WO2020069729A1

WO2020069729A1 - Method for preparing quantum-dot-in-layered-perovskite heterostructured energy funnels

Info

Publication number: WO2020069729A1
Application number: PCT/EP2018/076830
Authority: WO
Inventors: Sachin KINGE; F. Pelayo GARCIA DE ARQUER; Li Na QUAN; Liang Gao; Zhenyu Yang; Edward Sargent
Original assignee: Toyota Motor Europe; The Governing Council Of The University Of Toronto
Priority date: 2018-10-02
Filing date: 2018-10-02
Publication date: 2020-04-09

Abstract

The present invention relates to a method for preparing a quantum-dot-in- layered-perovskite structure comprising the following steps: (1) providing colloidal quantum dots (CQD) in a non-polar first solvent, in order to obtain a first mixture; (2) providing ligands in a polar second solvent, in order to obtain a second mixture, at least a part of said ligands being perovskite precursors; (3) mixing the first and the second mixture, in order to cap CQDs with the ligands, and to transfer the CQDs into the polar second solvent; (4) mixing the solution of capped CQDs obtained in step (3) with other perovskite precursors; and (5) adding a third solvent, which is orthogonal to the second solvent, to the mixture obtained in step (4), in order to trigger crystallization of a quantum-dot-in-layered-perovskite structure. The present invention also relates to quantum-dot-in-layered perovskite structure obtained by the method, and use thereof.

Description

Method for preparing quantum-dot-in-layered-perovskite heterostructured energy funnels

Field of Invention

The present application concerns the field of materials for optoelectronic applications and in particular light-emitting devices (LEDs).

Background Art

The development of bright, efficient, high color-purity and inexpensive LEDs is paramount for lighting, and display applications. Of special interest for communication, spectroscopies and health-related applications are LEDs that emit infrared light.

Efficient LEDs require good and balanced charge injection, good and balanced electrical transport and stability.

Current infrared LEDs rely on the use of epitaxial semiconductors such as III-V compounds which increase complexity and cost due to the necessity of high temperatures and high vacuum for epitaxial crystal growth.

Colloidal Quantum dots (CQDs) stand out as a prominent candidate to enable next-generation infrared LEDs. To date, infrared CQD LEDs that simultaneously achieve good passivation and rapid and balanced charge transport have not been realized, and this has precluded the realization of bright and efficient infrared LEDs using this material class.

CQDs can be prepared by a chemical synthesis approach. This might offer high tunability of size, composition and physico-chemical properties of the CQDs.

Quantum-dot-in-layered perovskite (QDiP) solids have been recently demonstrated as a promising platform for IR light emission. The perovskite matrix, a material with an ABX₃ crystal structure, provides better charge transport than pristine CQD solids while passivating the CQD surface. Current approaches to realize QDiP solids fail to introduce high fractions of CQDs into the host matrix, and this severely limits the attainable brightness.

Moreover, the lack of control over the energy tunneling from the perovskite matrix into the CQD emitting material, results in a limited transfer efficiency and charge accumulation within the CQDs, giving rise to deleterious non-radiative recombination in both media.

Currently, QDiP can be prepared by mixing pre-fabricated quantum dots with a solution of perovskite precursors. This active material is then deposited onto the desired substrate depending on the application. For optoelectronic devices these comprise charge transport/injecting electrodes such as TiC>2, ZnO, PEDOT:PSS (poly(3,4-ethylenedioxythiophene)-polystyrenesulfonate) etc., and/or charge carrier transport layers, such as poly(9,9-di-n-octylfluorenyl-2,7- diyl) (F8) or SiC>2. The resulting thin film is then crystallized in subsequent deposition steps (e.g. methylammonium iodide, or similar salt, soaked processes, comprising two steps; antisolvent solution, comprising only one step), (c.f. non-patent documents [1], [2] and [4]).

Perovskite materials can be applied as multi-layered perovskites which are ensembles of perovskite domains that are confined in one direction with different repeating units {/?}. Energy transfer in this system can be controlled by manipulating the energy and special distribution of the different {ri} domains. This allows ultrafast energy accumulation in the lowest energy domains (cf. non-patent documents [3] and [5]). This system can be applied for efficient energy funnelling from perovskite to QD.

Patent Literature [1] discloses a composite material of a pre-formed crystalline or polycrystalline semiconductor particles embedded in a crystalline or polycrystalline perovskite matrix material, wherein the crystalline matrix has a specific composition including 3D perovskite and bulky organic cations defining the layered perovskite structure. The pre-formed crystalline or polycrystalline semiconductor particles and crystalline or polycrystalline perovskite are selected so that any lattice mismatch between the two lattices does not exceed about 10%. The pre-formed crystalline or polycrystalline semiconductor particles and said crystalline or polycrystalline perovskite matrix material have lattice planes that are substantially aligned.

Current QDiP perovskite solids provided with excellent passivation of the CQD emitting material, fail to: (1) incorporate a large fraction of CQDs without QDiP deterioration; (2) control the energy transfer from the perovskite host to the CQD emitting material. This ultimately leads to unwanted recombination within the perovskite phase, limited energy transfer efficiency, and unbalanced charge accumulation into the CQDs that hampers radiative efficiency (efficiency to emit light) at high radiances.

The present invention relates to a practical method to achieve QDs/perovskite Heterostructure Energy Funnels (QPH-EF) that simultaneously allows:

(1) the incorporation of a high loading of CQDs into QPH-EF;

(2) the ordered incorporation of such QDs;

(3) the active manipulation and programming of the energy transfer from the perovskite into the QD; and

(4) excellent energy transfer, good passivation, high brightness, and record performance infrared light emitting diodes.

To date, infrared CQD LEDs that simultaneously achieve good passivation and rapid and balanced charge transport have not been realized, and this has precluded the realization of bright and efficient infrared LEDs using this material class.

