KR20230036920A - Ⅱ-Ⅵ based non-Cd quantum dots, manufacturing method thereof and QLED using the same - Google Patents

Abstract

Provided are an II-VI series non-Cd blue light emitting quantum dot which improves a wide full width at half maximum and poor quantum efficiency, and a manufacturing method thereof. In addition, a quantum dot-light emitting device using the II-VI series non-Cd blue light emitting quantum dot is also provided. The quantum dot according to the present invention is a ZnSeTe/ZnSe/ZnS core/shell/shell quantum dot which comprises: II-VI ternary ZnSeTe core with a nominal Te/Se ratio of 0.01-0.05; a ZnSe inner shell surrounding the core and having a thickness of 1-3 nm; and a ZnS outer shell surrounding the ZnSe inner shell and having a thickness of 0.5-2 nm. Accordingly, QLEDs can be manufactured with a solution method using the quantum dot.

Description

II-VI based non-Cd quantum dots, manufacturing method thereof and quantum dot-light emitting device using the same {II-VI based non-Cd quantum dots, manufacturing method thereof and QLED using the same}

The present invention relates to a non-Cd composition quantum dot (QD), a manufacturing method thereof, and a device using the same, and more particularly, to a II-VI blue light emitting QD, a manufacturing method thereof, and a QD-light emitting device using the same.

As a strong competitor of organic light-emitting diodes (OLEDs), QD display devices are liquid crystal display devices (LCD)-based QD-enhancement film (QDEF) non-self-emissive method to OLED-driven QD pixels ( Commonly known as QD-OLED), it is developing into a self-luminous method. Although the two QD display devices have different operating principles, the former utilizes the color conversion of green and red emitting QDs by excitation of a blue light source provided by an InGaN blue LED chip and the latter by a blue OLED. In addition to the above color conversion QD devices, QD electroluminescence (EL) devices (also known as QD light emitting diodes, QLEDs) have been considered the best self-emissive platforms. Thanks to outstanding advances in heterostructure design of core/shell QDs, charge transport materials and device architecture, outstanding external quantum efficiencies (EQEs) of up to 20% or more have been achieved in Cd (Se, S) and even non-Cd composition based QLEDs. .

Non-Cd type III-V type InP QDs that do not use heavy metals are in the limelight as environmentally friendly light emitting materials for displays and lighting. However, InP QDs appear to be only effective in green and red, and their blue luminescence is significantly inferior to both photoluminescence (PL) wavelength and quantum yield (QY). Since InP QDs have a relatively low-energy bulk band gap of 1.35 eV, in order to emit high-energy blue light, InP QDs must be very small or located in a very strong quantum confinement region. However, it is difficult to develop a synthesis method for controlling it. In the meantime, several types of InP blue light-emitting QDs have been proposed, but the emission wavelength is >468 nm and the QY of blue InP QDs is less than 45%, making it difficult to commercialize them.

Among the non-Cd composition candidates, the bulk band gap (2.69 eV) of II-VI ZnSe QDs is close to blue. In order to produce a more appropriate blue wavelength (450-460 nm), the size of ZnSe QDs must be very large, but it is also difficult to develop a synthesis method for size control. For this reason, most of the ZnSe QDs developed to date have been reported with intermediate wavelengths between violet (420 nm) and blue (440 nm). Alloying Te to ZnSe QDs is the most feasible means to gradually extend the PL towards longer wavelengths. However, the wide full width at half maximum (FWHM) and poor QY must be improved.

Regarding the existing blue light-emitting ZnSeTe QDs, the PL QY is 70% and the EQE of the EL device is 4.2% (Jang, E.-P.; Han, C.-Y.; Lim, S.-W. ;Jo, J.-H.; Jo, D.-Y.; Lee, S.-H.; Yoon, S.-Y.; Yang, H. Synthesis of Alloyed ZnSeTe Quantum Dots as Bright, Color-Pure Blue Emitters. ACS Appl. Mater. Interfaces 2019 , 11 , 46062-46069). There are also results that achieved 9.5% EQE in blue (445 nm) QD synthesis and QLED with PL QY of 84% (Han, C.-Y.; Lee, S.-H.; Song, S.-W.; Yoon , S.-Y.; Jo, J.-H.; Jo, D.-Y.; Kim, H.-M.; Lee, B.-J.; Kim, H.-S.; Yang, H.; More Than 9% Efficient ZnSeTe Quantum Dot-Based Blue Electroluminescent Devices. ACS Energy Lett. 2020 , 5 , 1568-1576).

There are also results for the synthesis of cube-shaped blue (457 nm) QDs (Kim, T.; Kim, K.-H.; Kim, S.; Choi, S.-M.; Jang, H.; Seo, H.- K.; Lee, H.; Chung, D.-Y.; Jang, E. Efficient and Stable Blue Quantum Dot Light-Emitting Diode. Nature 2020 , 586 , 385-389). Cube-shaped QDs were prepared in a precise but complex two-step manner by adding hydrofluoric acid (HF) and ZnCl ₂ to eliminate stacking faults in the ZnSe inner shell, followed by separate organic-inorganic ligand exchange (i.e., oleic acid → Cl ^- ) should be performed. The QD surface was treated with Cl ⁻ and the fabricated blue QLED showed a high EQE of 14.3%.

For the commercialization of II-VI blue light-emitting QDs, a QD manufacturing method with further improved emission characteristics is required.

The problem to be solved by the present invention is to provide a II-VI non-Cd blue light emitting quantum dot and a manufacturing method thereof while improving a wide half width and poor quantum efficiency.

Another problem to be solved by the present invention is to provide a quantum dot-light emitting device using such II-VI non-Cd blue light emitting quantum dots.

In order to solve the above problems, the quantum dot according to the present invention is a II-VI ternary ZnSeTe core having a nominal Te/Se ratio of 0.01-0.05; a ZnSe inner shell surrounding the core and having a thickness of 1 nm to 3 nm; and a ZnS outer shell surrounding the ZnSe inner shell and having a thickness of 0.5 nm to 2 nm.

The core diameter may be 5 nm-8 nm.

The quantum dots emit blue light.

In order to solve the above problems, the quantum dot manufacturing method according to the present invention comprises the steps of injecting a Se precursor and a Te precursor into a first mixed solution containing a Zn precursor and a solvent to form a II-VI ternary ZnSeTe core; A Zn source solution capable of forming a ZnSe shell and a Se precursor are injected into the first mixed solution in which the ZnSeTe core is formed to form a ZnSe inner shell surrounding the ZnSeTe core, thereby forming a ZnSeTe/ZnSe core/shell quantum dot. forming a; preparing a ZnSeTe/ZnSe core/shell quantum dot dispersion after separating the ZnSeTe/ZnSe core/shell quantum dots from the first mixed solution; And by injecting a Zn raw material solution capable of forming a ZnS shell and an S precursor into the second mixed solution including the ZnSeTe/ZnSe core/shell quantum dot dispersion to form a ZnS outer shell surrounding the ZnSe inner shell, thereby forming a ZnSeTe/ and forming ZnSe/ZnS core/shell/shell quantum dots.

In the method for manufacturing a quantum dot according to the present invention, the forming of the ZnSeTe core may include: firstly heating a first mixed solution including a Zn precursor and a solvent; Secondarily heating the first mixed solution to a temperature higher than the first heating temperature; and injecting and reacting the Se precursor and the Te precursor into the first mixed solution.

In this case, during the step of injecting and reacting the Se precursor and the Te precursor into the first mixed solution, a third step of heating at a temperature higher than the second heating temperature may be further included.

In the step of forming the ZnSeTe/ZnSe core/shell quantum dots, the Zn raw material solution and the Se precursor capable of forming a ZnSe shell during a reaction time of 60 minutes or more and 180 minutes or less in the first mixed solution in which the ZnSeTe core is formed It may include the step of injecting.

Mutual diffusion between the ZnSeTe core and the ZnSe inner shell may be performed during the forming of the ZnSeTe/ZnSe core/shell quantum dots.

During the step of forming the ZnSeTe/ZnSe core/shell quantum dots, the temperature of the first mixed solution is preferably maintained at 300° C. or higher.

The Zn raw material solution capable of forming the ZnSe shell may be prepared by dissolving a Zn precursor in a solvent containing fatty acid and at least one of TOP, TBP, and TOA.

In the quantum dot manufacturing method according to the present invention, the forming of the ZnSeTe / ZnSe / ZnS core / shell / shell quantum dots,

heating a second mixed solution containing a Zn precursor and a solvent; secondarily heating the second mixed solution to a temperature higher than the first heating temperature after injecting the ZnSeTe/ZnSe core/shell quantum dot dispersion into the second mixed solution; and injecting and reacting a Zn source solution capable of forming a ZnS shell with an S precursor into the second mixed solution.

The Zn raw material solution capable of forming the ZnS shell may be prepared by dissolving a Zn precursor in a solvent containing fatty acid, primary amine, and TOA.

At this time, the temperature during the secondary heating of the secondary heating of the second mixed solution may be 320-350 °C.

In the quantum dot manufacturing method according to the present invention, the nominal Te/Se ratio of the ZnSeTe core is 0.01-0.05, the thickness of the ZnSe inner shell is 1 nm-3 nm, and the thickness of the ZnS outer shell is 0.5 nm. -2nm can be made.

The core diameter may be 5 nm-8 nm.

The present invention also provides a quantum dot-light emitting device comprising the quantum dots according to the present invention. This quantum dot-light emitting device includes a hole transport layer, a quantum dot light emitting layer, and an electron transport layer, and the quantum dot light emitting layer includes the quantum dots according to the present invention.

The quantum dot-light emitting device according to the present invention further includes an anode, a hole injection layer, and a cathode, and the hole transport layer and the hole injection layer are poly(ethylenedioxythiophene):polystyrene sulphonate (PEDOT:PSS), poly[(9, 9- dioctyl-fluorenyl-2, 7-diyl)-co-(4,4'-(N-(p-butylphenyl))diphenylamine)](TFB), poly(9-vinlycarbazole)(PVK), N, N, N , N', N'-tetrakis (4-methoxyphenyl) -benzidine (TPD), poly-TPD, 4, 4', 4''-tris (N-carbazolyl) -triphenylamine (TCTA), N, N'-bis (naphthalen-1-yl)-N, N'bis(phenyl)-9, 9-spiro-bifluorene (spiro-NPB), dipyrazino[2, 3-f:2', 3'-h]quinoxaline-2, It may be any one selected from 3, 6, 7, 10, 11-hexacarbonitrile (HATCN), 1, 1-bis[(di-4-tolylamino)phenylcyclohexane (TAPC), p-type metal oxides, and combinations thereof. .

