WO2023274486A1

WO2023274486A1 - Quantum dot with a core-shell structure, method of preparing them and electronic device

Info

Publication number: WO2023274486A1
Application number: PCT/EP2021/000075
Authority: WO
Original assignee: Fraunhofer Gesellschaft zur Förderung der angewandten Forschung e.V.
Priority date: 2021-06-30
Filing date: 2021-06-30
Publication date: 2023-01-05
Also published as: DE112021007912T5; KR20240046437A

Abstract

The present invention relates to quantum dots with a core-shell structure, comprising a core and at least two shells, wherein at least one of the two shells comprises or consists of an In-Zn-P-Se semiconductor material.

Description

Quantum dot with a core-shell structure, method of preparing them and elec tronic device

The present invention relates to quantum dots with a core-shell structure, com- prising a core and at least two shells, wherein at least one of the two shells comprises or consists of an In-Zn-P-Se semiconductor material.

Recently, InP quantum dots (QDs) materials have received a lot of attention in the display field because of their bandgap engineering properties, theoretically obtainable narrow Full Width at Half Maximum (FWHM), and relatively eco- friendly character as compared to CdSe type QDs. (Chemistry of Materials 28(8): 2491-2506.)

Technologically important, environmentally friendly InP QDs typically used as green and red emitters in display devices can have outstanding photolumines cence (PL) quantum yields (QYs) of near-unity (95-100%) when the state-of-the- art core/shell heterostructure of ZnSe inner/ZnS outer shell elaborately applied (ACS. Appl. Nano. Mater. 2019, 2, 1496-1504, Nature 575(7784): 634-638). Also, Ligand engineering on the surface of QD and synthetic routes of mass- production such as heating up and one-pot synthesis were attributable to the stability and resin compatibility, which strongly promote the rapid commercial ization of InP QD. (Nanotechnol. 2012, 7, 577-82, Chem. Mater. 2016, 28, 2491- 2506)

In the QD-display, the degree of blue absorption is extremely important factor to decide whether the efficiency in photoluminescence devices is high or not (Figure 1: Both display of QD-LCD and QD-CF uses 450nm blue LED as excitation source for green and red QD emitters). The absorption property of InP/ZnSe/ZnS QD applied in the display industry has originally down-coverted emission character, in which InP Green QD inherently has weak absorption properties in 450 nm blue regime relative to CdSe QD because of their energy separation in bandgap structure (Figure 2: (a) UV-absorption spectra and PL in tensity of InP/ZnSe/ZnS QD and (b) less blue-absorbing InP QD relative to CdSe QD(b), in which InP QD mainly absorbs UV light (purple line) better than blue light (blue line)).

CdSe QD has been researched one of the materials to be able to be applied in the display industry, but Cd contents, which is known well for an extremely harmful element to humans and the environment, do not satisfy RoHS interna tional regulation. As a result, less blue light (450nm) absorption of green emit ters relative to UV light makes trouble with lower price competitiveness in the display industry because more QDs have to be filled in QD-LCD film to satisfy the luminance of display panels such as cell phone and TV.

Figure 3 shows (a) InP Green QD film layer in (b) QD-CF display, and (c) their simulation result of predicted blue absorption and EQE in QD-CF film layer rel ative to different film thickness, in which the concentration of InP green is equally applied to 3 wt%.

There have been efforts to increase the blue absorption of InP green QD layer in the QD-CF display, such as the increase of (1) film thickness, (2) the number of InP green particles, and (3) ZnSe shell thickness. Despite diverse trials to im prove blue absorption, all scenarios including above mentioned three kinds of methods have a negative trade-off in their efficiency. (1) The increase of InP green film thickness accompanies by higher manufactur ing prices. And when it considers the whole structure of QD-CF, a thicker layer over lOum is hard to apply because of their total panel thickness. (2) A large number of InP green particle applied in QD-CF film induces self-ab sorption between QDs and backward emission, by which max EQE can be dropped even in high concentrated QD-CF film.

(3) Thicker ZnSe shell onto InP core is not effective on 450nm blue regime be- cause band gap of ZnSe is matched to near 400nm area.