Patent Literature Reference

[1] WO 2016/109902

Non-Patent Literature References [1] Z. Ning, X. Gong, R. Comin, G. Walters, F. Fan, 0. Voznyy, E. Yassitepe, A. Buin, S. Hoogland, E. H. Sargent: Nature Letter, Vol. 523 (2015), p. 324-328

[2] X. Gong, Z. Yang, G. Walters, R. Comin, Z. Ning, E. Beauregard, V. Adinolfi, 0. Voznyy, E. H. Sargent: Nature Photonics Letters, Vol. 10 (2016), p. 253-257

[3] M. Yuan, L. N. Quan, R. Comin, G. Walters, R. Sabatini, 0. Voznyy, S. Hoogland, Y. Zhao, E. M. Beauregard, P. Kanjanaboos, Z. Lu, D. H. Kim, E. H. Sargent: Nature Nanotechnology, Vol. 11 (2016), p. 872-877

[4] Z. Yang, 0. Voznyy, G. Walters, J. Z. Fan, M. Liu, S. Kinge, S. Hoogland, E. H. Sargent: ACS Photonics, Vol. 4 (2017), p. 830-836

[5] L. N. Quan, Y. Zhao, F. P. Garcia de Arquer, R. Sabatini, G. Walters, 0. Voznyy, R. Comin, Y. Li, J. Z. Fan, H. Tan, 3. Pan, M. Yuan, 0. M. Bakr, Z. Lu, D. H. Kim, E. H. Sargent: Nano Letters, Vol. 17 (2017), 3701-3709

Summary of the Invention

In one aspect, the present invention relates to a method for preparing a quantum-dot-in-layered-perovskite structure comprising the following steps:

(1) providing colloidal quantum dots (CQD) in a non-polar first solvent, in order to obtain a first mixture;

(2) providing ligands in a polar second solvent, in order to obtain a second mixture, at least a part of said ligands being perovskite precursors;

(3) mixing the first and the second mixture, in order to cap CQDs with the ligands, and to transfer the CQDs into the polar second solvent;

(4) mixing the solution of capped CQDs obtained in step (3) with other perovskite precursors; and

adding a third solvent, which is orthogonal to the second solvent, to the mixture obtained in step (4), in order to trigger crystallization of a quantum- dot-in-layered-perovskite structure. In another aspect, the represent invention relates to the quantum-dot-in- layered perovskite (QDiP) structure obtained by the method of the present invention.

In still another aspect, the present invention relates to a use of the quantum-dot-in-layered perovskite structure according to the present invention in optoelectronic applications, and in light-emitting devices (LED).

Brief Description of the Figures

Figure 1 is a schematic representation of precusor composition, unit cell structure, High-Resolution Transmission Electron Microscopy (HRTEM) image and absorption of Quantum-dot/Perovskite Heterostructure Energy Funnels (QPH-EF).

a: QDs stabilized in Cs-based perovskite Dimethylsulfoxide (DMSO) solution by PEA (Phenylethylamine). The effect is shown on the QD dispersion,

b: Unit cell structure of QDiP and energy diagram

c: HRTEM image of QPH-EF nanoplatelet.

d: Absorption spectra of QPH-EF films with different QDs concentrations showing the contribution of the perovskite and quantum dot phases.

Figure 2 presents ordered incorporation of CQDs into QPH-EF at high loadings.

a: Photoluminescence Quantum Yield (PLQY) of different concentration 935 nm QDs in different perovskites.

b: Photoluminescence excitation (PLE) spectra of QDs in different perovskite films with the highest PLQY.

c: Azimuthally integrated one-dimensional (ID) Grazing-Incidence Small-Angle X-ray Scattering (GISAXS) intensities of QPH-EF film with different concentration of QDs. The ID GISAXS plots of QPH-EF films with 20% and 30% QDs inside respectively show sharp intensity peaks of QDs at q = 1.43 nm ^_1 with Full-Width at Half Maximum (FWHM) of 1.03 nm ¹ and 0.92 nm ¹, indicating QDs are highly ordered in these films and inter-QDs spacing is about 4.4 nm.

d: 2D GIWAXS image of QPH-EF films with 20% QDs concentration. QPH-EF layers are homogeneously oriented in 20% and 30% QDs hybrid film.

Figure 3 presents efficient carrier-exciton dynamics (energy transfer) and high PLQY of in QPH-EF films,

a: PL

b: PLE spectra of QPH-EF films with different QDs concentration,

c: PLQY

d: Transfer efficiency of QPH-EF films with different sizes of QDs. The QDs concentration is set to 30%.

Figure 4 presents LED device performance of QPH-EF.

a: LED device structure.

b: EL spectra of QMLP with different sizes of QDs.

c: EQE versus current density

d: Radiance versus voltage characteristics of QMLP with different concentration of QDs. The emission peak is around 986 nm.

Figure 5 presents an absorption spectra of PbS QDs exchanged by PEABr or PbBr₂ in DMF.

Figure 6 presents an absorption spectra of PbS QDs in octane, DMSO with or without PEA.

The redshift and broadening of QD absorption peak indicate that the Cs- component solvent DMSO makes liquid-exchanged PEABr-capped QDs aggrerated. The phenylethylamine (PEA) was used to stabilize QDs in that perovskite solution with a sharper aborption peak with a little redshift. The QDs are monodispersed in the DMSO solution.

Figure 7 presents a PL spectra of perovskites made in different solutions. 0.4 mmol PEABr and 0.2 mmol PbBr₂ were dissolved in 1 mL DMF to make pure PEA₂PbBr₄ film, which has a sharp PL peak at 408 nm. In order to clarify the ion exchange, PEA and BTA were respectively used as solvents to make MAPbBr₃ and PEA₂PbBr₄ films. The PL peak at 409 nm shows PEA exchanges MA⁺ to make a nearly PEA₂PbBr₄. The wide and asymetrical PL peak at 420 nm shows a 10 nm redshift form the pure PEA₂PbBr₄ film, indicating BTA could exchange PEA⁺.

Figure 8 presents perovskite PL intensity of excitation with 420 nm, wherein perovskite PL was quenched in PbS QD/perovskite hybrid sample due to the energy transfer.

Figure 9 presents PL intensity evolution of PbS QDs and perovskite, PbS QDs stabilized with different ligands.

Figure 10 presents an AFM image of QDLP film.

