Quantum dot film, preparation method thereof and light-emitting device

Abstract

The invention discloses a quantum dot film and a preparation method thereof, a light-emitting device and a display device, the quantum dot film comprises multiple sub-film layers, the multiple sub-film layers comprise a first sub-film layer, a second sub-film layer,..., an (N-1) th sub-film layer and an Nth sub-film layer which are sequentially laminated, N is an integer greater than or equal to 2, and n is an integer greater than or equal to 1. The direction from the first sub-film layer to the Nth sub-film layer is defined as a first direction; materials of the sub-film layers respectively and independently comprise quantum dots and ligands connected to the surfaces of the quantum dots, and the intrinsic dipole moment of the ligands contained in the sub-film layers is gradually reduced in the first direction. The energy levels of the quantum dot thin film are in gradient distribution, so that the quantum dot thin film can be suitable for being used between two film layers with large potential barrier difference, and the effect of adjusting the energy levels and potential barriers is achieved.

Description

Technical Field

[0001] The present application relates to the field of semiconductor materials, and particularly to a quantum dot thin film, a preparation method thereof, and a light-emitting device. Background Art

[0002] Quantum dot light-emitting diodes (QLEDs) use inorganic quantum dots as light-emitting materials, and have the advantages of wide color gamut coverage, high color purity, ultra-thin and light weight, bendable and rollable, etc., and have received extensive attention.

[0003] At present, most of the existing QLED devices have the problem of unbalanced carrier injection, which greatly affects the lifespan of the devices. Summary of the Invention

[0005] In view of this, the present application provides a quantum dot thin film, a preparation method thereof, and a light-emitting device.

[0006] The embodiments of the present application are implemented as follows:

[0007] In a first aspect, an embodiment of the present application provides a quantum dot thin film, including multiple sub-film layers, the multiple sub-film layers including a first sub-film layer, a second sub-film layer... an (N-1)th sub-film layer, and an Nth sub-film layer that are sequentially stacked, where N is an integer greater than or equal to 2, and a direction from the first sub-film layer to the Nth sub-film layer is defined as a first direction;

[0008] The materials of the sub-film layers each independently include quantum dots and ligands connected to the surfaces of the quantum dots, and along the first direction, the intrinsic dipole moments of the ligands contained in the sub-film layers gradually decrease.

[0009] In a second aspect, the present application provides a method for preparing a quantum dot thin film, including the following steps:

[0010] Prepare multiple sub-film layers in sequence according to a preset direction to obtain a quantum dot thin film, the multiple sub-film layers including a first sub-film layer, a second sub-film layer... an (N-1)th sub-film layer, and an Nth sub-film layer that are stacked, where N is an integer greater than or equal to 2, and the preset direction is a first direction along from the first sub-film layer to the Nth sub-film layer, or a second direction from the Nth sub-film layer to the first sub-film layer;

[0011] The materials of the sub-film layers each independently include quantum dots and ligands connected to the surfaces of the quantum dots, and along the first direction, the intrinsic dipole moments of the ligands contained in the sub-film layers gradually decrease.

[0012] In a third aspect, the present application provides a light-emitting device, including an anode, a light-emitting layer, and a cathode, where the light-emitting layer includes the quantum dot thin film described above, or is prepared by the preparation method described above;

[0013] The first sub-film layer is disposed close to the anode.

[0014] The quantum dot thin film of the present application is arranged as a multi-layer film, and the intrinsic dipole moments of the ligands contained in the multi-layer film gradually decrease along the first direction, which plays a role in adjusting the energy level barrier; when it is applied in a light-emitting device, it helps to improve the carrier balance, thereby improving the light-emitting efficiency and lifespan of the device. Description of the Drawings

[0015] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the following drawings are only some embodiments of the present application. For those skilled in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0016] Figure 1 It is a schematic structural diagram of an embodiment of a light-emitting device provided by the present application;

[0017] Figure 2 It is a schematic structural diagram of another embodiment of a light-emitting device provided by the present application;

[0018] Reference numerals: Light-emitting device 100; Anode 10; Cathode 20; Electron transport layer 30; Light-emitting layer 40; Sub-film layer 41; Single-layer film 42; Hole injection layer 50; Hole transport layer 60. Detailed Embodiments

[0019] Next, the technical solutions in the embodiments of the present application will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative efforts belong to the scope of protection of the present application. In addition, it should be understood that the specific implementation manners described herein are only used to illustrate and explain the present application, and are not used to limit the present application. In the present application, unless otherwise stated, the orientation terms such as "upper" and "lower" specifically refer to the drawing directions in the drawings. In addition, in the description of the present application, the term "including" means "including but not limited to". The various embodiments of the present application may exist in the form of a range; it should be understood that the description in the form of a range is only for convenience and brevity, and should not be construed as a rigid limitation on the scope of the present application; therefore, it should be considered that the described range description has specifically disclosed all possible sub-ranges and single values within that range. For example, it should be considered that the range description from 1 to 6 has specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., and single numbers within that range, such as 1, 2, 3, 4, 5, and 6, regardless of the range. In addition, whenever a numerical range is indicated herein, it means including any cited number (fraction or integer) within the indicated range.

[0020] In the present application, "and / or" describes the association relationship of associated objects, indicating that three relationships may exist. For example, A and / or B may represent: A exists alone, A and B exist simultaneously, and B exists alone. Wherein A and B may be singular or plural.

[0021] In the present application, "at least one" means one or more, and "a plurality" means two or more. "At least one kind", "at least one of the following items (pieces)" or similar expressions refer to any combination of these items, including any combination of single items (pieces) or plural items (pieces). For example, "at least one of a, b, or c", or, "at least one of a, b, and c" can both represent: a, b, c, a - b (that is, a and b), a - c, b - c, or a - b - c, where a, b, and c can be single or multiple respectively.

[0022] In a first aspect, an embodiment of the present application provides a quantum dot thin film. Please refer to Figure 1, the quantum dot thin film includes multiple sub-film layers 41, and the multiple sub-film layers 41 include a first sub-film layer, a second sub-film layer... an (N - 1)th sub-film layer and an Nth sub-film layer which are stacked in sequence, where N is an integer greater than or equal to 2. The materials of the sub-film layers 41 each independently include quantum dots and ligands connected to the surfaces of the quantum dots. In this article, the direction from the first sub-film layer to the Nth sub-film layer is defined as the first direction. Along the first direction, the intrinsic dipole moment of the ligands contained in the sub-film layer 41 gradually decreases.

[0023] The "first direction" described in this application is the direction in which the film layers of the quantum dot thin film are stacked. Specifically, taking the orientation when the quantum dot thin film is laid flat on a horizontal plane as an example, the multiple sub-film layers 41 can be composed of a first sub-film layer, a second sub-film layer... an (N - 1)th sub-film layer and an Nth sub-film layer that are stacked from bottom to top in sequence. At this time, the first direction is the direction from bottom to top; the multiple sub-film layers 41 can also be composed of an Nth sub-film layer, an (N - 1)th sub-film layer... a second sub-film layer and a first sub-film layer that are stacked from bottom to top in sequence. At this time, the first direction is the direction from top to bottom. The quantum dot thin film can be applied in the light-emitting device 100 as the light-emitting layer 40; for example, please refer to Figure 1 , the quantum dot thin film is composed of two film layers stacked from bottom to top (from the anode 10 to the cathode 20), and its first direction is the direction from bottom to top.

[0024] The quantum dot thin film in the embodiment of this application has multiple sub-film layers 41, and the material of each sub-film layer 41 includes quantum dots and ligands connected to the surfaces of the quantum dots. It can be understood that the quantum dots contained in the multiple sub-film layers 41 can be the same or different; the ligands contained in the multiple sub-film layers 41 are distributed according to the following rule: along the first direction, the intrinsic dipole moment of the ligands contained in the multiple sub-film layers 41 gradually decreases. In the quantum dot thin film provided in some embodiments, in the first direction, the intrinsic dipole moment of the ligands contained in any one film layer of the previous sub-film layer 41 is greater than the intrinsic dipole moment of the ligands contained in any one film layer of the next sub-film layer 41. In this way, the energy band position of the previous sub-film layer 41 moves up, and the energy band position of the next sub-film layer 41 moves down, so that the energy levels of the quantum dot thin film are distributed in a gradient.

