection and enhancing device efficiency and operational stability. This objective is achieved through the following technical solutions: The electron transport layer material of this application includes n-type metal oxide nanoparticles and photosensitive ligands modified on the surface of the n-type metal oxide nanoparticles; The photosensitive ligand includes an anchoring group capable of binding to the surface of the n-type metal oxide nanoparticles and a photosensitive group capable of undergoing cross-linking, polymerization, or photochemical reactions under light and / or heating conditions. The anchoring group is selected from one or more of carboxyl, phosphate, phosphonic acid, amino, and silane groups; The photosensitive group is selected from one or more of azophenyl, cinnamic acid group, anthracene group, diarylvinyl group, azido group, acrylate group, and methacrylate group.

[0006] In one embodiment, the material of the n-type metal oxide nanoparticles is selected from ZnO, SnO2, TiO2, ZnMgO, IZO and combinations thereof.

[0007] In one embodiment, the photosensitive ligand is selected from one or more of the following: carboxylated azobenzene derivatives, phosphate-containing azobenzene derivatives, cinnamic acid derivatives, anthracene derivatives, diarylethylene derivatives, azide derivatives, and silane-containing photosensitive monomers.

[0008] In one embodiment, the photosensitive ligand is bound to the surface of the n-type metal oxide nanoparticles via ligand exchange or dispersed between the n-type metal oxide nanoparticles via blending.

[0009] This application also discloses an electron transport layer, which is obtained by forming a cross-linked network structure from the aforementioned electron transport layer material through film formation and light irradiation and / or heat treatment; The n-type metal oxide nanoparticles are dispersed in the cross-linked network structure.

[0010] In one embodiment, the thickness of the electron transport layer is 10 nm to 60 nm.

[0011] This application further provides a quantum dot light-emitting diode device, comprising a substrate, an anode, a hole injection layer, a hole transport layer, a quantum dot light-emitting layer, an electron transport layer, and a cathode arranged sequentially. The electron transport layer is the electron transport layer described in claim 5 or 6.

[0012] In one embodiment, the hole injection layer includes one or more of MoOx, VOx, WOx, CrOx, CuO, MoS2, MoSe2, WS2, WSe2, CuS, and PEDOT; The hole transport layer includes one or more of PVK, TFB, and PTAA; The quantum dot light-emitting layer includes one or more of group II-VI and group III-V semiconductor nanocrystals.

[0013] This application also provides a method for preparing an electron transport layer, comprising the following steps: S1. Preparation of n-type metal oxide nanoparticle dispersion; S2. Add a photosensitive ligand to the n-type metal oxide nanoparticle dispersion, so that the photosensitive ligand binds to the n-type metal oxide nanoparticles through ligand exchange or blending. S3. Deposit the obtained dispersion onto the substrate surface to form an electron transport layer precursor film; S4. The electron transport layer precursor film is subjected to light irradiation and / or heat treatment to cause the photosensitive ligand to undergo cross-linking or polymerization reactions, thereby forming an electron transport layer.

[0014] In one embodiment, the illumination process uses a light source with a wavelength of 250nm to 450nm, preferably a 365nm light source or a 405nm light source. And / or, The heat treatment temperature shall not exceed 120°C; The deposition method includes one or more of spin coating, spray coating, blade coating, inkjet printing, and photolithography.

[0015] Compared with the prior art, this application has the following beneficial effects: This application achieves effective passivation of surface defects by binding the anchoring groups of photosensitive ligands to the surface of n-type metal oxide nanoparticles, reducing non-radiative recombination losses and improving the luminous efficiency of quantum dot light-emitting diodes. By binding anchoring groups such as carboxyl, phosphate, phosphonic, amino, or silane groups to the surface of metal oxide nanoparticles, unsaturated coordination sites on the nanoparticle surface are covered, reducing the defect state density. The photosensitive ligands are introduced into the nanoparticle surface before film formation through ligand exchange or blending, achieving defect passivation from the material level to the device level, providing a structural basis for improving device luminous efficiency.

[0016] In this application, the photosensitive ligand undergoes a cross-linking reaction after film formation via photo or thermal treatment, forming a cross-linked network structure in which n-type metal oxide nanoparticles are dispersed. This cross-linked network reduces the density of electron transport channels between nanoparticles through physical confinement, and also blocks electron transport through the insulating properties of the ligand molecules themselves, effectively reducing the mobility of the electron transport layer. By forming a cross-linked network structure, this application reduces the excess electron injection capability and promotes balanced recombination of holes and electrons, thereby suppressing efficiency roll-off at high current densities.

[0017] After photosensitive ligands bind to the surface of metal oxide nanoparticles through anchoring groups, they can adjust the energy level positions of the nanoparticles, optimize the electron injection barrier, and further promote balanced carrier injection. After the anchoring groups (such as carboxyl groups, phosphate groups, etc.) bond to the surface of the metal oxide nanoparticles, they change the surface potential and energy level arrangement of the nanoparticles through the interfacial dipole effect, causing the conduction band energy level to shift upwards appropriately or the electron injection barrier to increase appropriately. This allows for the control of electron injection capability while passivating defects, further assisting the cross-linked network in achieving balanced carrier injection. Furthermore, the photosensitive ligand cross-linked network formed after photo- or thermal treatment in this application fixes the nanoparticles in a three-dimensional mesh structure. This not only enhances the connection strength between the nanoparticles but also acts as a barrier against external water and oxygen, reducing the adsorption and erosion of the nanoparticle surface. The cross-linked network structure further improves the chemical and mechanical stability of the electron transport layer, helping to extend the device's operating life. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the quantum dot light-emitting diode device in this application; Figure 2 This is a schematic flowchart of a method for preparing an electron transport layer according to an embodiment of this application; Figure 3 This is a schematic diagram illustrating the principle of the formation of the electronic transport layer in this application.

