PEROVSKITE NANOCRYSTALS WITH IMPROVED COLLOIDAL STABILITY AND METHOD FOR PRODUCING SAME

Abstract

A perovskite nanocrystal having improved colloidal stability and a preparation method thereof are proposed. The perovskite nanocrystal includes a CsPbX.sub.3 (X is halogen) perovskite nanocrystal and a hydrazinium (NH.sub.2NH.sub.3.sup.+) ligand bonded to a surface of the CsPbX.sub.3 perovskite nanocrystal.

Claims

1. A perovskite nanocrystal with h improved colloidal stability, the perovskite nanocrystal comprising: a CsPbX.sub.3 (X is halogen) perovskite nanocrystal; and a hydrazinium (NH.sub.2NH.sub.3.sup.+) ligand bound to a surface of the CsPbX.sub.3 perovskite nanocrystal.

2. The perovskite nanocrystal with improved colloidal stability of claim 1, wherein the perovskite nanocrystal with improved colloidal stability has a particle size of 10 nm or less.

3. The perovskite nanocrystal with improved colloidal stability of claim 1, wherein the perovskite nanocrystal with improved colloidal stability has an interparticle distance of 1.8 nm or less.

4. The perovskite nanocrystal with improved colloidal stability of claim 1, wherein the perovskite nanocrystal with improved colloidal stability has a zeta potential () of 10 mV or more.

5. The perovskite nanocrystal with improved colloidal stability of claim 1, wherein the perovskite nanocrystal with improved colloidal stability has a Br/Cs atomic ratio in a range of 2.9 to 3.5.

6. The perovskite nanocrystal with improved colloidal stability of claim 1, wherein the perovskite nanocrystal with improved colloidal stability has a carrier trapping activation energy (E.sub.trap) value of 60 meV or more.

7. A method for producing a perovskite nanocrystal with improved colloidal stability, the method comprising: a) preparing a CsPbX.sub.3 (X is halogen) perovskite nanocrystal dispersion; and b) injecting the CsPbX.sub.3 perovskite nanocrystal dispersion into a hydrazinium halide reaction solution and stirring the mixture.

8. The method of claim 7, wherein the CsPbX.sub.3 perovskite nanocrystal dispersion has a concentration of 0.5 to 50 mg/mL.

9. The method of claim 8, wherein the hydrazinium halide reaction solution has a concentration that is higher than 0 nm and lower than 10 nm.

10. The method of claim 7, wherein the CsPbX.sub.3 perovskite nanocrystal dispersion is prepared by: (i) preparing a precursor solution A containing PbX.sub.2, a fatty acid, and a fatty amine; (ii) mixing cesium oleate with the precursor solution A; (iii) collecting nanocrystals from the resulting mixture of (ii); and (iv) dispersing the nanocrystals in a dispersion medium.

11. The method of claim 10, wherein the fatty acid is at least one selected from saturated fatty acids having 14 to 24 carbon atoms and unsaturated fatty acids having 14 to 24 carbon atoms.

12. The method of claim 10, wherein the fatty amine is at least one selected from saturated fatty amines having 14 to 24 carbon atoms and unsaturated fatty amines having 14 to 24 carbon atoms.

13. A light emitting device comprising the perovskite nanocrystal with improved colloidal stability of claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] FIG. 1 is a diagram schematically showing surface ligand changes and colloidal stability enhancement of CsPbBr.sub.3 nanocrystals (NCs) treated with hydrazine monohydrobromide (N.sub.2H.sub.5Br, HZBr) according to an example of the present disclosure;

[0033] FIG. 2 is a diagram schematically showing the structural characteristics of (a) pure CsPbBr.sub.3 NCs and (b) CsPbBr.sub.3 NCs (HZBr-L) treated with 0.5 mM of HZBr, with each inset showing transmission electron microscopy (TEM) image with a scale bar of 50 nm;

[0034] FIG. 3 is data showing the particle size distribution of pure CsPbBr.sub.3 NCs and HZBr-L;

[0035] FIG. 4 is data showing absorbance and PL spectra of pure CsPbBr.sub.3 NCs and HZBr-L;

[0036] FIG. 5 is data showing the PL intensity, PL decay time, and zeta potential of CsPbBr.sub.3 NCs according to the HZBr treatment concentration (0 to 4 mM);

[0037] FIG. 6 shows (a) absorbance, (b) PL intensity of a solution, (c) PL intensity of a film, and (d) PLQY analysis data according to the HZBr treatment concentration (0 to 4 mM) and the content of CsPbBr.sub.3 NCs;

[0038] FIG. 7 is a PL intensity analysis result according to HZBr treatment concentration (0 to 4 mM) and aging time (0 to 7 days) after HZBr treatment;

[0039] FIG. 8 shows 2D pseudocolor mapping data of temperature-dependent PL characteristics in a range of 80 K to 300 K for (a) pure CsPbBr.sub.3 NC, (b) HZBr-L, and (c) CsPbBr.sub.3 NC treated with 4 mM HZBr (HZBr-H);

[0040] FIG. 9 shows temperature-dependent PL intensities in a range of 80 K to 300 K for (a, e) pure CsPbBr.sub.3 NC, (b, f) HZBr-L, (c, g) CsPbBr.sub.3 NC treated with 1 mM HZBr (HZBr-M), and (d, h) HZBr-H;

[0041] FIG. 10 shows PL energy spectra according to temperature of pure CsPbBr.sub.3 NC, HZBr-L, HZBr-M, and HZBr-H, in which a dotted line is a linear fitting result for region I;

[0042] FIG. 11 shows full-width half-maximum (FWHM) energy values extracted from PL emission spectra of pure CsPbBr.sub.3 NC (blue circles) and HZBr-H (red circles), according to temperatures;

[0043] FIG. 12 shows full-width half-maximum (FWHM) energy values extracted from PL emission spectra of (a) pure CsPbBr.sub.3 NC, (b) HZBr-L, (c) HZBr-M, and (d) HZBr-H according to temperatures;

[0044] FIG. 13 is analysis data of longitudinal optical (LO) phonon energy (top) and exciton-LO phonon coupling coefficient (bottom) extracted from FIG. 12;

[0045] FIG. 14 shows integrated PL intensity analysis data according to temperature of pure CsPbBr.sub.3 NC, HZBr-L, HZBr-M, and HZBr-H, and integrated PL intensity analysis according to 1/T of pure CsPbBr.sub.3 NC and HZBr-L, in which a circle represents experimental results, and a line represents fitting results; The inset of FIG. 14 (right) represents E.sub.trap value determined by the fitting curve;