Figure 11 presents Ex-situ 2D GISAXS images of QDLP films with different concentration of QDs.

2D GISAXS images were azimuthally integrated to obtain this ID plot of Intensity vs. q. Here, signatures of quantum dots presence in the "No QDs" sample was not observed. "1% QDs" and "5% QDs" show presence of QDs which are not highly ordered. "20% QDs" sample shows the highest intensity peak of ordered QDs at q = 1.43 nm ¹ with FWHM of 1.03 nm ¹, corresponding to 4.4 nm inter-QD spacing. "30% QDs" sample also shows the scattering from ordered QDs at q = 1.43 nm ¹ with FWHM of 0.92 nm^'1. QDs are not highly ordered in "60% QDs" sample as it shows a broad and weak scattering peak.

Figure 12 presents an ex-situ 2D GIWAXS images of QDLP films with different concentrations of QDs.

The 2D GIWAXS images show quasi-2D perovskite nanoplatelets are homogeneously oriented in 20% and 30% QDs hybrid film, other than in hybrid films with higher or lower QDs concentration.

Figure 13 presents in-situ 2D GIWAXS images of QDLP films with different concentration of QDs. Figure 14 presents a transient absorption (TA) spectra of QDLP films with different concentration of QDs.

The transient absorption (TA) spectra illustrate the typical bleaches of different <n> quasi-2D perovskite nanoplatelets in the hybrid film according to QD concentration. The QDs induce further phase separation of quasi-2D perovskite, evidenced by the evolution of <n> distribution. The strong bleaches belong to low <n> nanoplatelets again confirm QDs-induced quasi-2D perovskite growth process: the CsPbBr3 component epitaxially grows on the QDs at first, then the PEA₂PbBr₄ component grows to cap the 3D matrix or form pure 2D perovskite nanoplatelets.

Figure 15 presents an absorption spectra of QDLP films with different QD concentrations.

Figure 16 presents a: PLQY of different concentration 935 nm QDs in different types of perovskites. b: Photoluminescence excitation (PLE) spectra of QDs with different perovskite heterostructure films with the highest PLQY.

Figure 17 presents a PL spectra of QDLP films with different size of QDs.

Figure 18 presents a: PLQY and b: Transfer efficiency of QDLP films for different QD sizes. QD concentration in perovskite precursor solutions was fixed to 30%.

Calculation of energy transfer efficiency from perovskite host to QDs:

In the equations, PLQYQDshort and PLQYqo-iong are respectively PLQYs of QDs excited at 420 and 675 nm; A_QD-Short and Ar_O-/_0P9 are respectively the absorptions of QDs at 420 and 675 nm; A_perovskite-short is the absorption of perovskite at 420 nm; h _trans is the energy transfer efficiency from perovskite to QDs. Comparing the absorption spectra of pure QDs film and 30% QDLP film, it Can be concluded that

Figure 19 presents a photo-excited stability of QDs-in-different perovskite films.

The excitation wavelength is 675 nm and the power density is 10 mW cm ². After 14-hour illumination in air, the PLQY of QDs in quasi-2D perovskite almost stays the same, while the PLQY of QDs in 2D and 3D perovskite respectively degrades by 10% and 40%. For the QDLP film, the perovskite matrix is double-side passivated by long-chain ligands, and QDs is lattice- matched capped by the perovskite matrix.

Perovskite PL decays of QDLP films with different concentration of QDs. Figure 20 presents the PL lifetime decreases from 12.86 ns of pure quasi-2D perovskite to 0.72 ns of 50% QDs hybrid film, which transfer is much faster than 2 ns of QDs in 3D perovskite. Higher QDs concentration means more possibility of confined excitons in quasi-2D perovskite nanoplatelet funneled into NIR recombination. The rapid energy transfer process of QDLP film can essentially preclude the exciton quenching effect presented in previously reported pure quasi-2D perovskite. The whole carrier-exciton dynamics are in the way of efficient exciton energy transfer in the QDLP NIR emitting layer, which avoids exciton dissociation as much as possible.

Figure 21 presents QDs PL spectra of QDLP films with 20% QDs under different excitation. The carrier-exciton dynamics in the 20% QDs hybrid film was investigated using time-resolved PL decay. Irrespective of whether only QDs excited by 723 nm or both materials excited by 374 nm, the decay traces could be double-component fitted. The fast component is ascribed to fast exciton dissociation into neighboring QDs, and the longer decay is attributed to radiative recombination. Due to the energy-transfer delay, the two components respectively increase from 93 ns and 598 ns under 723 nm excitation to 145 ns and 822 ns under 374 nm excitation. The QDs exciton decay time is much longer than the transfer time in quasi-2D perovskite, indicating excitons confined in quasi-2D perovskite would most likely be transferred into QDs. The comparison with QDs in 3D perovskite (20 ns and ~80 ns) shows a rapid slowdown of exciton-dissociation dynamics and a great improvement of radiative recombination of QDs in quasi-2D perovskite film.

Figure 22 presents a band alignment of QDLP LED device at zero bias.

Figure 23 presents LED EQE based on QDLP films with different mass ratio of 935 nm QDs to PbBr₂ in perovskite precursor.

As QDs concentration has a critical effect on the quality of QDs in quasi- 2D perovskite film, the LED performance was performed on different 935 nm QD concentrations. When QD concentration increases, the LED EQE dramatically increases to a peak of 4.5% at the QDs concentration of 35%, and then goes down.

Figure 24 presents LED EQE statistics based on QDLP films with different <n>. This optimization is based on 935 nm PbS QDs.

The <n> is the nominal ratio of CsPbBr₃ to PEA₂PbBr₄ composing the perovskite precursor. When <n> equals 3, QDs-in-PEA₂Cs₂Pb₃Bri₀ based devices achieve the highest EQE, a little higher than <n> = 4 devices.

Figure 25 presents LED EQE statistics based on QDLP films with different sizes of QD. The EQE statistics corresponding to different EL peaks were performed for more than 30 devices. The EQE tendency as QDs size is highly consistent with the value of PLQY ^c transfer efficiency (supplementary Figure 14). All EQE peaks are over 3% and the highest EQE is 8.08% at 986 nm emission. Around 1310 nm and 1550 nm for fiber communication, the EQEs reach up to 5.96% and 3.52%, respectively.