[0025] The quantum dot thin film of the present application is configured as a multi-layer film, and the intrinsic dipole moments of the ligands contained in the multi-layer film gradually decrease along the first direction, so that the energy levels of the quantum dot thin film are distributed in a gradient manner, enabling this quantum dot thin film to be applicable between two film layers with a large potential barrier difference and playing a role in adjusting the energy level potential barrier. When applied in the light-emitting device 100, it can play a role in reducing the carrier injection potential barrier between the light-emitting layer 40 and the adjacent two film layers on both sides, helping to promote carrier transport, increasing the exciton density in the light-emitting layer 40, improving carrier balance, and thus enhancing the light-emitting efficiency and lifespan of the device.

[0026] Please refer to Figure 2 , in some embodiments, the sub-film layer 41 can be a single film layer or composed of multiple single film layers 42. For example, each sub-film layer 41 can include two, three, or more than three single film layers 42; it can be understood that the number of single film layers 42 contained in each of the multiple sub-film layers 41 can be the same or different. For example, in one embodiment, the quantum dot thin film includes three sub-film layers 41, namely the first sub-film layer, the second sub-film layer, and the third sub-film layer. Among them, the first sub-film layer is composed of one single film layer 42, the second sub-film layer is composed of two single film layers 42, and the third sub-film layer is composed of three single film layers 42. The total number of single film layers 42 contained in the quantum dot thin film is 6; in another embodiment, the quantum dot thin film includes three sub-film layers 41, namely the first sub-film layer, the second sub-film layer, and the third sub-film layer. Among them, the first sub-film layer is composed of two single film layers 42, the second sub-film layer is composed of two single film layers 42, and the third sub-film layer is composed of two single film layers 42. The total number of single film layers 42 contained in the quantum dot thin film is 5. Further, in some embodiments, the single film layer 42 can be a film formed by single-layer quantum dots, and the ratio of its thickness to the average particle size of the quantum dots used can be 0.9 - 1.1, for example, 0.9, 0.95, 0.98, 1, 1.1, and values between any two of the above.

[0027] When the sub-film layer 41 has a plurality of single-layer films 42, in the sub-film layer 41, the intrinsic dipole moments of the ligands contained in each of the plurality of single-layer films 42 may all be equal or may not be equal. Specifically, in some embodiments, in the sub-film layer 41, the intrinsic dipole moments of the ligands contained in the plurality of single-layer films 42 are equal. In this way, the energy band positions are the same between any two adjacent single-layer films 42, but there is an energy level difference between two adjacent sub-film layers 41. In other embodiments, in the sub-film layer 41, along the first direction, the intrinsic dipole moments of the ligands contained in the plurality of single-layer films 42 gradually decrease. In this way, not only is there an energy level difference between two adjacent sub-film layers 41, but there is also an energy level difference between any two adjacent single-layer films 42. Along the first direction, the intrinsic dipole moment of the ligand gradually becomes smaller from the first layer to the last layer, that is, the energy band positions of the quantum dot thin film are distributed in a gradually decreasing manner layer by layer. Thus, when the prepared quantum dot thin film is used as the light-emitting layer 40, the energy level barriers between the light-emitting layer 40 and the adjacent film layers (the light-emitting layer 40 and the electron transport layer 30, and the light-emitting layer 40 and the hole transport layer 60) can be effectively adjusted, the carrier transport can be promoted to obtain a high exciton density, and high-efficiency device performance can be achieved. At the same time, the carrier balance of the formed device is easier to control, which is of great significance for improving the performance of long-life devices.

[0028] In some embodiments, in the quantum dot thin film, the total number of the sub-film layers 41 is 2 to 8; for example, the number can be 2, 3, 4, 5, 6, 7, or 8.

[0029] In some embodiments, the average thickness of the sub-film layer 41 is 5 to 20 nm; for example, it can be 5 to 7 nm, 6 to 9 nm, 8 to 10 nm, 9 to 13 nm, 12 to 15 nm, 13 to 16 nm, 14 to 18 nm, 17 to 20 nm, and so on.

[0030] In some embodiments, the average thickness of the quantum dot thin film is 10 to 100 nm; for example, it can be 10 to 20 nm, 15 to 25 nm, 24 to 35 nm, 30 to 40 nm, 38 to 46 nm, 45 to 55 nm, 53 to 68 nm, 67 to 75 nm, 74 to 85 nm, 83 to 95 nm, 94 to 100 nm, and so on.

[0031] In some embodiments, among the multiple sub-film layers 41, the absolute value of the difference in valence band energy levels between two adjacent sub-film layers 41 is less than or equal to 0.2 eV. For example, it can be 0 eV, 0.001 eV, 0.1 eV, 0.15 eV, 0.2 eV, and values between any two of the above. Since each sub-film layer 41 has the same or different intrinsic dipole moment ligands, the energy level difference between two adjacent sub-film layers 41 is 0 or greater than 0. In this embodiment, controlling the energy level difference within a range less than or equal to 0.2 eV helps reduce the potential barrier when carriers transport in the film layer, which is beneficial to the flow of carriers within the quantum dot thin film. When the quantum dot thin film is used as the light-emitting layer 40 of the light-emitting device 100, it helps the flow of carriers between the light-emitting layers 40.

[0032] The "intrinsic dipole moment" described in this application refers to the product of the distance r between the positive and negative charge centers and the electric charge q carried by the charge center, which is approximately equal to half of the magnitude of the electric moment caused by the positive and negative charge distribution in the molecule and can characterize the unevenness of the positive and negative charge distribution in the molecule. Generally speaking, the dipole moment is related to the polarity of the molecule. The greater the polarity, the greater the dipole moment. In some embodiments, the ligand is derived from a ligand compound, and the ligand compound includes one or more of a thiol compound, a primary amine compound, a thiocyanate compound, and an organic halide. Among them, the thiol compound is connected to the surface of the quantum dot through a sulfur atom, and the corresponding ligand can be the group formed when a hydrogen atom is removed from the thiol compound; the primary amine compound is connected to the surface of the quantum dot through a nitrogen atom, and the corresponding ligand can be the group formed when a hydrogen atom is removed from the primary amine compound; the ligand corresponding to the thiocyanate compound is a thiocyanate group; the ligand corresponding to the organic halide is a halogen atom, such as F, Cl, Br, or I. By utilizing the interfacial polarization effect between the quantum dot and the surface ligand, the energy band position of the quantum dot in the light-emitting layer 40 is adjusted, effectively adjusting the energy level barrier with the adjacent charge transport layer, promoting carrier transport to obtain a high exciton density, and achieving high-efficiency device performance. In addition, since such ligands have relatively smaller steric hindrance compared to the initial ligand (the initial ligand will not be explained here for the time being and will be elaborated later), it is beneficial to improve the surface defect passivation effect of the quantum dot, thereby enhancing the PLQY and fluorescence stability of the thin film; in addition, the introduction of such ligands helps to adjust the solubility of the quantum dot, making it have a certain solvent resistance, so that when constructing a multi-layer film, it can be formed smoothly and avoid damaging the previous film layer.

[0033] In some embodiments, the thiol compound includes one or more of benzenethiol, 1,2-benzenedithiol, 1,3-benzenedithiol, 1,4-benzenedithiol, 1,2-ethanethiol, 3-mercaptopropionic acid; the primary amine compound includes one or more of 1,2-ethylenediamine, ethylamine; the thiocyanate compound includes one or more of ammonium thiocyanate, potassium thiocyanate, sodium thiocyanate, cobalt thiocyanate, cuprous thiocyanate; the organic halide includes one or more of tetrabutylammonium iodide, tetrabutylammonium bromide, tetrabutylammonium chloride, tetrabutylammonium fluoride, trityl bromide, trityl chloride, and trityl iodide.

[0034] Further, the intrinsic dipole moment of the above-listed ligand compounds is sorted as follows: benzenethiol (BT), ethylamine, 1,2-ethylenediamine (EDA), 3-mercaptopropionic acid (MPA), 1,2-ethanethiol (EDT), tetrabutylammonium fluoride (TBAF), 1,2-benzenedithiol (1,2-BDT), 1,3-benzenedithiol (1,3-BDT), 1,4-benzenedithiol (1,4-BDT), ammonium thiocyanate, potassium thiocyanate, sodium thiocyanate, cobalt thiocyanate, cuprous thiocyanate, tetrabutylammonium chloride (TBACl), trityl chloride, tetrabutylammonium iodide (TBAI), trityl iodide, tetrabutylammonium bromide (TBABr), trityl bromide, and their intrinsic dipole moments decrease in turn.

[0035] In some embodiments, the average particle size of the quantum dots is 5 to 15 nm; for example, it can be 5 to 7 nm, 6 to 9 nm, 8 to 10 nm, 9 to 13 nm, 12 to 15 nm, 13 to 15 nm, and so on.