[0046] FIG. 15 shows integrated PL intensity analysis data according to 1/T of (a) pure CsPbBr.sub.3 NC, (b) HZBr-L, (c) HZBr-M, and (d) HZBr-H, in which a circle represents experimental results, and a line represents fitting results;

[0047] FIG. 16 shows time resolved PL (TRPL) spectra of HZBr-L (top) and PL decay times of pure CsPbBr.sub.3 NC, HZBr-L, HZBr-M, and HZBr-H (bottom);

[0048] FIG. 17 shows TRPL spectra according to temperature of (a) pure CsPbBr.sub.3 NC, (b) HZBr-L, (c) HZBr-M, and (d) HZBr-H;

[0049] FIG. 18 shows TEM images of (a) pure CsPbBr.sub.3 NC, (b) HZBr-L, (c) HZBr-M, and (d) HZBr-H;

[0050] FIG. 19 is graph showing the particle size and interparticle distance of nanocrystal particles of pure CsPbBr.sub.3 NC, HZBr-L, HZBr-M, and HZBr-H;

[0051] FIG. 20 shows analysis data of the particle size and interparticle distance of (a, e) pure CsPbBr.sub.3 NC, (b, f) HZBr-L, (c, g) HZBr-M, and (d, h) HZBr-H;

[0052] FIG. 21 shows PL (a, d), TRPL mapping (b, e), and TRPL spectra (c, f), in which a to c represent data for pure CsPbBr.sub.3 NC films and d to f represent data for HZBr films;

[0053] FIG. 22 shows AFM topography images (500500 nm.sup.2, height contrast (top), 3D image (bottom)) of (a) pure CsPbBr.sub.3 NC, (b) HZBr-L, (c) HZBr-M, and (d) HZBr-H;

[0054] FIG. 23 is X-ray diffraction pattern (XRD) analysis data of pure CsPbBr.sub.3 NC, HZBr-L, HZBr-M, and HZBr-H;

[0055] FIG. 24 is FT-IR analysis data of pure CsPbBr.sub.3 NC, HZBr-L, HZBr-M, and HZBr-H;

[0056] FIG. 25 shows XPS analysis of (a) C 1s, (b) N 1s, (c) O 1s, (d) Pb 3d, and (e) Br 3d of pure CsPbBr.sub.3 NC, HZBr-L, HZBr-M, and HZBr-H;

[0057] FIG. 26 shows atomic ratio analysis data of elements (Cs, Br, Pb, and N) obtained from XPS spectra of pure CsPbBr.sub.3 NC, HZBr-L, HZBr-M, and HZBr-H;

[0058] FIG. 27 is an energy level diagram of an LED according to an example of the present disclosure;

[0059] FIG. 28 is EL spectrum analysis data at an application voltage of 5.0 V to 8.0 V) of the LED;

[0060] FIG. 29 is a current density curve according to driving voltage of an LED device fabricated using pure CsPbBr.sub.3 NC or HZBr-treated CsPbBr.sub.3 NC, in which the inset is a cross-sectional SEM image;

[0061] FIG. 30 is a current efficiency curve according to driving voltage of an LED device fabricated using pure CsPbBr.sub.3 NC or HZBr-treated CsPbBr.sub.3 NC;

[0062] FIG. 31 is an EQE curve according to current density of an LED device fabricated using pure CsPbBr.sub.3 NC or HZBr-treated CsPbBr.sub.3 NC;

[0063] FIG. 32 is a luminance curve according to driving voltage of an LED device fabricated using pure CsPbBr.sub.3 NC or HZBr-treated CsPbBr.sub.3 NC; and

[0064] FIG. 33 shows current density curves according to driving voltages of (a) an electron-only device (ITO/TPBi/perovskite QD film/TPBi/LiF/Al) and (b) an hole-only device (ITO/PEDOT:PSS/poly-TPD/perovskite QD film/TPBi/MoO.sub.3/Ag) which are fabricated using fabricated using pure CsPbBr.sub.3 NCs or HZBr-treated CsPbBr.sub.3 NCs in dark conditions, in which the inset shows the structure of each device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0065] Herein after, perovskite nanocrystals with improved colloidal stability and a method for producing the same will be described in detail. The accompanying drawings are provided as examples to sufficiently convey the spirit of the present disclosure to those skilled in the art. Accordingly, the present disclosure is not limited to the drawings and may be embodied in other forms, and the drawings may be exaggerated to clarify the spirit of the present disclosure. In the flowing description, unless otherwise defined, all terms including technical and scientific terms used herein have the same meaning as commonly understood by those who are ordinarily skilled in the art to which this disclosure belongs. Further, when it is determined that the detailed description of the known art related to the present disclosure might obscure the gist of the present disclosure, the detailed description thereof will be omitted.

[0066] Referring to FIG. 1, in the case of perovskite nanocrystals to the surface of which hydrazinium (NH.sub.2NH.sub.3.sup.+) ligands are not bound, oleylamine and oleic acid ligands are bound to the surface of the CsPbX.sub.3 perovskite nanocrystals, resulting in large steric hindrance and low colloidal dispersion stability. Therefore, there is a problem that pinholes occur during film formation, or the film thickness uniformity is low due to aggregation of the nanocrystals.

[0067] Accordingly, the present inventors have consistently researched and studied a method for improving dispersion stability of nanocrystals while reducing steric hindrance by using relatively short ligands instead of conventional oleylamine and oleic acid ligands, and finally have found that a hydrazinium (NH.sub.2NH.sub.3.sup.+) ligand is used, colloidal dispersion stability can be improved by increasing the surface zeta potential and lowering steric hindrance. On the basis of the finding, the present disclosure hac been made.

[0068] Specifically, a first aspect of the present disclosure relates to a perovskite nanocrystal having improved colloidal stability, the perovskite nanocrystal including: a CsPbX.sub.3 (X is halogen) perovskite nanocrystal; and a hydrazinium (NH.sub.2NH.sub.3.sup.+) ligand bonded to a surface of the nanocrystal.

[0069] The CsPbX.sub.3 perovskite nanocrystal with improved colloidal stability according to the present disclosure has a hydrazinium (NH.sub.2NH.sub.3.sup.+) ligand bonded to the surface thereof instead of a long carbon chain. Therefore, the CsPbX.sub.3 perovskite nanocrystals have an improved zeta potential value than pristine CsPbX.sub.3, resulting in improved colloidal stability. In addition, the problem of pinhole generation during film formation is prevented, and the film thickness uniformity can be improved.

