CN114520290A

CN114520290A - Quantum dot light-emitting diode and preparation method thereof

Info

Publication number: CN114520290A
Application number: CN202011294178.3A
Authority: CN
Inventors: 聂志文
Original assignee: TCL Technology Group Co Ltd
Current assignee: TCL Technology Group Co Ltd
Priority date: 2020-11-18
Filing date: 2020-11-18
Publication date: 2022-05-20

Abstract

The invention discloses a quantum dot light-emitting diode and a preparation method thereof, wherein the quantum dot light-emitting diode comprises a first functional layer, a second functional layer and a quantum dot layer arranged between the first functional layer and the second functional layer, the quantum dot layer is formed by quantum dots of which the surfaces are combined with a first ligand and a second ligand, the first ligand is positioned on one side where the first functional layer is positioned, the polarity of the first ligand is the same as that of the first functional layer, the second ligand is positioned on one side where the second functional layer is positioned, and the polarity of the second ligand is the same as that of the second functional layer. Through the mode, the invention effectively improves the compatibility between the quantum dot layer and the functional layer, reduces the surface contact angle between the quantum dot layer and the functional layer, reduces the generation of leakage current and obviously improves the luminous performance of the device on the premise of keeping the original fluorescence efficiency of the quantum dot not to be lost.

Description

Quantum dot light-emitting diode and preparation method thereof

Technical Field

The invention relates to the field of quantum dot light-emitting devices, in particular to a quantum dot light-emitting diode and a preparation method thereof.

Background

Quantum Dots (QDs) are usually composed of tens to millions of atoms, and have a geometric size similar to that of excitons, which partially inherit the characteristics of bulk semiconductors and show unique photoelectric properties, specifically expressed as: high color purity, continuous and adjustable luminescence spectrum along with size and components, narrow half-peak width, high fluorescence efficiency, long service life, excellent monodispersity and photo-thermal stability, excellent solution processability and the like. The quantum dot has wide application prospects in the fields of display, laser, photovoltaic, biomarkers and the like, wherein the application of the quantum dot in the field of display, particularly QD-LCD televisions which are greatly popularized by manufacturers such as TCL (transmission control language) and Samsung and the like, also marks the initial commercialization of the quantum dot.

With the continuous improvement of quantum dot synthesis technology, the continuous optimization of device structure and the continuous deep theoretical research on the service life problem of QLED (quantum dot light emitting diode) devices, the efficiency and the service life of the devices are greatly improved. Especially, the device performance of the red and green QLEDs can be compared with that of the existing OLED (organic light emitting diode) which is widely applied, and the solid step is taken for really realizing the commercialization of the QLED. On the one hand, the existing high-performance QLED generally uses ZnO having high electron mobility as an electron transport layer. The ZnO is generally prepared by a low-temperature solution method, and the surface of the ZnO presents polarity. When a ZnO layer is deposited on the quantum dot light emitting layer, the contact angle between the polar surface of ZnO and the nonpolar surface of the quantum dot is large, the film forming property of ZnO nanoparticles is deteriorated, and electron injection becomes difficult. On the other hand, the existing ligand exchange technology is generally carried out based on a solution method, and the principle of universality is to replace the original weak ligand on the surface of the quantum dot with a strong ligand. The method can realize effective exchange of ligands to a certain extent, but the exchange usually occurs on the surface of the whole quantum dot layer, which causes the self-fluorescence efficiency of the quantum dots to be greatly reduced, and greatly limits the wide application of the QLED.

Accordingly, the prior art is yet to be improved and developed.

Disclosure of Invention

In view of the above disadvantages of the prior art, the present invention is directed to a quantum dot light emitting diode and a method for fabricating the same, which aims to solve the problem that the self-fluorescence efficiency of quantum dots is greatly reduced due to the occurrence of ligand exchange on the surface of the entire quantum dot layer.

The technical scheme of the invention is as follows:

in a first aspect of the present invention, there is provided a quantum dot light emitting diode comprising a first functional layer, a second functional layer, and a quantum dot layer disposed between the first functional layer and the second functional layer,

the quantum dot layer is formed of quantum dots to the surface of which a first ligand and a second ligand are bound,

the first ligand is positioned at one side of the first functional layer, the polarity of the first ligand is the same as that of the first functional layer,

the second ligand is positioned on one side of the second functional layer, and the polarity of the second ligand is the same as that of the second functional layer.

The surface of the quantum dot layer on one side of the first functional layer is coordinated to form the first ligand, the polarity of the first ligand is the same as that of the first functional layer, the surface of the quantum dot layer on one side of the second functional layer is coordinated to form the second ligand, and the polarity of the second ligand is the same as that of the second functional layer, so that the compatibility between the quantum dot layer and the first functional layer and the compatibility between the quantum dot layer and the second functional layer are effectively improved, the surface contact angle between the quantum dot layer and the functional layer is reduced, gaps and defects between film layers are filled, the occurrence of non-radiative recombination is effectively avoided, the generation of leakage current is reduced, and the luminous performance of a device is remarkably improved.

In a second aspect of the present invention, a method for preparing a quantum dot light emitting diode is provided, wherein the method comprises the steps of:

providing a functional layer, the functional layer being one of a first functional layer and a second functional layer;

forming a quantum dot layer on the surface of the functional layer, wherein a ligand with the same polarity as that of the functional layer is combined on the surface of the quantum dot layer;

covering another ligand on the surface of the quantum dot layer to perform ligand exchange with the ligand to obtain a quantum dot layer with the surface combined with the other ligand;

forming the other of the first functional layer and the second functional layer having the same polarity as the other ligand on the surface of the quantum dot layer to which the other ligand is bonded.