Figure 26 presents working stability of LEDs based on QD-in-different- perovskite films. This comparison is based on 845 nm PbS QDs.

The QDLP shows a coutineously working stability about 1 h at a current density of 10 mA cm ², which is much better than QDs in other perovskite devices.

Detailed Description of the Invention

< Definition >

The term "Colloidal Quantum dots (CQDs)", as used herein, refers to semiconductor nanoparticles having a diameter of less than 100 nm, and preferably 2 nm or more and 50 nm or less. The bandgap of these materials can be tuned from the visible to the infrared via size tuning. They stand out as a prominent candidate to enable next-generation infrared LEDs.

The term "perovskites", as used herein, refers to ABX₃ materials with A= cation groups such as methylammonium (MA), butylammonium (BA), formamidinium (FA), Cs, and their mixtures; M is a metal such as Pb, Sn, Bi, Cu, Ag and their mixtures; and X are halides such as Cl, Br, I and their mixtures. Layered perovskites contain additional bulkier molecules such as phenethylammonium (PEA), hexylammonium (HA) and longer, which confine different perovskite domains into different repeating units. Specific examples given here comprise inorganic metal-halide multi-layered perovskites. These examples are non-limiting. In the context of the present invention, the abbreviations MA, BA, FA, PEA and HA may be used to refer to the non-protonated amines (such as methylamine, butylamine...) or to the protonated ammonium forms (methylammonium, butylammonium...).

The term "Quantum dots in perovskite (QDiP)", as used herein, refers to epitaxially-aligned heterocrystals where quantum dots are embedded in a perovskite material host matrix.

The term "Quantum dots in layered perovskite", as used herein, refers to epitaxially-aligned heterocrystals where quantum dots are embedded in a perovskite material host matrix, wherein the perovskite is a layered perovskite.

The term "antisolvent", as used herein, refers to a solvent in which a target material to be precipitated is less soluble than it is in its current solvent. In particular, an antisolvent of a polar solvent may be a non-polar solvent, and an antisolvent of a non-polar solvent may be a polar solvent.

The term "QDiP", as used herein, refers to a subset of QPH-EF where the dots are embedded in the perovskite and energy transfer may not be actively controlled.

The term "QPH-EF", as used herein, refers to nanoparticles (in particular quantum dots) are not necessarily embedded in the perovskite, but can be outside of a perovskite domain (on top, side etc.), which might be observed, for example, with Transmission Electron Microscopy.

The term "ligand", as used herein, refers to molecules that are grafted to the nanocrystal QD surface.

The term "grafted", as used herein, refers to the formation of chemical bonds between the ligand and the nanocrystal QD surface.

The term "chemical bond", as used herein, refers to ionic or covalent bond.

The term "cation A", as used herein, refers to a monovalent cation. <Method for preparing a quantum-dot-in-layered-perovskite structure>

The method for preparing a quantum-dot-in-layered-perovskite structure of the present invention comprises the following steps:

(1) providing colloidal quantum dots (CQD) dispersed in a non-polar first solvent, in order to obtain a first mixture;

(2) providing ligands dispersed in a polar second solvent, in order to obtain a second mixture, at least a part of said ligands being perovskite precursors;

(5) adding a third solvent, which is orthogonal to the second solvent, to the mixture obtained in step (4), in order to trigger crystallization of a quantum-dot-in-layered-perovskite structure.

CQD materials comprise metal-chalcogenides and their core-shells, and tellurides, such as: PbS, PbSe, PbTe, CdSe, CdSe, CdTe, Mo₂S, WS₂, WTe₂, HgSe, HgTe, HgCdTe; and more complex compounds including anion mixtures such as CdSei-xSx, wherein 0 < x < 1. CQD materials can optionally have additional layers in a core-shell structure (e.g. CdS/CdSe, PbS/ZnS).

In the present invention, the CQD can preferably be selected from the group consisting of: PbS, PbSe, PbTe, CdSe, CdSe and CdTe.

During the crystallization process, the CQDs act as seeds that induce, in a preferred embodiment, CsPbBr3 growth on their surface until PEA⁺ termination determines the number of perovskite-repeating units.

Preferably, CQDs can be treated with CdCI₂, in order to have fewer surface defects and to be brighter.

< First Solvent> A first solvent in which QDs are dispersed is a non-polar solvent.

A first solvent can be, for examples, hydrocarbons, such as C5-C10 alkanes, aromatic hydrocarbons and cyclic hydrocarbons.

A first solvent can be, preferably, toluene, octane, chlorobenzene, ethyl acetate or a mixture thereof.

A second solvent, in which perovskite precursors are dissolved is a polar solvent.

A second solvent can be, for example, dimethylsulfoxide (DMSO), N,N'- dimethylformamide (DMF), gamma-butyrolactone (GBL), N-Methyl-2- pyrrolidone (NMP) or a mixture thereof.

A third solvent, which is orthogonal to the second solvent can be a non- polar solvent.

A third solvent can be, for example, hydrocarbons, such as C5-C10 alkanes, aromatic hydrocarbons and cyclic hydrocarbons.

A third solvent can be, preferably, toluene, octane, chlorobenzene, ethyl acetate or a mixture thereof.

A third solvent can be the same as the first solvent or different from the first solvent.

< Passivation agent>

In step (5), a passivation agent can also be added to the mixture obtained in step (4) together with a third solvent, in order to passivate the surface of QDs.

A passivation agent can be, for example, triphenylphosphine oxide (TPPO), tributylphosphine oxide (TBPO), trioctyl phosphine oxide (TOPO).

A preferred passivation agent is triphenylphosphine oxide (TPPO).

A passivation agent can be added separately from a third solvent in step (5), or can be mixed with a third solvent in advance. < Ligands provided in step (2) >

In step (2) of the process, perovskite precursors added in the form of e.g. MAI, MABr, MACI, PEAI, PEABr and PEACI, can be used as ligands. A PEA halide is a preferred precursor to be used in step (2). It is thus possible to add these perovskite precursors in a charged form (e.g. phenylethylammonium (with a - NH₃ ⁺ group)).