[0036] In some embodiments, the quantum dots are selected from at least one of single-structure quantum dots, core-shell structure quantum dots, and perovskite semiconductor materials. The shell layer of the core-shell structure quantum dots includes one or more layers. Among them, the single-structure quantum dots are selected from at least one of II-VI group compounds, IV-VI group compounds, III-V group compounds, and I-III-VI group compounds. The II-VI group compounds are selected from at least one of CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, ZnO, HgS, HgSe, HgTe, CdSeS, CdSeTe, CdSTe, ZnSeS, ZnSeTe, ZnSTe, HgSeS, HgSeTe, HgSTe, CdZnS, CdZnSe, CdZnTe, CdHgS, CdHgSe, CdHgTe, HgZnS, HgZnSe, HgZnTe, CdZnSeS, CdZnSeTe, CdZnSTe, CdHgSeS, CdHgSeTe, CdHgSTe, HgZnSeS, HgZnSeTe, and HgZnSTe. The IV-VI group compounds are selected from at least one of SnS, SnSe, SnTe, PbS, PbSe, PbTe, SnSeS, SnSeTe, SnSTe, PbSeS, PbSeTe, PbSTe, SnPbS, SnPbSe, SnPbTe, SnPbSSe, SnPbSeTe, and SnPbSTe. The III-V group compounds are selected from at least one of GaN, GaP, GaAs, GaSb, AlN, AlP, AlAs, AlSb, InN, InP, InAs, InSb, GaNP, GaNAs, GaNSb, GaPAs, GaPSb, AlNP, AlNAs, AlNSb, AlPAs, AlPSb, InNP, InNAs, InNSb, InPAs, InPSb, GaAlNP, GaAlNAs, GaAlNSb, GaAlPAs, GaAlPSb, GaInNP, GaInNAs, GaInNSb, GaInPAs, GaInPSb, InAlNP, InAlNAs, InAlNSb, InAlPAs, and InAlPSb. The I-III-VI group compounds are selected from at least one of CuInS, CuInSe, and AgInS. The core of the core-shell structure quantum dots is selected from any one of the above single-structure quantum dots, and the shell layer material of the core-shell structure quantum dots is selected from at least one of CdS, CdTe, CdSeTe, CdZnSe, CdZnS, CdSeS, ZnSe, ZnSeS, and ZnS.

[0037] As an example, the quantum dots of the core-shell structure may be selected from but not limited to at least one of CdZnSe / CdZnSe / ZnSe / CdZnS / ZnS, CdZnSe / CdZnSe / CdZnS / ZnSCdSe / CdSeS / CdS, InP / ZnSeS / ZnS, CdZnSe / ZnSe / ZnS, CdSeS / ZnSeS / ZnS, CdSe / ZnS, CdSe / ZnSe / ZnS, ZnSe / ZnS, ZnSeTe / ZnS, CdSe / CdZnSeS / ZnS, and InP / ZnSe / ZnS.

[0038] It should be noted that for the materials of the aforementioned single-structure quantum dots, or the core materials of the core-shell structure quantum dots, or the shell materials of the core-shell structure quantum dots, the chemical formulas provided only indicate the elemental composition and do not indicate the content of each element. For example, CdZnSe only indicates that it is composed of three elements, Cd, Zn, and Se. If the content of each element is to be expressed, it corresponds to Cd x Zn1-xSe, where 0 < x < 1.

[0039] The perovskite semiconductor material is selected from doped or undoped inorganic perovskite semiconductors, or organic-inorganic hybrid perovskite semiconductors; the general structural formula of the inorganic perovskite semiconductor is AMX3, where A is a Cs + ion, M is a divalent metal cation, selected from at least one of Pb 2+ , Sn 2+ , Cu 2+ , Ni 2+ , Cd 2+ , Cr 2+ , Mn 2+ , Co 2+ , Fe 2+ , Ge 2+ , Yb 2+ , Eu 2+ , and X is a halogen anion, selected from at least one of Cl - , Br - , I - ; the general structural formula of the organic-inorganic hybrid perovskite semiconductor is BMX3, where B is an organic amine cation, selected from CH3(CH2) n-2 NH3 + or [NH3(CH2) n NH3] 2+ , where n ≥ 2, M is a divalent metal cation, selected from at least one of Pb 2+ , Sn 2+ , Cu 2+ , Ni 2+ , Cd 2+ , Cr2+ 、Mn 2+ 、Co 2+ 、Fe 2+ 、Ge 2+ 、Yb 2+ 、Eu 2+ at least one of, X is a halogen anion, selected from Cl - 、Br - 、I - at least one of.

[0040] In a second aspect, the present application also provides a method for preparing a quantum dot thin film, the preparation method comprising the following steps:

[0041] S100, preparing multiple sub-film layers 41 in sequence according to a preset direction to obtain a quantum dot thin film, the multiple sub-film layers 41 comprising a first sub-film layer, a second sub-film layer... a (N - 1)th sub-film layer and an Nth sub-film layer arranged in a stacked manner, where N is an integer greater than or equal to 2;

[0042] wherein, the preset direction is a first direction from the first sub-film layer to the Nth sub-film layer, or a second direction from the Nth sub-film layer to the first sub-film layer;

[0043] The materials of the multiple sub-film layers 41 each independently comprise quantum dots and ligands connected to the surfaces of the quantum dots, and along the first direction, the intrinsic dipole moments of the ligands contained in the multiple sub-film layers 41 gradually decrease.

[0044] Specifically, in some embodiments, the preparation method may be to prepare multiple sub-film layers 41 in sequence according to the first direction, specifically including: using a first quantum dot and a first ligand compound to prepare a first sub-film layer, the first sub-film layer containing the first quantum dot and the first ligand connected to the first quantum dot, the first ligand being derived from the first ligand compound; using a second quantum dot and a second ligand compound to prepare a second sub-film layer on the first sub-film layer, the second sub-film layer containing the second quantum dot and the second ligand connected to the second quantum dot, the second ligand being derived from the second ligand compound; and so on... Finally, using an Nth quantum dot and an Nth ligand compound to prepare an Nth sub-film layer on the (N - 1)th sub-film layer (N is an integer greater than 1), the Nth sub-film layer containing the Nth quantum dot and the Nth ligand connected to the Nth quantum dot, the Nth ligand being derived from the Nth ligand compound. The above ligand compounds satisfy: the intrinsic dipole moment of the first ligand compound, the intrinsic dipole moment of the second ligand compound... the intrinsic dipole moment of the Nth ligand compound gradually decreases, that is, the intrinsic dipole moment of the Nth ligand compound is less than the intrinsic dipole moment of the (N - 1)th ligand compound.

[0045] In some other embodiments, the preparation method may be to sequentially prepare multiple sub-film layers 41 in the second direction, specifically including: using the Nth quantum dots and the Nth ligand compound to prepare the Nth sub-film layer, where the Nth sub-film layer contains the Nth quantum dots and the Nth ligands connected to the Nth quantum dots, and the Nth ligands are derived from the Nth ligand compound; using the (N - 1)th quantum dots and the (N - 1)th ligand compound to prepare the (N - 1)th sub-film layer on the Nth sub-film layer, where the (N - 1)th sub-film layer contains the (N - 1)th quantum dots and the (N - 1)th ligands connected to the (N - 1)th quantum dots, and the (N - 1)th ligands are derived from the (N - 1)th ligand compound; and so on... Using the first quantum dots and the first ligand compound to prepare the first sub-film layer on the second sub-film layer, where the first sub-film layer contains the first quantum dots and the first ligands connected to the first quantum dots, and the first ligands are derived from the first ligand compound. The above ligand compounds satisfy that the intrinsic dipole moment of the first ligand compound, the intrinsic dipole moment of the second ligand compound... the intrinsic dipole moment of the Nth ligand compound gradually decreases, that is, the intrinsic dipole moment of the Nth ligand compound is less than the intrinsic dipole moment of the (N - 1)th ligand compound.

[0046] In some embodiments, the preparation method of the nth sub-film layer (n is a positive integer) may be: providing quantum dots with the nth ligands connected to their surfaces, and depositing the quantum dots with the nth ligands connected to their surfaces to form the nth sub-film layer.