According to the invention, the surface of the quantum dot layer on the side of the first functional layer is coordinated with the ligand with the same polarity as the first functional layer, and the surface of the quantum dot layer on the side of the second functional layer is coordinated with the ligand with the same polarity as the second functional layer, so that the compatibility between the quantum dot layer and the first functional layer and the compatibility between the quantum dot layer and the second functional layer are effectively improved, the surface contact angle between the quantum dot layer and the functional layer is reduced, the gap and the defect between the film layers are filled, the non-radiative recombination is effectively avoided, the generation of leakage current is reduced, and the luminous performance of the device is obviously improved.

Drawings

Fig. 1 is a schematic structural diagram of a quantum dot light emitting diode according to an embodiment of the present invention.

Fig. 2 is a schematic flow chart of a method for manufacturing a quantum dot light emitting diode according to an embodiment of the present invention.

FIG. 3 is a schematic diagram of a quantum dot layer before and after ligand exchange in an embodiment of the invention.

Detailed Description

The invention provides a quantum dot light-emitting diode and a preparation method thereof, and the invention is further described in detail below in order to make the purpose, technical scheme and effect of the invention clearer and clearer. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

The embodiment of the invention provides a quantum dot light emitting diode, which comprises a first functional layer, a second functional layer and a quantum dot layer arranged between the first functional layer and the second functional layer,

the quantum dot layer is formed of quantum dots having a first ligand and a second ligand bound to the surface thereof,

In this embodiment, the surface of the quantum dot layer refers to a surface in contact with the first functional layer and a surface in contact with the second functional layer. That is, in the quantum dot layer, the surface in contact with the first functional layer coordinates (i.e., binds) to the first ligand, and the surface in contact with the second functional layer coordinates to the second ligand. In addition, the quantum dots between the two surfaces of the quantum dot layer may be both coordinated as the first ligand, may also be both coordinated as the second ligand, and may also be partially coordinated as the first ligand (located on the side where the first functional layer is located) and partially coordinated as the second ligand (located on the side where the second functional layer is located), and the kind of the ligands is determined specifically according to the actual situation. In other words, if the quantum dot layer is divided into two partial regions: and the quantum dots in the lower part area are coordinated as a first ligand, the quantum dots in the upper part area are coordinated as a second ligand, and the size of each part area is determined according to the actual situation.

In this embodiment, the surface of the quantum dot layer on the side of the first functional layer is coordinated as a first ligand, the polarity of the first ligand is the same as that of the first functional layer, the surface of the quantum dot layer on the side of the second functional layer is coordinated as a second ligand, and the polarity of the second ligand is the same as that of the second functional layer, so that the compatibility between the quantum dot layer and the first functional layer and the compatibility between the quantum dot layer and the second functional layer are effectively improved, the surface contact angle between the quantum dot layer and the functional layer is reduced, gaps and defects between the film layers are filled, the occurrence of non-radiative recombination is effectively avoided, the generation of leakage current is reduced, and the light emitting performance of the device is remarkably improved.

In one embodiment, one of the first functional layer and the second functional layer is a hole transport layer, the other is an electron transport layer, the hole transport layer is non-polar, and the electron transport layer is polar.

When the first functional layer is a hole transport layer, the second functional layer is an electron transport layer, the hole transport layer is nonpolar, the electron transport layer is polar, the first ligand is a nonpolar ligand, and the second ligand is a polar ligand.

When the first functional layer is an electron transport layer, the second functional layer is a hole transport layer, the hole transport layer is nonpolar, the electron transport layer is polar, the first ligand is a polar ligand, and the second ligand is a nonpolar ligand.

In this embodiment, the surface of the quantum dot layer on the side of the hole transport layer is coordinated with a non-polar ligand having the same polarity as the hole transport layer, and the surface of the quantum dot layer on the side of the electron transport layer is coordinated with a polar ligand having the same polarity as the electron transport layer. Therefore, the compatibility between the quantum dot layer and the hole transmission layer and the compatibility between the quantum dot layer and the electron transmission layer are effectively improved, the surface contact angle between the quantum dot layer and the functional layer is reduced, gaps and defects between film layers are filled, the non-radiative recombination is effectively avoided, the leakage current is reduced, and the luminous performance of the device is obviously improved.

In one embodiment, one end of the carbon chain of the polar ligand in the first ligand and the second ligand is a thiol group, and the other end is a polar group, and the polar ligand is bonded to the surface of the quantum dot layer through the thiol group.

In one embodiment, the polar group is selected from one of carboxyl, thiol, amine, phosphoric acid, and the like, but is not limited thereto. The polar group has good polarity, so that the surface contact angle between the quantum dot layer and the electron transmission layer can be effectively reduced, the gap and the defect between film layers are filled, the non-radiative recombination is effectively avoided, the leakage current is reduced, and the luminous performance of the device is remarkably improved.

In one embodiment, the nonpolar ligand of the first ligand and the second ligand is selected from one or more of an organic carboxylic acid having 8 or more carbon atoms, a primary amine having 8 or more carbon atoms, a secondary or tertiary amine having 4 or more branched carbon atoms, an organic phosphine having 4 or more branched carbon atoms, and the like.

In one embodiment, the nonpolar ligand of the first ligand and the second ligand is selected from one or more of an organic carboxylic acid having 8 or more carbon atoms, a primary amine having 8 or more carbon atoms, a secondary or tertiary amine having 4 or more branched carbon atoms, an organic phosphine having 4 or more branched carbon atoms, and the like, but is not limited thereto. The specific species are described below and will not be described further herein.

In one embodiment, the length of the first ligand carbon chain is the same as the length of the second ligand carbon chain, which is effective for ensuring the fluorescence efficiency of the quantum dot.