[0070] In addition, when the film is applied to a light emitting device, a light emitting device having improved light emission stability can be provided.

[0071] Hereinafter, a perovskite nanocrystal with improved colloidal stability according to an example of the present disclosure will be described in detail.

[0072] As described above, a perovskite nanocrystal with improved colloidal stability according to an example of the present disclosure is a CsPbX.sub.3 (X is a halogen) perovskite nanocrystals having a surface to which a hydrazinium (NH.sub.2NH.sub.3.sup.+) ligand is bound.

[0073] In the CsPbX.sub.3 perovskite nanocrystal, X may be a halogen, for example F, Br or Cl. Preferably, X may be Br.

[0074] The hydrazinium (NH.sub.2NH.sub.3.sup.+) ligand is bound to the surface of the CsPbX.sub.3 perovskite nanocrystal to improve colloidal dispersion stability and to reduce steric hindrance between colloidal nanocrystals. For example, the hydrazinium (NH.sub.2NH.sub.3.sup.+) ligand may be a ligand derived from one or more hydrazinium halides selected from H.sub.2NNH.sub.2.Math.HF, H.sub.2NNH.sub.2.Math.HBr and H.sub.2NNH.sub.2.Math.HCl.

[0075] In order to secure excellent colloidal dispersion stability of CsPbX.sub.3 perovskite nanocrystals having hydrazinium (NH.sub.2NH.sub.3.sup.+) ligands bound to the surface, the perovskite nanocrystals need to have an appropriate zeta potential value. Specifically, for example, the zeta potential (Z) of the perovskite nanocrystal with improved colloidal stability may be 10 mV or more, and may be more preferably in a range of 15 mV to 30 mV. In the zeta potential is within the specified range, good colloidal dispersion stability can be secured.

[0076] The perovskite nanocrystal improved colloidal stability may have a particle size of 10 nm or more and an interparticle distance of 1.8 nm or less because the zeta potential value thereof is 10 mV or more. More specifically, the perovskite nanocrystal having improved colloidal stability may have a particle size of 9 to 10 nm and an interparticle distance of 1.6 to 1.8 nm. It can be confirmed that the colloidal dispersion stability is excellent from the fact that the perovskite nanocrystal has such a particle size and interparticle distance.

[0077] In addition, the perovskite nanocrystal having improved colloidal stability have a Br/Cs atomic ratio of 2.9 to 3.5. When the Br/Cs atomic ratio is less than 2.9, the steric hindrance may still be high because the hydrazinium (NH.sub.2NH.sub.3.sup.+) ligand is not sufficiently bonded to the CsPbX.sub.3 perovskite nanocrystal, whereas the Br/Cs atomic ratio greater than 3.5 is not good because Br vacancies increase, resulting in many defects in the [PbBr.sub.6].sup.4 octahedral structure, and formation of nanocrystal aggregates.

[0078] In addition, the perovskite nanocrystal with improved colloidal stability may have a carrier-trap activation energy (E.sub.trap) value of 60 meV or more, more preferably 80 meV or more, and most preferably 90 to 120 meV. When the activation energy value falls within the specified range, the hydrazinium (NH.sub.2NH.sub.3.sup.+) ligand is sufficiently bound, thereby preventing Br vacancies and the related defects on the surface.

[0079] In addition, a second aspect of the present disclosure relates to a method of producing the perovskite nanocrystal with improved colloidal stability, the method including: a) preparing a CsPbX.sub.3 (X is a halogen) perovskite nanocrystal dispersion; and b) injecting the CsPbX.sub.3 perovskite nanocrystal dispersion into a hydrazinium halide reaction solution and stirring the mixture.

[0080] First, step a) of preparing the CsPbX.sub.3 (X is a halogen) perovskite nanocrystal dispersion is performed. Specifically, the CsPbX.sub.3 perovskite nanocrystal dispersion is prepared by (i) preparing a precursor solution A containing PbX.sub.2, a fatty acid, and a fatty amine, (ii) mixing cesium oleate with the precursor solution A, (iii) collecting nanocrystals from the reactant products of step (ii), and (iv) dispersing the nanocrystals in a dispersion medium.

[0081] In one example of the present disclosure, the PbX.sub.2 is a precursor material of CsPbX.sub.3. Specifically, it is at least one selected from PbF.sub.2, PbBr.sub.2, and PbCl.sub.2, and it is preferably PbBr.sub.2.

[0082] The fatty acid may be at least one selected from saturated fatty acids having 14 to 24 carbon atoms and unsaturated fatty acids having 14 to 24 carbon atoms. Specifically, for example, the fatty acid may be at least one selected from oleic acid, palmitoleic acid, vaccenic acid, and paullinic acid. Such a fatty acid may be added in an amount of 0.001 to 1 mol, preferably in an amount of 0.005 to 0.01 mol, per 1 mol of PbX.sub.2.

[0083] The fatty amine may be at least one selected from saturated fatty amines having 14 to 24 carbon atoms and unsaturated fatty amines having 14 to 24 carbon atoms, and specifically, for example, may be one or more selected from hexylamine, octylamine, decylamine, dodecylamine, and oleylamine. Such a fatty amine may be added in an amount of 0.001 to 1 mol, preferably in an amount of 0.005 to 0.01 mol, per 1 mol of PbX.sub.2.

[0084] When preparing the precursor solution A, a solvent may be further used. The solvent may be used without limitation as long as it is non-reactive. For example, the solvent is 1-octadecene or the like. The solvent may be added in an amount of 5 to 100 ml, and preferably 10 to 50 ml, per 1 mmol of PbX.sub.2.

[0085] After the precursor solution A is prepared, cesium oleate may be mixed and reacted with the precursor solution A. In this case, the reaction temperature is 70 C. to 200 C. and more preferably 100 C. to 160 C., and the reaction time is 2 to 20 seconds, and preferably 4 to 8 seconds. With the ranges being satisfied, nanocrystals can be effectively synthesized. With the ranges being satisfied, nanocrystals can be effectively synthesized.

[0086] Thereafter, a step of collecting the synthesized nanocrystals and dispersing the collected nanocrystals in a dispersion medium may be performed. In this case, the dispersion medium may be at least one selected from toluene and acetone, but the dispersion medium is not necessarily limited thereto. The concentration of the CsPbX.sub.3 perovskite nanocrystal dispersion may be 0.5 to 50 mg/ml and more preferably 1 to 30 mg/ml.

[0087] Next, step b) of injecting the nanocrystal dispersion into the hydrazinium halide reaction solution and stirring may be performed.