The electron transport layer includes metal oxide nanoparticles, and the metal oxide nanoparticles include Zn-containing Mg oxide nanoparticles having a composition of Zn _1-x Mg _x O (0≤x≤0.5); and a Mg ion surface treatment layer formed on the surface of the nanoparticles.

According to the present invention, QDs with excellent PL QYs of up to 96% and moderate deep blue (especially >450 nm) luminescence are provided. And using these QDs, QLEDs can be manufactured by a solution method.

The blue QLED manufactured according to the present invention shows high electroluminescence performance of luminance of 6107-12654 cd/m ² and external quantum efficiency of 5.3-18.6% depending on the ZnSe inner shell, ZnS outer shell thickness, and ZnSeTe core composition.

1 is a schematic diagram of a QD according to one embodiment of the present invention.
2 is a flow chart of a QD manufacturing method according to an embodiment of the present invention.
3 is a cross-sectional view of a QD-light emitting device according to an embodiment of the present invention.
Figure 4 shows (a) PL spectrum as a function of ZnSe shell growth time, (b) changes in PL peak wavelength, QY, and FWHM, (c) absorption spectrum of ZnSeTe/ZnSe QD, (d) TEM image, and (e) It is a schematic diagram depicting the enlargement of the ZnSeTe core during the ZnSe shelling process.
5 is a PL spectral decomposition diagram of (a) ZnSeTe, (b) ZnSeTe/thin-ZnSe, (c) ZnSeTe/medium-ZnSe, (d) ZnSeTe/thick-ZnSe QDs, (e) are (a) to (d) ), which summarizes the results obtained from
6 shows (a) TEM image of ZnSeTe/ZnSe/ZnS QD, (b) XRD pattern of ZnSeTe core, ZnSeTe/ZnSe/ZnS QD, (c) normalized PL, (d) PL peak wavelength, QY, FWHM and (e) PL decay profile.
7 shows (a) EDS spectra and actual compositions of ZnSeTe cores synthesized with a nominal Te/Se molar ratio of 0.035, (b) ZnSeTe/thin-ZnSe/ZnS QDs, ZnSeTe/medium-ZnSe/ZnS QDs, and ZnSeTe/thick -EDS spectrum of ZnSe/ZnS QD, (c) Composition (atomic %) of Zn, Se, and S.
8 is a comparison of absorption spectra between core/shell and core/shell/shell QDs, in the case of (a) thin ZnSe, (b) medium ZnSe, and (c) thick ZnSe.
9 is a comparison of PL decay profiles between core/shell and core/shell/shell QDs, in the case of (a) thin ZnSe, (b) medium ZnSe, and (c) thick ZnSe.
10 shows (a) a schematic diagram of a multilayer blue QLED device, (b) a cross-sectional TEM image of a blue QLED fabricated with ZnSeTe/mid-ZnSe/ZnS QDs (inset: higher magnification corresponding to the dot region), and (c) multilayer EQE-current density relationship with energy level of blue QLED, (d) normalized EL spectrum (collected at 8 V), (e) current density-voltage, (f) luminance-current density and (g) current efficiency.
11 is an enlarged cross-sectional TEM image of blue QLEDs fabricated with (a) ZnSeTe/thin-ZnSe/ZnS and (b) ZnSeTe/thick-ZnSe/ZnS QDs.
12 is a spectral comparison of solution PL (top) vs. 8 V driven EL (bottom): (a) ZnSeTe/thin-ZnSe/ZnS, (b) ZnSeTe/medium-ZnSe/ZnS, (c) ZnSeTe/thick-ZnSe This is the case for /ZnS QDs.
13 shows absorption and PL spectra of (a) ZnSeTe core and (b) ZnSeTe/mid-ZnSe core/shell QDs with Te/Se molar ratios of 0.023, 0.035, and 0.047.
14 shows (a) absorption and PL spectra, (b) comparison of PL peak wavelength, QY, and FWHM of blue ZnSeTe/intermediate ZnSe/ZnS QDs synthesized with different Te/Se molar ratios of 0.023, 0.035, and 0.047, and (c) room illumination. and a photograph under a UV lamp.
15 shows changes in PL peak wavelength, QY, and FWHM according to the shelling step, with Te/Se ratios of (a) 0.023, (b) 0.035, and (c) 0.047.
16 shows (a) current density-voltage, (b) luminance-current density, and (d) current efficiency of blue devices fabricated with ZnSeTe/Medium ZnSe/ZnS QDs with Te/Se molar ratios of 0.023, 0.035, and 0.047; EQE-current density relationship, (d) voltage-dependent EL spectrum evolution of a device with a Te/Se molar ratio of 0.023, (e) voltage-dependent EL spectrum evolution of a device with a Te/Se molar ratio of 0.035, (f) Te/Se Voltage-dependent EL spectral evolution of a device with a molar ratio of 0.047, (g) CIE color coordinates recorded at 8 V.
17 shows (a) TEM images of ZnSeTe/Medium-ZnSe/Thin-ZnS and ZnSeTe/Medium-ZnSe/Thick-ZnS QDs based on Te/Se=0.035, (b) Absorption, (c) ZnS outer shell thickness dependent core Comparison of PL spectra of /shell/shell QDs, (d) current density and luminance-voltage as a function of ZnS outer shell thickness, (f) current density-voltage characteristics of EOD and HOD as a function of ZnS outer shell thickness, ( g) It is a schematic diagram depicting the degree of injection of electrons and holes (difference in charge balance) differently according to the thickness of the ZnS outer shell.
18 is a graph comparing PL peak wavelength, QY, and FWHM of ZnS outer shell thickness dependent core/shell/shell QDs.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, but will be implemented in a variety of different forms, only these embodiments will complete the disclosure of the present invention, and will fully cover the scope of the invention to those skilled in the art. It is provided to inform you.

Ternary ZnSeTe QDs are the most promising non-Cd blue emitters for the fabrication of self-emissive QD display devices or QLEDs. In the present invention, a high-quality blue ZnSeTe QD with a double shell structure of a ZnSe inner shell and a ZnS outer shell can be provided by a unique two-step approach.

1 is a schematic diagram of a QD according to one embodiment of the present invention.

The QD 100 according to an embodiment of the present invention surrounds the II-VI ternary ZnSeTe core 10, the ZnSe inner shell 20 surrounding the core 10, and the ZnSe inner shell 20. includes a ZnS outer shell 30.

In the ZnSeTe core 10, the Te/Se ratio is adjusted to achieve a blue (450 nm or more) emission band. In realizing a blue wavelength that is difficult to implement through alloying with ZnTe (2.3eV), which has a smaller bulk band gap than ZnSe (2.69eV), the band gap changes according to the Te/Se ratio. A desired target wavelength can be expressed by appropriately adjusting the Te/Se ratio. The Te/Se ratio of the core of the present invention should be understood in consideration of the fact that the size and shape of the QDs vary slightly depending on the manufacturing method, and thus the band gaps of the QDs of the same composition are different.

The Te/Se ratio also affects the current efficiency. The higher the Te/Se ratio, the higher the current efficiency over the entire current density range. The Te/Se ratio also affects the FWHM. In order for the ZnSeTe core 10 to have a wavelength of 451 nm-463 nm and a FWHM of 18 nm-38 nm, the nominal Te/Se ratio in the ZnSeTe core 10 is preferably 0.01-0.05. Outside of the 0.01-0.05 ratio, it is difficult to achieve a blue (450 nm or more) emission band. More preferably, it is 0.023-0.047. As the nominal Te/Se ratio changes from 0.023 to 0.047, the PL peak wavelength expands from 451 to 463 nm and the FWHM from 18 to 38 nm in the ZnSeTe/ZnSe/ZnS core/shell/shell QD (100). Outside the 0.023-0.047 ratio, the PL peak wavelength and FWHM are different.

The ZnSe inner shell 20 surrounds and covers the ZnSeTe core 10 . The ZnS outer shell 30 surrounds and covers the ZnSe inner shell 20 . Various defects may exist on the surface of the ZnSeTe core 10, and these defects act as non-radiative relaxation sites, resulting in inferior QY. The ZnSe inner shell 20 and the ZnS outer shell 30 cap surface defects of the ZnSeTe core 10 to have a more improved QY and a narrower half width.

The diameter of the core 10 may be greater than 5 nm. For example, the diameter of the core 10 may be 5 nm-8 nm. A QD 100 having a core/shell/shell structure including a core 10 of this diameter may have a diameter of 10 nm or more. For example, a core/shell/shell QD 100 may have a diameter of 10 nm-12 nm. These QDs (100) can emit blue light.

A larger ZnSeTe core diameter is desirable because it becomes a non-radiative pathway and makes Auger recombination, which is well-known the most detrimental factor in QLED efficiency, difficult. However, it is difficult to increase the ZnSeTe core diameter to 5 nm or more in a controllable manner using conventional manufacturing methods. According to one embodiment of the present invention, the diameter of the ZnSeTe core 10 can be made 5 nm or more. The QD 100 including such a core 10 has excellent PL characteristics.

According to one embodiment of the present invention, the size of the core 10 can be increased to 5 nm or more and can be made up to about 10 nm, but it may be difficult to smoothly form shells with a uniform thickness subsequent to the large-diameter core. . If the shells are not well formed, the PL characteristics of the QD are rather deteriorated. Therefore, the size of the core 10 can be 8 nm or less.

The thickness of the ZnSe inner shell 20 may be 1 nm-3 nm. The thickness of the ZnS outer shell 30 may be 0.5 nm-2 nm. This thickness is a thickness for exhibiting a satisfactory level of PL or EL characteristics. When the thickness of the ZnSe inner shell 20 is less than 1 nm and the thickness of the ZnS outer shell 30 is less than 0.5 nm, the passivation degree of the core 10 is insufficient and the FWHM cannot be narrowed, which is not preferable. When the thickness of the ZnSe inner shell 20 is greater than 3 nm and the thickness of the ZnS outer shell 30 is greater than 2 nm, PL QY is reduced, which is not preferable.