In one embodiment, the first functional layer is a hole transport layer, the second functional layer is an electron transport layer, the first ligand is a non-polar ligand, and the second ligand is a polar ligand.

In this embodiment, in the quantum dot layer, the surface in contact with the hole transport layer is coordinated as a first ligand, the first ligand is a nonpolar ligand, the surface in contact with the electron transport layer is coordinated as a second ligand, and the second ligand is a polar ligand. In addition, the quantum dots between the two surfaces of the quantum dot layer may be both coordinated as the first ligand, or a part of the quantum dots may be coordinated as the first ligand (located on the side where the hole transport layer is located) and the other part may be coordinated as the second ligand (located on the side where the electron transport layer is located). Further, the quantum dots between both surfaces of the quantum dot layer are each coordinated as a first ligand. That is, in the quantum dot layer, the surface in contact with the electron transport layer is coordinated as the second ligand, and the remaining region of the quantum dot layer is the first ligand.

In the existing quantum dot light-emitting diode, a ligand coordinated by a quantum dot is usually a nonpolar ligand, the whole surface of a quantum dot layer is in a nonpolar state, and the surface of an electron transmission layer contacted with the quantum dot layer is in a polar state, so that the quantum dot layer and the electron transmission layer are not tightly connected due to large polarity difference, a large number of defects exist, electron injection is difficult, nonradiative recombination is serious, and the luminous performance of a device is low. In order to solve the technical problem, the conventional method firstly adopts a form of quantum dot solution to perform ligand exchange, and then deposits the ligand-exchanged quantum dot solution to form a quantum dot layer, so as to improve the compatibility between the quantum dot layer and the electron transport layer. Although this method can achieve efficient exchange of ligands, ligand exchange occurs on all quantum dot surfaces, which results in a large reduction in the autofluorescence efficiency of the quantum dots.

In the quantum dot light-emitting diode of the embodiment, the region of the quantum dot layer close to the electron transport layer is coordinated to be the polar ligand, and the rest regions of the quantum dot layer are still kept to be the non-polar ligand of the quantum dot, so that the surface of the quantum dot layer in contact with the electron transport layer is in a polar state, and the rest regions of the quantum dot layer are still kept to be in an original non-polar state. Meanwhile, the compatibility between the quantum dot layer and the electron transmission layer is effectively improved, the surface contact angle between the quantum dot layer and the electron transmission layer is reduced, gaps and defects between film layers are filled, non-radiative recombination is effectively avoided, leakage current is reduced, and the luminous performance of the device is remarkably improved.

In one embodiment, the length of the carbon chain of the first ligand is the same as the length of the carbon chain of the second ligand. For example, when the first ligand is undecanoic acid, the second ligand is 11-mercaptoundecanoic acid.

In this embodiment, the length of the first ligand carbon chain is the same as that of the second ligand carbon chain, so that the problem of greatly reduced autofluorescence efficiency of the quantum dot due to conventional ligand replacement can be further avoided.

In one embodiment, the first ligand is selected from one or more of an organic carboxylic acid having 8 or more carbon atoms, a primary amine having 8 or more carbon atoms, a secondary or tertiary amine having 4 or more branched carbon atoms, an organic phosphine having 4 or more branched carbon atoms, and the like.

In one embodiment, the first ligand is one or more selected from an organic carboxylic acid having 8 or more and 20 or less carbon atoms, a primary amine having 8 or more and 20 or less carbon atoms, a secondary or tertiary amine having 4 or more and 20 or branched carbon atoms, an organic phosphine having 4 or more and 20 or branched carbon atoms, and the like.

The organic carboxylic acid having 8 or more and 20 or less carbon atoms is selected from one or more of octanoic acid, nonanoic acid, decanoic acid, undecanoic acid, dodecanoic acid, tridecanoic acid, tetradecanoic acid, hexadecanoic acid, octadecanoic acid, undecylenic acid, dodecenoic acid, tridecenoic acid, tetradecanoic acid, pentadecenoic acid, hexadecenoic acid, heptadecenoic acid, and octadecenoic acid.

For example, the primary amine having 8 or more and 20 or less carbon atoms is one or more selected from octylamine, nonylamine, decylamine, dodecylamine, tetradecylamine, hexadecylamine, octadecylamine, and the like.

For example, the secondary or tertiary amine having 4 or more and 20 or less carbon atoms in a branched chain is one or more selected from tributylamine, trihexylamine, triheptylamine, trioctylamine, trinonyl amine, tridecylamine, and the like.

For example, the organic phosphine having 4 or more and 20 or less carbon atoms in a branched chain is one or more selected from tributylphosphine, trihexylphosphine, triheptylphosphine, trioctylphosphine, trinonylphosphine, tridecylphosphine, and the like.

In one embodiment, the carbon chain of the second ligand is a thiol at one end and a polar group at the other end, and the second ligand is bonded to the surface of the quantum dot layer through the thiol.

In this embodiment, the quantum dot light emitting diode is divided into two types: the quantum dot light emitting diode of each structure can have various forms. The structure of the quantum dot light-emitting diode and the material selection thereof in this embodiment will be described below by taking the quantum dot light-emitting diode of the positive type structure shown in fig. 1 as an example. As shown in fig. 1, the quantum dot light emitting diode includes, from bottom to top, a substrate, an anode, a hole injection layer, a hole transport layer, a quantum dot layer, an electron transport layer, and a cathode in this order; and a second ligand is combined on the surface of the quantum dot layer on the side where the electron transport layer is located, and the second ligand is a polar ligand.