[0019] Explanation of reference numerals in the attached figures: 100, substrate; 200, anode; 300, hole injection layer; 400, hole transport layer; 500, quantum dot light-emitting layer; 600, electron transport layer; 700, cathode. Detailed Implementation

[0020] To make the above-mentioned objectives, features, and advantages of this application more apparent and understandable, the specific embodiments of this application will be described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are for illustrative purposes only and are not intended to limit the scope of this application. Furthermore, it should be noted that, for ease of description, only the parts relevant to this application are shown in the accompanying drawings, not the entire structure. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without inventive effort are within the scope of protection of this application.

[0021] The terms “comprising” and “having”, and any variations thereof, used in this application are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or apparatus that includes a series of steps or units is not limited to the steps or units listed, but may optionally include steps or units not listed, or may optionally include other steps or units inherent to such process, method, product, or apparatus.

[0022] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0023] To address the issues of excessive electron injection, charge accumulation, and efficiency roll-off in quantum dot light-emitting diodes (QLEDs) described in the background art, caused by the significant difference in carrier mobility between the electron transport layer and hole transport layer, this application provides a modified electron transport layer material, an electron transport layer based on this material, a quantum dot light-emitting diode device containing this electron transport layer, and a method for fabricating the same. Specifically, this application introduces photosensitive ligands containing anchoring and photosensitive groups onto the surface of n-type metal oxide nanoparticles (e.g., ZnO, SnO2, TiO2, ZnMgO, IZO, etc.), and after film formation, triggers crosslinking or polymerization reactions through light irradiation and / or heat treatment to form an electron transport layer with a crosslinked network structure. This crosslinked network structure, on the one hand, passivates surface defects of nanoparticles through ligand anchoring, reducing nonradiative recombination; on the other hand, it reduces electron mobility and improves carrier injection balance through the physical confinement and insulating barrier effects of the crosslinked network; simultaneously, the crosslinked structure enhances the density and environmental stability of the film, helping to suppress efficiency roll-off during device operation and extend its operating life. The technical solution and its effects of this application are described in detail below with reference to specific embodiments and experimental data. The electron transport layer material of this application includes n-type metal oxide nanoparticles and photosensitive ligands modified on the surface of the n-type metal oxide nanoparticles; the photosensitive ligands include anchoring groups capable of binding to the surface of the n-type metal oxide nanoparticles and photosensitive groups capable of undergoing crosslinking, polymerization, or photochemical reactions under light and / or heating conditions; the anchoring groups are selected from one or more of carboxyl, phosphate, phosphonic, amino, and silane groups; the photosensitive groups are selected from one or more of azophenyl, cinnamic acid, anthracene, diarylvinyl, azide, acrylate, and methacrylate groups.

[0024] In the electron transport layer material provided in this application, the surface of n-type metal oxide nanoparticles is modified with a photosensitive ligand. This photosensitive ligand simultaneously contains an anchoring group capable of binding to the nanoparticle surface and a photosensitive group capable of cross-linking or polymerization under light and / or heating conditions. The n-type metal oxide nanoparticles are selected from ZnO, SnO2, TiO2, ZnMgO, IZO, and combinations thereof. Taking ZnO as an example, it possesses high electron mobility, a wide bandgap (3.37 eV), and good transparency and hole blocking properties, thus it is widely used as an electron transport layer material for QLED devices. The electron mobility of ZnO nanoparticles can reach 200–300 cm² / V·s (bulk material), far exceeding that of organic hole transport materials (such as TFB, approximately 10). - 2 cm² / V·s, PVK approximately 10 -6 This mobility difference (cm² / V·s) means that electron injection in the device is usually greater than hole injection, which is the fundamental reason for the unbalanced carrier injection. In addition, the inherent defect states on the surface of ZnO nanoparticles, such as oxygen vacancies and hydroxyl groups, can act as trap sites to induce exciton quenching, reducing the effective radiative recombination efficiency.

[0025] The photosensitive ligand includes an anchoring group capable of binding to the surface of n-type metal oxide nanoparticles. This anchoring group is selected from one or more of the following: carboxyl (–COOH), phosphate (–PO3H2), phosphonate (–PO(OH)2), amino (–NH2), and silyl (–Si(OR)3). The main functions of the anchoring group are: to achieve stable binding of the photosensitive ligand to the nanoparticle surface, ensuring that the ligand is not easily detached during solution processing and film formation; to passivate surface defects by covering unsaturated coordination sites on the nanoparticle surface, reducing non-radiative recombination losses; and the bonding between the anchoring group and the nanoparticle surface can also modulate the energy level positions of the nanoparticles through the interfacial dipole effect, optimizing the electron injection barrier.