In another preferred embodiment, it is also possible to add these perovskite precursors in a non-protonated amine form e.g. PEA as a preferred precursor for step (2) in the form of phenylethylamine (with a -NH₂ group).

At least a part of ligands which are provided in step (2) and cap the CQDs in step (3) are a perovskite precursor, and therefore enables to better control the synthesis of QDiP.

The ligands which are provided in step (2) and cap the CQDs in step (3) could in general be the bulk organic cation employed in the perovskite.

The ligands which are provided in step (2) and cap the CQDs in step (3) can be selected from the group consisting of molecules with at least one of the following functional end groups: thiol, carboxylate and ammonium.

The preferred ligands which are provided in step (2) and cap the CQDs can be selected from the group consisting of alkylammonium cations such as phenylethylammonium, butylammonium, hexylammonium, and more preferably, MAI, MABr, MACI, PEAI, PEABr and PEACI.

PEA is the most preferred ligand which is provided in step (2) and caps CQDs in step (3) since it allows a precise control over the perovskite funnel.

PEA has a strong binding energy with PbS QDs compared to the DMSO solvent. The addition of PEA resulted in more stable PbS QD colloids that could withstand incorporation in the perovskite phase.

PEA is at the same time functions as a ligand and as a precursor of perovskite. During the fast crystallization, the PEA plays a crucial role in enabling a uniform incorporation of the CQDs into the resulting QPH-EF solid. The QPH-EF film shows sub-nanometer smoothness (~0.8 nm).

PEA further improves the solubility of QDs in certain solvents, and in particular, improves the solubility of the PbS nanocrystals in the perovskite precursor solution.

The other perovskite mixed in step (4) precursors comprises at least one metal halide salts of Pb and at least one precursors of the cation A (typically in a halide formulation as well).

Preferably, the other perovskite precursors mixed in step (4) can be selected from the group consisting of MAI, Pbl2, MABr, PbBr₂, MACI, PbCI₂, PEAI, PEABr, PEACI.

More preferably, the other perovskite precursors mixed in step (4) are MAI and Pbl₂.

Due to this step (4), it is possible to obtain a quantum-dot-in-layered structure with higher QD loadings, higher order, higher stability, higher performance, with an excitonic matric, which can give an optoelectronic device with higher brightness.

Additional steps>

The method of the present invention can further comprise one or more of the following steps:

- one or more purification steps, conducted between step (3) and (4), consisting in adding a cleaning solution to the bottom solution and then shaking; here, the cleaning solvent can be identical to or different from the non-polar first solvent, and can be, for example, octane, hexane, pentane, toluene, chlorobenzene, or a mixture thereof;

- a post-anti so I vent treatment, consisting in the deposition and removal of a solvent; - a thermal treatment, consisting in the annealing of sample with substrate at high temperature (40 - 120°C).

< Application >

The quantum-dot-in-layered perovskite structure of the present invention can be used in optoelectronic applications.

Preferably, the quantum-dot-in-layered perovskite structure of the present invention can be used in light-emitting devices (LED). Technical effect of the present invention>

The single-step deposition method of the present invention allows one to realize heterostructures comprising CQDs and multilayered perovskites (QD/perovskite Heterostructured Energy Funnels). The method of the present invention may enable judicious modification of the surface chemistry of the CQD to allow their incorporation into the multilayered perovskite precursors and their ultimate assembly into CQD/perovskite heterostructure.

The method of the present invention allows one to achieve the ordered incorporation of CQDs in the QPH-EF at high loadings, and the manipulation of the energy landscape of the multilayered perovskite host to maximize energy transfer into the CQDs.

The method of the present invention has successfully modulated the

QPH-EF, achieving a high-energy transfer efficiency (80%) and a high photoluminescence efficiency (45%). The resulting NIR LEDs exhibit a record external quantum efficiency of 8.08% a high radiances (7.38 W Sr ¹ m ²) and a much improved stability.

The method of the present invention allows one to realize quantum- dot/perovskite heterostructured energy funnels (QDP-EH) with a high and ordered loading of QDs. In this platform, the judicious tuning of the multilayered perovskite phase allows maximized and balanced energy transfer, leading to highly efficient infrared emitting materials. The present method may enable the modification of the CQD surface by appropriate organic cations that provide a high solubility of the surface-modified CQDs within the multilayered perovskite precursors.

The energy landscape of the host perovskite material in the final QDP-EH solid can be controlled by modifying the ratio of the precursors. The energy landscape can be modulated ad hoc to maximize energy transfer into the CQD light-emitting material.

The amount of the CQDs in the final QDP-EH can tailored by controlling the ratio of the CQDs to the perovskite precursors in solution.

The QDP-EH precursor solution is deposited onto a substrate. The QDP- EH is crystalized in a single-step method via antisolvent casting (e.g. toluene). The surface modification of the CQD might be the key to enable the ordered incorporation of CQDs into the heterostructure during crystallization. Notably, the QDs retain 100% of their original photoluminescence quantum efficiency from the colloid phase (45%) at much higher loadings (lOx) compared to prior methods.

Examples

Example 1: QPH-EF system wherein the perovskite is PEA2Cs_n-iPbnBr3n_+l and the CQD is PbS

All chemicals used are commercially available from Sigma-Aldrich (or otherwise specified) and were used without any additional purification steps: lead(II) oxide (PbO, 99.99 %, from Alfa Aesar), cadmium chloride (99.99 %), bis(trimethylsilyl)sulfide (synthesis grade), oleic acid (OA, tech. 90 %), 1- octadecene (ODE, > 95%), oleylamine (> 98%), dimethylformamide (DMF, 99 %), octane (anhydrous, > 99%), butylamine (BA, 99.5%), pentylamine (PTA, 99.5%), hexylamine (HXA, 99%), lead(II) iodide (from Alfa Aesar, 99.999 %, ultra dry), methylammonium iodide (MAI, from Dyesol Inc., 99.9 %), tetrabutylammonium iodide (TBAI,

98%), 3-mercaptopropionic acid (MPA, ^ 99%), 8-mercaptooctanoic acid (MOA, 95%), toluene anhydrous, methanol anhydrous, acetone, distilled in glass (Caledon).