In one embodiment, the substrate may be a rigid substrate or a flexible substrate, and the substrate is selected from glass, a silicon wafer, polycarbonate, polymethyl methacrylate, polyethylene terephthalate, polyethylene naphthalate, polyamide, polyether sulfone, a combination thereof, or the like.

In one embodiment, the material of the anode may be selected from nickel, platinum, vanadium, chromium, copper, zinc, gold, or alloys thereof; the material of the anode can also be selected from one or more of zinc oxide, indium oxide, tin oxide, indium zinc oxide, indium-doped tin oxide, fluorine-doped tin oxide, antimony-doped tin oxide, aluminum-doped zinc oxide and the like; the material of the anode may be any two or a combination of two or more of the above.

In one embodiment, the material of the hole injection layer may be selected from materials having good hole injection properties, such as but not limited to one or more of poly (3, 4-ethylenedioxythiophene) -polystyrenesulfonic acid (PEDOT: PSS), copper phthalocyanine (CuPc), 2,3,5, 6-tetrafluoro-7, 7',8,8' -tetracyanoquinodimethane (F4-TCNQ), 2,3,6,7,10, 11-hexacyano-1, 4,5,8,9, 12-Hexaazatriphenylene (HATCN), doped or undoped transition metal oxides, doped or undoped metal chalcogenide compounds; wherein the transition metal oxide includes, but is not limited to, MoO₃、VO₂、WO₃One or more of CuO and CuO; metal chalcogenide compounds including but not limited to MoS₂、MoSe₂、WS₂、WSe₂And CuS. In one embodiment, the hole injection layer has a thickness of 10 to 150 nm.

In one embodiment, the material of the hole transport layer may be selected from organic materials having good hole transport ability, such as but not limited to Poly (9, 9-dioctylfluorene-CO-N- (4-butylphenyl) diphenylamine) (TFB), Polyvinylcarbazole (PVK), Poly (N, N 'bis (4-butylphenyl) -N, N' -bis (phenyl) benzidine) (Poly-TPD), Poly (9, 9-dioctylfluorene-CO-bis-N, N-phenyl-1, 4-Phenylenediamine) (PFB), 4', 4 ″ -tris (carbazol-9-yl) triphenylamine (TCTA), 4' -bis (9-Carbazole) Biphenyl (CBP), N '-diphenyl-N, N' -bis (3-methylphenyl) -1,1 '-biphenyl-4, 4' -diamine (TPD), N '-diphenyl-N, N' - (1-naphthyl) -1,1 '-biphenyl-4, 4' -diamine (NPB), doped graphene, undoped graphene, C60. In one embodiment, the hole transport layer has a thickness of 10 to 150 nm.

In one embodiment, the quantum dots can be selected from groups II-VI CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, ZnO, HgS, HgSe, HgTe, CdSeS, CdSeTe, CdSTe, ZnSeS, ZnSeTe, ZnSTe, HgSeS, HgSeTe, HgSTe, CdZnS, CdZnSe, CdZnTe, cdhghgss, CdHgSe, CdHgTe, HgZnSe, HgZnTe, cdznese, CdZnSeTe, CdHgSTe, HgZnSeTe; or group III-V GaN, GaP, GaAs, GaSb, AlN, AlP, AlAs, AlSb, InN, InP, InAs, InSb, GaNP, GaNAs, GaNSb, GaGaAs, GaPSb, AlNP, AlNAs, AlNSb, AlPAs, AlPSb, InNP, InNAs, InNSb, InPAs, InPSb, GaAlNP, GaAlNAs, GaAlNSb, GaAlPAs, GaAlPSb, GaInNP, GaInNAs, GaInNSb, GaInPAs, GaInPSb, InInInAlN, InLNAs, InAsInNSb, InAlGaAs, InLPSb; or group IV-VI SnS, SnSe, SnTe, PbS, PbSe, PbTe, SnSeS, SnSeTe, SnSTe, PbSeS, PbSeTe, PbSTe, SnPbS, SnPbSe, SnPbTe, SnPbSSe, SnPbSeTe, SnPbSTe; or a combination of any one or more of the above.

In one embodiment, the electron transport layer may be selected from ZnO, TiO₂、Alq3、SnO、ZrO、AlZnO、ZnSnO、BCP、TAZ、PBD、TPBI、Bphen、CsCO₃One or more of (a). In one embodiment, the thickness of the electron transport layer is 5 to 100 nm.

In one embodiment, the cathode may be selected from metals or alloys thereof, such as magnesium, calcium, sodium, potassium, titanium, indium, yttrium, lithium, gadolinium, aluminum, silver, tin, lead, cesium, barium, or alloys thereof; the cathode may also be composed of a multi-layered structure of materials, such as a first layer of an alkali metal halide, an alkaline earth metal halide, an alkali metal oxide, or a combination thereof, and a second layer of an alkaline earth metal, a group 13 metal, or a combination thereof, on the first layer. For example, the cathode is LiF/Al, LiO₂Al, LiF/Ca, Liq/Al, and BaF₂But not limited thereto,/Ca.

The embodiment of the invention provides a preparation method of a quantum dot light-emitting diode, which comprises the following steps of:

s1, providing a functional layer, wherein the functional layer is one of a first functional layer and a second functional layer;

s2, forming a quantum dot layer on the surface of the functional layer, wherein ligands with the same polarity as the functional layer are combined on the surface of the quantum dot layer;

s3 at<5×10^-4Covering another ligand on the surface of the quantum dot layer under the vacuum condition of Pa so as to perform ligand exchange with the ligand, and obtaining the quantum dot layer with the surface combined with the other ligand;

and S4, forming the other one of the first functional layer and the second functional layer, which has the same polarity as the other ligand, on the surface of the quantum dot layer bonded with the other ligand.