[0088] The hydrazinium halide is to provide a hydrazinium ligand. The hydrazinium halide is at least one selected from H.sub.2NNH.sub.2.Math.HF, H.sub.2NNH.sub.2.Math.HBr, and H.sub.2NNH.sub.2.Math.HCl. Preferably, it is H.sub.2NNH.sub.2.Math.HBr. In this case, the hydrazinium halide reaction solution may further include a solvent, and the solvent may be at least one selected from toluene and acetone, but is not necessarily limited thereto. In addition, the concentration of the hydrazinium halide reaction solution may be higher than 0 and lower than 10 mM, may be preferably 0.1 to 5 mM, and may be more preferably 0.2 to 1 mM. With the specific range being satisfied, perovskite nanocrystals with improved colloidal stability and higher zeta potential can be effectively synthesized.

[0089] A further aspect of the present disclosure relates to a light emitting device including the perovskite nanocrystal having improved colloidal stability. As described below, the perovskite nanocrystals with improved colloidal stability can be used to form a perovskite layer of an existing light emitting device. The perovskite nanocrystal has excellent colloidal dispersion stability, pinhole generation can be prevented during film formation, and the uniformity of film thickness can be improved. Due to the advantages, when the film is applied to a light emitting device, the light emitting device has improved light emission stability.

[0090] Of course, the other layers of the device may be the same as those of an existing light emitting device.

[0091] Herein after, perovskite nanocrystals with improved colloidal stability and a method for producing the same according to the present disclosure will be described in detail, with reference to examples. However, the examples described below are presented only for illustrative purposes and are not intended to limit the scope of the present disclosure. The examples can be embodied in other various forms.

[0092] In addition, unless otherwise defined, all technical and scientific terms have the same meaning as that is generally understood by the ordinarily skilled in the art to which the present disclosure pertains. The terminology used in the description herein is merely to effectively describe specific embodiments and examples and is not intended to limit the present disclosure. In addition, the unit of each of the component to be added, which are not specifically described in the specification, may be % by weight (represented as wt %).

[0093] HZBr is an interesting additive because of its unique chemical constituents consisting of a hydrazinium (NH.sub.2NH.sub.3.sup.+) cation and a Br.sup. anion. As illustrated in FIG. 1, the hydrogen bonding between the NH.sub.3.sup.+ moiety and the Br.sup. ion promotes efficient ligand exchange between the relatively long-chain amine ligand with the relatively short-chain NH.sub.2NH.sub.3.sup.+ ion. In particular, [PbBr.sub.6].sup.4 octahedra along with the surface-bound ligands can be effectively stripped as more Pb.sup.2+ ions are reduced in the presence of excess hydrazinium, which acts as a reducing agent. As a result, a net positive charge on the NC surface can be induced by the uncoordinated Pb atoms, which provide (i) attractive sites for coordination with electron-rich molecules (i.e., Lewis bases) such as amine groups (NH.sub.2) having a lone pair of electrons (FIG. 5, calculated electrostatic potential profile of N.sub.2H.sub.5.sup.+) and (ii) electrostatic stabilization of colloidal CsPbBr.sub.3 NCs.

[0094] On the other hand, a stabilized NC surface with controlled defect sites related to V.sub.Br.sub. (i.e., non-radiative recombination centers) can be achieved by strong affinity between NH.sub.3.sup.+ and [PbBr.sub.6].sup.4 octahedral structure and excess Br.sup. anions. This effectively passivates the Br.sup. vacancies and enhances the radiative recombination of charge carriers. In this study, the inventors hypothesizes that the ionic reactivity between HZBr and CsPbBr.sub.3 NC surface moieties governs the structural integrity of CsPbBr.sub.3 NCs having highly improved colloidal stability, which ultimately improves the performance of CsPbBr.sub.3 NC-based PeLEDs.

[Preparation Example 1] Preparation of Cesium Oleate

[0095] Cesium oleate was separately prepared according to a previously reported protocol (Nano Lett. 2015, 15 (6), 3692-3696). 1-Octadecene (ODE, 20 mL) and oleic acid (OA, 1.25 mL) were placed in a 50 mL two-necked round bottom flask equipped with a magnetic stirrer. The flask was heated to 120 C. and held under nitrogen flow for 1 hour. Cesium carbonate (Cs.sub.2CO.sub.3, 0.407 g) was put into a flask and stirred at 150 C. until Cs.sub.2CO.sub.3 completely reacted with the OA, to prepare cesium oleate (Cs-oleate).

[Preparation Example 2] Preparation of CsPbBr.SUB.3 .NCs

[0096] CsPbBr.sub.3 NCs were synthesized through a hot injection method. 0.069 g of PbBr.sub.2 (0.188 mmol) and ODE (5.0 ml) were placed in a 50 ml flask and dried at 120 C. for 1 hour under vacuum conditions. Oleic acid (OA, 0.5 mL) and OlAm (0.5 mL) were put into a flask at 120 C., and the temperature of the flask was raised to 160 C. under N.sub.2 flow. Cs-oleate (0.4 mL) as a solid at ambient conditions was preheated to 100 C. and rapidly injected into the reaction mixture with vigorous stirring. The reaction mixture was cooled after 5 seconds by immersing the flask in an ice bath. The cooled reaction mixture was centrifuged at 6000 rpm for 5 minutes to remove NC aggregates. NCs were collected by adding ethyl acetate in an amount of 2.5 times the volume of the NC reaction mixture and centrifuging the resulting mixture at 8000 rpm for 10 minutes. The supernatant was discarded, and the precipitate was collected and then dispersed in toluene (1 mL) for the subsequent use. The concentration was determined by the weight of the solid residue after drying a certain volume of the solution until reaching a certain mass at 100 C.

[Example 1] Surface Modification of CsPbBr.SUB.3 .NCs with Hydrazine Monohydrobromide

[0097] For NC surface modification by hydrazine monohydrobromide (N.sub.2H.sub.5Br, HZBr), HZBr dissolved in acetone and toluene was injected into the NC solution while vigorously stirring. The toluene:acetone volume ratio was maintained at 10:1, and the concentrations of NC and HZBr in the final sample were adjusted in the range of 1 to 9 mg/mL to 0.1 to 4 mmol/mL.

[0098] Representative samples of CsPbBr.sub.3 NCs treated with various amounts of HZBr were labeled (1) HZBr-L (0.5 mM), (2) HZBr-M (1 mM), and (3) HZBr-H (4 mM). Unless otherwise specified, the concentration of NCs is 3 mg/mL and the concentration of HZBr is 0.5 mM.