[0026] Specifically, taking carboxyl anchoring groups as an example, the connection between carboxyl groups and ZnO quantum dots is more stable than that between amine groups, and the adsorption energy of dissociated carboxyl groups on the 1010 surface of ZnO can reach 1.39 eV. Carboxyl ligands with multi-branched structures (such as EDTA and ethylenediaminetetraacetic acid) can coordinate with ZnO through all four branches. The steric hindrance effect brought about by numerous branches is beneficial to the dispersion of ZnO quantum dots, resulting in a more uniform particle size distribution and effectively alleviating the aggregation of nanoparticles. Taking phosphate / phosphonic acid groups as an example, phosphonic acid groups can form strong bonds with the surfaces of various metal oxides (including ZnO) through bidentate or tripentate coordination. The P=O bonds in organic molecules containing phospho groups (such as TSPO1) can bind with uncoordinated Zn² groups on the ZnO surface. +Stable coordination is formed, achieving passivation and energy level modulation at the ZnO interface. The aforementioned anchoring groups enable stable attachment of ligands to the nanoparticle surface, passivation of surface defects, and modulation of electron mobility.

[0027] The photosensitive ligand also includes a photosensitive group capable of undergoing cross-linking, polymerization, or photochemical reactions under light and / or heating conditions. This photosensitive group is selected from one or more of azophenyl, cinnamic acid, anthracene, diarylvinyl, azide, acrylate, and methacrylate groups. The main functions of the photosensitive group include: triggering cross-linking or polymerization reactions after film formation via light or heat treatment to form a cross-linked network structure, immobilizing nanoparticles within it; reducing electron mobility through physical confinement and insulating barrier effects, improving carrier injection balance; the cross-linked network acts as a barrier against external water and oxygen, reducing adsorption and erosion of the nanoparticle surface; enhancing the film stability and chemical stability of the electron transport layer; and some photosensitive groups themselves possess certain charge transport characteristics, which can further optimize energy level matching.

[0028] Taking azophenyl as an example, the scheme of binding azophenyl ligands to the surface of quantum dots allows these ligands to transform from a trans structure to a cis structure under ultraviolet irradiation, thereby modulating surface polarity and interfacial contact angle, resulting in more uniform charge transport and improved device luminescence performance. In the electron transport layer, in addition to photoisomerization, the end groups of azophenyl can undergo further functionalization to achieve cross-linking reactions. Taking acrylate and methacrylate groups as examples, ultraviolet photopolymerization based on zinc acrylate and acrylic acid is an effective way to prepare ZnO thin films. The photosensitive groups, through different cross-linking reaction mechanisms, are transformed into a cross-linked network structure after film formation by photo-irradiation and / or heat treatment. In this cross-linked network structure, n-type metal oxide nanoparticles are dispersed, thereby achieving multiple effects such as passivation of defects, reduction of electron mobility, and enhancement of film stability.

[0029] Photosensitive ligands can bind to the surface of n-type metal oxide nanoparticles via ligand exchange or disperse between n-type metal oxide nanoparticles through blending. Ligand exchange involves replacing the existing ligands (such as oleic acid or ethanol) on the nanoparticle surface with photosensitive ligands. This method allows for a tighter and more uniform coating of the photosensitive ligands on the nanoparticle surface, facilitating more efficient defect passivation and more controllable cross-linking reactions. Blending, on the other hand, involves directly mixing the photosensitive ligands with the nanoparticle dispersion without replacing the existing ligands. This method simplifies the process, is suitable for large-scale solution processing, and allows the photosensitive ligands to form a cross-linked network between the nanoparticles, providing physical confinement.

[0030] Examples of specific ligand chemical formulas are as follows: After the electron transport layer material is deposited into a film, photosensitive ligands undergo cross-linking or polymerization reactions under light and / or heat treatment to form a cross-linked network structure, in which n-type metal oxide nanoparticles are dispersed. The thickness of the electron transport layer is 10 nm to 60 nm. In this structure, the cross-linked network, on the one hand, restricts the migration and aggregation of nanoparticles through physical confinement, ensuring the uniformity of the film morphology; on the other hand, the organic components in the cross-linked network have insulating properties, which can effectively regulate the density of electron transport channels between nanoparticles, reduce electron mobility, and thus improve carrier injection balance. The cross-linked network enhances the compactness and environmental stability of the film, and improves the electrochemical stability of the ZnO electron transport layer, enabling QLEDs to have higher efficiency and longer operating life.

[0031] This application also discloses an electron transport layer, which is obtained by forming a cross-linked network structure through photo-irradiation and / or heat treatment after the aforementioned electron transport layer material is deposited into a film. The n-type metal oxide nanoparticles are dispersed in the cross-linked network structure. After film formation, the photosensitive ligands in the electron transport layer material trigger a cross-linking or polymerization reaction through photo-irradiation and / or heat treatment to form a three-dimensional cross-linked network structure. The n-type metal oxide nanoparticles are dispersed in this cross-linked network structure, and the thickness of the electron transport layer is 10 nm to 60 nm. The cross-linking reaction of photosensitive ligands can be achieved through various mechanisms: taking cinnamic acid as an example, it can undergo a [2+2] cycloaddition reaction under light irradiation, and photocrosslinking can be carried out without photosensitizers, avoiding the influence of photosensitizers on material stability and avoiding the damage to device performance caused by thermal crosslinking methods; taking acrylate and methacrylate as examples, organic molecules containing such photosensitive groups can undergo free radical polymerization under ultraviolet light irradiation to form a cross-linked network structure. Studies have shown that using ultraviolet light to irradiate the photosensitive organic molecule layer can cause small organic molecules to undergo photocrosslinking reactions to generate polymers with network structures; taking azide as an example, it can decompose under light or heating conditions to generate azene intermediates, and then undergo insertion reactions with neighboring molecules to form cross-linked structures.