In this embodiment, when another ligand is added to the surface of the quantum dot layer having the ligand, under the dual induction of the concentration difference of the ligand and the interaction between the cation on the surface of the quantum dot and the functional group of the other ligand, the other ligand is directly diffused along the functional layer, thereby inducing ligand exchange of a part of the ligands on the surface of the quantum dot layer. Meanwhile, in order to prevent the ligand exchange from occurring in the entire quantum dot film layer due to an excessive concentration difference, a vacuum treatment is performed on the quantum dot film layer (the vacuum condition is preferably a vacuum condition)<5×10^-4Pa) is added. Under the condition of vacuum pumping, an upward pulling force is generated on the ligand to be exchanged, so that the ligand to be exchanged is better balanced to be generated in the whole quantum dot film layer due to overlarge concentration difference. While the region in contact with the functional layer is not subject to ligand exchange, this region remains the original ligand of the quantum dot. By adopting the ligand exchange method, the coordination of the surface of the quantum dot layer at one side of the first functional layer to the ligand with the same polarity as the first functional layer can be realized, the coordination of the surface of the quantum dot layer at one side of the second functional layer to the ligand with the same polarity as the second functional layer can be realized, thus effectively improving the compatibility between the quantum dot layer and the first functional layer and the compatibility between the quantum dot layer and the second functional layer, reducing the surface contact angle between the quantum dot layer and the functional layer, filling the gaps and defects between the film layers, effectively avoiding the occurrence of non-radiative recombination and reducing the generation of leakage current,thereby remarkably improving the luminous performance of the device.

In step S3, in one embodiment, the ratio of the volume of the other ligand to the volume of the quantum dots is 0.1-10: and 1, covering the other ligand on the surface of the quantum dot layer.

In one embodiment, the step of covering the surface of the quantum dot layer with the other ligand to perform ligand exchange with the ligand comprises:

and after the surface of the quantum dot layer is covered with the other ligand to carry out ligand exchange, washing by using an organic solvent, and drying to obtain the quantum dot layer with the surface combined with the other ligand.

In one embodiment, the length of a carbon chain of a ligand is the same as the length of a carbon chain of another ligand. For example, when the ligand is undecanoic acid, another ligand is 11-mercaptoundecanoic acid. The length of the ligand carbon chain is the same as that of the other ligand carbon chain, which is equivalent to the in-situ ligand exchange on the surface of the quantum dot layer, so that the problem of greatly reduced fluorescence efficiency of the quantum dot caused by conventional ligand exchange is effectively avoided on the premise of no loss of fluorescence of the original quantum dot.

In one embodiment, one of the ligand and the further ligand is polar and the other is non-polar.

In one embodiment, the polar ligand carbon chain has a sulfhydryl group at one end and a polar group at the other end.

In one embodiment, the polar group is selected from one of carboxyl, thiol, amine, phosphate, and the like.

In step S1, in one embodiment, the functional layer is a first functional layer, the first functional layer is a hole transport layer, the second functional layer is an electron transport layer, the ligand is a nonpolar ligand, and the other ligand is a polar ligand. That is to say, the method for manufacturing the quantum dot light emitting diode of the embodiment includes the steps of:

s10, providing a hole transport layer;

s20, forming a quantum dot layer on the surface of the hole transport layer, wherein the surface of the quantum dot layer is combined with a nonpolar ligand;

s30, covering a polar ligand on the surface of the quantum dot layer for ligand exchange to obtain the quantum dot layer with the surface combined with the polar ligand;

and S40, forming an electron transport layer on the surface of the quantum dot layer bonded with the polar ligand.

In this embodiment, when a polar ligand is added to the surface of the quantum dot layer having a nonpolar ligand, the polar ligand is directly diffused along the direction of the hole transport layer by the dual induction action of the difference in the concentration of the ligand and the interaction between the cations on the surface of the quantum dot and the functional groups of the polar ligand, thereby inducing ligand exchange of a part of the ligands on the surface of the quantum dot layer (the surface in contact with the electron transport layer). Meanwhile, in order to prevent the ligand exchange from occurring in the entire quantum dot film layer due to an excessive concentration difference, a vacuum treatment is performed on the quantum dot film layer (the vacuum condition is preferably a vacuum condition)<5×10^-4Pa) is added. Under the condition of vacuum pumping, an upward pulling force is generated on the ligand to be exchanged, so that the ligand to be exchanged is better balanced to be generated in the whole quantum dot film layer due to overlarge concentration difference. While the region in contact with the hole transport layer, which is still the original ligand of the quantum dot, is not subject to ligand exchange. As shown in fig. 3, the surfaces of all quantum dots in the quantum dot layer 1 before ligand exchange are coordinated with nonpolar ligands, the surface of the quantum dot layer 2 after ligand exchange, which is close to the electron transport layer, is coordinated with polar ligands, and the rest regions retain the original nonpolar ligands. By adopting the ligand exchange method, the region in the quantum dot layer, which is in contact with the hole transport layer, is still kept in the original nonpolar state, and the region in the quantum dot layer, which is in contact with the electron transport layer, is in the polar state. By the method, on the premise of keeping the original quantum dot fluorescence efficiency without loss, the compatibility between the quantum dot layer and the electron transmission layer is effectively improved, the surface contact angle between the quantum dot layer and the electron transmission layer is reduced, gaps and defects between film layers are filled, non-radiative recombination is effectively avoided, leakage current is reduced, and the luminous performance of the device is remarkably improved.