The synthesis and the solution CdCI₂ treatment of PbS QDs followed published methods disclosed in

- Adv. Mater. 2003, 15, 1844-1849 (M. A. Hines, Gd. D. Scholes: Colloidal PbS Nanocrystals with Size-Tunable Near-Infrared Emission: Observations of Post-Synthesis Self- narrowing of the Particle Size Distribution) (hereafter referred as Hines et al.); and in

- ACS Nano 2015, 9, 12327- 12333 (Z. Yang, O. Voznyy, M.Liu, M.Yuan, A.

H. Ip, 0. S. Ahmed, L. Levina, S. Kinge, S.Hoogland, E. H. Sargent: All- Quantum-Dot Infrared Light-Emitting Diodes) (hereafter referred to as Yang et al.).

PbS CQD Synthesis and Cadmium Chloride Treatment

PbS CQDs (absorption maximum > 900 nm) were synthesized using a well-established hot injection method with some modifications: ( Thon et a/. ACS Nano, 2013, 7(9), pp 7680-7688).

The synthesis and solution CdCI₂ treatment of PbS QDs were carried out following the published method (non patent document [4], ACS Nano, 2013, 7(9), pp 7680-7688).

0.45 g of PbO, 1.5 mL of OA, 18 mL of ODE, and 0.5 mL of oleylamine were loaded in 250 mL 3-neck round-bottom flask, and then the mixture was pumped at 100 °C for 60 min. After the solution turned clear, the temperature was set to 110°C. When the solution temperature was stable at the predesigned reaction temperature, bis(trimethylsilyl)sulfide ODE solution (0.083 M) was rapidly injected into reaction flask. The heating mantle was turned off (but was not removed) to provide slow cooling. The dots were isolated by addition of 80 mL of acetone and redispersed in anhydrous toluene.

The CdCI₂ treatment was carried out during the slow cooling process following a recently published method.fnon patent document G41). 1 mL of CdCI₂ (0.06 M) oleyamine solution was injected into the CQD reaction flask during the slow cooling process. At temperature below 40°C, QDs were precipitated by the addition of ~50 mL of acetone and separated by ultracentrifugation. The supernatant was decanted, and the nanoparticles were redispersed in 2 mL of toluene and transferred into a glovebox. Inside the glovebox, the CQDs were reprecipitated by the addition of an ethanol/methanol mixture (1:1 volume ratio). After the centrifugation, the supernatant was decanted and particles were dried in vacuum for 1 h and then redispersed in octane at a concentration of 50 mg mL^-1. < Preparation >

(a) PbS CQDs capped with oleic acid dissolved in the hydrophobic solvent toluene was provided. More precisely, a total of 0.45 g PbO, 1.5 mL of OA, 18 mL of ODE, and 0.5 mL of oleylamine were loaded in a 250 mL 3-neck round-bottom flask. The mixture was then pumped at 100 °C for 60 min. After the mixed solution turned transparent, the temperature was set to 110 °C. When the solution temperature was stable at the predesigned reaction temperature, bis(trimethylsilyl)sulfide ODE solution (0.083 M) was rapidly injected into the reaction flask. The heating mantle was then turned off (but not removed) to provide slow cooling. The CdCI₂ treatment was carried out during the slow cooling process following a recently published method (non patent document [4]). A 1 mL aliquot of CdCI₂ (0.06 M) oleylamine solution was injected into the QD reaction flask during the slow cooling process. At temperatures below 40 °C, QDs were precipitated by the addition of ~50 mL of acetone and separated by ultracentrifugation. The supernatant was decanted and the nanoparticles were re-dispersed in 2 mL of toluene and transferred into a glovebox.

PEA (phenylethylamine) dissolved in DMSO was then employed to replace the oleic acid ligands that originally capped the CQDs to allow the re- dispersion of CQDs into a hydrophilic solvent such as DMSO.

(b)The CQDs solution obtained in step (a) was mixed with perovskite precursors: PbBr₂ CsBr and PEABr dispersed in DMSO.

(c) Octane, a solvent orthogonal to the one employed in the CQD/perovskite precursor colloid (DMSO), is used to wash away unreacted precursors and excess DMSO. This triggers the crystallization of the QD/perovskite heterostructure. Other solvents, such as hexane, toluene, chlorobenzene or ethyl acetate, can also be used.

The resulting heterostructure exhibits exceptional photo-stability and a good lattice match between the PbS QDs and the host perovskite (Fig 1 a, b).

In this example, the resulting QPH-EF film is composed of different {n} layered perovskites as indicated in XRD measurement. High-resolution transmission electron microscopy (FIRTEM) images show that the PbS QDs are highly lattice-matched with the CsPbBr₃ between two layers PEA⁺ (Figure 1,2).

It can be noted that when {/?} is lower than a threshold determined by the CQD diameter (4 in this case), the perovskite layers are too thin to cap the QDs inside. This results in a heterostructure where CQDs are adjacent to the perovskite layer. When {n} is larger than this threshold CQDs are prone to be incorporated into the layered perovskite in a core-shell fashion.

In the resulting QPFI-EF, energy transfer takes place from the larger bandgap domains (lower n values) into the CQDs directly or sequentially through smaller bandgap, larger n domains. The excitons and chargers are ultimately confined in the CQD where they recombine. PLQY of the QPH-EF was measured and compared to previous QDiP systems (MAPbBr_3/ PEA₂PbBr₄, QPH-EF = PEA₂Cs2Pb₃Brio). The QPH-EF shows a significant PLQY improvement even at high QD concentrations (20-30%).

Photoluminescence excitation (PLE) spectra was used to demonstrate the energy transfer from the perovskite host into the QDs emitting material in QPH- EF (Figure 3). The increase of the PL intensity at excitations below the wavelength of the perovskite absorption edge indicate that efficient energy transfer from the perovskite into the CQD.

In this example, QPH-EF shows a high energy transfer efficiency (80%) and a high photoluminescence efficiency (45%).