In the experimental example, the highest QY (40%) was achieved when a 2.16 nm thick ZnSe inner shell 20 was formed for a ZnSeTe core having a size of 5.38 nm. It can be estimated that the critical thickness of the ZnSe inner shell 20, which can grow consistently without significant interfacial stress, is 2.16 nm. Further formation of the ZnS outer shell 30 further increases PL QY. For a ZnSeTe core with a size of 5.38 nm, the thickness of the ZnSe inner shell 20 may be 1.14 nm-2.47 nm. And the thickness of the ZnS outer shell 30 may be 0.52nm-1.00nm. In this way, as the ZnSe inner shell 20 and the ZnS outer shell 30 are included, the QD 100 according to an embodiment of the present invention may exhibit an excellent PL QY of up to 96%.

QD (100) according to an embodiment of the present invention having such a configuration is a blue light-emitting QD. In particular, QDs 100 according to an embodiment of the present invention possess appropriate deep blue (particularly >450 nm) luminescence.

In this double-shell ZnSeTe / ZnSe / ZnS heterostructure QD (100), by controlling the thickness of the ZnSe inner shell 20 and the ZnS outer shell 30 as well as the Te / Se composition of the ZnSeTe core 10, It is possible to adjust the PL of ) and the EL performance of a device including these QDs (100). As a result of a nominal Te/Se ratio of 0.023–0.047 in the ZnSeTe core (10), the deep blue ZnSeTe/ZnSe/ZnS QDs exhibit tunable PL wavelengths of 451–463 nm and narrow bandwidths of 18–38 nm, with a ZnSe inner shell (20 ), its properties are controlled by the composition of the ZnSeTe core 10 together with the thickness.

2 is a flow chart of a QD manufacturing method according to an embodiment of the present invention.

The present invention proposes a method for synthesizing blue QDs of the ZnSeTe/ZnSe/ZnS structure as described above by a two-step approach. Here, we propose to transfer continuously grown single-shell ZnSeTe/ZnSe QDs with controllable ZnSe thickness to a separate pot and place them on the next ZnS shelling. Extensive synthesis of blue QDs can be performed by this two-step approach. And, it is possible to synthesize high-efficiency QDs in a very simple way.

Referring to FIG. 2, first, a II-VI ternary ZnSeTe core is synthesized (step S10).

In the synthesis of the ZnSeTe core, a heat-up method of forming a core by mixing a precursor material at a low temperature and then heating it to a high temperature is most preferred. However, the present invention is not limited thereto, and a hot-injection method may be used to form a core by injecting a precursor material at a high temperature.

For example, a ZnSeTe core is synthesized by reacting a Zn precursor, a Se precursor, and a Te precursor. First, the first mixed solution containing the Zn precursor and the solvent may be primarily heated to a low temperature, for example, 120° C., and degassed. The solvent of the first mixed solution may be oleic acid (OA) or 1-octadecene (ODE).

Zn precursor may be zinc acetate [Zn(Ac) ₂ ]. In addition to preparing the Zn precursor, Zn metal powder, ZnO, zinc chloride or zinc stearate may be included.

Then, the first mixed solution may be secondarily heated to a temperature higher than the first heating temperature. For example, the temperature of the first mixed solution may be raised to 210°C. After the temperature is stabilized, the Se precursor and the Te precursor may be injected into the first mixed solution at a desired Te/Se ratio.

The Se precursor may be Se powder or a Se stock solution prepared by dissolving Se powder. Likewise, the Te precursor may be a Te stock solution. The solvent of the stock solution may include one or more of diphenylphosphine (DPP), trioctylphosphine (TOP), tributylphosphine (TBP), and tri-phenylphosphine (TPP). For example, the Se precursor may be Se-DPP, and the Te precursor may be Te-TOP.

After injecting the Se precursor and the Te precursor, it may be maintained for a certain period of time so that the reaction is sufficiently performed. At this time, the step of performing third heating at a temperature higher than the second heating temperature may be further included. That is, the step of raising the temperature of the first mixed solution of 210 ° C., for example, to 300 ° C., and maintaining the temperature for a certain period of time may be further included. By this method, the ZnSeTe core is synthesized in the first mixed solution. The size of the ZnSeTe core can be controlled by the reaction temperature and time.

Next, by forming a ZnSe inner shell (corresponding to 20 in FIG. 1) surrounding the core, a ZnSeTe/ZnSe core/shell quantum dot is formed (step S20). At this time, without taking out the ZnSeTe core from the first mixed solution in which the ZnSeTe core is synthesized, that is, continuously with the ZnSeTe core synthesizing step, a ZnSe shell is formed for the first mixed solution in which the ZnSeTe core is synthesized. It is important to form the ZnSe inner shell 20 surrounding the ZnSeTe core by injecting a possible Zn raw material solution and Se precursor. The Zn raw material solution and the Se precursor may be referred to together as a ZnSe stock solution.

For example, if the final temperature at the time of synthesizing the ZnSeTe core in the previous step S10 is 300 ° C, preferably, the Zn raw material solution capable of forming the ZnSe shell and the Se precursor are added to the first mixed solution while maintaining the temperature continuously. inject At this time, a Zn raw material solution capable of forming a ZnSe shell may be prepared by dissolving a Zn precursor in a solvent containing fatty acid and at least one of TOP, TBP, and TOA. The fatty acid may be palmitic acid, myristic acid (stearic acid) or oleic acid (OA). For example, a solution prepared by dissolving Zn(Ac) ₂ in OA, TOP, and trioctylamine (TOA) may be used (hereinafter referred to as Zn(OA) ₂ solution).

As in the previous example, if ZnSe shelling is performed at a high temperature of 300 ° C, there is a very high possibility of mutual diffusion between the pre-grown ZnSeTe core and the ZnSe shell during shelling, and through this, the effective area of the ZnSeTe core can be expanded to some extent. The area of the ZnSeTe core 10 of QD 100 of 1 can be defined. As shelling progresses, since the content of Te to Se in the enlarged ZnSeTe core is lowered, the tail emission is mitigated to further narrow the spectral symmetry of the entire PL or to improve color purity.

ZnSe shelling can be further improved by proceeding at a higher temperature of 300 ° C or higher. Therefore, it is preferable to set the reaction temperature to 300°C or higher by using a solvent having a boiling point higher than 300°C for the Zn raw material solution capable of forming the ZnSe shell. In this embodiment, TOA may be used as the solvent. Since TOA has a boiling point of 365° C., ZnSe shelling may be increased to 300° C. or higher.

As such, in the present invention, the reaction temperature for performing the ZnSe shelling may be increased to 300° C. or more, and the spectral symmetry of the entire PL may be further narrowed or the color purity may be further increased. The upper limit of the reaction temperature may be the boiling point of the solvent.

The amount and injection speed of the Zn raw material solution and the Se precursor may be appropriately controlled to form the ZnSe inner shell 20 having a desired thickness. For example, as the amount of the Zn raw material solution and the Se precursor increases, the thickness of the ZnSe inner shell 20 may increase.

For example, when using a Zn(OA) ₂ solution as a Zn raw material solution and Se-TOP as a Se precursor, compared to the case of injecting 10 mL of Zn(OA) ₂ solution and 2.5 mL of Se-TOP, Zn ( When OA) ₂ solution 20mL and Se-TOP 5.0mL are injected, the thickness of the ZnSe inner shell 20 is greater. Similarly, the thickness of the ZnSe inner shell 20 is greater when 30mL of Zn(OA) ₂ solution and 7.5mL of Se-TOP are injected than when 20mL of Zn(OA) ₂ solution and 5.0mL of Se-TOP are injected. .

The time for injecting the Zn raw material solution and the Se precursor into the first mixed solution, that is, the reaction time for forming the ZnSe inner shell 20 also affects the thickness of the ZnSe inner shell 20 . As the reaction time increases, the thickness of the ZnSe inner shell 20 increases. Preferably, the reaction time is 60 minutes or more and 180 minutes or less.

As the reaction time increases, as the thickness of the ZnSe inner shell 20 increases, the peak wavelength in the ZnSeTe core gradually redshifts. However, after a certain period of time, the redshift does not occur even if the reaction time is further increased. As the reaction time increases, the PL QY also changes. PL QY may gradually increase and then decrease as the reaction time increases. If an excessively thick ZnSe inner shell 20 is formed due to a reaction time longer than appropriate, PL QY is reduced due to an increase in interfacial stress.

In the experimental example, the highest QY (40%) was achieved when a 2.16 nm thick ZnSe inner shell 20 was formed for a ZnSeTe core having a size of 5.38 nm. The reaction time to form the ZnSe inner shell 20 of this thickness was 120 minutes. It can be estimated that the critical thickness of the ZnSe inner shell 20, which can grow consistently without significant interfacial stress, is 2.16 nm. And, if all other conditions are the same, it can be seen that the reaction time is preferably around 120 minutes.

As described above, in addition to changing the PL wavelength and QY according to the thickness of the ZnSe inner shell 20, the FWHM narrows as the thickness of the ZnSe inner shell 20 increases by affecting the PL bandwidth.

In addition, Auger recombination also becomes highly dependent on QD heterostructure details. The thicker the ZnSe inner shell 20 slows Auger recombination. In this context, in the case of blue QLEDs, where QDs are negatively charged during device operation due to asymmetrically excessive electron injection, QDs with a thick ZnSe inner shell (20) can be superior in suppressing Auger recombination, leading to improved device efficiency. It is highly likely that And the thicker the ZnSe inner shell 20 is, the longer the average lifetime of the QD 100 is.

Therefore, in consideration of all these conditions, the thickness of the ZnSe inner shell 20 can be adjusted in order to exhibit an appropriate PL wavelength, QY, and FWHM, and in the present invention, the amount of Zn raw material solution and Se precursor, and / or reaction time We suggest that you can use it to adjust.

After completion of injection of the Zn raw material solution and the Se precursor, the first mixed solution is cooled to room temperature, and the resultant ZnSeTe/ZnSe core/shell QD (that is, a structure in which the ZnSe core 10 is surrounded by the ZnSe inner shell 20) ) is separated from the first mixed solution, and then dispersed in a solvent such as hexane to prepare a ZnSeTe/ZnSe QD dispersion (step S30).

Next, by using the ZnSeTe / ZnSe QD dispersion, a ZnS outer shell 30 is formed on the surface of the ZnSeTe / ZnSe QD to form ZnSeTe / ZnSe / ZnS core / shell / shell quantum dots (step S40).