The existing ligand exchange occurs on the surface of all quantum dots, which leads to a great reduction in the autofluorescence efficiency of the quantum dots. Different from the existing ligand exchange, the embodiment effectively avoids the problem of greatly reduced fluorescence efficiency of the quantum dots caused by the existing ligand exchange on the premise of keeping the original fluorescence of the quantum dots from being lost, and realizes the ligand exchange of partial areas on the surface of the quantum dot layer. Meanwhile, the problem that the film forming quality of the electron transmission layer is poor due to poor compatibility caused by large polarity difference between the quantum dot layer and the electron transmission layer is solved. In addition, the method has the advantages of simplicity, mildness, effectiveness, rapidness, strong universality and the like, and is very suitable for future large-scale application of the quantum dots.

In step S20, the surface of the quantum dot layer is bonded with a nonpolar ligand, which is an original ligand of the quantum dot, and the surface of the original quantum dot layer away from the hole transport layer assumes a nonpolar state. For specific classes of nonpolar ligands, see above, further description is omitted here. In this step, the nonpolar ligand is bonded to the surfaces of all the quantum dots in the quantum dot layer, and thus the surfaces of the entire quantum dot layer are in a nonpolar state.

In one embodiment, step S20 specifically includes: and spin-coating the prepared quantum dot solution on the hole transport layer, and then carrying out thermal annealing treatment to obtain the quantum dot layer. Wherein the film thickness can be controlled by adjusting the concentration of the solution, the spin-coating speed and the spin-coating time. The quantum dots are quantum dots with nonpolar ligands bonded on the surfaces. In one embodiment, the quantum dot layer may have a thickness of 20-60nm, such as 30 nm.

In one embodiment, the concentration of the quantum dot solution is 10-30 mg/ml. In one embodiment, the solvent for dispersing the quantum dots is selected from at least one of n-octane, n-hexane, cyclohexane, cyclooctane, etc., but is not limited thereto.

In one embodiment, step S30 includes: adding polar ligand to the surface of quantum dot layer, and placing<5×10^-4And (3) carrying out spin coating under the vacuum of Pa, after the spin coating is finished, cleaning the surface of the quantum dot layer, and finally drying to obtain the quantum dot layer with the polar ligand combined on the surface.

In one embodimentIn step S30, specifically, the method includes: dripping polar ligand on the surface of quantum dot layer, and placing on<5×10^-4And (3) carrying out spin coating under the vacuum of Pa, wherein the spin coating rotation speed is 100-2000rpm, the spin coating time is 10s-2min, after the completion, adding a small amount of isopropanol to clean the surface of the quantum dot layer, and finally drying at the temperature of 50-120 ℃ for 30min-4h to obtain the quantum dot layer with the surface combined with the polar ligand. In one embodiment, the volume ratio of isopropanol to polar ligand is from 1 to 20: 1.

in one embodiment, the ratio of the polar ligand to the quantum dot by volume is from 0.1 to 10: and 1, covering a polar ligand on the surface of the quantum dot layer for ligand exchange.

In one embodiment, the polar ligand carbon chain is the same length as the nonpolar ligand carbon chain. For example, when the nonpolar ligand is undecanoic acid, the polar organic ligand is 11-mercaptoundecanoic acid. The length of the nonpolar ligand carbon chain is the same as that of the polar ligand carbon chain, which is equivalent to the in-situ ligand exchange on the surface of the quantum dot contacted with the electron transport layer, so that the problem of greatly reduced fluorescence efficiency of the quantum dot caused by conventional ligand exchange is effectively avoided on the premise of no loss of fluorescence of the original quantum dot.

In one embodiment, one end of the carbon chain of the polar ligand is a thiol group, and the other end is a polar group, and the polar ligand is bonded to the surface of the quantum dot layer through the thiol group. Because the bonding force between the sulfydryl and the quantum dots is greater than the bonding force between the carboxyl, the amino, the phosphoric acid and the like and the quantum dots, the coordination capacity of the polar ligand is stronger than that of the nonpolar ligand, the ligand exchange is facilitated, and the exchange efficiency is improved.

In one embodiment, the polar group is selected from one of carboxyl, thiol, amine, and phosphoric acid.

In this embodiment, when the polar ligand is added to the surface of the quantum dot layer, under the dual inducing action of the concentration difference of the ligand and the interaction between the cations on the surface of the quantum dot and the thiol groups of the polar ligand, the polar ligand is directly diffused along the direction of the hole transport layer, so as to induce the surface of the quantum dot layerLigand exchange occurs at a portion of the ligands (at the surface in contact with the electron transport layer). Meanwhile, in order to prevent the ligand exchange from occurring in the entire quantum dot film layer due to an excessive concentration difference, a vacuum treatment is performed on the quantum dot film layer (the vacuum condition is preferably a vacuum condition)<5×10^-4Pa) is added. Under the condition of vacuum pumping, an upward pulling force is generated on the ligand to be exchanged, so that the ligand to be exchanged is better balanced to be generated in the whole quantum dot film layer due to overlarge concentration difference. While the region in contact with the hole transport layer, which is still the original ligand of the quantum dot, is not subject to ligand exchange. By adopting the ligand exchange method, the region in contact with the hole transport layer in the quantum dot layer can still be kept in the original nonpolar state, and the region in contact with the electron transport layer is in the polar state. By the method, on the premise of keeping the original quantum dot fluorescence efficiency without loss, the compatibility between the quantum dot layer and the electron transmission layer is effectively improved, the surface contact angle between the quantum dot layer and the electron transmission layer is reduced, gaps and defects between film layers are filled, non-radiative recombination is effectively avoided, leakage current is reduced, and the luminous performance of the device is remarkably improved.

In this embodiment, the obtained quantum dot light emitting diode may be packaged. The packaging process can adopt common machine packaging or manual packaging. In one embodiment, the oxygen content and water content are both below 0.1ppm in the environment of the encapsulation process to ensure device stability.