[0032] The three-dimensional network structure formed after the above crosslinking reaction encapsulates and fixes the n-type metal oxide nanoparticles within it. The crosslinking network in this application effectively suppresses the aggregation of nanoparticles during film formation and migration during device operation through physical confinement, ensuring the uniformity of film morphology and reducing leakage current channels and exciton quenching sites caused by particle aggregation, thereby providing a structural basis for subsequent improvement of device optoelectronic performance.

[0033] n-type metal oxide nanoparticles (taking ZnO as an example) exhibit high electron mobility, while organic hole transport materials have relatively low hole mobility. This mobility difference is one of the main reasons for excessive electron injection, carrier imbalance, and efficiency roll-off in QLED devices. In this application, the three-dimensional network structure formed by photosensitive ligand crosslinking regulates electron mobility in two ways: First, the organic components in the crosslinked network have certain insulating properties, and the physical barriers they form between nanoparticles increase the resistance to electron transport between adjacent nanoparticles, thereby reducing the effective electron mobility of the electron transport layer. This application first achieves a reduction in electron mobility without introducing additional dopants by forming a crosslinked network structure, thereby improving the carrier injection balance. After the anchoring groups in the photosensitive ligands bind to the surface of the n-type metal oxide nanoparticles, they also passivate surface defects, reducing excess electron injection induced by defect states. This application achieves a synergistic effect between the regulation of electron mobility by the cross-linked network structure and the passivation effect of surface defects by the ligand anchoring groups, thereby promoting the balanced injection of electrons and holes, reducing the accumulation of excess charge in the light-emitting layer, suppressing the efficiency roll-off phenomenon under high current density, and helping to improve the stability and service life of the device during operation.

[0034] In unmodified ZnO nanoparticle films, ligands are prone to detachment during storage or device operation, and the adhesion of external water and oxygen further affects the film morphology. This application utilizes a three-dimensional network structure formed by photosensitive ligand crosslinking to physically block external water and oxygen, while simultaneously fixing the nanoparticles within the network, reducing the likelihood of ligand detachment during device operation. The crosslinked network structure in this application enhances the chemical and mechanical stability of the electron transport layer, helping to reduce performance degradation caused by electron transport layer degradation during long-term device operation. The electron transport layer provided in this application forms a three-dimensional network structure through photosensitive ligand crosslinking, with n-type metal oxide nanoparticles dispersed within this network. This achieves dispersion and fixation of the nanoparticles to ensure film morphology uniformity. The organic insulating network increases the electron transport resistance between nanoparticles to regulate electron mobility and promote balanced carrier injection. Furthermore, the physical barrier and enhanced chemical stability of the crosslinked network extend the device's lifetime.

[0035] Please see Figure 1This application further provides a quantum dot light-emitting diode device, comprising a substrate 100, an anode 200, a hole injection layer 300, a hole transport layer 400, a quantum dot light-emitting layer 500, an electron transport layer 600, and a cathode 700 sequentially disposed therefrom, wherein the electron transport layer 600 is the aforementioned electron transport layer 600. The hole injection layer 300 is located between the anode 200 and the hole transport layer 400, and it lowers the hole injection barrier from the anode 200 to the hole transport layer 400, promoting hole injection from the anode 200 into the device interior. The hole injection layer 300 material in this application includes one or more of MoOx, VOx, WOx, CrOx, CuO, MoS2, MoSe2, WS2, WSe2, CuS, and PEDOT:PSS. All of the above materials can be prepared by solution methods and are suitable for solution processing technologies such as spin coating, blade coating, and inkjet printing. The hole transport layer 400 is located between the hole injection layer 300 and the quantum dot emitting layer 500, receiving holes from the hole injection layer 300 and transporting them to the quantum dot emitting layer 500. The hole transport layer 400 in this application is made of one or more of PVK, TFB, and PTAA.

[0036] The quantum dot luminescent layer 500 is located between the hole transport layer 400 and the electron transport layer 600, and is the core region for exciton radiative recombination luminescence. In this application, the quantum dot luminescent layer 500 includes one or more of group II-VI and group III-V semiconductor nanocrystals. The group II-VI semiconductor nanocrystals include, but are not limited to, CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, HgS, HgSe, HgTe, PbS, PbSe, PbTe, and other binary, ternary, and quaternary group II-VI compounds; the group III-V semiconductor nanocrystals include, but are not limited to, InP. Quantum dots are semiconductor nanocrystals with a particle size of 1–50 nm. Because the particle size of quantum dots is smaller than their exciton Bohr radius, electrons and holes are effectively confined in three-dimensional space, producing a quantum confinement effect, exhibiting properties such as continuously tunable emission wavelength with size and composition, narrow emission spectrum, and high luminous efficiency.

[0037] This application modifies the electron transport layer 600 through photosensitive ligand crosslinking, effectively controlling electron injection capability and passivating interface defects. This improves the balanced injection of electrons and holes and suppresses charge accumulation in the quantum dot emitting layer 500, regardless of the quantum dot material system used. Therefore, the above-mentioned range of selectable materials for the quantum dot emitting layer 500 in this application is applicable.

[0038] Please see Figures 2-3 This application also provides a method for preparing an electron transport layer 600, comprising the following steps: S1. Preparation of n-type metal oxide nanoparticle dispersion; S2. Add a photosensitive ligand to the n-type metal oxide nanoparticle dispersion, so that the photosensitive ligand binds to the n-type metal oxide nanoparticles through ligand exchange or blending. S3. Deposit the obtained dispersion onto the substrate surface to form an electron transport layer precursor film; S4. The electron transport layer precursor film is subjected to photo-irradiation and / or heat treatment to cause the photosensitive ligand to undergo a cross-linking or polymerization reaction, thereby forming the electron transport layer 600.