To perform this step, for example, the second mixed solution including the Zn precursor and the solvent may be first heated to a low temperature, for example, 120° C., and then degassed. The solvent of the second mixed solution may be, for example, OA, 1-hexadecylamine (HDA), or TOA.

After the temperature is stabilized, the ZnSeTe/ZnSe QD dispersion is injected into the second mixed solution. Then, the second mixed solution may be secondarily heated to a temperature higher than the first heating temperature. For example, it can be raised to 330 ° C. Subsequently, a Zn raw material solution capable of forming a ZnS shell and an S precursor are injected into the second mixed solution and reacted. The Zn raw material solution and the S precursor capable of forming a ZnS shell may also be referred to as a ZnS stock solution.

At this time, a Zn raw material solution capable of forming a ZnS shell may be prepared by dissolving a Zn precursor in a solvent containing fatty acid, primary amine, and TOA. The fatty acid may be palmitic acid, myristic acid or OA. The primary amine may be oleyl amine, octyl amine, or HAD. For example, as the Zn raw material solution capable of forming the ZnS shell, the same Zn(OA) ₂ solution used in step S20 may be used. The S precursor may be a S stock solution. As mentioned above, the solvent of the stock solution may be TOP, TBP, TPP, DPP, or the like. For example, the S precursor may be S-TOP.

The amount and injection speed of the Zn raw material solution and the S precursor can be appropriately controlled to suitably form the ZnS outer shell 30 having a desired thickness. For example, as the amount of the Zn raw material solution and the S precursor increases, the thickness of the ZnS outer shell 30 may increase.

For example, when a Zn(OA) ₂ solution is used as a Zn raw material solution and S-TOP is used as an S precursor, compared to the case of injecting 2 mL of Zn(OA) ₂ solution and 0.5 mL of S-TOP, Zn ( When OA) ₂ solution 4mL and S-TOP 1.0mL are injected, the thickness of the ZnS outer shell 30 is greater. Similarly, the thickness of the ZnS outer shell 30 is greater when 6mL of Zn(OA) ₂ solution and 1.5mL of S-TOP are injected than when 4mL of Zn(OA) ₂ solution and 1.0mL of S-TOP are injected. .

The QD 100 including the ZnS outer shell 30 is blue-shifted compared to the structure in which the ZnSe inner shell 20 surrounds the ZnSeTe core 10, which is due to the strengthening of quantum confinement and the increase in band gap. The ZnS outer shell 30 can build a high energy barrier against delocalization of the electronic wavelength function. Forming the ZnS outer shell 30 increases PL QY. This is because the formation of the ZnS outer shell 30 according to the present invention provides very effective surface passivation. By using TOA having a high boiling point (365° C.) as a solvent, the reaction temperature for forming the ZnS outer shell 30 can be raised to, for example, 335° C. As such, it is easy to form the ZnS outer shell 30 uniformly and thickly at a relatively high temperature, thereby manufacturing a QD 100 having an increased PL QY.

Increasing the thickness of the ZnS outer shell 30 makes injection of electrons rather than holes more impeded, which leads to better charge balance in a device comprising QDs 100. In this regard, as the thickness of the ZnS outer shell 30 increases, it may show superiority in terms of charge balance and device efficiency. The thicker the ZnS outer shell 30 is, the higher the luminance is. However, the thicker the ZnS outer shell 30 is, the lower the current density.

Therefore, considering all these conditions, the thickness of the ZnS outer shell 30 can be adjusted, and the present invention proposes that it can be adjusted using the amount of the Zn raw material solution and the S precursor, and/or the reaction temperature.

After completion of the injection of the Zn raw material solution and the S precursor, the second mixed solution is cooled to room temperature, and the resulting ZnSeTe/ZnSe/ZnS core/shell/shell QD (ie, the ZnSeTe core 10 is replaced with the ZnSe inner shell 20) This surrounds the ZnSe inner shell 20 and the ZnS outer shell 30 surrounds the QD 100 according to an embodiment of the present invention.

The QD 100 manufactured through this method may be redispersed in a solvent such as, for example, octane, and stored, distributed, or used to manufacture a device by a solution method.

As described above, the ZnSe inner shell 20 surrounds the ZnSeTe core 10, and the ZnSe inner shell 20 is surrounded by the ZnS outer shell 30, QD 100 according to an embodiment of the present invention In , the ZnSeTe core 10 and the ZnSe inner shell 20 are continuously formed in one pot. Since the ZnS outer shell 30 is formed in another pot, the ZnSe inner shell 20 and the ZnS outer shell 30 are formed discontinuously. That is, it is a unique two-step approach where the step of forming the inner shell 20 proceeds in one port and the step of forming the outer shell 30 proceeds in the other port.

Since the ZnS outer shell 30 is not continuously formed on the ZnSe inner shell 20 within one process and the process is terminated after forming the ZnSe inner shell 20 of a desired thickness, in a subsequent process, the ZnS outer shell During the formation of (30), the change in the thickness of the ZnSe inner shell 20 is small, and accordingly, the PL of the ZnSeTe QD 100 and the EL characteristics of the device including the same can be manufactured as desired.

In the present invention, a high-quality blue ZnSeTe QD (100) with a double shell structure of ZnSe inner shell (20) and ZnS outer shell (30) can be fabricated with a unique two-step approach. In the ZnSeTe/ZnSe/ZnS heterostructure, changing the thickness of the ZnSe inner shell 20 changes not only the thickness of the ZnS outer shell 30 but also PL characteristics such as peak wavelength, color purity related bandwidth, and quantum efficiency. To obtain a satisfactory level of PL characteristics, the thickness of the ZnSe inner shell 20 and the Te/Se ratio of the ZnSeTe core 10 may be adjusted. In particular, by separately proceeding the step of forming the ZnS outer shell 30, the ZnS outer shell 30 without a significant change in the thickness of the ZnSe inner shell 20 and the Te / Se ratio of the ZnSeTe core 10 adjusted in the previous step. ) can be formed. In addition, since the formation of the ZnS outer shell 30 can be performed at a high temperature, perfect capping can be achieved.

Despite the fact that additives (HF, ZnCl ₂ ) that can dramatically improve PL QY by 50-93% in the papers of Kim, T., etc. mentioned in the prior art are not introduced, one embodiment of the present invention The QD (100) synthesized according to can show a PL QY of 89% or more, and according to the experimental example, it showed a particularly high PL QY up to 96%.

QD (100) according to an embodiment of the present invention can be used to manufacture a blue QD-light emitting device. That is, the QD having the ZnSeTe/ZnSe/ZnS core/shell/shell composition according to the present invention prepared by the above method can be applied as a QD light emitting layer (EML) to a blue QLED for display. At this time, the blue QLED structure may have a positive structure of anode/hole injection layer (HIL)/hole transport layer (HTL)/blue QD EML/electron transport layer (ETL)/cathode, or an inverse structure opposite to the above stacking order. A blue QLED for display in which the QD having the ZnSeTe/ZnSe/ZnS core/shell/shell composition according to the present invention is applied to the QD EML can be applied as a blue excitation light source of a color conversion device. By using this QLED as a blue excitation light source and color-converting, for example, green and red InP QDs, a self-luminous display device implementing blue, green, and red may be manufactured.

3 is a cross-sectional view of a QD-light emitting device according to an embodiment of the present invention.

The example shown in FIG. 3 is the QLED 200 . The QLED 200 according to an embodiment of the present invention includes a hole transport layer 140 (HTL), a QD light emitting layer 150 (EML), and an electron transport layer 160 (ETL). The QD light emitting layer 150 is a layer that emits light by combining holes and electrons coming from the hole transport layer 140 and the electron transport layer 160, respectively. Such a multilayer structure may be formed on the substrate 110 serving as a mechanical support, and includes an anode 120 for hole injection, a cathode 170 for electron injection, and the anode 120 and the hole transport layer. A hole injection layer 130 (HIL) may be further included between the layers 140.

The substrate 110 may be a transparent glass substrate or a transparent plastic substrate having a flat surface. The substrate 110 may be used after ultrasonic cleaning with a solvent such as isopropyl alcohol (IPA), acetone, or methanol and UV-ozone treatment to remove contaminants.

The anode 120 and the cathode 170 include metals, metal oxides suitable for each transparent/opaque condition, or other non-oxide inorganic materials. For bottom light emission, the anode 120 may be made of a transparent conductive metal such as ITO, IZO, ITZO, or AZO, and the cathode 170 may be a metal having a small work function such as I, Ca, Ba, Ca/Al, LiF/Ca, LiF/Al, BaF ₂ /Al, BaF ₂ /Ca/Al, Al, Mg, Ag:Mg alloy, and the like can be used.

The hole injection layer 130 and the hole transport layer 140 facilitate hole injection from the anode 120 and serve to transfer holes to the QD light emitting layer 150 . Organic or inorganic materials can be applied to form them, and poly(ethylenedioxythiophene):polystyrene sulphonate (PEDOT:PSS), poly[(9, 9-dioctyl-fluorenyl-2, 7-diyl)-co-(4, 4' -(N-(p-butylphenyl))diphenylamine)](TFB), poly(9-vinlycarbazole)(PVK), N, N, N, N', N'-tetrakis(4-methoxyphenyl)-benzidine (TPD) , poly-TPD, 4, 4', 4''-tris(N-carbazolyl)-triphenylamine (TCTA), N, N'-bis(naphthalen-1-yl)-N, N'bis(phenyl)-9 , 9-spiro-bifluorene (spiro-NPB), dipyrazino[2, 3-f:2', 3'-h]quinoxaline-2, 3, 6, 7, 10, 11-hexacarbonitrile (HATCN), 1, 1 It may be any one selected from -bis[(di-4-tolylamino)phenylcyclohexane (TAPC), p-type metal oxides, and combinations thereof. The p-type metal oxide may be, for example, NiO, MoO ₃ , or WO ₃ . According to a preferred embodiment, the hole transport layer 140 is an organic material. In one specific example, the hole injection layer 130 is PEDOT:PSS, the hole transport layer 140 is PVK or TFB.