In the previous report, thicker ZnSe shell is only attributable to absorption of the 400nm wavelength regardless of whether ZnSe is alloyed with ZnS or not. Figure 4 shows (a) Overall Se/(Se+S) ratio (dashed line) and PL QY (solid line) of

Gradient shell (GS)- and Discrete shell(DS)-QDs and the schematic drawing of the stepwise shell growth of GS- and DS-QDs. (b and C) Steady-state absorption (dashed line) and PL emission (solid line) spectra of GS-QDs (blue) and DS-QDs (green) at each growth step in toluene. (Nanoscale, 2019, 11, 23251-23258)

As a result, the proper solution has not been established to date, which is one of a huge hence to the commercialization in QD-CF display. There are other ef forts to find the post-lnP, such as Perovskite, ZnTeSe, and AglnS, which has dif ferent composition relative to InP, but their optical properties and stability in the device do not reach to the grade of commercialization.

Accordingly, it was the technical objective of the present invention to provide quantum dots that have an enhanced or an improved blue absorption and en hanced optical properties to provide sufficient stability to allow commercializa- tion.

The solution to this technical objective is solved by the quantum dot according to claim 1, a method for preparing the quantum dot according to claim 11 as well as the electronic device according to claim 12. The dependent claims de scribe advantageous embodiments. Accordingly, the present invention provides a quantum dot with a core-shell structure, comprising a core and at least two shells, said at least two adjacent shells surrounding the core, wherein said core and the at least two shells comprise or consist of semiconducting materials, characterized in that at least one of the at least two shells comprises or consists of an In-Zn-P-Se semiconductor material.

Surprisingly, it has been found by the inventors that the quantum dot according to the present invention are able to absorb in the 450nm blue area of the optical spectrum. The observed high gamma values (g) around 1.487 could be observed (ratio of optical density in 450nm regime to an optical density of Is peak of the core) allow the manufacturing of technically important QD-LCDs at a high cost-effectiveness.

According to a preferred embodiment the quantum dot comprises one shell comprising or is made of an In-Zn-P-Se semiconductor material adjacent to the core. Accordingly, said shell adjacent to the core is the inner-shell, surrounding the core, whereas the at least one further shell is aligned on top of the shell adjacent to the core and thus can be regarded as at least one outer-shell.

According to a further improved embodiment, a shell adjacent to the core com prises or is made of an In-Zn-P-Se semiconductor material and the at least one further shell comprises or is made of a semiconductor material selected from the group consisting of ZnS, ZnSe, ZnTeSe, ZnMgSe, CdS, ZnCdS.

Furthermore, it is preferred that the core comprises or consists of a semicon ductor material selected from the group consisting of InP, ln(Zn)P, InGaP.

Especially preferred, the core is formed of InP.

Preferably, the quantum dot has a diameter in between 1.00 and 15.00 nm, preferably in between 1.50 and 7.50 nm, especially preferred in between 2.50 and 5.00 nm.

The diameter can be obtained by evaluating the size distribution of the quan tum dot with the TEM image analysis, in which a sufficient large number of quantum dots (e.g. 150 or more quantum dots) are analysed with regards to their size, or diameter, respectively, and the average value of the size distribu tion is taken as diameter.

Furthermore, it is possible that the core has a diameter in between 0.1 and 10.0 nm, preferably in between 1.0 and 7.5 nm, especially preferred in between 1.75 and 3.0 nm.

In addition, the shell adjacent to the core can have a thickness in between 0.1 and 12 nm, preferably in between 1.0 and 10 nm, especially preferred in be tween 3.0 and 8.0 nm.

According to a further specific embodiment, the shell adjacent to the shell com prising or consisting of an In-Zn-P-Se semiconductor material preferably has a thickness in between 0.01 and 6.0 nm, preferably in between 0.1 and 3.0 nm, especially preferred in between 1.0 and 1.5 nm.

According to a specific embodiment, the core is formed of In-P semiconductor material, a shell adjacent to the core is made of an In-Zn-P-Se semiconductor material and an outer shell is made of Zn-Se semiconductor material or Zn-S semiconductor material.

Preferably, the quantum dot has an absorption at 450nm in the optical spec trum which is a ratio of optical density in 450nm wavelength to optical density of InP Is peak is over 1.0.