[0099] After the HZBr injection, the samples were stirred for 20 seconds, and then film-casted or processed for characterization. For AFM characterization, the prepared NC solution was spin-coated on poly(ethylenedioxythiophene):polystyrene-sulfonate (PEDOT:PSS) at 3500 rpm for 60 seconds. For XRD and XPS characterization, the NC solution was centrifuged at 11000 rpm for 30 minutes to collect NCs. The collected NCs were dried at 120 C. or drop-casted onto glass slides or Si wafers. All optical measurements on the NC solutions, including PL, absorbance, and zeta potential, were performed in freshly prepared and diluted colloidal solutions to prevent reabsorption of PL.

Evaluation of Properties

1) Analysis Method:

[0100] Transmission electron microscopy (TEM) images of NCs were acquired using a JEM 2010 electron microscope operated at 200 kV. X-ray diffraction (XRD) patterns were collected using a Bruker AXS D8 diffractometer using Cu-K radiation at =1.54 . X-ray photoelectron spectroscopy (XPS) spectra of NCs were collected using a Thermofisher Scientific/K-Alpha+X-ray photoelectron spectrometer equipped with a monochromatic X-ray source from Al K (h=1486.6 eV). The solid-to-organic surfactant ratio was determined using thermogravimetric analysis (TGA/DSCl, Mettler-Toledo) under N.sub.2 up to 800 C. at a heating rate of 10 C./min. Fourier transform infrared spectroscopy (FTIR) spectra (400 to 4000 cm.sup.1) of NC solutions were obtained using a Bruker ALPHA-P. UV-vis absorption and photoluminescence spectra were recorded under ambient conditions using a Shimadzu UV-2600 UV-vis and Hitachi F-7000 fluorescence spectrometer, respectively. Time-resolved PL (TRPL) decay spectra were obtained using Fluorolog3 with time-correlated single photon counting (TCSPC, Horiba Scientific), with a 375 nm laser excitation source. The fluorescent decay curves were fitted by triexponential fitting.

[00001]$\begin{array}{cc}I\left(t\right)=C+{}_1\exp \left(-\frac{t}{{}_1}\right)+{}_2\exp \left(-\frac{t}{{}_2}\right)+{}_3\exp \left(-\frac{t}{{}_3}\right)& \left(1\right)\end{array}$

[0101] Here, I(t) is the PL intensity, t is the lifetime, is the pre-exponential factor, and C is a constant. The average lifetime (.sub.avg) was calculated as the average of the tri-exponential decay.

[00002]$\begin{array}{cc}{}_{\mathrm{avg}}=\frac{{}_1{}_1^2+{}_2{}_2^2+{}_3{}_3^2}{{}_1{}_1+{}_2{}_2+{}_3{}_3}=f_1{}_1+f_2{}_2+f_3{}_3+A& \left(2\right)\end{array}$

[0102] Here, f is the fractional contribution of each decay component and A is a constant. The accuracy of the fitting was determined to be 10.2 by .sup.2. The zeta potential of the surface-modified NC solution interface was measured using a Zetasizer Nano ZS (Malvern). Cryogenic PL spectroscopic measurements were performed in the temperature range of from 80 K 300 K using a spectrofluorometer FS5 (Edinburgh Instruments, United Kingdom).

[0103] The electro-potential map of hydrazine bromide was obtained by optimizing the molecular structure and using the density functional theory (DFT) and the B3LYP method. Models were created and visualized using Gaussian 09 software and the GaussianView 6.0 interface.

[0104] PL and TRPL mapping measurement: Steady-state PL and TRPL mappings were obtained using micro-PL/Raman spectroscopy (XperRam-RF, Nanobase) at room temperature. The sample was excited with a picosecond pulsed diode laser (LDH-DC-405, PicoQuant) having an excitation wavelength of 405 nm (FWHM <50 ps) and a repetition rate of 40 MHz. The excitation laser was modulated in continuous wave mode and pulse mode according to the PL and TRPL measurements. The laser power and exposure time for PL and TRPL measurements were set to 500 nW and 1 ms, respectively, using an objective (40, NA 0.75, MPlanFLN, Olympus). For the PL measurement, the grating was set to 600 gr/mm. TRPL signals were detected using single photon avalanche diodes (SPAD, PMD series, PicoQuant) and time-correlated single photon counting (TCSPC, TimeHarp 260, PicoQuant).

[0105] Low-noise (LN) AFM measurement: A custom LN AFM system for use at a temperature of 21.80.1 C. and a humidity level of 210.5% was developed by the Korea Research Institute of Standards and Science (KRISS). The temperature can be controlled by circulating a temperature control liquid. The sample surface was examined in tapping mode using a high-density carbon probe (SuperSharpStandard-NCHR; Nanotools, Germany)) a normal probe radius of about 2 nm and a cantilever spring constant of 40 N/m. The cantilever vibrated at 3.18 nm (free air amplitude) with a Q value of 533. The set point for the distance between the probe and the sample was 2.31 nm. The set point was maintained at 7 nm to ensure a large distance to reduce probe damage that may be caused when the AFM probe was brought into contact with the sample. The probe then slowly approached the sample by a distance of 0.1 nm using large proportional and integral gain factors.

2) Analysis;

[0106] FIG. 2 is a diagram schematically showing the structural characteristics and optical characteristics of (a) pristine CsPbBr.sub.3 and (b) CsPbBr.sub.3 NCs (HZBr-L) treated with 0.5 mM of HZBr. In both cases, ammonium groups are well coordinated with the Br.sup. ions at the surface of the perovskite lattice. FIG. 3 shows the average edge lengths of pristine CsPbBr.sub.3 and HZBr-L measured on the TEM images of FIG. 2 (insets of FIG. 2), in which the pristine CsPbBr.sub.3 and HZBr-L are uniform cube-shaped nanocrystals with average edge lengths of 9.11.6 nm and 9.51.6 nm, respectively. FIG. 4 shows absorbance and PL emission spectra of pristine CsPbBr.sub.3 and HZBr-L, and similar absorbance and PL emission spectra were observed from both the samples.

[0107] To understand how the presence of HZBr affects PL emission and charge relaxation, the maximum PL intensity, PLQY, and decay time were observed for various HZBr concentrations (0 to 4 mM) (see FIGS. 5 and 6).