The QD light emitting layer 150 is a layer formed by filling the QDs 100 according to the present invention. Here, the QD light emitting layer 150 is formed by, for example, a solution process of coating a dispersion containing the QDs 100 in a solvent. After coating on the transport layer 140, it may be formed by volatilizing a solvent. The coating method may be, for example, drop casting, spin coating, dip coating, spray coating, flow coating, screen printing or inkjet. Printing and the like may be used alone or in combination.

The electron transport layer 160 facilitates injection of electrons from the cathode 170 and serves to transport electrons to the QD light emitting layer 150 . The electron transport layer 160 includes metal oxide nanoparticles. For example, the electron transport layer 160 may be formed by coating the QD light emitting layer 150 in a solution process of coating a dispersion containing the metal oxide nanoparticles in a solvent and then volatilizing the solvent.

The metal oxide nanoparticles may include Zn-containing Mg oxide nanoparticles having a composition of Zn _1-x Mg _x O (0≤x≤0.5); and a Mg ion surface treatment layer formed on the surface of the nanoparticles. The charge imbalance phenomenon can be improved through the Mg ion surface treatment layer. Since balanced injection of electrons and holes into the QD light emitting layer 150 is possible, performance of the QLED 200, particularly luminance and efficiency, can be improved.

In this way, the blue QLED 200 can be manufactured with a process using a solution. In the experimental example, the effect of the thickness of the ZnSe inner shell 20 and the ZnS outer shell 30 on the device performance was investigated to improve the charge balance. As a result, in the optimal QD heterostructure with a relatively thick outer shell, It was confirmed that a high EQE of 18.6% could be achieved.

Hereinafter, the present invention will be described in more detail by explaining the experimental examples of the present invention in detail. However, the present invention is not limited to the experimental examples below.

Through experiments, it was confirmed that changing the thickness of the ZnSe inner shell in the ZnSeTe/ZnSe/ZnS heterostructure resulted in a change in PL characteristics such as peak wavelength, color purity related bandwidth, and QY as well as the thickness of the ZnS outer shell. To prove the medium-level PL modulator in the deep blue region, the Te/Se ratio of the ZnSeTe core can be precisely controlled, and the effect of the thickness of the ZnS outer shell on the QLED performance is examined.

ingredient

Zinc acetate (Zn(Ac) ₂ , ≥ 99.9%), selenium (Se, 99.999% powder), 1-hexadecylamine (HDA, 90%) can be purchased from Alfa Aesar. Sulfur (S, 99.998%, on a metal basis), tellurium (Te, 99.8%, powder), oleic acid (OA, 90%) 1-octadecene (ODE, 90%), PVK (Mw = ~25000-50000) are It can be purchased from Sigma Aldrich. TPBi (99.9%), CBP (99.9%) and Molybdenum Oxide (MoO ₃ , 99.95%) can be purchased from OSM. Diphenylphosphine (DPP, >98%) and trioctylphosphine (TOP, ≥99%) are available from Lake Materials. Trioctylamine (TOA, 97%) is available from Acros Organics. All of the above chemicals were used in experiments as received without further treatment.

characterization

UV-Vis absorption and PL spectra of the QDs were recorded with an absorption spectrometer (Shimadzu, UV-2450) and a spectrophotometer (PSI Inc., Darsa Pro-5200) equipped with a 500 W xenon lamp, respectively. The absolute value of the PL quantum efficiency of the diluted QD dispersion was evaluated with a PL QY measurement system (Otsuka, QE-2000). TEM images of QD particles and cross-sectional QLEDs were obtained using a JEM-2100F (JEOL Ltd.). Supplementary information on the heterostructure evolution of the QDs was obtained using a powder X-ray diffractometer (Rigaku, Ultima IV) of Cu Kα radiation operating at 40 kV and 30 mA. The elemental composition of the QDs was evaluated by an electroluminescence scanning electron microscope (JEOL JSM-7800F) equipped with an energy dispersive X-ray spectrometer (Oxford Instruments X-MAX 80) operating at 15 kV. The VBM level of the QDs was estimated with a UV photoelectron spectrometer (Kratos Inc. AXIS Ultra DLD). The PL decay profiles of the QDs were recorded by time-correlated single photon counting (TCSPC) method on a spectrophotometer (Edinburg Instruments FS5) equipped with a picosecond pulsed diode laser (EPL-375). The EL spectrum and current-voltage-luminance (I-V-L) characteristics of the QLEDs were measured with a Konica-Minolta CS-2000A spectroluminometer and a Keithley 2400 voltage-current power supply.

Synthesis of ZnSeTe/ZnSe/ZnS QDs

To synthesize a blue light-emitting ZnSeTe core, a mixture containing ₂ mmol of Zn(Ac) 2 , 2 mL of OA, and 15 mL of ODE was heated to 120 °C and degassed for 1 hour in a 100 mL three-necked flask. The reaction flask was then heated to 210 °C in a N ₂ atmosphere. After the temperature stabilized, 0.5 mL of 2 M Se-DPP and 0.75 mL of 0.047 M Te-TOP solution (i.e., nominal Te/Se ratio = 0.035) were sequentially injected into the reaction flask. The growth of the ZnSeTe core was maintained at this temperature for 30 minutes, then increased to 300 °C and maintained for 1 hour.

1.2 M Se-TOP 2.5, 5.0, 7.5 mL and 10, 20, 30 mL of Zn(OA) ₂ solution (prepared by dissolving 60 mmol of Zn(Ac) ₂ in 40 mL of OA, 20 mL of TOP, and 20 mL of TOA) respectively were added dropwise together with a syringe pump at 300 ° C to form thin, medium, and thick ZnSe inner shells, respectively, to form QDs having ZnSe inner shells of different thicknesses (injection rate: 2.5 mL for Se-TOP) /h, 10 mL/h for Zn(OA) ₂ ).

After completion of the injection, the reaction flask was cooled to room temperature, and the resulting ZnSeTe/ZnSe core/shell QDs were precipitated with ethanol and redispersed in 9 mL of hexane to prepare a ZnSeTe/ZnSe QD dispersion.

To form the ZnS outer shell, a mixture composed of 2 mmol of Zn(Ac) ₂ , 4 mmol of HDA, 2 mL of OA, and 20 mL of TOA was degassed at 120° C. for 10 minutes in a 50 mL three-necked flask. After backfilling with N ₂ , 3 mL of the ZnSeTe/ZnSe QD dispersion was injected and the temperature was raised to 330 °C. Subsequently, 0.5, 1.0, and 1.5 mL of 1.2 M S-TOP were continuously injected together with ₂ , 4, and 6 mL of Zn(OA) 2 and reacted for 1 hour to produce thin, medium, and thick ZnS, respectively. The resulting ZnSeTe/ZnSe/ZnS core/shell/shell QDs were repeatedly purified by centrifugation using a mixed solvent of hexane and excess acetone. The purified QDs were dispersed in octane for characterization and EML spin casting.

ZnMgO nanoparticle (NP) formation

ZnMgO nanoparticles were synthesized through a solution-precipitation chemistry method.

A clear solution containing 10 mmol of tetramethylammoniumhydroxide (TMAH) and 10 mL of ethanol was dissolved in a cationic solution of 8.5 mmol of Zn acetate dihydrate and 1.5 mmol of Mg acetate tetrahydrate in 30 mL of dimethyl sulfoxide (DMSO) at room temperature. After adding slowly, the temperature was maintained for 1 hour. The synthesized ZnMgO nanoparticles were precipitated with an excess of acetone and redispersed in ethanol for ETL spin coating.

Fabrication of QLEDs

A blue light-emitting QLED with a multilayer architecture was fabricated by sequential solution deposition for all layers except for the Al cathode. The patterned ITO glass was ultrasonically cleaned with acetone and methanol for 10 minutes each and then treated with UV-ozone for 30 minutes. A 30 nm thick PEDOT:PSS (Heraeus Clevios AI 4083) was spin-coated at 3000 rpm for 30 seconds and then baked at 150° C. for 30 minutes to form an HIL. HTL with a thickness of 25 nm using a PVK solution (10 mg/mL of PVK in chlorobenzene) was spin-coated onto the HIL at 3000 rpm for 30 seconds followed by baking at 150° C. for 30 minutes.

Then, the ZnSeTe/ZnSe/ZnS QD dispersion dispersed in octane at a concentration of 7.5 mg/mL was spin-coated at 3000 rpm for 20 seconds to form a nearly monolayer-thick QD EML, which was then baked at 70 °C for 10 minutes. After formation of the QD EML, ZnMgO NP ETL with a thickness of 40 nm was formed by spin-coating an ethanol dispersion of ZnMgO nanoparticles (concentration: 28-30 mg/mL) at 3000 rpm for 30 seconds.

Finally, the fabrication of the device was completed by forming an Al cathode with a thickness of 100 nm by thermal evaporation through a linear metal mask.

EOD and HOD were also fabricated to further investigate the effect of ZnS outer shell thickness on the charge balance. The structure of EOD is ITO/ZnMgO NP (40 nm)/QDs/TPBi (40 nm)/Al, and the structure of HOD is ITO/PEDOT:PSS (30 nm)/PVK (25 nm)/QD/CBP (40 nm)/MoO ₃ ( 10 nm)/Al.

result

Similar to InP heterostructure QD fabrication, usually ZnSeTe core/shell QDs have been fabricated in a one-port or two-step method. In a typical two-step induction multishell InP or ZnSeTe QD, a double shell is continuously formed on a pre-grown core extracted in a core growth solution. Unlike these existing two-step protocols, the present invention proposes a method of forming the final ZnS outer shell 30 by separating the ZnSeTe/ZnSe QDs having the inner shell 20 continuously synthesized in the same port into another port. .

Figure 4 shows (a) PL spectrum as a function of ZnSe shell growth time, (b) changes in PL peak wavelength, QY, and FWHM, (c) absorption spectrum of ZnSeTe/ZnSe QD, (d) TEM image, and (e) It is a schematic diagram depicting the enlargement of the ZnSeTe core during the ZnSe shelling process.

When the growth of the ZnSeTe core was completed at 300 ° C., the ZnSe stock solution was added dropwise for a long time and continuously injected. Figure 4(a) shows the evolution of the PL spectra of ZnSeTe/ZnSe QDs as a function of inner shell growth time up to 180 min. The optical densities of all samples for PL measurement (Fig. 4(a)) were equally adjusted to 0.05 at 370 nm.