According to a further aspect, the present invention relates to a method of pre paring quantum dot according to the present invention, comprising the follow ing steps:

• provision of a first solution comprising precursor materials for the sem iconductor of the core and reacting the precursor materials to form a dispersion comprising semiconductor nanoparticles of the core,

• provision of a second solution comprising precursor materials for the In- Zn-P-Se semiconductor, addition of the second solution to the disper sion comprising nanoparticles of the core and reacting the precursor materials to form the In-Zn-P-Se semiconductor shell adjacent to the core,

• provision of a second solution comprising precursor materials for the In- Zn-P-Se semiconductor, addition of the third solution to the dispersion obtained in the preceding step and reacting the precursor materials to form a shell comprising a semiconductor material or consisting thereof on the In-Zn-P-Se semiconductor shell adjacent to the core.

According to an additional aspect, the present invention relates to an electronic device comprising quantum dots as described in the foregoing. Specifically, the electronic device e.g. can be selected from the group consisting of displays, especially TV or computer displays.

The present invention will be described in greater detail in the following with out being delimited to the described specific embodiments.

Production examples of quantum dots according to the present invention

Example 1

A InP/ZnSe quantum dots having a y value of 0 61, as a index indicating 450nm wavelength absorption rate, is manufactured as follows.

Step 1-1: 480nm InP

Step 1-2: 517nm In P/ZnSe Example 2

A InP/lnZnPSe/ZnSe quantum dots having a y value of 1.963, as a index indicat ing 450nm wavelength absorption rate, is manufactured as follows.

Step 2-1: 480nm InP Step 2-2: 529nm I n P/lnZn PSe

Step 2-3: 534nm InP/lnZnPSe/ZnSe Example 3

A InP/lnZnPSe/ZnSe/ZnS quantum dots having a y value of 1.963, as an index indicating 450nm wavelength absorption rate, is manufactured as follows.

Step 2-l:480nm InP Step 2-2: 529nm InP/lnZnPSe Step 2-3: 534nm InP/lnZnPSe/ZnSe Step 2-4: 529nm InP/lnZnPSe/ZnSe/ZnS the steps of manufacturing the quantum dots according to Example 1 though 3 will now be described in detail.

Example 1.

First, the core materials of 480nm InP according to the step 1-1 is prepared as follows.

1. 2 millimoles (mmol) of Indium acetate, 4 mmol Zinc stearate, and 1ml of oleic acid are put in a 1000ml 3-neck round flask reactor and heated at 130°C under vacuum for 3 hours. After the evacuation, the temperature cooled down to 60°C, 10ml octadecene is injected in-situ, and then nitrogen is backfilled into the reactor and maintained the temperature at 60°C

2. Next a mixed solution of tris(trimethylsilyl)phosphine (TMS3P, 2mmol) and 1-octadecene 5ml is injected into the reactor at 60°C and rapidly heated up to 300°C at a rate of 20°C/min to synthesize a 480nm InP core material according to the step 1-1.

3. 6mmol of Zinc acetate, 12mmol of oleic acid, and 12ml of 1-octadecene are mixed in a 100ml round bottom flask and heated at 130°C under vacuum for 3 hours. And 6mmol of Selenium and 6ml of Trioctylphosphine are mixed in a 50ml round bottom flask and heated at 120°C under vacuum for 2 hours. Then the reactor is cooled down to room temperature to prepare a 0.5M Zinc oleate stock solution and 1 M TOPSe stock solution.

4. For ZnSe shell, the reactor is heated up to 300°C and introduced 12ml of 0.5M Zinc oleate and 6ml of 1 M TOPSe stock solution at the same time. For the shell growth, the temperature of reactor is maintained at 300°C for 1 hour to syn thesize a 517nm InP/ZnSe material according to the step 1-2 Example 2.

First, the core materials of 480nm InP according to the step 2-1 is prepared as follows.

1. 2 millimoles (mmol) of Indium acetate, 4 mmol Zinc stearate, and 1ml of oleic acid are put in a 100ml 3-neck round flask reactor and heated at 130°C under vacuum for 3 hours. After the evacuation, the temperature cooled down to 60°C, 10ml octadecene is injected in-situ, and then nitrogen is backfilled into the reactor and maintained the temperature at 60°C

2. Next a mixed solution of tris(trimethylsilyl)phosphine (TMS3P, 2mmol) and 1-octadecene 5ml is injected into the reactor at 60°C and rapidly heated up to 300°C at a rate of 20°C/min to synthesize a 480nm InP core material according to the step 2-1.