[0047] In some other embodiments, the preparation method of the nth sub-film layer (n is a positive integer) may be: providing the nth first solid film containing the nth quantum dots; providing the nth ligand compound, contacting the nth first solid film with the nth ligand compound for ligand exchange to obtain the nth sub-film layer. Specifically, in implementation, the nth first solid film can be immersed in a mixed solution containing the nth ligand compound and left standing for a period of time for ligand exchange; alternatively, the above mixed solution can be spin-coated on the surface of the nth first solid film, left standing for a period of time for ligand exchange, and then washed with a polar solvent. Compared with the method of first performing ligand exchange to prepare quantum dot materials and then depositing them into a film, the method of solid film ligand exchange can avoid the problem of the decline in film-forming effect caused by the improved anti-solvent property of quantum dots after ligand exchange, helps to improve the film-forming effect, gives full play to the characteristics of quantum dots after ligand exchange, and enables the film to have a high PLQY, fluorescence stability, and carrier mobility.

[0048] Among them, the solvent in the mixed solution can be a polar solvent, and the polar solvent can be one or more of C1-C5 alcohol solvents, including but not limited to one or more of methanol, ethanol, isopropanol, n-butanol, and pentanol. The standing time is 3-15 min; for example, it can be 3 min, 4 min, 5 min, 6 min, 7 min, 8 min, 9 min, 10 min, 11 min, 12 min, 13 min, 14 min, 15 min, and values between any two of the above.

[0049] Among them, the nth first solid film containing the nth quantum dot can be prepared by the following method: Disperse the nth quantum dot in a non-polar solvent to form a quantum dot solution, deposit the quantum dot solution, and after forming a liquid film, remove the solvent to obtain the nth first solid film containing the nth quantum dot. The non-polar solvent can be one or more of alkane solvents with 5-16 carbon atoms, for example, it can be one or more of n-pentane, n-hexane, n-heptane, n-octane, tridecane, tetradecane, cyclooctane, cycloheptane, cyclohexane, cyclopentane, 2-methyloctane, 3-ethylheptane, 2,2-dimethyloctane, 1-cyclohexyldecane. The method for removing the solvent can be heating to volatilize the solvent, or placing the liquid film in a flowing gas environment to accelerate the solvent volatilization, or allowing the liquid film to stand in a vacuum environment to volatilize the solvent.

[0050] In some embodiments, in the quantum dot solution, the concentration of the quantum dot is 5-50 mg / mL; for example, it can be 5 mg / mL, 6 mg / mL, 7 mg / mL, 8 mg / mL, 9 mg / mL, 10 mg / mL, 11 mg / mL, 12 mg / mL, 13 mg / mL, 14 mg / mL, 15 mg / mL, 18 mg / mL, 20 mg / mL, 25 mg / mL, 30 mg / mL, 35 mg / mL, 40 mg / mL, 45 mg / mL, 50 mg / mL, and values between any two of the above.

[0051] To ensure the dispersibility of quantum dots, initial ligands are usually connected to the surface of the quantum dots. The initial ligands can be derived from at least one of substituted or unsubstituted C10-C24 chain organic carboxylic acids, substituted or unsubstituted C10-C24 chain organic amines, substituted or unsubstituted C10-C24 chain organic phosphines, and substituted or unsubstituted C10-C24 chain organic phosphine oxides. Among them, "substituted or unsubstituted" means that the defined group can be substituted or not. When the defined group is substituted, it should be understood that the defined group can be substituted by one or more substituents; the substituents are selected from at least one of C1-C8 alkyl groups, C1-C8 alkoxy groups, and halogens; optionally, the substituents are selected from at least one of C1-C5 alkyl groups, C1-C5 alkoxy groups, and halogens; or the substituents are selected from at least one of C1-C3 alkyl groups, C1-C3 alkoxy groups, and halogens. Specifically, the organic carboxylic acid includes one or more of decanoic acid (CAS: 334-48-5), undecylenic acid (CAS: 112-38-9), myristic acid (CAS: 544-63-8), oleic acid (CAS: 112-80-1), and stearic acid (CAS: 57-11-4); the organic amine includes one or more of oleylamine (CAS: 112-90-3) and octadecylamine (CAS: 124-30-1); the organic (oxy)phosphine includes one or more of trioctylphosphine (CAS: 4731-53-7) and trioctyloxophosphine (CAS: 78-50-2). In the preparation method of the present application, by contacting the corresponding ligand compound with the solid film, ligand exchange occurs between the initial ligand on the surface of the quantum dots in the solid film and the ligand compound, and the ligand compound replaces the initial ligand and is connected to the surface of the quantum dots.

[0052] It can be understood that the "solution" described in the present application can be a mixed system in which the material is completely dissolved in the solvent, or a dispersion system in which the material is dispersed in the solvent in a dispersed state. The concentration of the solution refers to the concentration of the material in the entire system.

[0053] In some embodiments, the ligand is derived from a ligand compound, and the ligand compound includes one or more of a thiol compound, a primary amine compound, a thiocyanate compound, and an organic halide; the thiol compound includes one or more of benzenethiol, 1,2-benzenedithiol, 1,3-benzenedithiol, 1,4-benzenedithiol, 1,2-ethanethiol, and 3-mercaptopropionic acid; the primary amine compound includes one or more of 1,2-ethylenediamine and ethylamine; the thiocyanate compound includes one or more of ammonium thiocyanate, potassium thiocyanate, sodium thiocyanate, cobalt thiocyanate, and cuprous thiocyanate; the organic halide includes one or more of tetrabutylammonium iodide, tetrabutylammonium bromide, tetrabutylammonium chloride, tetrabutylammonium fluoride, trityl bromide, trityl chloride, and trityl iodide. Moreover, the intrinsic dipole moments of the ligand compounds are sorted in descending order as follows: benzenethiol, ethylamine, 1,2-ethylenediamine, 3-mercaptopropionic acid, 1,2-ethanethiol, tetrabutylammonium fluoride, 1,2-benzenedithiol, 1,3-benzenedithiol, 1,4-benzenedithiol, ammonium thiocyanate, potassium thiocyanate, sodium thiocyanate, cobalt thiocyanate, cuprous thiocyanate, tetrabutylammonium chloride, trityl chloride, tetrabutylammonium iodide, trityl iodide, tetrabutylammonium bromide, and trityl bromide.

[0054] Among them, when the thiocyanate compound and the organic halide are connected to the quantum dots, they help to improve the anti-solvent property and can be used as the ligand compounds for the first to the Nth sub-film layers; when the primary amine compound containing only one amino group and the thiol compound containing only one mercapto group are connected to the quantum dots, the solubility of the quantum dots is relatively good, and they can be used as the ligand compounds for the Nth sub-film layer (for example, in the process of sequentially preparing multiple film layers from bottom to top, the topmost layer); when the primary amine compound containing two or more amino groups and the thiol compound containing two or more mercapto groups are connected to the quantum dots, due to having more coordination groups, at least two quantum dots can be connected simultaneously, enabling the formation of a network between the quantum dots to construct an anti-solvent network, and such compounds can be used as the ligand compounds for the first to the Nth sub-film layers.

[0055] It should be noted that when the trityl halide is used as a ligand compound, after ligand exchange with the solid film, the film also needs to be treated with UV light. The conditions of the light treatment can be: the light wavelength is 254 - 450 nm, and the irradiation dose is 50 - 400 mJ / cm 2 。

[0056] In some embodiments, the sub-film layer 41 includes a plurality of single-layer films 42 stacked in sequence along a first direction. Thus, when preparing each sub-film layer 41, the preparation steps of the sub-film layer 41 can be: sequentially preparing multiple single-layer films 42 along a preset direction to obtain the sub-film layer 41.

[0057] In some embodiments, in the sub-membrane layer 41, the intrinsic dipole moments of the ligands contained in the multiple single-layer membranes 42 are equal; correspondingly, the preparation of the sub-membrane layer 41 specifically includes: sequentially preparing multiple single-layer membranes 42 along a preset direction to obtain the sub-membrane layer 41, and the intrinsic dipole moments of the ligands contained in the multiple single-layer membranes 42 are equal, that is, after each single-layer membrane 42 is made into a solid film, the same ligand compound is contacted with the corresponding solid film for ligand exchange to make the single-layer membrane 42.

[0058] In the sub-membrane layer 41, along the first direction, the intrinsic dipole moments of the ligands contained in the multiple single-layer membranes 42 gradually decrease; correspondingly, the preparation of the sub-membrane layer 41 specifically includes: sequentially preparing multiple single-layer membranes 42 along a preset direction to obtain the sub-membrane layer 41, wherein the intrinsic dipole moments of the ligands contained in the multiple single-layer membranes 42 gradually decrease along the first direction. The preparation of the multiple single-layer membranes 42 can refer to the preparation steps of the above multiple sub-membrane layers 41. Specifically, when preparing the Mth single-layer membrane (M is an integer greater than 1), first use the Mth quantum dot to prepare the Mth sub-solid film on the (M - 1)th single-layer membrane, contact the Mth sub-solid film with the Mth ligand compound for ligand exchange to obtain the Mth single-layer membrane; the Mth single-layer membrane contains the Mth quantum dot and the Mth ligand connected to the Mth quantum dot, and the Mth ligand is derived from the Mth ligand compound. Assuming the direction from the first single-layer membrane to the Mth single-layer membrane is the first direction, the intrinsic dipole moment of the Mth ligand compound is less than that of the (M - 1)th ligand compound.