[0108] Referring to FIG. 5, both the PL intensity and decay time increased significantly when initially exposed to a low concentration of HZBr (<1 mM). This indicates that the addition of HZBr successfully improves the radiative recombination of the CsPbBr.sub.3 NCs and that the hydrazinium ligand with a lower steric hindrance can be densely coordinated at the surface of the [PbBr.sub.6].sup.4 octahedral by replacing OlAm with a large steric hindrance. However, as the HZBr concentration increased, both values recovered to their initial levels with considerable aggregation of CsPbBr.sub.3 NCs. By monitoring their absorbance and PL intensity, the concentration of the colloidal CsPbBr.sub.3 NC solution was carefully controlled to be nearly identical to avoid possible effects such as the dilution-induced formation of nanoplates (NPLs) (FIG. 6).

[0109] To investigate the stability of colloidal dispersion during the HZBr resurfacing process, we measured the zeta potentials (Z) of the CsPbBr.sub.3 NCs depending on the HZBr concentration (FIG. 5). Initially, an abrupt increase in the positive value from +8 mV to +27 mV was observed for the HZBr-L, confirming the adsorption of NH.sub.2NH.sub.3.sup.+ ions and the stronger electrostatic repulsion on the NC surface. This suggests that the colloidal stability for film fabrication stems from increased surface charge density, despite considerably lower steric hindrance of short surface-bound ligands.

[0110] In addition, the dispersion of HZBr-treated CsPbBr.sub.3 NCs maintained the initial PL intensity and full-width half-maximum (FWHM) for more than one week, indicating relatively good long-term PL stability (FIG. 7). However, after the HZBr concentration increased above 1 mM (HZBr-M), it rapidly decreased to negligible values, indicating poor colloidal stability and aggregation. Therefore, optimized HZBr concentration is required to achieve a colloidal dispersion of the CsPbBr.sub.3 NCs with the shell of HZBr ligands.

[0111] Cryogenic PL spectroscopic measurements were performed on HZBr-treated CsPbBr.sub.3 NCs to elucidate exciton properties and exciton-phonon interactions based on the universal thermal expansion model (FIG. 8). For II-VI and III-V semiconductor quantum dots, red-shift of the PL peak wavelength can be observed with increasing temperature. This is due to the lattice deformation potential and exciton-phonon coupling, as described by an empirical Varshni relation. However, the 2-D pseudo-color plots of the temperature-dependent PL emission spectrum show a blue-shift and peak broadening as the temperature increases from 100 to 300 K for all samples (FIGS. 8 and 9). The PL peak energies exhibit a temperature dependence arising from the lattice thermal expansion and electron-phonon interaction. The increase in the bandgap energy indicates that the bandgap deformation potential is positive as the lattice expands at a higher temperature. The lattice expansion reduces the hybridization between the 4p orbital of Br and the 6s orbital of Pb, corresponding to the valence band maximum states of perovskite. The out-of phase band-edge states can be stabilized, increasing the bandgap energy. FIG. 10 shows that the PL energy can be approximated by a linear function with a slope of 282.8 eV/K below 225 K (region I). However, it exhibits a sublinear behavior (region II) above 225 K, indicating the dominant role of the exciton-phonon coupling at higher temperatures.

[0112] The effect of HZBr treatment on the defect density on CsPbBr.sub.3 NCs and on exciton-phonon coupling can be investigated by fitting temperature-dependent PL spectra. FIGS. 11 and 12 show the PL bandwidth of HZBr-treated CsPbBr.sub.3 NCs as a function of temperature. The temperature dependence is attributable to phonons and exciton scattering using longitudinal optical (LO) phonons:

[00003]$\begin{array}{cc}\left(T\right)={}_{\mathrm{inh}}+{}_{\mathrm{AC}}T+{{}_{\mathrm{LO}}\left(e^{h{}_{\mathrm{LO}}/k_{B}T}-1\right)}^{-1}& \left(1\right)\end{array}$

[0113] where .sub.inh is the temperature-independent inhomogeneous line width arising from variation of NC size, shape, and composition, as well as the scattering process associated with disorder and imperfections of the lattice, .sub.AC is the exciton acoustic phonon coupling coefficient, k.sub.B is the Boltzmann constant, .sub.LO is the exciton-LO phonon coupling coefficient, and .sub.LO is the LO phonon energy.

[0114] The linear .sub.AC component dominates when the temperature is lower than 75 K (T<75 K) due to low-energy acoustic phonons, whereas the LO phonon energy (.sub.LO) component is significant when the temperature is higher than 75 K (T>75 K) dominated by Bose-Einstein statistics. The extracted values of .sub.LO and .sub.LO are shown in FIG. 13 and Table 1 below.

TABLE-US-00001 TABLE 1 Pure CsPbBr.sub.3 HZBr-L HZBr-M HZBr-H .sub.inh 51.5 56.5 55.2 55.0 0.005 0.005 0.005 0.005 .sub.LO 228.1 170.0 148.2 97.8 E.sub.LO 37.4 35.0 31.5 26.1

[0115] .sub.LO is 37.4 meV for pristine CsPbBr.sub.3 and decreases slightly to 26.1 meV (HZBr-H) with increasing HZBr concentration. These values are in a range of from 15.5 to 52.4 meV for CsPbBr.sub.3 NCs and are consistent with the results of previous studies. Meanwhile, Two significantly decreases 97.8 meV from .sub.LO=228.1 meV with increasing HZBr concentration, suggesting that the contribution of exciton-LO phonon coupling to the broadening of PL becomes weaker. Similarly, the (3-aminopropyl)triethoxysilane ligands of CsPbBr.sub.3 NCs act as a mechanical damper and inhibits lattice vibrations, resulting in weak exciton-phonon coupling strength. This shows that the hydrazinium ligands coordinated on the surface of the [PbBr.sub.6].sup.4 octahedron effectively suppress the formation of permanent and transient exciton traps by reducing the exciton-LO phonon coupling on the surface of CsPbBr.sub.3 NCs.

[0116] The localization depth of excitons associated with non-radiative processes on the surface of NCs can be estimated from the temperature dependence of the integrated PL intensity as represented by the following equation:

[00004]$\begin{array}{cc}I\left(T\right)={I_0\left(1+{\mathrm{Ae}}^{-E_{\mathrm{trap}}/k_{B}T}\right)}^{-1}& \left(2\right)\end{array}$ [0117] where I.sub.0 is the integrated PL intensity at T=0 K, E.sub.trap is the activation energy for carrier trapping, and A is the frequency factor related to the trap state.