Referring to (a) of FIG. 4, when the ZnSe inner shell reaction time is continuously increased up to 120 minutes, the PL changes from 440 nm to 458 nm for the ZnSeTe core in a given composition (ie, nominal Te/Se molar ratio of 0.035). gradually redshifted.

After that, a longer growth time did not induce any more PL shift (Fig. 4(b)).

This PL shift was also in good agreement with the absorption spectrum result (Fig. 4(c)).

The quasi-type II band alignment of the ZnSeTe/ZnSe heterostructure causes the hole wavefunction to be relatively confined to the core region and the electron wavefunction to be significantly delocalized on the ZnSe inner shell (Fig. 4(c) inset).

This leakage of electrons throughout the shell, accompanied by a reduction of the quantum confinement effect, is responsible for the band gap reduction. An increase in the thickness of the ZnSe inner shell (which subsequently induces electron delocalization) induced a gradual PL redshift at a maximum peak wavelength of 458 nm upon 120 min growth. The spectral change then reached saturation, indicating no further electron delocalization when growing the thick inner shell through further extended growth times up to 180 min (Fig. 4(b)).

Referring to (b) of FIG. 4, PL QY gradually increased from 18% in the ZnSeTe core to a maximum of 40% in the ZnSeTe / ZnSe QD shelled for 120 minutes, and then decreased to 25% after shelling for a longer period of 180 minutes. This PL QY reduction is related to the formation of an excessively thick ZnSe inner shell due to the growth time longer than 120 min, which makes the interfacial stress more pronounced. Here, for convenience, ZnSe shells with growth times of 60 minutes, 120 minutes, and 180 minutes are denoted as thin ZnSe, medium ZnSe, and thick ZnSe, respectively.

Compared with the transmission electron microscopy (TEM) image (Fig. 4(d)), the sizes are 5.38 nm for ZnSeTe core, 7.67 nm for ZnSeTe/thin-ZnSe, and 9.71 nm for ZnSeTe/mid-ZnSe, ZnSeTe/thick-ZnSe of 10.31nm steadily increased. The thin ZnSe, medium ZnSe, and thick ZnSe shells are approximately 1.14, 2.16, and 2.47 nm thick, respectively. Considering that middle-ZnSe with 120 min of growth showed the highest PL QY (i.e., 40%), it can be estimated that the critical thickness of the ZnSe inner shell, which enables consistent growth without significant interfacial stress, is about 2.16 nm. .

Binary ZnSe QDs from near-UV to purple emission can show a very narrow PL bandwidth of less than 20 nm, whereas Te alloying ZnSeTe QDs inevitably causes inhomogeneous spectral broadening, which is also comparable to that of blue-emitting ZnSeTe QDs. appears to be proportional to the relative Te content. Spectral broadening is likely due to the presence of trap levels formed in the Te alloy and lattice defects (induced by large differences in ionic radii between Se and Te) or surface impurities (introduced during the shell forming operation) for the individual QDs. can In addition to the above changes in PL wavelength and QY, the thickness of the ZnSe inner shell also affects the PL bandwidth, resulting in FWHM of 52 nm for ZnSeTe core, 28 nm for ZnSeTe/thin-ZnSe, and 26 nm for ZnSeTe/medium-ZnSe to ZnSeTe/thick- The spectrum was consistently narrowed to 24 nm of ZnSe (Fig. 4 (a), (b)).

5 is a PL spectral decomposition diagram of (a) ZnSeTe, (b) ZnSeTe/thin-ZnSe, (c) ZnSeTe/medium-ZnSe, (d) ZnSeTe/thick-ZnSe QDs, (e) are (a) to (d) ), which summarizes the results obtained from

Among these four QD samples, PL was resolved into two subspectrals composed of high-energy excitation and low-energy tail (or trap) emission components, showing a continuous decrease in the spectral contribution of the latter with increasing ZnSe shell thickness (Fig. 5). . This spectral contraction eventually reduces the overall PL asymmetry and narrows the emission half-width.

Considering that the ZnSe shelling in the present invention proceeds at a high temperature (300 ° C) for a long period of time, the possibility of interdiffusion between the pre-grown ZnSeTe core and the ZnSe shell during shelling is very high, and through this, the effective area of the ZnSeTe core is reduced to some extent. It can be extended (see Fig. 4(e)). Therefore, as the shelling proceeds, the content of Te relative to Se in the enlarged ZnSeTe core is lowered, so that the tail emission is mitigated to further improve the spectral symmetry of the entire PL or to further increase the color purity.

6 shows (a) TEM image of ZnSeTe/ZnSe/ZnS QD, (b) XRD pattern of ZnSeTe core, ZnSeTe/ZnSe/ZnS QD, (c) normalized PL, (d) PL peak wavelength, QY, FWHM and (e) PL decay profile. 7 shows (a) EDS spectra and actual compositions of ZnSeTe cores synthesized with a nominal Te/Se molar ratio of 0.035, (b) ZnSeTe/thin-ZnSe/ZnS, ZnSeTe/medium-ZnSe/ZnS, and ZnSeTe/thick-ZnSe. /EDS spectrum of ZnS QDs, (c) Composition (atomic %) of Zn, Se, and S.

The ZnSeTe QDs with the above thin, medium and thick ZnSe shells extracted from the growth solution were equally placed on the final ZnS shells in a two-step method. As a result, the average sizes of the ZnSeTe/thin, medium, and thick-ZnSe/ZnS core/shell/shell QDs were 9.66, 10.93, and 11.36 nm, and the thicknesses of the ZnS outer shell were 1.00, 0.61, and 0.52 nm, respectively. It was estimated (Fig. 6 (a)). Since the same ZnS shelling is applied, the thick ZnSe inner shell QD is large and the surface area is wide, so the thickness of the ZnS outer shell is naturally thin.

Figure 6(b) shows X-ray diffraction (XRD) patterns of ZnSeTe/ZnSe core/shell and ZnSeTe/ZnSe/ZnS core/shell/shell as a function of ZnSe thickness together with ZnSeTe core. Regardless of the ZnSe thickness, the reflection peaks of all ZnSeTe/ZnSe QDs and ZnSeTe cores closely match the zinc blend ZnSe phase due to the low Te content of the ZnSeTe core, which does not induce a noticeable lattice change. It was confirmed that the nominal Te/Se molar ratio (0.035) used in the synthesis of the ZnSeTe core was almost identical to the actual molar ratio (0.036) in energy dispersive spectroscopy (EDS) analysis (Fig. 7(a)).

Upon ZnS shelling, the reflection peaks of all core/shell QDs slightly shifted to large 2θ (i.e., bulk ZnS phase), indicating that the ZnS shell was entirely grown on the core/shell QDs. On the other hand, it can be seen that the XRD peak shift between the core/shell and the core/shell/shell is small even when the thickness of the ZnSe inner shell is increased. This is because the thicker the ZnSe inner shell, the thinner the ZnS outer shell, and the TEM image and core/shell/shell QDs shown in FIG. c)) matches.

Fig. 6(c) shows the normalized PL spectra of core/shell/shell QDs with various ZnSe inner shell thicknesses. Shows the central wavelength (FIG. 6(d)). For reference, the PL peak of these core/shell/shell QDs is slightly blue-shifted by 1-3 nm compared to each core/shell QD, which is due to the strengthening of quantum confinement and the increase in band gap.

8 is a comparison of absorption spectra between core/shell and core/shell/shell QDs, in the case of (a) thin ZnSe, (b) medium ZnSe, and (c) thick ZnSe.

The ZnS outer shell can build a high energy barrier against delocalization of the electronic wave function. This band gap increase after forming the ZnS outer shell can be further verified by comparing the absorption spectra between core/shell and core/shell/shell QDs (Fig. 8). PL QY increased rapidly from 25-40% of core/shell QDs (Fig. 4(b)) to 90-96% of core/shell/shell (depending on ZnSe inner shell thickness) (Fig. 6(d) )), suggesting that the ZnS outer shell formation according to the present invention provided very effective surface passivation. This near-perfect shelling can be achieved by forming the ZnS outer shell at a relatively high temperature of 335 °C, which was possible by using TOA with a high boiling point (365 °C) as a solvent.

As a comparative example, as a result of ZnS shelling of ZnSeTe/intermediate ZnSe QDs by replacing TOA with an ODE with a low boiling point of 315 ° C. and lowering the shell growth temperature to 300 ° C., the prepared core / shell / shell QDs are in accordance with the examples of the present invention. (93%), the PL QY was significantly lower at 80%.

On the other hand, the PL QY of the core/shell/shell QDs steadily decreased from 96% in the case of thin ZnSe to 90% in the case of thick ZnSe (Fig. This is because the possibility of complete passivation of the core/shell QD surface is lowered. In addition, the FWHM values (23–27 nm) of the series of core/shell/shell QDs narrowed slightly by 1 nm compared to each core/shell due to non-radiative recombination inhibition activated by effective surface passivation with ZnS (Fig. (d)). The FWHM of the core/shell/shell QDs developed here was slightly lower than the precedent results where PL QY performed best (with HF/ZnCl ₂ , PL peak 457 nm, FWHM 36 nm), but the deep blue to date (especially >450 nm) ) is the narrowest among ZnSeTe emitters.

9 is a comparison of PL decay profiles between core/shell and core/shell/shell QDs, in the case of (a) thin ZnSe, (b) medium ZnSe, and (c) thick ZnSe.

In Fig. 9, the PL decay lifetime generally increased from core/shell to core/shell/shell QDs. This PL decay due to the formation of the ZnS outer shell is a result of the significant suppression of non-radiative charge recombination by the ZnS outer shell on the core/shell QD surface. Referring to (e) of FIG. 6, it can be seen that the average lifetime of 54.3ns of the thin ZnSe shell increases to the average lifetime of 67.9ns of the thick ZnSe shell. Considering the trend of PL QY according to the increase in ZnSe shell thickness (Fig. 6(d)), the thicker the ZnSe inner shell, the longer the QD average lifetime and the lower the PL QY. From this, it can be seen that electron delocalization dominates the radiative/non-radiative recombination process in determining the overall PL decay.