[0059] In a third aspect, the present application also proposes a light-emitting device 100, which can be a QLED, etc. Please refer to Figure 1 , the light-emitting device 100 includes an anode 10, a light-emitting layer 40, and a cathode 20. The light-emitting layer 40 includes the quantum dot thin film as described above or is prepared by the preparation method as described above; assuming that the quantum dot thin film has a first side and a second side opposite to each other in the first direction, and the first direction is set as the direction from the first side to the second side (i.e., the direction from the anode 10 to the cathode 20 in the figure), then, when preparing the quantum dot thin film, the first side faces the anode 10.

[0060] In some embodiments, the light-emitting device 100 further includes a hole functional layer disposed between the light-emitting layer 40 and the anode 10. The hole functional layer includes, but is not limited to, one or both of a hole transport layer 60 and a hole injection layer 50. When the hole functional layer includes the hole transport layer 60 and the hole injection layer 50, the hole transport layer 60 and the hole injection layer 50 are stacked, and the hole injection layer 50 is located between the hole transport layer 60 and the anode 10. The hole functional layer can be prepared from hole functional materials known in the art for optoelectronic devices and having hole transport performance or hole injection performance. Specifically, the materials of the hole transport layer 60 include, but are not limited to, 4,4'-N,N'-dicarbazolyl-biphenyl (CBP), poly[(9,9'-dioctylfluorene-2,7-diyl)-co-(4,4'-(N-(4-sec-butylphenyl)diphenylamine))] (TFB), N,N'-diphenyl-N,N'-bis(1-naphthyl)-1,1'-biphenyl-4,4”-diamine (α-NPD), N,N'-diphenyl-N,N'-bis(3-methylphenyl)-(1,1'-biphenyl)-4,4'-diamine (TPD), N,N'-bis(3-methylphenyl)-N,N'-bis(phenyl)-spiro(spiro-TPD), N,N'-bis(4-(N,N'-diphenyl-amino)phenyl)-N,N'-diphenylbenzidine (DNTPD), tris(3-methylphenylphenylamino)-triphenylamine (m-MTDATA), poly[(9,9'-dioctylfluorene-2,7-diyl)-co-(4,4'-(N-(4-sec-butylphenyl)diphenylamine))] (TFB) and poly(4-butylphenyl-diphenylamine) (poly-TPD), polyaniline, polypyrrole, poly(p-phenylene vinylene) (PPV), poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV), poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylene vinylene] (MOMO-PPV), copper phthalocyanine, aromatic tertiary amines, polynuclear aromatic tertiary amines, 4,4'-bis(p-carbazolyl)-1,1'-biphenyl compounds, N,N,N',N'-tetraarylbenzidine, PEDOT:PSS and its derivatives, poly(N-vinylcarbazole) (PVK) and its derivatives, polymethacrylate and its derivatives, poly(9,9-octylfluorene) and its derivatives, poly(spirofluorene) and its derivatives, N,N'-bis(naphthalen-1-yl)-N,N'-diphenylbenzidine (NPB), spiro-NPB, or one or more of them.The material of the hole injection layer 50 includes but is not limited to at least one of 2,3,6,7,10,11-hexacyano-1,4,5,8,9,12-hexaazatriphenylene, PEDOT, PEDOT:PSS, derivatives of PEDOT:PSS doped with s-MoO3, 4,4',4'-tris(N-3-methylphenyl-N-phenylamino)triphenylamine, tetracyanoquinodimethane, copper phthalocyanine, nickel oxide, molybdenum oxide, tungsten oxide, vanadium oxide, molybdenum sulfide, tungsten sulfide, and copper oxide.

[0061] The light-emitting device 100 further includes an electron functional layer disposed between the light-emitting layer 40 and the cathode 20. The electron functional layer includes one or both of an electron transport layer 30 and an electron injection layer. When the electron functional layer includes the electron transport layer 30 and the electron injection layer, the electron transport layer 30 and the electron injection layer are stacked, and the electron injection layer is located between the electron transport layer 30 and the cathode 20. The electron functional layer can be prepared using electron functional materials known in the art for optoelectronic devices and having electron transport performance or electron injection performance. Specifically, the material of the electron transport layer 30 includes one or more of metal oxides, doped metal oxides, IIB-VIA group materials, IIIB-VA group materials, and IB-IIIB-VIA group materials; the metal oxides include one or more of ZnO, TiO2, and SnO2; the metal oxides in the doped metal oxides include one or more of ZnO, TiO2, and SnO2, and the doping elements include one or more of Al, Mg, Li, In, and Ga; the IIB-VIA group materials include one or more of ZnS, ZnSe, CdS, and CdSe; the IIIB-VA group materials include one or more of InP and GaP; the IB-IIIB-VIA group materials include one or more of CuInS and CuGaS; the material of the electron injection layer includes at least one of cesium carbonate, cesium fluoride, cesium azide, and lithium fluoride.

[0062] In one embodiment, the anode 10 and the cathode 20 are independently selected from metal electrodes, carbon electrodes, doped or undoped metal oxide electrodes, and composite electrodes; wherein, the material of the metal electrode is selected from at least one of Al, Ag, Cu, Mo, Au, Ba, Ca, Ni, Ir, and Mg; the material of the carbon electrode is selected from at least one of graphite, carbon nanotubes, graphene, and carbon fibers; the material of the doped or undoped metal oxide electrode is selected from at least one of ITO, FTO, ATO, AZO, GZO, IZO, MZO, ITZO, ICO, AMO, SnO2, In2O3, Cd:ZnO, Ga:SnO2; the material of the composite electrode is selected from one of AZO / Ag / AZO, AZO / Al / AZO, ITO / Ag / ITO, ITO / Al / ITO, ZnO / Ag / ZnO, ZnO / Al / ZnO, TiO2 / Ag / TiO2, TiO2 / Al / TiO2, ZnS / Ag / ZnS, and ZnS / Al / ZnS. Among them, " / " represents a laminated structure. For example, the composite electrode AZO / Ag / AZO represents an electrode with a three-layer laminated composite structure composed of an AZO layer, an Ag layer, and an AZO layer.

[0063] It can be understood that in addition to the above functional layers, the light-emitting device 100 may also be provided with some functional layers that are commonly used in the light-emitting device 100 and are helpful for improving the performance of the light-emitting device 100, such as an electron blocking layer, an electron injection layer, a hole blocking layer, and / or an interface modification layer, etc.

[0064] It can be understood that the materials and thicknesses of the various layers of the light-emitting device 100 can be set and adjusted accordingly according to the light-emitting requirements of the light-emitting device 100.

[0065] In some embodiments, the light-emitting device 100 further includes a substrate (not shown in the figure), and the substrate can also be referred to as a substrate. The substrate can be a rigid substrate or a flexible substrate. The rigid substrate can be a ceramic material or various glass materials, etc. The flexible substrate can be formed of materials such as polyimide film (PI) and its derivatives, polyethylene naphthalate (PEN), phosphoenolpyruvate (PEP), or polyphenylene ether resin. In one embodiment, the material of the substrate includes glass, silicon wafer, polycarbonate, polymethyl methacrylate, polyethylene terephthalate, polyethylene naphthalate, polyamide, polyethersulfone, or a combination thereof.

[0066] It can be understood that the light-emitting device 100 can be a normal light-emitting device or an inverted light-emitting device. When the light-emitting device 100 is a normal light-emitting device, the substrate is bonded to the side of the anode 10 away from the light-emitting layer 40. When the light-emitting device 100 is an inverted light-emitting device, the substrate is bonded to the side of the cathode 20 away from the light-emitting layer 40.