[0039] The preparation of the n-type metal oxide nanoparticle dispersion in this application can be achieved using various synthetic methods. Taking ZnO nanoparticles as an example, zinc acetate can be used as the zinc source, and potassium hydroxide or tetramethylammonium hydroxide as the alkali source. These solutions are dissolved in polar solvents such as methanol or ethanol to prepare a precursor solution of a certain concentration. The solutions are then mixed at temperatures ranging from 50°C to 80°C, and after nucleation growth and centrifugation and washing, nanoparticles that can be stably dispersed in a specific solvent can be obtained. This dispersion is typically dissolved in alcohol solvents such as ethanol, isopropanol, and n-butanol, facilitating subsequent solution film formation processing.

[0040] Photosensitive ligands are added to the above-mentioned nanoparticle dispersion, allowing the photosensitive ligands to bind to the n-type metal oxide nanoparticles through ligand exchange or blending. Ligand exchange refers to replacing the original ligand molecules (such as oleic acid or residual organic molecules from the synthesis process) on the nanoparticle surface with photosensitive ligands. This method allows for a tighter chemical bond between the photosensitive ligands and the nanoparticle surface, facilitating more uniform surface coverage and more efficient cross-linking reactions. Blending, on the other hand, involves directly mixing the photosensitive ligands with the nanoparticle dispersion without replacing the original ligands. This method simplifies the process and is suitable for rapid screening and batch preparation.

[0041] The anchoring groups in photoligands (such as carboxyl, phosphate, phosphonic, amino, and silane groups) can be stably linked to the surface of nanoparticles through chemical bonding, covering unsaturated coordination sites on the nanoparticle surface and passivating surface defects and reducing nonradiative recombination losses. The photosensitive groups in the photoligands (such as azophenyl, cinnamic acid, anthracene, diarylvinyl, azido, acrylate, and methacrylate groups) provide reactive sites for subsequent crosslinking reactions, enabling the electron transport layer material to possess "crosslinkable" functionality.

[0042] A dispersion is deposited onto a substrate surface to form an electron transport layer precursor film. The deposition method includes one or more of spin coating, spray coating, blade coating, inkjet printing, and photolithography. These methods are all solution processing techniques, suitable for large-area, low-cost preparation, transforming liquid nanoparticle / photosensitive ligand dispersions into solid continuous films. The electron transport layer precursor film is then subjected to photo-irradiation and / or heat treatment to induce cross-linking or polymerization of the photosensitive ligands, thereby forming the electron transport layer 600. The photo-irradiation treatment can use a light source with a wavelength of 250 nm to 450 nm, preferably a 365 nm or 405 nm light source; the heat treatment temperature does not exceed 120 °C. The cross-linking reaction of the photosensitive ligands is triggered to form a three-dimensional network structure. The cross-linked photosensitive ligands bind to the surface of the metal oxide nanoparticles through anchoring groups, exerting a defect passivation effect, reducing the trapped state density on the nanoparticle surface, and reducing non-radiative recombination of excitons at the interface between the electron transport layer 600 and the luminescent layer. The three-dimensional cross-linked network structure reduces electron mobility through physical confinement and insulating barrier effects, promoting balanced carrier injection. By controlling the thickness of the electron transport layer 600 to within the range of 10 nm to 60 nm, a better charge transport effect can be obtained.

[0043] The electron transport layer 600 preparation method provided in this application achieves synergistic control over the microstructure and macroscopic properties of the electron transport layer 600 through a technical route of "nanoparticle synthesis—photosensitive ligand introduction—solution film formation—photo / thermal crosslinking". This method features good process compatibility and strong adaptability to solution processing, and is applicable to various solution-based film formation processes such as spin coating and inkjet printing, providing a technological foundation for large-scale preparation. The electron transport layer 600 obtained through this method has n-type metal oxide nanoparticles dispersed in a crosslinked network structure, thereby achieving multiple effects such as defect passivation, reduced electron mobility, and improved film stability.

[0044] In specific embodiments, the illumination treatment uses a light source with a wavelength of 250nm to 450nm, preferably a 365nm or 405nm light source. In this application, the aforementioned photosensitive ligand can complete the cross-linking reaction at a lower temperature by selecting a photosensitive group responsive to a 405nm light source, thereby reducing adverse effects on other functional layers of the device (especially the temperature-sensitive organic hole transport layer 400). It should be noted that the effect of the illumination treatment is directly related to the absorption wavelength range of the photosensitive group in the photosensitive ligand. Different photosensitive groups have different optimal excitation wavelengths. The wavelength range of 250nm to 450nm covers the effective excitation range of the photosensitive group used, providing a wider process window.

[0045] The heat treatment temperature is no higher than 120°C. The temperature control of the heat treatment (i.e., thermal annealing or crosslinking-initiated heat treatment) affects the device performance. A heat treatment temperature of no higher than 120°C matches the temperature tolerance of the organic hole transport layer 400 (such as TFB, PVK, PTAA, etc.) in the QLED device, avoiding thermal damage or deformation caused by high-temperature processing to this type of organic layer. This temperature range is compatible with the crosslinking reaction conditions of the aforementioned photosensitive ligands - photosensitive groups such as azide groups and cinnamic acid groups can undergo effective thermally induced crosslinking reactions or photothermal synergistic crosslinking reactions below 120°C, without the need for higher thermal initiation temperatures. The processing temperature below 120°C has good process compatibility with the flexible display substrate 100 (such as PI, etc.), providing a preparation window for the application of QLED devices in the field of flexible displays.