[0118] The normalized PL area as a function of temperature is shown along with the fitting curves associated with FIGS. 14 and 15. FIG. 14 clearly shows that HZBr-L exhibits reduced thermal quenching properties compared to pristine CsPbBr.sub.3. The E.sub.trap value determined by the fitting curve is the same as the inset value in FIG. 14. For pristine CsPbBr.sub.3, the trapping activation energy is 57.4 meV, which is comparable to the value for the shallow state of the Br vacancy. In addition, the PL quenching process with an activation energy of about 30 meV is usually observed through a transition between intrinsic and shallow defect states. Interestingly, in the case of HZBr-L, the E.sub.trap value increased to 101.9 meV, indicating that the surface Br vacancies and related defect states were well passivated. After HZBr treatment, an increase in the relative Br/Cs atomic ratio was also confirmed by X-ray photoelectron spectroscopy (XPS) analysis, and it will be more clearly described later.

[0119] Time-resolved PL (TRPL) decay curves were obtained to understand carrier recombination dynamics depending on the temperature (FIGS. 16 and 17). The PL lifetime increased monotonically for all samples as the temperature increased from 80 K to 300 K. The short PL lifetime at low temperatures indicates that free excitons are efficiently captured by trap states. The prolonged carrier decay process at higher temperatures is attributable to exciton thermal dissociation which hinders recombination.

[0120] Transmission electron microscope (TEM) images and atomic force microscope (AFM) images were obtained to investigate the surface characteristics of a film of CsPbBr.sub.3 NCs according to HZBr treatment (FIGS. 18 to 20 and 22).

[0121] FIG. 18 shows TEM images of CsPbBr.sub.3 NCs treated with various concentrations of HZBr. While the overall square shape of the NCs was maintained, the size slightly increased from 9.2 nm to 10.6 nm. The interparticle distance decreased from about 1.8 nm to 1.4 nm as the concentration of HZBr increased (FIGS. 19 and 20). It is postulated that further growth to maintain the chemical potential of NCs in the solution medium occurs due to redissolution of the exfoliated [PbBr.sub.6].sup.4 octahedra in the presence of excess HZBr. The partial size reduction of CsPbBr.sub.3 NCs is confirmed on the basis of the blue shift of the PL peak energy, which is exhibited by CsPbBr.sub.3 NCs when exposed to HZBr (FIG. 10) because this often leads to such quantum confinement effects in PL spectra.

[0122] Uniform PL emission and lifetime of the CsPbBr.sub.3 nanocrystalline film were confirmed from images obtained by PL and TRPL mapping (FIG. 21). The corresponding AFM images show the surface topology of the uniform NC film with low root mean square roughness (R.sub.q) of 2.384, 2.230 nm, 2.253 nm, and 2.067 nm for pristine CsPbBr.sub.3, HZBr-L, HZBr-M, and HZBr-H, respectively (FIG. 22). XRD patterns show that all NCs have a well-defined orthorhombic CsPbBr.sub.3 phase (PDF #01-072-7929) with a space group of Pnma (a=8.165, b=8.425, c=12.011 ) (FIG. 23).

[0123] The presence of NH.sub.2NH.sub.3.sup.+ ions was confirmed through Fourier Transform Infrared Spectroscopy (FT-IR) (FIG. 24). Upon exposure to HZBr, CsPbBr.sub.3 NCs clearly exhibited an evolution of the rocking .sub.1 (NH.sub.2) at 951 cm.sup.1, the stretching .sub.2 (NN) at 1138 cm.sup.1, and the bending .sub.3 (NH) at 1577 cm.sup.1. In particular, a significant shift of the NN stretching mode (.sub.2) to a higher wavenumber was observed while the NH.sub.2 rocking and the NH bending modes (.sub.1 and .sub.3) showed a minimal peak shift compared to the pure HZBr. In addition, unlike pristine CsPbBr.sub.3 that exhibits an NH.sub.2 rocking at 970 cm.sup.1 in HZBr and OlAm, the HZBr-treated sample exhibited an additional NH.sub.2 rocking .sub.1 at 951 cm.sup.1. The origin of the shift in the vibrational band can be linked to the weaker vibrational motion of NH.sub.2NH.sub.3.sup.+ ions when they strongly interact with the surface coordination sites of the CsPbBr.sub.3 NCs. This suggests that the addition of an appropriate amount of HZBr effectively passivates and stabilizes the surface of the CsPbBr.sub.3 NCs.

[0124] The unique properties of CsPbBr.sub.3 NCs are closely related to the chemical reaction with the surface containing the [PbBr.sub.6].sup.4 octahedron composed CsPbBr.sub.3 NCs, Pb.sup.2+ ions, and Br.sup. vacancies when HZBr is added. The surface chemistry of CsPbBr.sub.3 NCs was further investigated by XPS. FIG. 25 shows a representative high-resolution XPS spectrum of CsPbBr.sub.3 NCs treated with HZBr. The C 1s spectrum of pristine CsPbBr.sub.3 shows prominent shoulder peaks at 287.9 (OCO) and 285.6 eV (CO). This indicates that organic ligands such as oleic acid are present on the surface of CsPbBr.sub.3 NCs ((a) of FIG. 25). However, the shoulder peak at 287.9 eV (OCO) completely disappeared as the HZBr concentration increased, suggesting that the HZBr treatment could effectively reduce the amount of oleic acid (inset, (a) in FIG. 25). Similarly, well-resolved signals corresponding to NH.sub.3.sup.+ (401.2 eV) and NH.sub.2 (398.6 eV) were observed from the XPS spectrum of N 1s ((b) in FIG. 25). After the HZBr treatment, the N 1s peak corresponding to a protonated amine group (NH.sub.3.sup.+) is dominated by a sub-peak (NH.sub.2) that is negligible at a lower binding energy (398.6 eV). This indicates the presence of NH.sub.2NH.sub.3.sup.+ ions on the surface of CsPbBr.sub.3 NCs. These results confirm that relatively short NH.sub.2NH.sub.3.sup.+ ions replace relatively long hydrocarbon chains. In the O 1s spectrum of pristine CsPbBr.sub.3, an additional shoulder peak appeared at 530.7 eV due to the 0-Pb bond, but the shoulder peak significantly decreased as the HZBr concentration increased ((c) in FIG. 25). These results indicate that HZBr can successfully promote the surface reorganization by dissociating Pb.sup.2+RCOO.sup. and forming Pb.sup.2+Br.sup. coordination, resulting in exfoliation of surface-bound ligands. On the other hand, the amine groups (NH.sub.2) having a line pair of electrons, of NH.sub.2NH.sub.3.sup.+ ions can preferentially coordinate with uncoordinated Pb atoms, thereby electrostatically stabilized CsPbBr.sub.3 NCs consistent with zeta potential measurements (FIG. 5). However, it should be noted that excessive amounts of HZBr (i.e., HZBr-M and HZBr-H) can severely etch the surface of CsPbBr.sub.3 NCs, resulting in poor colloidal dispersion of the aggregates.