[0067] The quantum dot thin film of the present application is set as a multi-layer film, and the intrinsic dipole moments of the ligands contained in the multi-layer film gradually decrease along the first direction, so that the energy levels of the quantum dot thin film are distributed in a gradient manner, enabling such a quantum dot thin film to be applicable between two film layers with a large potential barrier difference and playing a role in adjusting the energy level potential barrier; when it is applied in the light-emitting device 100, the film layer on the side of the light-emitting layer 40 close to the anode 10 uses a ligand with a larger intrinsic dipole moment, which can offset the interfacial dipole moment generated between it and the surface of the quantum dots, resulting in an upward shift of the energy band position of the corresponding film layer, thereby reducing the hole injection barrier between the light-emitting layer 40 and the hole transport layer 60; the film layer on the side close to the cathode 20 uses a ligand with a smaller intrinsic dipole moment, resulting in a lower energy band position of the corresponding film layer. Thus, the electron injection barrier between the light-emitting layer 40 and the electron transport layer 30 can be reduced, thereby playing a role in reducing the carrier injection barrier between the light-emitting layer 40 and the adjacent two film layers, contributing to promoting carrier transport, increasing the exciton density in the light-emitting layer 40, improving carrier balance, and further enhancing the light-emitting efficiency and lifetime of the device. In addition, the connected ligand has a smaller steric hindrance relative to the initial ligand (the initial ligand will not be explained here for the time being and will be elaborated later), which is beneficial to improving the surface defect passivation effect of the quantum dots, further enhancing the PLQY and fluorescence stability of the light-emitting layer 40, and further improving the light-emitting efficiency and lifetime of the device.

[0068] It can be understood that the preparation methods of the respective film layers in the light-emitting device 100 provided in the present application, including the anode 10, cathode 20, light-emitting layer 40, hole functional layer, electron functional layer, and other film layers, can be realized by conventional techniques in the art, such as chemical methods or physical methods. Among them, chemical methods include chemical vapor deposition, sequential ionic layer adsorption and reaction, anodic oxidation, electro-deposition, co-precipitation. Physical methods include physical coating methods and solution methods. Among them, physical coating methods include: thermal evaporation coating, electron beam evaporation coating, magnetron sputtering, multi-arc ion plating, physical vapor deposition, atomic layer deposition, pulsed laser deposition, etc.; solution methods can be spin coating, printing, inkjet printing, blade coating, printing, dip coating, immersion, spraying, roll coating, casting, slot die coating, and bar coating, etc.

[0069] It can be understood that the light-emitting device 100 may further include a packaging layer (not shown in the figure) to isolate water and oxygen (for example, to make the concentrations of oxygen and water lower than 0.1 ppm), thereby improving the performance stability of the device. Specifically, the packaging material used to form the packaging layer may be selected from at least one of UV glue, metal thin film, and glass glue. In a specific embodiment, the packaging material may be acrylic resin or epoxy resin.

[0070] This application also relates to a display device, which includes the light-emitting device 100 provided by this application. The display device can be any electronic product with a display function. The electronic product includes but is not limited to smartphones, tablet computers, laptop computers, digital cameras, digital video cameras, smart wearable devices, smart weighing electronic scales, in-vehicle displays, televisions, or e-book readers. Among them, the smart wearable device can be, for example, a smart bracelet, a smart watch, a virtual reality (VR) helmet, etc.

[0071] The following will specifically illustrate this application through specific embodiments. The following embodiments are only partial embodiments of this application and do not limit this application. The raw materials used in the following embodiments are all commercially available products unless otherwise specified.

[0072] Example 1

[0073] Red quantum dots: CdZnSe / CdZnS / ZnS were synthesized. The original ligand was oleic acid, the particle size was about 14 nm, and the band gap of the quantum dots was 1.97 eV.

[0074] The quantum dot thin film is composed of a first film layer and a second film layer. The preparation steps of the quantum dot thin film are as follows:

[0075] (1) Spin-coat a 10 mg / ml red quantum dot n-octane solution on a glass substrate, spin-coat into a film at a speed of 2000 rpm, and anneal at 80 °C for 10 min to form a first film layer with a thickness of 14 nm;

[0076] (2) Spin-coat the first ligand compound solution on the first film layer, let it stand for 5 min, and then wash it three times with ethanol to obtain the treated first film layer. Among them, the first ligand compound solution is an ethanol solution of EDT, and the concentration of EDT is 2 mg / ml;

[0077] (3) Spin-coat a 10 mg / ml red quantum dot n-octane solution on the treated first film layer, spin-coat into a film at a speed of 2000 rpm, and anneal at 80 °C for 10 min to form a second film layer with a thickness of 14 nm;

[0078] (4) Spin-coat a second ligand compound solution on the second film layer, let it stand for 5 min, and then wash it three times with ethanol to obtain the treated second film layer. Among them, the second ligand compound solution is an ethanol solution of TBACl, and the concentration of TBACl is 2 mg / ml.

[0079] Example 2

[0080] This example is basically the same as Example 1, except that in this example, the second ligand contained in the second film layer is 1,2-BDT; correspondingly, in step (4), the second ligand compound solution is changed to an ethanol solution of 1,2-BDT.

[0081] Example 3

[0082] This example is basically the same as Example 1, except that in this example, the second ligand contained in the second film layer is potassium thiocyanate; correspondingly, in step (4), the second ligand compound solution is changed to an ethanol solution of potassium thiocyanate.

[0083] Example 4

[0084] This example is basically the same as Example 1, except that in this example, the first ligand contained in the first film layer is EDA; correspondingly, in step (2), the first ligand compound solution is changed to an ethanol solution of EDA.

[0085] Example 5

[0086] This example is basically the same as Example 1, except that in this example, the quantum dot thin film is composed of a first film layer, a second film layer and a third film layer. Correspondingly, the preparation steps of the quantum dot thin film are as follows:

[0087] (1) Spin-coat a 10 mg / ml red quantum dot n-octane solution on a glass substrate, spin-coat to form a film at a speed of 2000 rpm, and anneal at 80 °C for 10 min to form a first film layer with a thickness of 14 nm;

[0088] (2) Spin-coat a first ligand compound solution on the first film layer, let it stand for 5 min, and then wash it three times with ethanol to obtain the treated first film layer. Among them, the first ligand compound solution is an ethanol solution of 1,2-BDT, and the concentration of 1,2-BDT is 2 mg / ml;

[0089] (3) Spin-coat a 10 mg / ml red quantum dot n-octane solution on the treated first film layer, spin-coat to form a film at a speed of 2000 rpm, and anneal at 80 °C for 10 min to form a second film layer with a thickness of 14 nm;

[0090] (4) Spin-coat a second ligand compound solution on the second film layer, let it stand for 5 min, and then wash it three times with ethanol to obtain the treated second film layer. The second ligand compound solution is an ethanol solution of 1,4-BDT, and the concentration of 1,4-BDT is 2 mg / ml.

[0091] (5) Spin-coat a mixed solution on the treated second film layer, spin-coat to form a film at a speed of 2000 rpm, and then irradiate it with ultraviolet light of 254 nm for 5 min to form a third film layer with a thickness of 14 nm. The mixed solution is a solution formed by dispersing red quantum dots and triphenylmethyl chloride in n-octane. In the solution, the concentration of red quantum dots is 10 mg / ml, and the concentration of triphenylmethyl chloride is 2 mg / ml.

[0092] Example 6

[0093] This example is basically the same as Example 5, except that in this example, the second ligand contained in the second film layer is triphenylmethyl chloride. Correspondingly, steps (3) and (4) are changed to:

[0094] Spin-coat a mixed solution on the treated first film layer, spin-coat to form a film at a speed of 2000 rpm, and then irradiate it with ultraviolet light of 254 nm for 5 min to form a second film layer with a thickness of 14 nm. The mixed solution is a solution formed by dispersing red quantum dots and triphenylmethyl chloride in n-octane. In the solution, the concentration of red quantum dots is 10 mg / ml, and the concentration of triphenylmethyl chloride is 2 mg / ml.

[0095] Example 7

[0096] This example is basically the same as Example 1, except that in this example, the quantum dots are changed to blue quantum dots: CdZnSe / ZnSe / ZnS, the original ligands are oleic acid and oleylamine, the particle size is about 12 nm, and the band gap of the quantum dots is 2.63 eV.

[0097] Correspondingly, the thicknesses of the first film layer and the second film layer become 12 nm.

[0098] Comparative Example 1

[0099] This comparative example is basically the same as Example 1, except that in this comparative example, the ligand contained in the quantum dot film is EDT. Correspondingly, in the preparation method, the preparation of the second film layer is omitted, and the thickness of the first film layer is doubled to 28 nm. Specifically, the preparation steps of the quantum dot film in this comparative example are as follows:

[0100] (1) Spin-coat a 10 mg / ml red quantum dot n-octane solution on a glass substrate, spin-coat to form a film at a speed of 2000 rpm, and anneal at 80 °C for 10 min to form a film layer with a thickness of 28 nm.