10 shows (a) a schematic diagram of a multilayer blue QLED device, (b) a cross-sectional TEM image of a blue QLED fabricated with ZnSeTe/mid-ZnSe/ZnS QDs (inset: higher magnification corresponding to the dot region), and (c) multilayer EQE-current density relationship with energy level of blue QLED, (d) normalized EL spectrum (collected at 8 V), (e) current density-voltage, (f) luminance-current density and (g) current efficiency. 11 is an enlarged cross-sectional TEM image of blue QLEDs fabricated with (a) ZnSeTe/thin-ZnSe/ZnS and (b) ZnSeTe/thick-ZnSe/ZnS QDs. 12 is a spectral comparison of solution PL (top) vs. 8 V driven EL (bottom): (a) ZnSeTe/thin-ZnSe/ZnS, (b) ZnSeTe/medium-ZnSe/ZnS, (c) ZnSeTe/thick-ZnSe This is the case for /ZnS QDs.

Blue-emitting multilayer QLEDs were fabricated using thin, medium, and thick ZnSe inner shells (a series of core/shell/shell QDs with thick, medium, and thin ZnS outer shells, respectively). It has the structure of ITO anode/ poly(ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) HIL/poly(9-vinylcarbazole) (PVK) HTL/QD EML/ZnMgO NP ETL/Al cathode (FIG. 10(a)). As can be seen in the cross-sectional TEM image (Fig. 10(b)) of a QLED fabricated with ZnSe/Medium-ZnSe/ZnS QDs, the QD EML is similar to that of the other two EL devices constructed using QDs with a thin and thick inner shell. As in Fig. 11), it consists of almost a monolayer.

An energy level diagram is presented in (c) of FIG. 10 . The valence band maximum (VBM) and conduction band minimum (CBM) levels of the QD EML were measured by UV photoelectron spectroscopy (UPS).

Figure 10 (d) shows the 8V driven standardized EL spectrum in three QLEDs. Peak wavelengths of 455, 457, and 459 nm appear for thin, medium, and thick ZnSe, respectively. All EL spectra were red-shifted by 2-3 nm relative to PL (FIG. 12) due to the field-induced quantum confined Stark effect. For QDs with a thick ZnS inner shell followed by formation of a thin ZnS outer shell, the current density (J) of the resulting device tended to increase (Fig. 10(e)). The high bandgap ZnS outer shell of the QD heterostructure sets a high energy barrier for charge injection from the HTL and ETL to the QD EML, so reducing its thickness makes the injection of electrons and holes relatively easy, increasing the current density of the device. It rises.

Consistent with the JV results above, devices with thicker ZnSe inner shells (or devices with thinner ZnS outer shells) produced higher luminance (L), especially for QDs with thin, medium, and thick ZnSe inner shells of 7200 and 7200, respectively. It showed peak values of 7841 and 10020 cd/m ² (FIG. 10(f)). The peak current efficiency and EQE were 10, 8.9, 9.8 cd/A, 15.3, 13.7, and 12.7% for the thin, medium, and thick ZnSe inner shell-based QLEDs, respectively, at high luminance levels (current density of 1925–3522 cd/m ² ). ca.20-40 mA/cm ² in the region) was achieved (Fig. 10(g)).

Despite this ZnSe inner shell thickness dependence of current efficiency and EQE, the device-to-device imbalance was not significant and the overall device efficiency was fairly similar across the current densities. In this regard, the device efficiency was found not to be highly sensitive to QD heterostructure details (i.e., inner and outer shell thickness). This can be understood by considering the effects of two aspects, namely the ZnS outer shell and ZnSe inner shell thickness, on charge injection and non-radiative Auger recombination, respectively. First, increasing the ZnS outer shell thickness reduces the injection of charge from the HTL/ETL to the QD EML, but the degree of injection reduction may vary depending on the charge type. Increasing the thickness of the ZnS outer shell hinders electron injection rather than hole, leading to better charge balance of the device. In this regard, QDs based on the thin ZnSe inner shell with the thickest ZnS outer shell can show superiority in terms of charge balance and device efficiency. Second, Auger recombination, which is well known to be the most detrimental factor in QLED efficiency by being a non-radiative pathway, becomes highly dependent on the QD heterostructure details. The thicker ZnSe inner shell slows Auger recombination. In this context, in the case of blue QLEDs, where QDs are negatively charged during device operation due to asymmetrically excessive electron injection, QDs with thick ZnSe inner shells may be superior in suppressing Auger recombination, which is likely to lead to improved device efficiency. high. However, ZnSeTe QDs with a thick ZnSe inner shell had a thinner ZnS outer shell (see Fig. 6(a)). This had an adverse effect on the charge balance (as mentioned earlier). Therefore, the similarity of the device efficiency for QDs with different inner and outer shell thicknesses (Fig. 6(g)) shows the negative effect of the thin ZnS outer shell on the charge balance and the positive effect of the thick ZnSe inner shell on Auger recombination. can be understood as the coincident result of

13 shows absorption and PL spectra of (a) ZnSeTe core and (b) ZnSeTe/mid-ZnSe core/shell QDs with Te/Se molar ratios of 0.023, 0.035, and 0.047. 14 shows (a) absorption and PL spectra, (b) comparison of PL peak wavelength, QY, and FWHM of blue ZnSeTe/intermediate ZnSe/ZnS QDs synthesized with different Te/Se molar ratios of 0.023, 0.035, and 0.047, and (c) room illumination. and a photograph under a UV lamp.

In the synthesis of the ZnSeTe core, the nominal Te/Se ratio was further changed from 0.023–0.047 to tune the blue-based emission, while a growth time of 120 min was applied for ZnSe inner shelling, i.e., intermediate ZnSe. An increase in the Te/Se ratio results in systematic reddening of absorption and PL in all phases of the ZnSeTe core, ZnSeTe/mid-ZnSe core/shell (Fig. 13) and ZnSeTe/mid-ZnSe/ZnS core/shell/shell QDs (Fig. 14). It led to sideways.

The detailed evolution of the PL characteristics (ie, peak wavelength, QY, FWHM) of the Te/Se ratio variation ZnSeTe QDs with the shelling step is also summarized in FIG. 15 . 15 shows changes in PL peak wavelength, QY, and FWHM according to the shelling step, with Te/Se ratios of (a) 0.023, (b) 0.035, and (c) 0.047.

16 shows (a) current density-voltage, (b) luminance-current density, and (d) current efficiency of blue devices fabricated with ZnSeTe/Medium ZnSe/ZnS QDs with Te/Se molar ratios of 0.023, 0.035, and 0.047; EQE current density relationship, (d) voltage-dependent EL spectrum evolution of the device with Te/Se molar ratio of 0.023, (e) voltage-dependent EL spectrum evolution of the device with Te/Se molar ratio of 0.035, (f) Te/Se molar evolution Voltage-dependent EL spectral evolution of the device with a ratio of 0.047, (g) CIE color coordinates recorded at 8 V.

As the Te/Se ratio increased from 0.023 to 0.047, the PL peak wavelength of the final core/shell/shell QD increased from 451 to 463 nm and the FWHM increased from 18 to 38 nm (Fig. 14(b)). Although the effective core diameter enlarged during the long-term (i.e., 120 min) formation of the ZnSe inner shell, which was equally effective for all QDs towards a narrow emission half-width (Fig. 16), spectral broadening was common with increasing Te/Se ratio , the higher Te content of the ZnSeTe core inevitably includes more defect levels in the bandgap, thus increasing the spectral contribution from the tail emission.

As can be seen in the QD diffuse illumination image under room lighting (Fig. 14(c)), all core/shell/shell QDs exhibited PL QY levels of 89-93%, regardless of the Te/Se ratio (Fig. 14(c)). (b)). Then, core/shell/shell QDs with different Te/Se ratios but the same thickness of ZnSe inner shell and ZnS outer shell were individually tested for the following blue QLED fabrication.

The resulting three blue QLEDs showed almost the same J-V results (Fig. 16(a)), which is due to the minute change in charge injection (ie, Te/Se = 0.023-0.047), resulting in the composition (or energy) of the ZnSeTe core. level), but is mostly controlled by the shell thickness.

On the other hand, in this blue region, the luminance became remarkably bright as the Te/Se ratio of ZnSeTe QDs increased due to the combined effect of high visibility for longer emission wavelengths and wider emission range (advantage of enlarged FWHM), and the Te/ For the Se ratios of 0.023, 0.035, and 0.047, the peak values were 6538, 7841, and 12654 cd/m ² ((b) of FIG. 16). Therefore, the higher the Te/Se ratio, the higher the current efficiency in the entire current density range, and the peak value increased from 7.2 cd/A for Te/Se = 0.023 to a maximum of 14.8 cd/A for Te/Se = 0.047.

However, the overall EQEs were not significantly different, yielding similar peak EQEs of 13.4–13.7% (obtained at high luminance levels 2661, 3522, and 4775 cd/m ² for Te/Se ratios of 0.023, 0.035, and 0.047, respectively). This similarity of the EQEs is due to the almost identical PL QY (i.e., 89–93%) and the same thickness of the ZnSe/ZnS double shells (leading to identicality of charge injection and Auger recombination).

For the three blue QLEDs with Te/Se ratios of 0.023, 0.035, and 0.047, respectively, the EL peaks at 453, 457, and 465 nm were well maintained regardless of the applied voltage ((d)-(f) in FIG. 16). Corresponds to Internationale de l'Eclairage (CIE) color coordinates (0.146, 0.045), (0.140, 0.059), and (0.128, 0.109) (Fig. 16(g)). Such spectral stability over voltage suggests that the current blue QD heterostructure is resistant to field-induced exciton polarization because of the presence of a shell thick enough to block the external electric field.

As mentioned above in Fig. 6(a), the final thickness of the ZnS outer shell depended on the thickness of the ZnSe inner shell when the same ZnS shelling precursor was applied. To further examine the effect of the thickness of the ZnS outer shell independently on the EL performance, proportionally modulating the ZnS shelling precursor by varying the thickness with ZnSeTe/mid-ZnSe core/shell QDs based on Te/Se = 0.035, standard Experiments were conducted with 50% reduction and increase.

17 shows (a) TEM images of ZnSeTe/Medium-ZnSe/Thin-ZnS and ZnSeTe/Medium-ZnSe/Thick-ZnS QDs based on Te/Se = 0.035, (b) Absorption, (c) ZnS outer shell thickness dependent core. Comparison of PL spectra of /shell/shell QDs, (d) current density and luminance-voltage as a function of ZnS outer shell thickness, (f) current density-voltage characteristics of EOD and HOD as a function of ZnS outer shell thickness, ( g) It is a schematic diagram depicting the degree of injection of electrons and holes (difference in charge balance) differently according to the thickness of the ZnS outer shell.