[0046] The deposition methods include one or more of spin coating, spray coating, blade coating, inkjet printing, and photolithography. All of these deposition methods are solution processing techniques, suitable for large-area, low-cost QLED device fabrication, and exhibit good process compatibility with the solution dispersion system (ethanol, isopropanol, n-butanol, and other alcohol solvents) of the electron transport layer 600 material in this application. While each deposition method differs in film uniformity, processing efficiency, material utilization, patterning resolution, and equipment cost, all can achieve effective film formation of the electron transport layer 600 material in this application. In practical applications, one or a combination of multiple deposition methods can be selected based on the device structure design, display resolution requirements, and production scale.

[0047] This application provides a controllable process window for the crosslinking reaction of photosensitive ligands in the electron transport layer 600 material by limiting the illumination wavelength to 250nm to 450nm (preferably 365nm or 405nm), the heat treatment temperature not exceeding 120℃, and various solution deposition methods including spin coating, spray coating, blade coating, inkjet printing, and photolithography. At the same time, it takes into account the temperature tolerance of the organic functional layer in the device and the adaptability of the film deposition process to different application scenarios. Specific Implementation The following will further introduce some specific implementation methods to provide a more detailed explanation of the technical solution of this application.

[0049] Example 1 This embodiment provides a QLED device that uses azobenzene carboxylic acid to modify ZnO nanoparticles and forms an electron transport layer through photocuring.

[0050] First, the pre-prepared ITO glass substrate was ultrasonically cleaned sequentially with deionized water, acetone, and isopropanol for 15 min, and then treated with ultraviolet ozone for 20 min before use. A TFB hole transport layer solution (concentration 8 mg / mL) was then spin-coated onto the ITO substrate at 3000 rpm for 40 s, followed by annealing at 150℃ for 30 min to form a hole transport layer approximately 35 nm thick.

[0051] Subsequently, a CdSe / ZnS core-shell structured red quantum dot luminescent layer was spin-coated at 2000 rpm to form a luminescent layer approximately 25 nm thick, and then annealed at 80 °C for 10 min.

[0052] Weigh 100 mg of ZnO nanoparticles and disperse them in 10 mL of n-butanol. Add 2 mg of 4-(4-carboxyphenylazo)benzoic acid and stir at room temperature for 12 h to allow the carboxyl groups to adhere to the Zn²⁺ ions on the ZnO surface. + Coordination adsorption occurred at the site. After centrifugation and washing, it was redispersed in n-butanol and the concentration was adjusted to 20 mg / mL.

[0053] The above dispersion was spin-coated onto the surface of the quantum dot layer at 2500 rpm to obtain an electron transport layer precursor film with a thickness of approximately 35 nm. Subsequently, it was irradiated with 365 nm ultraviolet light for 120 s at an intensity of 10 mW / cm² to induce cis-trans isomerization and intermolecular cross-linking reactions of the azophenyl groups, forming a stable network structure.

[0054] Finally, a 100nm thick Al cathode was deposited by vacuum evaporation and encapsulated with a glass cover.

[0055] Because carboxyl groups can effectively passivate oxygen vacancy defects on the ZnO surface, and the azobenzene crosslinking network can restrict rapid electron transport, the electron and hole injection in the device tend to be in equilibrium. The resulting device achieves a maximum external quantum efficiency of over 18%, which is more than twice the lifetime of unmodified ZnO devices.

[0056] Example 2 In this embodiment, a cinnamic acid bifunctional molecule is used to modify the ZnO electron transport layer.

[0057] The ZnO nanoparticle dispersion was prepared in the same manner as in Example 1. 3 mg of ethyl p-carboxycinnamate was added to 100 mg of ZnO nanoparticles, and the mixture was stirred for 8 hours to complete surface ligand exchange.

[0058] After spin-coating the modified ZnO to form an electron transport layer, it was irradiated with 365nm ultraviolet light for 180s. A [2+2] photocycloaddition reaction occurred between the cinnamic acid groups, forming a cross-linked network.

[0059] Due to the high crosslinking efficiency of cinnamic acid groups, the surface roughness of the resulting electron transport layer is reduced to below 1.2 nm, the electron mobility is reduced by about an order of magnitude, the efficiency roll-off is significantly reduced when the device operates at a brightness of 1000 cd / m², and the T50 lifetime is increased by about 2.5 times.

[0060] Example 3 In this embodiment, an anthracene carboxylic acid derivative is used as a bifunctional ligand.

[0061] 9-Anthracenic acid was added to the ZnO nanoparticle dispersion at an amount of 3 wt% of the ZnO mass, and the mixture was stirred at room temperature for 10 h.

[0062] After the electron transport layer is formed, it is irradiated with 365nm ultraviolet light for 300s, and the anthracene groups undergo photodimerization to form a cross-linked structure.

[0063] The larger π-conjugated structure of the anthracene rings increases the potential barrier in the electron transport path, while the anthracene group crosslinks form a rigid network that effectively suppresses ZnO particle migration. The device's maximum brightness remains at 1×10⁻⁶. 5 With a cd / m² or higher, the service life is increased by about 3 times.

[0064] Example 4 In this embodiment, ZnO nanoparticles were modified with 4-vinylbenzylphosphonic acid.

[0065] 4-Vinylbenzylphosphonic acid was added to the ZnO dispersion at a concentration of 5 wt%, and stirred for 12 h.