3. 6mmol of Zinc acetate, 12mmol of oleic acid, and 12ml of 1-octadecene are mixed in a 100ml round bottom flask and heated at 130°C under vacuum for 3 hours. And 6mmol of Selenium and 6ml of Trioctylphosphine are mixed in a 50ml round bottom flask and heated at 120°C under vacuum for 2 hours. Then the reactor is cooled down to room temperature to prepare a 2M Zinc oleate stock solution and 1M TOPSe stock solution.

4. For the alloyed InZnPSe shell onto InP core, the reactor is heated at 150°C and introduced 12ml of 2M Zinc oleate stock solution and 6ml of 1M TOPSe stock solution. Then, reactor is gradually heated up to 210°C at a rate of l°C/min and maintained at 210°C during 2hours. And then, the reactor is slowly heated up to 280°C at a rate of l°C/min and maintained at 280°C for 2hour to synthesize alloyed InZnPSe muti-shell layer onto InP core, which is according to the step 2-2.

5. For the ZnSe overcoating on the surface of InP/lnZnPSe QD, the reactor is gradually heated up to 300°C at l°C/min and maintained at 300°C during 1 hour for ZnSe overcaoting to synthesize a 534nm InP/lnZnPSe/ZnSe material accord ing to step 2-3

Example 3.

First, the core materials of 480nm InP according to the step 3-1 is prepared as follows.

2. Next a mixed solution of tris(trimethylsilyl)phosphine (TMS3P, 2mmol) and 1-octadecene 5ml is injected into the reactor at 60°C and rapidly heated up to 300°C at a rate of 20°C/min to synthesize a 480nm InP core material according to the step 3-1.

3. 6mmol of Zinc acetate, 12mmol of oleic acd, and 12m of 1-octadecene are mixed in a 100ml round bottom flask and heated at 130°C under vacuum for 3 hours. And 6mmol of Selenium and 6ml of Trioctylphosphine are mixed in a 50ml round bottom flask and heated at 120°C under vacuum for 2 hours. Then the reactor is cooled down to room temperature to prepare a 2M Zinc oleate stock solution and 1M TOPSe stock solution.

4. For the alloyed InZnPSe shell onto InP core, the reactor is heated at 150°C and introduced 12ml of 2M Zinc oleate stock solution and 6ml of 1M TOPSe stock solution. Then, reactor is gradually heated up to 210°C at a rate of l°C/min and maintained at 210°C during 2hours. And then, the reactor is slowly heated up to 280°C at a rate of l°C/min and maintained at 280°C for 2hour to synthesize alloyed InZnPSe muti-shell layer onto InP core, which is according to the step 3-2.

5. For the ZnSe overcoating on the surface of InP/lnZnPSe QD, the reactor is gradually heated up to 300°C at l°C/min and maintained at 300°C during 1 hour for ZnSe overcoating to synthesize a 534nm InP/lnZnPSe/ZnSe material accord ing to step 3-3

6. For the ZnS shelling onto InP/lnZnPSe/ZnSe, previously prepared 12ml of 0.5 M zinc oleate stock solution and 6ml of 1 M TOPS stock solution, mixed 6mmol of sulfur with 6ml of trioctylphosphine and heated at 120°C for 2 hours under vacuum, are slowly introduced into the reactor at 300°C and maintained for lhour to synthesize a 529nm InP/lnZnPSe/ZnSe/ZnS material according to the step 3-4

Table 1 Optical properties of InP/ZnSe, InP/lnZnPSe/ZnSe, and

InP/lnZnPSe/ZnSe/ZnS

Discussion of the gamma value (v)

Gamma value (g) means the optical density ratio of 450nm blue regime to InP Is peak, by which it is possible to calculate how much blue light the InP QDs absorbs.

Figure 5 shows (a) UV-absorption spectra and PL intensity of InP/ZnSe/ZnS QD and description of gamma value, (b) External quantum efficiency (EQE) simula tion with different gamma values, and (c) EQE simulation with InP,

CdSe, and perovskite.

As above mentioned, InP QD is originally weak to absorb the blue light than CdSe and CsPbBr3 (perovskite) because of energy separation in band gap struc ture. Despite this instinctive weakness point in blue absorption, if higher blue absorbed InP QD having y value over 1.5 is made, over doubled EQE perfor mance in the same QY relative to 0.5 y value as seen in Figure 5b.