The present invention will be described in detail below with reference to examples.

EXAMPLE 1

1. And (3) carrying out ligand exchange on the CdZnSe/ZnSe/ZnS quantum dot layer by adopting 11-mercaptoundecanoic acid to obtain the quantum dot layer with a partial region of 11-mercaptoundecanoic acid.

First, a CdZnSe/ZnSe/ZnS quantum dot layer was deposited on the hole transport layer. The concentration of the quantum dots is 20mg/ml, the solvent is n-octane, and the volume is 40 microliters.

Then, 300. mu.l of 11-mercaptoundecanoic acid was dropped onto the surface of the quantum dot layer, followed by vacuum condition of 6X 10^-4Spin coating was carried out at Pa (speed 800rpm for 1 min). After completion, 1 ml of isopropanol was added to rinse the surface of the quantum dot layer.

And drying the obtained substrate at 60 ℃ for 40min to obtain the quantum dot layer with a partial region of 11-mercaptoundecanoic acid.

2. Preparing a quantum dot light-emitting diode:

this embodiment quantum dot light emitting diode includes from bottom to top in proper order: an anode, a hole injection layer, a hole transport layer, a quantum dot layer, an electron transport layer and a cathode are sequentially deposited on the substrate. Wherein the substrate is a glass substrate; the anode is ITO with the thickness of 120 nm; PSS, the thickness of the hole injection layer is 80 nm; the hole transport layer was TFB and was 70nm thick. The quantum dot layer was the quantum dot layer prepared in the above step 1, and had a thickness of 70 nm. The electron transmission layer is ZnO with the thickness of 50 nm; the cathode is Al and has a thickness of 60 nm.

Comparative example 1

Consistent with example 1, except that the quantum dot layer did not undergo the ligand exchange process in step 1 above.

Example 2

1. And carrying out ligand exchange on the CdZnSe/ZnSe/ZnS quantum dot layer by adopting 8-mercapto octanoic acid to obtain the quantum dot film with a partial region of 8-mercapto octanoic acid.

Then, 200. mu.l of 8-mercaptooctanoic acid was dropped onto the surface of the quantum dot layer, followed by vacuum at 6X 10^-4Spin coating was carried out at Pa (speed 1000rpm, time 40 s). After completion, 1.5 ml of isopropanol was added to rinse the surface of the quantum dot layer.

And drying the obtained substrate at 70 ℃ for 30min to obtain the quantum dot film with a partial region of 8-mercapto octanoic acid.

2. Preparing a quantum dot light-emitting diode:

Comparative example 2

Consistent with example 2, except that the quantum dot layer did not undergo the ligand exchange process in step 1 above.

Example 3

1. And (3) carrying out ligand exchange on the CdZnSe/ZnSe/ZnS quantum dot film by adopting 12-mercaptododecanoic acid to obtain the quantum dot film with a partial region of 12-mercaptododecanoic acid.

Then, 250 microliters of 12-mercaptododecanoic acid was dropped onto the surface of the quantum dot layer, and then under vacuum conditions of 6 × 10^- ⁴Spin coating was carried out at Pa (speed 1400rpm, time 30 s). After completion, 1.2 ml of isopropanol was added to rinse the surface of the quantum dot layer.

And drying the obtained substrate at 80 ℃ for 25min to obtain the quantum dot layer with partial region of 12-mercaptododecanoic acid.

2. Preparing a quantum dot light-emitting diode:

this embodiment quantum dot light emitting diode includes from bottom to top in proper order: an anode, a hole injection layer, a hole transport layer, a quantum dot layer, an electron transport layer and a cathode are sequentially deposited on the substrate. Wherein the substrate is a glass substrate; the anode is ITO with the thickness of 120 nm; PSS, the thickness of the hole injection layer is 80 nm; the hole transport layer was TFB and was 90nm thick. The quantum dot layer was prepared in the above step 1, and had a thickness of 60 nm. The electron transmission layer is ZnO with the thickness of 50 nm; the cathode is Al and has a thickness of 80 nm.

Comparative example 3

Consistent with example 3, except that the quantum dot layer did not undergo the ligand exchange process in step 1 above.

Example 4

1. For Cd_xZn_1-xSe/Cd_yZn_1-ySe/ZnSe/ZnS(0<x<1，0<y<1, and x<y) performing ligand exchange on the quantum dot layer by adopting 11-mercaptoundecanoic acid to obtain the quantum dot layer with a partial region of 11-mercaptoundecanoic acid.

First, Cd is deposited on the hole transport layer_xZn_1-xSe/Cd_yZn_1-ySe/ZnSe/ZnS(0<x<1，0<y<1, and x<y) a quantum dot layer. The concentration of the quantum dots is 20mg/ml, the solvent is n-octane, and the volume is 40 microliters.

2. Preparing a quantum dot light-emitting diode:

this embodiment quantum dot light emitting diode includes from bottom to top in proper order: an anode, a hole injection layer, a hole transport layer, a quantum dot layer, an electron transport layer and a cathode deposited on the substrate in sequence. Wherein the substrate is a glass substrate; the anode is ITO with the thickness of 120 nm; PSS, the thickness of the hole injection layer is 80 nm; the hole transport layer was TFB and was 70nm thick. The quantum dot light-emitting layer is the quantum dot layer prepared in the step 1 and has a thickness of 70 nm. The electron transmission layer is ZnO with the thickness of 50 nm; the cathode is Al and has a thickness of 60 nm.

Comparative example 4

Consistent with example 4, except that the quantum dot layer did not undergo the ligand exchange process in step 1 above.