The use of QPH-EF for LED applications (Fig. 4) was demonstrated, using an architecture consisting on: indium-doped tin oxide (GTΌ) / poly(3,4- ethylenedioxythiophene) poly(styrenesulphonate) (PEDOT:PSS)/QPH-EF emission layer (45 nm)/2,2',2"-(l,3,5-Benzinetriyl)-tris(l-phennyl-l-H- benzimidazole) (TPBi)/lithium fluoride (LiF 10 nm)/Aluminum (Al 100 nm).

It can be noted that the excellent charge-exciton dynamics and the thin and smooth emitting layer enable high injection current density accompanied by high EQE and radiance.

Different sizes of QDs were embedded into the best <n> = 3 multilayered perovskite. Figure 4b shows the normalized electroluminescence (EL) spectra of 35% QMLP with different sizes of QDs. The EL peaks are respectively at 986, 1097, 1210, 1301, 1467 and 1564 nm, all with a little more redshift than PL peaks. The EQE tendency as QDs size is highly consistent with the value of PLQY x transfer efficiency (Fig. 3c and 3d). All EQE peaks are over 3% and the highest EQE is 8.08% at 986 nm emission. Around 1310 and 1550 nm for fiber communication, the EQEs reach up to 5.96% and 3.52%, respectively.

Example 2 solution-phase ligand exchange on QD surfaces

QD ligand exchange procedure

(a) 0.1 g PEABr was dissolved in 0.5 mL DMF (1 mmol/mL PEABr DMF).

(b) 0.2 mL PEABr DMF was used for QD ligand exchange.

(c) 0.6 mL QD in octane was added into 0.2 mL PEABr DMF.

(d) The mixed solution was shaken for 3 minutes under all QDs were exchanged to the bottom DMF layer.

(e) The supernatant was taken out by pipette.

(f) 0.5 mL octane was added into the bottom solution to clean QDs by shaking.

(g) This clean process was repeated three times and QD-in-PEABr DMF was gathered (precursor 3). 10 v/v% PEA was added into precursor 3. QD-in-PVK-energy funnel precursor and anti-solvent preparation

(h) 1 mL DMSO was added into 0.22 g PbBr₂ + 0.852 g CsBr (precursor 1). The concentration of PbBr₂ was 0.6 M, the concentration of CsBr was 0.4 M.

(i) 100 pL precursor 1 was diluted by 300 pL DMSO to form 0.15 M PbBr₂ + 0.1 M CsBr (precursor 2).

(j) 44 pL precursor 3 was added into 400 pL precursor 2 to form QD in layered perovskite precursor.

(k) Then QD in layered perovskite precursor was filtered by hydrophilic filter.

(L) 1.2 mL toluene was added into 6 mg TPPO to form 5 mg/mL TPPO toluene as anti-solvent. Thin film preparation

(m) 100 pL QD in layered perovskite precursor was spin-coated in flowing IM₂ onto the substrate at 1000 rpm for 10 s and then 3000 rpm for 60 s.

(n) 20 pL of 5 mg/mL TPPO toluene was used as antisolvent and was dropped at 41 s of the 3000 rpm step. (o) The as-deposit film was annealed at 90 °C for 20 min in an N₂ glovebox to remove solvent completely.

Example 3

QD synthesis and solution exchange.

QDs were synthesized using methods previously reported (non patent document[4], ACS Nano, 2013, 7(9), pp 7680-7688). 3 mL of QDs dispersed in octane (110 mg mL ¹) were added into a 1 mL dimethylformamide (DMF) solution containing 202 mg of PEABr. After this solution was stirred vigorously for more than 6 min, QDs transferred from the top octane to the bottom DMF. The octane layer was then removed and the QDs DMF solution was washed with octane for three more times to remove any organic residue. 20 uL phenethylamine (PEA) was added into the QDs DMF solution.

Quantum-dot-in-layered-perovskite (QDLP) films and LED fabrication

110.1 mg of PbBr₂ and 42.6 mg of CsBr were dissolved in 1 mL dimethyl sulfoxide (DMSO). 202 mg of PEABr was dissolved in DMF. The desired amount of QDs in DMF containing PEABr and PEA was added into PbBr₂ and CsBr DMSO solution, and the PEABr DMF solution was used to make up a 3:2:2 molar ratio of PbBr₂, CsBr and PEABr. The solution was spin-coated in flowing N₂ onto the substrate at 1000 rpm for 10 s and then 3000 rpm for 60 s. Toluene was used as antisolvent dropped at 41 s of the 3000 rpm step. The as-deposit film was annealed at 90 °C for 20 min in an N₂ glovebox to remove solvent completely.

Pre-patterned GTO-coated glass substrates were treated using an oxygen plasma for 10 min immediately before use. PEDOT:PSS purchased from Heraeus was spin-coated at 5000 rmp for 60 s in air and then annealed at 150 °C for 30 min in an N₂ glovebox. The GD-QDiP film was spin-coated onto the PEDOT'.PSS layer. 40 nm TPBi, 10 nm LiF and 100 nm Al were deposit layer by layer using thermol evapotation. Each ITO substrate was patterned to yield 8 devices, each with an area of 6.14 mm². Film characterizations and optical property measurement

The film morphology and construction were characterized by Scan Asyst atomic force microscope (AFM) (Fig. 10), Hitachi scanning electron microscope (SEM), GISAXS (Fig. 11), GIWAXS (Fig. 12, 13) (Cornell High Energy Synchrotron Source) and FEI transmission electron microscope (TEM). The absorption spectra were obtained by PerkinElmer Lambda 950 (Fig. 5, 6, 15). TA measurements were conducted using a Ultrafast, Helios system (Fig. 14). PL measurements were performed using a Horiba Fluorolog system with a single grating and a time-correlated single-photon counting detector (Fig. 7, 8, 9, 17, 20, 21). A monochromatized Xe lamp was applied as excitation source for steady-state PL and PL excitation measurements. Pulsed laser diodes (l = 374 or 723 nm) were used for transient PL measurements. Standard PLQY measuments reported before were conducted in a Quanta-Phi integrating sphere coupled with optical fibers (Fig. 15, 16, 17, 18, 19). PL stability was measured under continuous 450 nm excitation from Xe lamp.