[0066] Phosphonic acid groups form Zn-OP chemical bonds with the ZnO surface. After spin coating, 0.5 wt% photoinitiator 1173 is added, and the film is irradiated with 365 nm ultraviolet light for 60 s.

[0067] Vinyl groups undergo free radical polymerization to form cross-linked networks.

[0068] Because the binding energy of Zn-OP bonds is higher than that of Zn-OOC bonds, the surface passivation effect is more significant. The leakage current of the device is reduced by more than 40%, and the operating stability is significantly improved.

[0069] Example 5 In this embodiment, ZnO is modified by grafting polyethyleneimine with azobenzene polymer.

[0070] PEI with a molecular weight of 25,000 was grafted with 4-carboxyazobenzene via an amidation reaction to obtain PEI-Azo polymer.

[0071] Add 2wt% PEI-Azo polymer to the ZnO dispersion, stir until homogeneous, and then spin-coat into a film.

[0072] Cured for 5 minutes under 405nm visible light irradiation.

[0073] The amino groups in the polymer can undergo Lewis acid-base interactions with the ZnO surface, while azobenzene provides photocrosslinking capability. The resulting polymer network uniformly immobilizes the ZnO particles.

[0074] The mechanical stability of the resulting electron transport layer is significantly improved, and it still maintains its complete structure after aging at 85℃ for 500 hours.

[0075] Example 6 In this embodiment, ZnO is modified with methacryloyloxypropyltrimethoxysilane (MPS).

[0076] MPS was added to the ZnO dispersion at a mass ratio of 3%.

[0077] During stirring, trimethoxysilane undergoes partial hydrolysis to generate Si-OH, which further condenses with the hydroxyl groups on the ZnO surface to form Zn-O-Si bonds.

[0078] After spin coating, the film was irradiated under 365nm ultraviolet light for 90s to initiate the polymerization of methacrylate double bonds.

[0079] The resulting organic-inorganic hybrid network exhibits excellent heat resistance and interfacial bonding. The device retains over 85% of its performance after being stored in a high-temperature, high-humidity environment for 1000 hours.

[0080] Example 7 In this embodiment, azidoethylphosphonic acid is used as a bifunctional ligand.

[0081] Azide ethylphosphonic acid was added to the ZnO nanoparticle dispersion at 4 wt% of the ZnO mass.

[0082] After spin coating, the film was irradiated with 254nm ultraviolet light for 30s.

[0083] The azide group decomposes under light to produce a nitrogen-based reactive intermediate, which further undergoes insertion and cross-linking reactions with surrounding molecules.

[0084] The formed cross-linked layer has a high cross-linking density, which can effectively suppress the aggregation of ZnO particles. The brightness retention rate of the device increases by approximately 35% after 100 hours of continuous operation.

[0085] Example 8 In this embodiment, ZnO is modified with diarylethylene carboxylic acid ligand.

[0086] 4-(4-carboxyphenyl)diarylethylene was added to the ZnO dispersion at a dosage of 5 wt%.

[0087] After spin coating to form the electron transport layer, the electron transport layer is irradiated with 365nm ultraviolet light for 120s.

[0088] Diarylethylene undergoes a reversible photocyclization reaction and forms a partially cross-linked structure.

[0089] Due to the excellent photostability of diarylethene, the electron transport layer formed remains structurally stable under long-term illumination. The resulting device achieves an initial external quantum efficiency of over 20% and a lifetime approximately four times greater than the comparative example.

[0090] Comparative Example 1 The electron transport layer was prepared directly using ZnO nanoparticles without any surface modification, and the remaining processes were the same as in Example 1.

[0091] Due to the abundance of oxygen vacancies and hydroxyl defects on the ZnO surface, excessively rapid electron injection leads to a severe imbalance between electrons and holes. This results in a significant efficiency roll-off, with an external quantum efficiency of only about 13% and a short lifetime.

[0092] Comparative Example 2 ZnO nanoparticles were modified with azobenzene carboxylic acid, but no photocuring treatment was performed after film formation. The rest of the process was the same as in Example 1.

[0093] Although surface defects are passivated to some extent, ZnO particles are prone to migration and aggregation due to the lack of a cross-linking network. The electron transport layer gradually deteriorates after continuous operation, resulting in limited lifetime improvement.

[0094] Comparative Example 3 An electron transport layer is formed by simply mixing a photosensitive polymer (polymethyl methacrylate grafted with cinnamic acid) without passivation groups with ZnO.

[0095] Although a cross-linked network can be formed after illumination, oxygen vacancies and surface defects still exist because they cannot form effective coordination with the ZnO surface. The device lifetime is slightly improved, but the electron injection imbalance problem is still serious, and both efficiency and lifetime are significantly lower than those of Examples 1-8.

[0096] As described above, this application provides an electron transport layer material for quantum dot light-emitting diodes, an electron transport layer, and a QLED device comprising the same. The electron transport layer material comprises n-type metal oxide nanoparticles (selected from ZnO, SnO2, TiO2, ZnMgO, IZO, and combinations thereof) and photosensitive ligands modified on the surface of the nanoparticles. The photosensitive ligands comprise anchoring groups (selected from one or more of carboxyl, phosphate, phosphonic, amino, and silyl groups) and photosensitive groups (selected from one or more of azophenyl, cinnamic acid, anthracene, diarylvinyl, azide, acrylate, and methacrylate groups).