Example 5

1. For Cd_xZn_1-xSe/Cd_yZn_1-ySe/ZnSe/ZnS(0<x<1，0<y<1, and x<y) ligand exchange is carried out on the quantum dot layer by adopting 8-mercapto octanoic acid, and the quantum dot layer with a partial region of 8-mercapto octanoic acid is obtained.

Then, 200. mu.l of 8-mercaptooctanoic acid was dropped onto the surface of the quantum dot layer, followed by vacuum condition of 6X 10^-4Spin coating was carried out at Pa (speed 1000rpm, time 40 s). After completion, 1.5 ml of isopropanol was added to rinse the surface of the quantum dot layer.

And drying the obtained substrate at 70 ℃ for 30min to obtain a quantum dot layer with a partial region of 8-mercapto octanoic acid.

2. Preparing a quantum dot light-emitting diode:

this embodiment quantum dot light emitting diode includes from bottom to top in proper order: an anode, a hole injection layer, a hole transport layer, a quantum dot layer, an electron transport layer and a cathode deposited on the substrate in sequence. Wherein the substrate is a glass substrate; the anode is ITO with the thickness of 120 nm; PSS, the thickness of the hole injection layer is 80 nm; the hole transport layer was TFB and was 70nm thick. The quantum dot layer was the quantum dot layer prepared in the above step 1, and had a thickness of 70 nm. The electron transmission layer is ZnO with the thickness of 50 nm; the cathode is Al and has a thickness of 60 nm.

Comparative example 5

Consistent with example 5, except that the quantum dot layer did not undergo the ligand exchange process in step 1 above.

Example 6

1. For Cd_xZn_1-xSe/Cd_yZn_1-ySe/ZnSe/ZnS(0<x<1，0<y<1, and x<y) ligand exchange is carried out on the quantum dot layer by adopting 12-mercaptododecanoic acid to obtain the quantum dot layer with a partial region of 12-mercaptododecanoic acid.

2. Preparing a quantum dot light-emitting diode:

Comparative example 6

Consistent with example 6, except that the quantum dot layer did not undergo the ligand exchange process in step 1 above.

Device EQE (%), prepared in Table 1, comparative example and examples 1-6

The quantum dot layers and quantum dot light emitting diodes prepared in comparative examples and examples 1 to 6 were subjected to the above performance tests by the following methods:

external quantum dot efficiency:

the ratio of the number of electrons-holes injected into the quantum dots to the number of emitted photons, the unit is%, is an important parameter for measuring the quality of the electroluminescent device, and can be obtained by measuring with an EQE optical measuring instrument. The specific calculation formula is as follows:

where η e is the light output coupling efficiency, η r is the ratio of the number of recombination carriers to the number of injection carriers, χ is the ratio of the number of excitons generating photons to the total number of excitons, K_RTo the rate of the radiation process, K_NRIs the non-radiative process rate.

In summary, the invention provides a quantum dot light emitting diode and a preparation method thereof. According to the invention, the region of the quantum dot layer close to the electron transmission layer is coordinated as the polar second ligand, and the rest regions of the quantum dot layer are still kept as the nonpolar first ligands of the quantum dots, so that the surface of the quantum dot layer in contact with the electron transmission layer is in a polar state, and the rest regions of the quantum dot layer are still kept in an original nonpolar state.

It is to be understood that the invention is not limited to the examples described above, but that modifications and variations may be effected thereto by those of ordinary skill in the art in light of the foregoing description, and that all such modifications and variations are intended to be within the scope of the invention as defined by the appended claims.

Claims

1. A quantum dot light emitting diode comprising a first functional layer, a second functional layer, and a quantum dot layer disposed between the first functional layer and the second functional layer,

the quantum dot layer is formed of quantum dots to the surfaces of which a first ligand and a second ligand are bound,

2. The quantum dot light-emitting diode of claim 1, wherein one of the first functional layer and the second functional layer is a hole transport layer and the other is an electron transport layer, the hole transport layer being non-polar and the electron transport layer being polar.

3. The qd-led of claim 2, wherein the carbon chain of the polar ligands in the first and second ligands has a thiol group at one end and a polar group at the other end, and the polar ligands are bonded to the surface of the qd layer through the thiol group.

4. The qd-led of claim 3, wherein the polar group is selected from one of carboxyl, mercapto, amine, and phosphoric acid.

5. The qd-led of claim 2, wherein the non-polar ligands in the first and second ligands are selected from one or more of organic carboxylic acids with a carbon number of 8 or more, primary amines with a carbon number of 8 or more, secondary or tertiary amines with a branched carbon number of 4 or more, and organic phosphines with a branched carbon number of 4 or more.

6. The quantum dot light-emitting diode of claim 1, wherein the length of the first ligand carbon chain is the same as the length of the second ligand carbon chain.

7. A method for preparing a quantum dot light-emitting diode according to any one of claims 1 to 6, comprising the steps of:

under the condition of vacuum pumping, covering another ligand on the surface of the quantum dot layer to perform ligand exchange with the ligand, so as to obtain the quantum dot layer with the surface combined with the other ligand;

8. The method of claim 7, wherein one of the ligand and the other ligand is polar and the other ligand is non-polar.

9. The method for preparing a quantum dot light-emitting diode according to claim 7, wherein the ratio of the other ligand to the quantum dot by volume is 0.1-10: and 1, covering the other ligand on the surface of the quantum dot layer.

10. The method as claimed in claim 7, wherein the step of covering the surface of the quantum dot layer with the other ligand to exchange the ligand with the ligand comprises:

and after the quantum dot layer surface is covered with the other ligand for ligand exchange, washing by using an organic solvent, and drying to obtain the quantum dot layer with the other ligand combined on the surface.