[0101] (2) Spin-coat the ligand compound solution on the film layer, let it stand for 5 min, and then wash it three times with ethanol to obtain the treated quantum dot thin film. Among them, the ligand compound solution is an ethanol solution of EDT, and the concentration of EDT is 2 mg / ml.

[0102] Comparative Example 2

[0103] This comparative example is basically the same as Comparative Example 1, except that in this comparative example, the quantum dots are changed to blue quantum dots: CdZnSe / ZnSe / ZnS, the original ligands are oleic acid and oleylamine, the particle size is about 12 nm, and the band gaps of the quantum dots are all 2.63 eV;

[0104] Correspondingly, the thicknesses of the first film layer and the second film layer become 12 nm.

[0105] Device Example 1

[0106] This device example provides a quantum dot light-emitting diode and its preparation method, which specifically includes the following steps.

[0107] Step 1: Spin-coat PEDOT:PSS on the ITO substrate, and then anneal it at 100 °C for 15 min to obtain a hole injection layer with a thickness of 25 nm;

[0108] Step 2: Spin-coat TFB (8 mg / mL) on the hole injection layer, and then anneal it at 100 °C for 15 min to obtain a hole transport layer with a thickness of 30 nm;

[0109] Step 3: Refer to the steps for preparing the quantum dot thin film in Example 1 to prepare a quantum dot thin film on the hole transport layer to obtain a light-emitting layer. Among them, the first film layer is formed on the hole transport layer;

[0110] Step 4: Spin-coat an ethanol solution of ZnO (30 mg / ml) on the light-emitting layer, and then anneal it at 80 °C for 10 min to obtain an electron transport layer with a thickness of 30 nm;

[0111] Step 5: Evaporate Ag on the electron transport layer by thermal evaporation to form a cathode with a thickness of 20 nm, and encapsulate it with epoxy resin to obtain a QLED device.

[0112] Its device structure is: ITO / PEDOT:PPS / TFB / QD / ZnO / Ag.

[0113] Device Examples 2 - 7

[0114] Device Example n is basically the same as Device Example 1, except that in Device Example n: when preparing the light-emitting layer in Step 3, refer to the steps for preparing the quantum dot thin film in Example n to prepare a quantum dot thin film on the hole transport layer to obtain a light-emitting layer, where n is an integer from 2 to 7.

[0115] Device Comparative Example 1

[0116] This device example is basically the same as Device Example 1, except that: when preparing the light-emitting layer in Step 3, referring to the steps of preparing the quantum dot thin film in Comparative Example 1, a quantum dot thin film is prepared on the hole transport layer to obtain the light-emitting layer.

[0117] Device Comparative Example 2

[0118] This device example is basically the same as Device Example 1, except that: when preparing the light-emitting layer in Step 3, referring to the steps of preparing the quantum dot thin film in Comparative Example 2, a quantum dot thin film is prepared on the hole transport layer to obtain the light-emitting layer.

[0119] Device Comparative Example 3

[0120] This device example is basically the same as Device Example 1, except that the first ligand contained in the first film layer and the second ligand contained in the second film layer are interchanged, that is, the first ligand compound is changed to TBACl and the second ligand compound is changed to EDT; correspondingly, in Step (2), the first ligand compound solution is changed to an ethanol solution of TBACl, and in Step (4), the second ligand compound solution is changed to an ethanol solution of EDT.

[0121] Experimental Example

[0122] (1) The luminescence properties of the thin films of Thin Film Examples 1 to 7 and Thin Film Comparative Examples 1 and 2 were detected using a steady-state fluorescence spectrometer from Edinburgh Instruments, and the results are shown in Table 1.

[0123] Table 1

[0124] PL (nm) FWHM (nm) PLQY (%) Film Example 1 629 22 72 Film Example 2 629 22 75 Film Example 3 629 22 70 Film Example 4 629 22 71 Film Example 5 629 22 72 Film Example 6 629 22 70 Film Example 7 471 17 60 Film Comparative Example 1 629 22 43 Film Comparative Example 2 471 17 30

[0125] As can be seen from the above table, the thin films of each thin film example have a significantly higher PLQY than the thin films of the comparative examples, indicating that the quantum dot thin film proposed in this application is beneficial to improving the surface defect passivation effect of the quantum dot thin film by connecting ligands with relatively smaller steric hindrance on the surface of the quantum dots, and has a gain effect on the PLQY of the thin film.

[0126] (2) The external quantum efficiency EQE test and the lifetime T95@1000nit test were carried out on the quantum dot light-emitting diodes of Device Examples 1-7 and Device Comparative Examples 1-3, and the test results are shown in Table 2.

[0127] 1. The test method for the external quantum efficiency EQE is as follows:

[0128] The ratio of the number of electron-hole pairs injected into the quantum dots to the number of emitted photons, in %, is an important parameter for measuring the quality of an electroluminescent device and can be obtained by measuring with an EQE optical test instrument. The specific calculation formula is as follows:

[0129]

[0130] where ηe is the optical output coupling efficiency, ηr is the ratio of the number of recombined carriers to the number of injected carriers, χ is the ratio of the number of excitons generating photons to the total number of excitons, K R is the rate of the radiative process, and K NR is the rate of the non-radiative process.

[0131] Test conditions: Conducted at room temperature with an air humidity of 30 - 60%.

[0132] 2. The test method for the lifetime T95@1000nit is as follows: The time required for the device, under a constant current or voltage drive, to reduce the brightness to a certain proportion of the maximum brightness. The time when the brightness drops to 95% of the maximum brightness is defined as T95, and this lifetime is the measured lifetime. To shorten the test cycle, the device lifetime test is usually carried out at high brightness by accelerating the device aging, and the lifetime at high brightness is obtained by fitting with an extended exponential decay brightness decay fitting formula. For example, the lifetime at 1000nit is denoted as T95@1000nit. The specific calculation formula is as follows:

[0133]

[0134] where T95 L is the lifetime at low brightness, T95 H is the measured lifetime at high brightness, L H is the device accelerated to the maximum brightness, L L is 1000nit, A is the acceleration factor, and in this experiment, the value of A is obtained as 1.7 by measuring the lifetimes of several groups of QLED devices at the rated brightness.

[0135] Table 2

[0136] EQE (%) T95@1000nit (h) Device Example 1 20 3800 Device Example 2 19 3600 Device Example 3 21 4200 Device Example 4 18 3500 Device Example 5 20.4 3900 Device Example 6 19.5 3700 Device Example 7 19 150 Device Comparative Example 1 15 2000 Device Comparative Example 2 10 30 Device Comparative Example 3 8 900

[0137] Device Examples 1 to 6 have significantly higher EQE and T95@1000nit than Device Comparative Example 1. At the same time, Device Example 7 has significantly higher EQE and T95@1000nit than Device Comparative Example 2, indicating that the light-emitting layer is set as a multi-layer film, and the intrinsic dipole moment of the ligands contained in the multi-layer film gradually decreases along the direction from the anode to the cathode. Since the steric hindrance of the ligand molecules is relatively smaller, it is beneficial to improve the passivation effect of surface defects of quantum dots, improve the PLQY and fluorescence stability of the light-emitting layer; at the same time, the ligand molecules reduce the transport impedance of carriers between the light-emitting layers, promote the transport and injection balance of charges in the light-emitting layer, reduce the probability of Auger recombination, and improve the efficiency stability of the device, thus macroscopically characterized by a significant improvement in the light-emitting efficiency and lifetime of the device;

[0138] In addition, by comparing Device Example 1, Device Comparative Examples 1 and 3, it can also be seen that although Device Comparative Example 3 uses the same two ligand compounds and the light-emitting layer is also set as a two-layer film, its EQE and T95@1000nit are lower than those of Device Comparative Example 1, indicating that limiting the intrinsic dipole moment of the ligands contained in the multi-layer film to gradually decrease along the direction from the cathode to the anode helps to reduce the transport impedance of carriers between the light-emitting layers, improve the carrier balance, and thus play a role in improving the light-emitting efficiency and lifetime of the device.

[0139] The technical solutions provided by the embodiments of the present application have been introduced in detail above. Specific examples are used in this article to elaborate on the principle and implementation manner of the present application. The description of the above embodiments is only used to help understand the method and its core idea of the present application; at the same time, for those skilled in the art, according to the idea of the present application, there will be changes in the specific implementation manner and application scope. In summary, the content of this specification should not be construed as a limitation to the present application.

