Patent Application
20240351906
The present application claims priority to Korean Patent Application No. 10-2023-0050712, filed Apr. 18, 2023, the entire contents of which is incorporated herein for all purposes by this reference.
The present disclosure relates to perovskite nanocrystals with improved colloidal stability and a method for producing the same.
Metal halide perovskites (MHP) is a promising candidates for a light-emitting materials owing to their wide color gamut, high color purity, high photoluminescence quantum yield (PLQY), and inexpensive solution processability.
Because of these properties, MHP has attracted development of efficient considerable attention to the perovskite light-emitting diodes (PeLEDs) with high maximum external quantum efficiencies (EQEs) for blue, green, and red PeLEDs.
The recent improvements of electroluminescence (EL) efficiency in PeLEDs (EQEs >20%) were achieved using anti-solvent crystallization for polycrystalline thin films with nanograins and adopting colloidal MHP nanocrystals (NCs). This significantly increases the radiative recombination rate of spatially confined excitons.
In addition, a versatile strategy to enhance stability reportedly reduces surface defects associated with the non-radiative recombination at grain boundaries and the interfaces between MHP NCs through the introduction of surface-binding ligands. Therefore, the interfacial interaction of MHP NCs with the functional groups of organic molecules plays a crucial role in the efficiency of injection charge and exciton recombination, which govern the overall performance of PeLEDs.
The surface-binding ligands on MHP NCs provide colloidal long-term stability and high PLQY. However, low structural integrity and poor luminescence are commonly observed in the MHP NC films during fabrication (i.e. spin-coating) because their stability relies on the rapid attachment-detachment dynamics of surface capping molecules.
In particular, the dynamic equilibrium associated with the formation of VBr— defects is limited owing to mismatched ligands on the surface of MHP NCs, resulting in the formation of non-radiative recombination centers.
Meanwhile, electrically insulating hydrocarbon chains (e.g. oleylamine (OlAm) and oleic acid (OA)) on the MHP surface inevitably hinder efficient charge injection and transportation, which are critical prerequisites for highly efficient PeLEDs.
Several approaches to overcome these challenges have been reported, including the partial or total replacement of these ligands with a combination of shorter fatty acids and amines or engineered surface ligands, such as zwitterionic capping ligands, aromatic ligands, organic halides, and inorganic ligands.
For example, Dong et. al. demonstrated that a bipolar shell consisting of an inner anion and outer cation can enable highly confined CsPbBr3 NCs for efficient green LEDs with an EQE of 22%. Zhao et. al. reported on bright PeLEDs based on pure-green emissive formamidinium lead bromide (FAPbBr3) NCs by substituting long-chain OA ligands with short aromatic molecules of 2-naphthalenesulfonic acid. Other approaches using capping ligands with more reactive functional groups can successfully reduce halide migrations via stronger chemical bonds with the MHP surfaces, exhibiting enhanced PeLED stability.
However, despite progress in the efficiency of PeLEDs via a rational design of surface ligands, many unresolved problems remain. Ligand strategies to minimize the interparticle spacing of MHP NCs significantly lower the steric hindrance between colloidal MHP NCs, and thus increase the diffusion rate related to further crystalline growth. This phenomenon is of critical importance for two-dimensional MHP nanoplatelets (NPLs) because it causes a red-shifted emission spectrum as a result of an increase in NPL thickness. Poor colloidal stability arising from low steric hindrance or electrostatic stabilization leads to another processing issue relevant to film morphology and uniformity, as well as pin-hole formation. Furthermore, locally aggregated ligands imposed by chemically reactive functional groups can result in internal Joule heat and Auger recombination at high current densities. As a result, precise control of the molecular structure of surface-binding ligands is required to overcome several shortcomings of commonly used ligands and their combinations.
In order to solve the above problems, an objective of the present disclosure is to provide perovskite nanocrystals with improved colloidal stability and significantly reduced steric hindrance between colloidal nanocrystal through reasonable design of surface ligands resin, and another objective of the present disclosure is to provide a method for producing the same perovskite nanocrystals.
However, the objectives are illustrative, and the technical spirit of the present disclosure is not limited thereto.
One a first aspect of the present disclosure for achieving the above object relates to a perovskite nanocrystal having improved colloidal stability, the perovskite nanocrystal including: a CsPbX3 (X is halogen) perovskite nanocrystal; and a hydrazinium (NH2—NH3+) ligand bonded to a surface of the nanocrystal.
In the first aspect, the perovskite nanocrystal having improved colloidal stability may have a particle size of 10 nm or less.
In the first aspect, the perovskite nanocrystal having improved colloidal stability may have an inter-particle distance of 1.8 nm or less.
In the first aspect, the perovskite nanocrystal having improved colloidal stability may have a zeta potential (Z) of 10 mV or more.
In the first aspect, the perovskite nanocrystal having improved colloidal stability may have a Br/Cs atomic ratio in a range of 2.9 to 3.5.
In the first aspect, the perovskite nanocrystal with improved colloidal stability may have a carrier-trap activation energy (ΔEtrap) value of 60 meV or more.
In addition, a second aspect of the present disclosure relates to a method of producing a perovskite nanocrystal with improved colloidal stability, the method including: a) preparing a CsPbX3 (X is a halogen) perovskite nanocrystal dispersion; and b) injecting the CsPbX3 perovskite nanocrystal dispersion into a hydrazinium halide reaction solution and stirring the mixture.
In the second aspect, the CsPbX3 perovskite nanocrystal dispersion has a concentration of 0.5 to 50 mg/ml.
In the second aspect, the hydrazinium halide reaction solution may have a concentration that is higher than 0 and lower than 10 mM.