[0097] Photosensitive ligands bind to the surface of n-type metal oxide nanoparticles through anchoring groups, covering unsaturated coordination sites, reducing defect state density, and decreasing nonradiative recombination losses. Through light and / or heat treatment, photosensitive ligands undergo cross-linking reactions to form a cross-linked network structure. This organic insulating network forms a physical barrier between nanoparticles, reducing the effective electron mobility of the electron transport layer, promoting carrier injection equilibrium, and suppressing efficiency roll-off. The cross-linked network structure enhances the environmental stability of the electron transport layer, reducing water and oxygen erosion of the nanoparticle surface and ligand detachment, thus helping to extend device lifespan. After the anchoring groups bond to the nanoparticle surface, the energy level positions are modulated through the interfacial dipole effect, optimizing the electron injection barrier and assisting in achieving balanced carrier injection.

[0098] This application also provides a method for preparing an electron transport layer, comprising: preparing an n-type metal oxide nanoparticle dispersion; adding a photosensitive ligand to the dispersion, and binding the photosensitive ligand to the nanoparticles through ligand exchange or blending; depositing the resulting dispersion on a substrate surface to form a precursor film; subjecting the precursor film to photo-irradiation (wavelength 250 nm to 450 nm) and / or heat treatment (temperature not exceeding 120 °C) to induce a crosslinking or polymerization reaction of the photosensitive ligand to form an electron transport layer. The deposition method includes one or more of spin coating, spray coating, blade coating, inkjet printing, and photolithography.

[0099] This application modifies the surface of n-type metal oxide nanoparticles with photosensitive ligands and forms a cross-linked network structure through photo- or thermal treatment after film formation. This achieves multiple technical effects, including surface defect passivation, reduced electron mobility, improved film stability, and energy level modulation. As a result, it improves the carrier balance injection of quantum dot light-emitting diodes, enhances the luminous efficiency and operational stability of the device, and provides a feasible technical solution for solving the problems of charge accumulation and efficiency roll-off caused by excessive electron injection in QLEDs.

[0100] The above is only one specific implementation of this application, and any other improvements made based on the concept of this application shall be considered within the scope of protection of this application.

Claims

1. An electron transport layer material, characterized in that, It includes n-type metal oxide nanoparticles and photosensitive ligands modified on the surface of the n-type metal oxide nanoparticles; The photosensitive ligand includes an anchoring group capable of binding to the surface of the n-type metal oxide nanoparticles and a photosensitive group capable of undergoing cross-linking, polymerization, or photochemical reactions under light and / or heating conditions. The anchoring group is selected from one or more of carboxyl, phosphate, phosphonic acid, amino, and silane groups; The photosensitive group is selected from one or more of azophenyl, cinnamic acid group, anthracene group, diarylvinyl group, azido group, acrylate group, and methacrylate group.

2. The electron transport layer material according to claim 1, characterized in that, The materials of the n-type metal oxide nanoparticles are selected from ZnO, SnO2, TiO2, ZnMgO, IZO and combinations thereof.

3. The electron transport layer material according to claim 1 or 2, characterized in that, The photosensitive ligand is selected from one or more of the following: carboxylated azobenzene derivatives, phosphate-containing azobenzene derivatives, cinnamic acid derivatives, anthracene derivatives, diarylethylene derivatives, azide derivatives, and silane-containing photosensitive monomers.

4. The electron transport layer material according to claim 3, characterized in that, The photosensitive ligands are bound to the surface of n-type metal oxide nanoparticles via ligand exchange or dispersed between n-type metal oxide nanoparticles via blending.

5. An electron transport layer, characterized in that, The electron transport layer is obtained by forming a cross-linked network structure from the electron transport layer material according to any one of claims 1 to 4 after film formation by light irradiation and / or heat treatment; The n-type metal oxide nanoparticles are dispersed in the cross-linked network structure.

6. The electron transport layer according to claim 5, characterized in that, The thickness of the electron transport layer is 10 nm to 60 nm.

7. A quantum dot light-emitting diode device, characterized in that, It includes a substrate, an anode, a hole injection layer, a hole transport layer, a quantum dot light-emitting layer, an electron transport layer, and a cathode arranged sequentially. The electron transport layer is the electron transport layer described in claim 5 or 6.

8. The quantum dot light-emitting diode device according to claim 7, characterized in that, The hole injection layer includes one or more of MoOx, VOx, WOx, CrOx, CuO, MoS2, MoSe2, WS2, WSe2, CuS, and PEDOT; The hole transport layer includes one or more of PVK, TFB, and PTAA; The quantum dot light-emitting layer includes one or more of group II-VI and group III-V semiconductor nanocrystals.

9. A method for preparing an electron transport layer, characterized in that, The steps include the following: S1. Preparation of n-type metal oxide nanoparticle dispersion; S2. Add a photosensitive ligand to the n-type metal oxide nanoparticle dispersion, so that the photosensitive ligand binds to the n-type metal oxide nanoparticles through ligand exchange or blending. S3. Deposit the obtained dispersion onto the substrate surface to form an electron transport layer precursor film; S4. The electron transport layer precursor film is subjected to light irradiation and / or heat treatment to cause the photosensitive ligand to undergo cross-linking or polymerization reactions, thereby forming an electron transport layer.

10. The preparation method according to claim 9, characterized in that, The illumination process uses a light source with a wavelength of 250nm to 450nm; And / or, The heat treatment temperature shall not exceed 120°C; The deposition method includes one or more of spin coating, spray coating, blade coating, inkjet printing, and photolithography.

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