Claims

1. A quantum dot thin film, characterized in that, It includes multiple sub-film layers, and the multiple sub-film layers include a first sub-film layer, a second sub-film layer... an (N - 1)th sub-film layer and an Nth sub-film layer which are stacked in sequence, where N is an integer greater than or equal to 2, and the direction from the first sub-film layer to the Nth sub-film layer is the first direction; The materials of the sub-film layers each independently include quantum dots and ligands connected to the surfaces of the quantum dots. Along the first direction, the intrinsic dipole moments of the ligands contained in the sub-film layers gradually decrease.

2. The quantum dot thin film according to claim 1, characterized in that, In the quantum dot thin film, the total number of the sub-film layers is 2 - 8; and / or, The average thickness of the quantum dot thin film is 10 - 100 nm; and / or, The average thickness of the sub-film layer is 5 - 20 nm; and / or, Among the multiple sub-film layers, the absolute value of the difference in valence band energy levels between two adjacent sub-film layers is less than or equal to 0.2 eV.

3. The quantum dot thin film according to claim 1 or 2, characterized in that, The ligand is derived from a ligand compound, and the ligand compound includes one or more of a thiol compound, a primary amine compound, a thiocyanate compound, and an organic halide; Optionally, the thiol compound includes one or more of benzenethiol, 1,2-benzenedithiol, 1,3-benzenedithiol, 1,4-benzenedithiol, 1,2-ethanethiol, and 3-mercaptopropionic acid; Optionally, the primary amine compound includes one or more of 1,2-ethylenediamine and ethylamine; the thiocyanate compound includes one or more of ammonium thiocyanate, potassium thiocyanate, sodium thiocyanate, cobalt thiocyanate, and cuprous thiocyanate; Optionally, the organic halide includes one or more of tetrabutylammonium iodide, tetrabutylammonium bromide, tetrabutylammonium chloride, tetrabutylammonium fluoride, triphenylmethyl bromide, triphenylmethyl chloride, and triphenylmethyl iodide.

4. The quantum dot thin film according to claim 1, characterized in that, The average particle size of the quantum dots is 5 - 15 nm; and / or, The quantum dots are selected from at least one of single-structure quantum dots, core-shell structure quantum dots, and perovskite semiconductor materials. The single-structure quantum dots are selected from at least one of II-VI group compounds, IV-VI group compounds, III-V group compounds, and I-III-VI group compounds. The II-VI group compounds are selected from at least one of CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, ZnO, HgS, HgSe, HgTe, CdSeS, CdSeTe, CdSTe, ZnSeS, ZnSeTe, ZnSTe, HgSeS, HgSeTe, HgSTe, CdZnS, CdZnSe, CdZnTe, CdHgS, CdHgSe, CdHgTe, HgZnS, HgZnSe, HgZnTe, CdZnSeS, CdZnSeTe, CdZnSTe, CdHgSeS, CdHgSeTe, CdHgSTe, HgZnSeS, HgZnSeTe, and HgZnSTe. The IV-VI group compounds are selected from at least one of SnS, SnSe, SnTe, PbS, PbSe, PbTe, SnSeS, SnSeTe, SnSTe, PbSeS, PbSeTe, PbSTe, SnPbS, SnPbSe, SnPbTe, SnPbSSe, SnPbSeTe, and SnPbSTe. The III-V group compounds are selected from at least one of GaN, GaP, GaAs, GaSb, AlN, AlP, AlAs, AlSb, InN, InP, InAs, InSb, GaNP, GaNAs, GaNSb, GaPAs, GaPSb, AlNP, AlNAs, AlNSb, AlPAs, AlPSb, InNP, InNAs, InNSb, InPAs, InPSb, GaAlNP, GaAlNAs, GaAlNSb, GaAlPAs, GaAlPSb, GaInNP, GaInNAs, GaInNSb, GaInPAs, GaInPSb, InAlNP, InAlNAs, InAlNSb, InAlPAs, and InAlPSb. The I-III-VI group compounds are selected from at least one of CuInS, CuInSe, and AgInS. The core of the core-shell structure quantum dots is selected from any one of the above single-structure quantum dots. The shell layer of the core-shell structure quantum dots includes one or more layers. The shell layer material of the core-shell structure quantum dots is selected from at least one of CdS, CdTe, CdSeTe, CdZnSe, CdZnS, CdSeS, ZnSe, ZnSeS, and ZnS. The perovskite semiconductor material is selected from doped or undoped inorganic perovskite semiconductors, or organic-inorganic hybrid perovskite semiconductors;The structural general formula of the inorganic perovskite semiconductor is AMX3, where A is Cs; + ion, M is selected from Pb 2+ , Sn 2+ , Cu 2+ , Ni 2+ , Cd 2+ , Cr 2+ , Mn 2+ , Co 2+ , Fe 2+ , Ge 2+ , Yb 2+ , Eu 2+ and at least one of them, X is selected from Cl - , Br - , I - and at least one of them; The structural general formula of the organic-inorganic hybrid perovskite semiconductor is BMX3, where B is selected from CH3(CH2) n-2 NH3 + or [NH3(CH2) n NH3] 2+ , where n≥2, M is selected from Pb 2+ , Sn 2+ , Cu 2+ , Ni 2+ , Cd 2+ , Cr 2+ , Mn 2+ , Co 2+ , Fe 2+ , Ge 2+ , Yb 2+ , Eu 2+ and at least one of them, X is selected from Cl - , Br - , I - and at least one of them.

5. A method for preparing a quantum dot thin film, characterized in that, It includes the following steps: Prepare multiple sub-film layers in sequence according to a preset direction to obtain a quantum dot thin film. The multiple sub-film layers include a first sub-film layer, a second sub-film layer... an (N - 1)th sub-film layer and an Nth sub-film layer which are stacked, where N is an integer greater than or equal to 2. The preset direction is the first direction along from the first sub-film layer to the Nth sub-film layer, or the second direction from the Nth sub-film layer to the first sub-film layer; The materials of the sub-film layers each independently include quantum dots and ligands connected to the surfaces of the quantum dots. Along the first direction, the intrinsic dipole moments of the ligands contained in the sub-film layers gradually decrease.

6. The preparation method according to claim 5, characterized in that, The preparation of each sub-film layer includes: providing a first solid film containing quantum dots; providing a ligand compound, contacting the first solid film with the ligand compound, and performing ligand exchange to obtain a sub-film layer.

7. The preparation method according to claim 5, characterized in that, When preparing multiple sub-film layers in sequence according to the first direction, multiple ligand compounds with gradually decreasing intrinsic dipole moments are used in sequence, or when preparing multiple sub-film layers in sequence according to the second direction, multiple ligand compounds with gradually increasing intrinsic dipole moments are used in sequence.

8. The preparation method according to any one of claims 5-7, characterized in that, The steps of providing the first solid-state film containing quantum dots include: dispersing quantum dots in a first solvent to form a quantum dot solution; depositing the quantum dot solution to form a liquid film, and then removing the first solvent to obtain the first solid-state film containing quantum dots; and / or, The steps of ligand exchange by contacting the first solid-state film with the ligand compound include: mixing the ligand compound and a second solvent to form a mixed solution; immersing the first solid-state film in the mixed solution, or coating the mixed solution on the surface of the first solid-state film, and then washing the first solid-state film with a third solvent after standing.

9. The preparation method according to claim 8, characterized in that, The first solvent includes one or more of alkane solvents having 5 to 16 carbon atoms, and the alkane solvents having 5 to 16 carbon atoms include one or more of n-pentane, n-hexane, n-heptane, n-octane, tridecane, tetradecane, cyclooctane, cycloheptane, cyclohexane, cyclopentane, 2-methyloctane, 3-ethylheptane, 2,2-dimethyloctane, 1-cyclohexyldecane; and / or, The second solvent includes one or more of C1 to C5 alcohol solvents, and the C1 to C5 alcohol solvents include one or more of methanol, ethanol, isopropanol, n-butanol, pentanol; and / or, The third solvent includes one or more of C1 to C5 alcohol solvents, and the C1 to C5 alcohol solvents include one or more of methanol, ethanol, isopropanol, n-butanol, pentanol; and / or, In the quantum dot solution, the concentration of the quantum dots is 5 to 50 mg / mL; and / or, The standing time is 3 to 15 min.

10. A light-emitting device, characterized in that, Comprising an anode, a light-emitting layer and a cathode, the light-emitting layer comprises the quantum dot thin film according to any one of claims 1 to 5, or is prepared by the preparation method according to any one of claims 6 to 9; The first sub-film layer is disposed close to the anode.

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