EL measurement and LED performance characterization

EL measurement was performed using a Keithley 2410 source meter and a NIR spectrophotometer (Ocean Optics, NIR-512) coupled with a set of lens and an optical fiber. The device stability was measured under 10 mA cm ² injection in N₂ and the EL spectra were continuously obtained. Current density- voltage (J-V) charateristics were monitored by a computer-controlled Keithley 2400 source meter in a N₂ atmosphere. In parallel, the in-situ EL radiance was recorded via a calibrated Ophir PD300-IR germanium photodiode (active area of 19.6 mm²). Lambertian emission was used to calculate EQE and radiance according to reported standard methods. Peak EQE was calculated as the number of emitted photons to the number of injected electrons. A minimum of thirty devices were tested for each sample type (Fig. 24-25). Comparative Example 1

Control samples comprising inorganically-passivated QDs (e.g. Pbl₂) show aggregation at the perovskite/DMSO precursor solutions, so that limiting the dispersion of the QDs in solution and deteriorate the kinetics of perovskite growth during the spin-coating processes.

Control samples were prepared in the same manner as in Example 1, apart from that the dots were treated instead in the method disclosed in Nature Materials volume 16, pages 258-263 (2017) (Liu et al.).

Comparative Example 2

Previous QDiP systems (MAPbBr3, PEA₂PbBr₄, QPH-EF = PEA₂Cs₂Pb3Bri₀)

A 70 pL aliquot of PbS QD BTL solution was transferred into a small centrifuge tube (1.5 mL size) and further diluted to 3.75 mg/mL by the addition of 70 pL of perovskite precursors (i.e., Pbl₂ and CH₃NH₃I with 1:1 molar ratio) in BA solution. The amount of the perovskite precursor added to the QD BTL solution was dependent on the final desired weight ratio between PbS and perovskite. For instance, for a 1:1 PbS/perovskite precursor mixed solution, 55.8 and 19.2 mg of Pbl₂ and CH₃NH₃I were dissolved in 10 mL of BA to form the perovskite precursors solution.

Claims

1. Method for preparing a quantum-dot-in-layered-perovskite structure comprising the following steps:

2. The method according to claim 1, wherein the CQD is selected from the group consisting of metal chalcogenides and tellurides, such as: PbS, PbSe, PbTe, CdSe, CdSe, CdTe, Mo₂S, WS₂, WTe₂, HgSe, HgTe, and HgCdTe; compounds including anion mixtures, optionally having additional layers in a core-shell structure such as CdS/CdSe, and PbS/ZnS.

3. The method according to claim 1 or 2, wherein the CQD is selected from the group consisting of PbS, PbSe and PbTe.

4. The method according to any one of claims 1 to 3, wherein the at least a part of the ligands provided in step (2) comprise any one of the functional groups selected from the group consisting of thiol, amine and carboxylate.

5. The method according to any one of claims 1 to 4, wherein the at least a part of the ligands provided in step (2) are selected from the group consisting of phenylethylammonium (PEA), butylammonium (BA), hexylammonium (HA) and methylammonium (MA).

6. The method according to any one of claims 1 to 5, wherein the at least a part of the ligands provided in step (2) are selected from the group consisting of MAI, MABr, MACI, PEAI, PEABr and PEACI.

7. The method according to any one of claims 1 to 6, wherein the at least a part of the ligands provided in step (2) is PEA.

8. The method according to any one of claims 1 to 7, wherein the other perovskite precursors mixed in step (4) comprise at least one metal halide salt of Pb and optionally a halide of Cs.

9. The method according to any one of claims 1 to 8, wherein the other perovskite precursors mixed in step (4) comprise at least one metal halide salt of Pb and at least one ammonium cation.

10. The method according to any one of claims 1 to 9, wherein the other perovskite precursors mixed in step (4) comprises at least one element selected from the group consisting of MAI, MABr, MACI, PEAI, PEABr and PEACI, and at least one element selected from the group consisting of PbCI₂, Pbl₂ and PbBr₂.

11. The method according to any one of claims 1 to 10, wherein the other perovskite precursors mixed in step (4) are MAI and Pbl₂.

12. The method according to any one of claims 1 to 11, wherein the nonpolar first solvent is a hydrocarbon, such as C5-C10 alkane, aromatic hydrocarbon and cyclic hydrocarbon.

13. The method according to any one of claims 1 to 11, wherein the nonpolar first solvent is selected from the group consisting of toluene, chlorobenzene, chloroform, dimethylchlororide, ethyl acetate and any of the combinations thereof.

14. The method according to claim 12 or 13, wherein the polar second solvent is selected from the group consisting of dimethyl sulfoxide (DMSO), dimethylformamide (DMF), gamma-butyrolactone (GBL), or any of the solvent combinations thereof.

15. The method according to claim 14, wherein the polar second solvent is DMSO.

16. The method according to any one of claims 1 to 15, wherein the third solvent orthogonal to the polar second solvent is a hydrocarbon, such as C5- C10 alkane, aromatic hydrocarbon and cyclic hydrocarbon.

17. The method according to any one of claims 1 to 16, wherein the third solvent orthogonal to the polar second solvent is selected from the group consisting of toluene, chlorobenzene, chloroform, dimethylchlororide, ethyl acetate and any of the combinations thereof.

18. The method according to any one of claims 1 to 17, wherein the method further comprises at least one of the following steps: - one or more purification steps, conducted between step (3) and (4), consisting in adding a cleaning solution to the bottom solution and then shaking;

- a post-antisolvent treatment, consisting in a deposition and removal of a solvent that can effectively remove unreacted compounds/ions; and

- a thermal treatment, consisting in the annealing of sample with substrate at high temperature (40 - 120°C).

19. Quantum-dot-in-layered perovskite (QDiP) structure obtained by the method according to any one of claims 1 to 18.

20. Use of the quantum-dot-in-layered perovskite structure according to claim 19 in optoelectronic applications.

21. Use of the quantum-dot-in-layered perovskite structure according to claim 20 in light-emitting devices (LED).