In the standard core/shell/shell QD (middle of Fig. 6(a)), the thickness of the ZnS outer shell was 0.61 nm, but due to the decrease and increase of the ZnS shell, the thickness of the ZnS outer shell was reduced to 0.44 nm and 1.06 nm as originally intended. Each became thinner and thicker (Fig. 17(a)). Such thick ZnS growth was also noticeable in the absorption spectrum (recorded at the same optical density (0.05) at 370 nm), and the absorbance below 340 nm gradually increased through the ZnS shelling precursor (Fig. 17(b)).

Fig. 17(c) shows the recorded PL spectrum of the same set of QDs as Fig. 17(b). The PL peak and FWHM were almost unchanged at 455 nm and 25–26 nm, respectively, and the PL QY increased from 90% of thin ZnS to 95% of thick ZnS core/shell/shell QDs, resulting in complete surface passivation by thick ZnS growth. indicates that this has been achieved (FIG. 18). 18 is a graph comparing PL peak wavelength, QY, and FWHM of ZnS outer shell thickness dependent core/shell/shell QDs.

Compared to the JVL results of the three blue QLEDs (FIG. 17(d)) as a function of the ZnS outer shell thickness, the thicker the ZnS outer shell, the lower the current density but the higher the luminance. The former is due to the suppression of injection of both charges from the HTL/ETL to the QD EML, while the latter is mainly related to the improved charge balance given the similar PL QY (90–95%). As a result, the device with thick ZnS-based QDs produced the highest EQE of 18.6% (corresponding to 12.9 cd/A at current efficiency) recorded at a high luminance level of 4366 cd/m ² (Fig. 17(e)). .

In order to verify the reproducibility of the device performance, 25 devices were repeatedly manufactured, and as a result, the average maximum EQE was 17.4%, and 6 devices showed more than 20%.

To elucidate how the thick ZnS outer shell contributes to better charge balance, electron-only devices (EODs) and hole-only devices (HODs) were fabricated, respectively, ITO/ZnMgO NPs/QDs/TPBi)/Al and ITO/PEDOT:PSS / PVK/QDs/CBP/MoO ₃ /Al. The JV characteristics of this single charge device show that the electron current density decrease is more dominant than the hole current density as the ZnS outer shell becomes thicker (FIG. 17(f)). That is, improved charge balance in QLEDs using thick ZnS-based QDs can be achieved by further suppressing electron injection in the current HTL/ETL-based blue device system (FIG. 17(g)).

In summary, we first varied the thickness of the ZnSe shell in the ZnSeTe/ZnSe QD heterostructure and investigated the effect on PL. As the ZnSe shell thickness increases, the PL of the core/shell QDs redshifts due to the delocalization of the electronic wavefunction in the quasi-type II band alignment, reducing the spectral contribution by tail emission enabled by the broadening of the effective core area. As a result, the FWHM decreased. After final ZnS shelling, the ZnSeTe/ZnSe/ZnS QDs showed tunable deep blue PL of 452–457 nm, excellent PL QY of 90–96% and narrow FHWM of 23–27 nm, depending on the ZnSe shell thickness. The above core/shell/shell QDs, which produced thin-, mid-, and thick-ZnSe inner shells (conversely forming thick-, mid-, and thin-ZnS outer shells) in blue QLED fabrication, respectively, show these inner and outer shells. It is shown that the overall device efficiency does not vary significantly by the combination of thicknesses. This is understood by the interaction between the negative effect of the thin ZnS outer shell on charge balance and the positive effect of the thick ZnSe inner shell on Auger recombination. The Te/Se ratio of the ZnSeTe core was changed for more noticeable emission modulation in the blue series. As a result of applying the same double shelling, each core/shell/shell QD showed an appropriate PL peak at 451–463 nm with a high PL QY of 89–93% depending on the Te/Se ratio variation. Each device employing these Te/Se ratio-varying QDs yielded equivalent EQEs (i.e., peak values of 13.4–13.7%) due to similar PL QYs and identical ZnSe/ZnS double shell thicknesses. ZnSeTe/Medium-ZnSe core/shell QDs were used to further control the ZnS outer shell thickness to investigate the effect on device efficiency. Increasing the thickness of the ZnS outer shell inhibits the injection of electrons compared to holes in favor of improved charge balance, resulting in significant efficiency improvements up to 18.6% at peak EQE.

Although the preferred embodiments of the present invention have been shown and described above, the present invention is not limited to the specific preferred embodiments described above, and without departing from the gist of the present invention claimed in the claims, in the technical field to which the present invention belongs Anyone skilled in the art can make various modifications, of course, and such changes are within the scope of the claims.

10: ZnSeTe core
20: ZnSe inner shell
30: ZnS outer shell
100: QD

Claims (17)

II-VI ternary ZnSeTe cores with a nominal Te/Se ratio of 0.01-0.05;
a ZnSe inner shell surrounding the core and having a thickness of 1 nm to 3 nm; and
ZnSeTe / ZnSe / ZnS core / shell / shell quantum dots, characterized in that it surrounds the ZnSe inner shell and comprises a ZnS outer shell having a thickness of 0.5 nm-2 nm. According to claim 1, ZnSeTe / ZnSe / ZnS core / shell / shell quantum dots, characterized in that the core diameter is 5nm-8nm. forming a II-VI ternary ZnSeTe core by injecting a Se precursor and a Te precursor into a first mixed solution containing a Zn precursor and a solvent;
A Zn source solution capable of forming a ZnSe shell and a Se precursor are injected into the first mixed solution in which the ZnSeTe core is formed to form a ZnSe inner shell surrounding the ZnSeTe core, thereby forming a ZnSeTe/ZnSe core/shell quantum dot. forming a;
preparing a ZnSeTe/ZnSe core/shell quantum dot dispersion after separating the ZnSeTe/ZnSe core/shell quantum dots from the first mixed solution; and
A Zn source solution capable of forming a ZnS shell and an S precursor are injected into the second mixed solution containing the ZnSeTe/ZnSe core/shell quantum dot dispersion to form a ZnS outer shell surrounding the ZnSe inner shell, thereby forming a ZnSeTe/ZnSe / ZnS core / shell / quantum dot manufacturing method comprising the step of forming a shell quantum dot. The method of claim 3, wherein forming the ZnSeTe core,
Primary heating of a first mixed solution containing a Zn precursor and a solvent;
Secondarily heating the first mixed solution to a temperature higher than the first heating temperature; and
Quantum dot manufacturing method comprising the step of injecting and reacting the Se precursor and the Te precursor in the first mixed solution. The method of claim 4 , further comprising performing third heating at a temperature higher than the second heating temperature during the step of injecting and reacting the Se precursor and the Te precursor into the first mixed solution. The method of claim 3, wherein the forming of the ZnSeTe / ZnSe core / shell quantum dots,
A quantum dot manufacturing method comprising the step of injecting a Zn raw material solution capable of forming a ZnSe shell and a Se precursor for a reaction time of 60 minutes or more and 180 minutes or less to the first mixed solution in which the ZnSeTe core is formed. . The method of claim 3 , wherein mutual diffusion between the ZnSeTe core and the ZnSe inner shell is performed during the forming of the ZnSeTe/ZnSe core/shell quantum dots. The method of claim 3 , wherein the temperature of the first mixed solution is maintained at 300° C. or higher during the forming of the ZnSeTe/ZnSe core/shell quantum dots. According to claim 6,
The Zn raw material solution capable of forming the ZnSe shell is prepared by dissolving the Zn precursor in a solvent containing fatty acid and at least one of TOP, TBP and TOA. The method of claim 3, wherein the forming of the ZnSeTe / ZnSe / ZnS core / shell / shell quantum dots,
heating a second mixed solution containing a Zn precursor and a solvent;
secondarily heating the second mixed solution to a temperature higher than the first heating temperature after injecting the ZnSeTe/ZnSe core/shell quantum dot dispersion into the second mixed solution; and
Quantum dot manufacturing method comprising the step of injecting and reacting a Zn source solution capable of forming a ZnS shell and an S precursor in the second mixed solution. According to claim 10,
The Zn raw material solution capable of forming the ZnS shell is prepared by dissolving the Zn precursor in a solvent containing fatty acid, primary amine and TOA. 11. The method of claim 10, wherein the temperature during the secondary heating is 320-350 °C. 4. The method of claim 3, wherein the nominal Te/Se ratio of the ZnSeTe core is 0.01-0.05, the thickness of the ZnSe inner shell is 1nm-3nm, and the thickness of the ZnS outer shell is 0.5nm-2nm. Quantum dot manufacturing method, characterized in that for. 14. The method of claim 13, wherein the core diameter is 5 nm-8 nm. A quantum dot-light emitting device comprising a hole transport layer, a quantum dot light emitting layer, and an electron transport layer, wherein the quantum dot light emitting layer includes the quantum dots according to claim 1 or claim 2. The method of claim 15, further comprising an anode, a hole injection layer and a cathode, wherein the hole transport layer and the hole injection layer are poly(ethylenedioxythiophene):polystyrene sulphonate (PEDOT:PSS), poly[(9, 9-dioctyl-fluorenyl- 2,7-diyl)-co-(4,4'-(N-(p-butylphenyl))diphenylamine)](TFB), poly(9-vinlycarbazole)(PVK), N, N, N, N', N'-tetrakis(4-methoxyphenyl)-benzidine(TPD), poly-TPD, 4,4',4''-tris(N-carbazolyl)-triphenylamine(TCTA), N,N'-bis(naphthalen-1 -yl)-N, N'bis(phenyl)-9, 9-spiro-bifluorene(spiro-NPB), dipyrazino[2, 3-f:2', 3'-h]quinoxaline-2, 3, 6, Quantum dots, characterized in that any one selected from 7, 10, 11-hexacarbonitrile (HATCN), 1, 1-bis [(di-4-tolylamino) phenylcyclohexane (TAPC), p-type metal oxides, and combinations thereof- light emitting element. [Claim 16] The method of claim 15, wherein the electron transport layer includes metal oxide nanoparticles, wherein the metal oxide nanoparticles include Zn _1-x Mg _x O (0≤x≤0.5) Zn-containing Mg oxide nanoparticles; and a Mg ion surface treatment layer formed on the surface of the nanoparticles.

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