This means that InP QD having a higher y value can cut the production cost by more than half because the meaning of EQE is the ratio of an emitted photon to an excited photon, which strongly depends on the total number of the ab sorbed blue photon.

Figure 6 shows the UV-absorption spectra and PL intensity of (a) InP/ZnSe QD (Example.1) and (b) InP/lnZnPSe/ZnSe QD (Example 2)

As seen in Figure 6b, the alloyed InZnPSe multi-shell layer between InP and ZnSe absorbs 450 nm blue wavelength better than InP/ZnSe QD because the position of band gap of InZnPSe muti-shell layer is located between InP and ZnSe. Figure 7 shows the blue absorption (g) value of InP core according to Example 1-1, 2-1, and 3-1.

Figure 8 shows TEM photographs, which were used in order to obtain the di ameter of the quantum dots.

Claims

1. Quantum dot with a core-shell structure, comprising a core and at least two shells, said at least two adjacent shells surrounding the core, wherein said core and the at least two shells comprise or consist of semiconducting materials, characterized in that at least one of the at least two shells comprises or consists of an In-Zn-P-Se semiconductor material.

2. Quantum dot according to claim 1, characterized in that the quantum dot comprises one shell comprising or is made of an In-Zn-P-Se semi conductor material adjacent to the core.

3. Quantum dot according to one of the preceding claims, characterized in that the quantum dot comprises at least two shells, wherein a shell adjacent to the core comprises or is made of an In-Zn-P-Se semicon ductor material and the at least one further shell comprises or is made of a semiconductor material selected from the group consisting of ZnS, ZnSe, ZnTeSe, ZnMgSe, CdS, ZnCdS.

4. Quantum dot according to one of the preceding claims, characterized in that the core comprises or consists of a semiconductor material selected from the group consisting of InP, ln(Zn)P, InGaP

5. Quantum dot according to one of the preceding claims, characterized by a diameter in between 1.00 and 15.00 nm, preferably in between 1.50 and 7.50 nm, especially preferred in between 2.50and 5.00 nm.

6. Quantum dot according to one of the preceding claims, characterized in that the core has a diameter in between 0.1 and 10.0 nm, preferably in between 1.0 and 7.5 nm, especially preferred in between 1.75 and 3.0 nm.

7. Quantum dot according to one of the preceding claims, characterized in that the shell adjacent to the core has a thickness in between 0.1 and 12 nm, preferably in between 1.0 and 10 nm, especially preferred in between 3.0 and 8.0 nm.

8. Quantum dot according to one of the preceding claims, characterized in that the shell being adjacent to the shell compring or consisting of an In-Zn-P-Se semiconductor material has a thickness in between 0.01 and 6.0 nm, preferably in between 0.1 and 3.0 nm, especially preferred in between 1.0 and 1.5 nm.

9. Quantum dot according to one of the preceding claims, comprising a core formed of In-P semiconductor material, a shell adjacent to the core is made of an In-Zn-P-Se semiconductor material and an outer shell is made of Zn-Se semiconductor material or Zn-S semiconductor material.

10. Quantum dot according to one of the preceding claims, characterized in that its 450 nm blue absorption (Gamma, g) which is a ratio of opti cal density in 450nm wavelength to optical density of InP Is peak is over 1.0.

11. Method of preparing a quantum dot according to one of the preceding claims, comprising the following steps:

• provision of a first solution comprising precursor materials for the semiconductor of the core and reacting the precursor materials to form a dispersion comprising semiconductor nanoparticles of the core,

• provision of a second solution comprising precursor materials for the In-Zn-P-Se semiconductor, addition of the second solution to the dispersion comprising nanoparticles of the core and reacting the precursor materials to form the In-Zn-P-Se semiconductor shell adjacent to the core,

• provision of a second solution comprising precursor materials for the In-Zn-P-Se semiconductor, addition of the third solution to the dispersion obtained in the preceding step and reacting the precur sor materials to form a shell comprising a semiconductor material or consisting thereof on the In-Zn-P-Se semiconductor shell adja cent to the core,

12. Electronic device comprising quantum dots according to one of the claims 1 to 11.

13. Electronic device according to the preceding claim selected from the group consisting of displays, especially TV or computer displays.