[0125] While there was no significant change in the XPS spectra of the Pb 4f and Br 3d features, HZBr-H exhibited there was a noticeable peak shift ((d, e) in FIG. 25). Both the Pb 4f.sub.5/2 and Pb 4f.sub.7/2 peaks shifted to higher binding energies, that is, from 138.2 eV and 143.1 eV to 138.7 eV and 143.9 eV, respectively. On the other hand, the Br 3d.sub.5/2 and 3d.sub.3/2 peaks shifted toward lower binding energies, that is, from 68.0 eV and 69.1 eV to 67.8 eV and 68.8 eV, respectively. These peak shifts may be due to the enhancement of the structural integrity of the [PbBr.sub.6].sup.4 octahedron as the Br vacancies are filled with an excess of Br ions due to a high concentration of HZBr. The Br/Cs atomic ratios were determined to be 2.83, 3.11, 3.17, and 3.66 for pristine CsPbBr.sub.3, HZBr-L, HZBr-M, and HZBr-H, respectively, thereby confirming that the Br vacancies were inhibited by the HZBr treatment. It was also observed that the nitrogen content continuously increased with increase in concentration of HZBr, from 0.27 to 0.55 (N/Cs atomic ratio) (FIG. 26).

[Fabrication Example 1] Fabrication of LED Made of CsPbBr.SUB.3 .NCs Treated with Hydrazine Monohydrobromide

[0126] A patterned ITO glass substrate (25 mm25 mm) was sonicated sequentially in deionized water, acetone, and isopropyl alcohol for 15 minutes. The substrate was subjected to N.sub.2 blowing, and then the substrate was further cleaned by UV-ozone treatment for 15 minutes. A poly-(ethylenedioxy thiophene):polystyrene-sulfonate (PEDOT:PSS) solution (Clevios P VP A14083 filtered through a 0.45 m PES filter) was spin-coated onto the substrate at 8000 rpm for 15 seconds. After annealing at 150 C. for 15 minutes, the substrate was transferred to a nitrogen-filled glove box (H.sub.2O and O.sub.2 are less than 1 ppm). To form a hole transport layer, poly(N,N-bis(4-butylphenyl)-N,N-bis (phenyl)-benzidine (poly-TPD, Sigma Aldrich, 1 mg/mL in chlorobenzene) and formamidinium bromide (7.5 mg/ml in dimethylformamide) was sequentially deposited by spin coating at 4000 rpm for 60 seconds. The prepared CsPbBr.sub.3 NC solution (HZBr-L, NC concentration of 20 mg/mL) was spin-coated at 2000 rpm for 15 seconds. The prepared substrate was transferred to a thermal evaporator. 1,3,5-tris(N-phenylbenzimidazol-2-yl) benzene (TPBi, 42.5 nm), LiF (2 nm), and Al (80 nm) layers were sequentially deposited under a high vacuum condition at a pressure of 2.010.sup.5 torr or higher, at rates of 0.35, 0.1, 2.0 /s, respectively. Finally, optical epoxy (NOA71) was used to encapsulate the device in the glove box. The active area (4 mm.sup.2) of the device is defined as the overlapping area between the ITO and Al electrodes.

Evaluation of Properties

1) Analysis Method:

[0127] Current-voltage (IV) characteristics were measured using a Keithley 2450 source measurer. Luminance and EL spectra were measured using an SR-3AR spectroradiometer (TOPCON). EQE was calculated from the obtained EL characteristics and Lambertian emission profiles. All instrumental measurements were performed under ambient conditions at room temperature.

2) Analysis:

[0128] A green LED based on CsPbBr.sub.3 NCs was fabricated with a flat band energy level diagram for all functional layers (FIG. 27). As fabricated in Fabrication Example 1, the device is composed of multiple layers including a patterned ITO anode, a PEDOT:PSS (30 nm) film, a poly-TPD (10 nm) film, a CsPbBr.sub.3 NC film (50 nm), a TPBi (42.5 nm) film, and a LiF/Al cathode (ITO/PEDOT:PSS/poly-TPD/CsPbBr.sub.3/TPBi/LiF/Al). The EL spectra of the HZBr-treated CsPbBr.sub.3 LED showed a single stable peak at 513 nm with a maximum EQE of 3.92% (FIG. 28). It was confirmed that the insulating ligands (i.e., oleylamine and oleic acid) were effectively replaced by shorter hydrazinium ligands as the current density of the HZBr-treated CsPbBr.sub.3 LED increased significantly at low voltage, and an efficient hole-electron injection into the CsPbBr.sub.3 NCs was facilitated (FIG. 29). That is, the LED using the HZBr-treated CsPbBr.sub.3 NCs exhibited an increased current efficiency of 12.16 cd/A compared to LEDs using pristine CsPbBr.sub.3 which had a current efficiency of 6.75 cd/A (FIG. 30). EQE was 2.17% in the case of using pristine CsPbBr.sub.3 but EQE was increased to 3.92% in the case of using the HZBr-treated CsPbBr.sub.3 (FIG. 31). The maximum luminance of the HZBr-treated CsPbBr.sub.3 LED was 4100 cd/m.sup.2 (FIG. 32).

[0129] In addition, a hole-only device using the HZBr-treated CsPbBr.sub.3 NCs exhibited a reduced current density due to a decrease in the trap filling voltage compared to an existing hole-only device using pristine CsPbBr.sub.3. However, there was no significant difference in current density between an existing electron-only device using pristine CsPbBr.sub.3 and an electron-only device using the HZBr-treated CsPbBr.sub.3 NCs (FIG. 33). This result suggests the dominant role of hole transport behavior in the LEDs using the HZBr treated CsPbBr.sub.3. Thus, these results clearly indicate that shorter HZBr ligand shells with higher grafting densities improve radiative recombination of charge carriers by passivating surface Br vacancies and the associated defect states without the usual compromise between charge transfer efficiency and insulating ligands. These results indicate that the charge injection efficiency is increased by promoting charge carrier delocalization on the surface of CsPbBr.sub.3 NCs.

[0130] The present disclosure has been described with reference to some specific examples and features. However, the specific examples and features are only for illustrative purposes and are not intended to limit the scope of the present disclosure, and it will be appreciated by those skilled in the art that various modifications and changes are possible on the basis of the description of the examples given above.

[0131] Therefore, the spirit of the present disclosure is not limited to the specific examples described above, and all forms defined by the appended claims and all equivalents and modifications thereto fall within the scope of the present disclosure.