In the second aspect, the CsPbX3 perovskite nanocrystal dispersion is produced by (i) preparing a precursor solution A containing PbX2, 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.
In the second aspect, 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.
In the second aspect, 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.
In addition, a third aspect of the present disclosure relates to a light emitting device including the perovskite nanocrystal having improved colloidal stability.
In the case of the CsPbX3 perovskite nanocrystal with improved colloidal stability according to the present disclosure, a hydrazinium (NH2—NH3+) ligand is bounded to the surface of the CsPbX3 perovskite nanocrystal, instead of a long carbon chain ligand. Therefore, the CsPbX3 perovskite nanocrystals have an improved zeta potential value compared to pure CsPbX3 nanocrystals, resulting in improved colloidal dispersion stability. In addition, since the steric hindrance between colloidal nanocrystals is reduced, the problem of pinhole generation during film formation is prevented, and the film thickness uniformity is improved.
In addition, when the film is applied to a light emitting device, a light emitting device having improved light emission stability can be provided.
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.
Referring to
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 (NH2—NH3+) 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.
Specifically, a first aspect of the present disclosure relates to a perovskite nanocrystal having improved colloidal stability, the perovskite nanocrystal including: a CsPbX3 (X is halogen) perovskite nanocrystal; and a hydrazinium (NH2—NH3+) ligand bonded to a surface of the nanocrystal.
The CsPbX3 perovskite nanocrystal with improved colloidal stability according to the present disclosure has a hydrazinium (NH2—NH3+) ligand bonded to the surface thereof instead of a long carbon chain. Therefore, the CsPbX3 perovskite nanocrystals have an improved zeta potential value than pristine CsPbX3, 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.
In addition, when the film is applied to a light emitting device, a light emitting device having improved light emission stability can be provided.
Hereinafter, a perovskite nanocrystal with improved colloidal stability according to an example of the present disclosure will be described in detail.
As described above, a perovskite nanocrystal with improved colloidal stability according to an example of the present disclosure is a CsPbX3 (X is a halogen) perovskite nanocrystals having a surface to which a hydrazinium (NH2—NH3+) ligand is bound.
In the CsPbX3 perovskite nanocrystal, X may be a halogen, for example F, Br or Cl. Preferably, X may be Br.
The hydrazinium (NH2—NH3+) ligand is bound to the surface of the CsPbX3 perovskite nanocrystal to improve colloidal dispersion stability and to reduce steric hindrance between colloidal nanocrystals. For example, the hydrazinium (NH2—NH3+) ligand may be a ligand derived from one or more hydrazinium halides selected from H2NNH2·HF, H2NNH2·HBr and H2NNH2·HCl.
In order to secure excellent colloidal dispersion stability of CsPbX3 perovskite nanocrystals having hydrazinium (NH2—NH3+) 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.
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.
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 (NH2—NH3+) ligand is not sufficiently bonded to the CsPbX3 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 [PbBr6]4− octahedral structure, and formation of nanocrystal aggregates.
In addition, the perovskite nanocrystal with improved colloidal stability may have a carrier-trap activation energy (ΔEtrap) 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 (NH2—NH3+) ligand is sufficiently bound, thereby preventing Br vacancies and the related defects on the surface.
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 CsPbX3 (X is a halogen) perovskite nanocrystal dispersion; and b) injecting the CsPbX3 perovskite nanocrystal dispersion into a hydrazinium halide reaction solution and stirring the mixture.
First, step a) of preparing the CsPbX3 (X is a halogen) perovskite nanocrystal dispersion is performed. Specifically, the CsPbX3 perovskite nanocrystal dispersion is prepared by (i) preparing a precursor solution A containing PbX2, 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.
In one example of the present disclosure, the PbX2 is a precursor material of CsPbX3. Specifically, it is at least one selected from PbF2, PbBr2, and PbCl2, and it is preferably PbBr2.
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 PbX2.
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 PbX2.
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 PbX2.
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.
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 CsPbX3 perovskite nanocrystal dispersion may be 0.5 to 50 mg/ml and more preferably 1 to 30 mg/ml.
Next, step b) of injecting the nanocrystal dispersion into the hydrazinium halide reaction solution and stirring may be performed.
The hydrazinium halide is to provide a hydrazinium ligand. The hydrazinium halide is at least one selected from H2NNH2·HF, H2NNH2·HBr, and H2NNH2·HCl. Preferably, it is H2NNH2·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.
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.
Of course, the other layers of the device may be the same as those of an existing light emitting device.
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.
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 %).
HZBr is an interesting additive because of its unique chemical constituents consisting of a hydrazinium (NH2—NH3+) cation and a Br− anion. As illustrated in
On the other hand, a stabilized NC surface with controlled defect sites related to VBr
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 (Cs2CO3, 0.407 g) was put into a flask and stirred at 150° C. until Cs2CO3 completely reacted with the OA, to prepare cesium oleate (Cs-oleate).
CsPbBr3 NCs were synthesized through a hot injection method. 0.069 g of PbBr2 (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 N2 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.
For NC surface modification by hydrazine monohydrobromide (N2H5Br, 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.
Representative samples of CsPbBr3 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.
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.
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-Kα radiation at λ=1.54 Å. 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 Kα (hν=1486.6 eV). The solid-to-organic surfactant ratio was determined using thermogravimetric analysis (TGA/DSCl, Mettler-Toledo) under N2 up to 800° C. at a heating rate of 10° C./min. Fourier transform infrared spectroscopy (FTIR) spectra (400 to 4000 cm−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.
Here, I(t) is the PL intensity, t is the lifetime, α is the pre-exponential factor, and C is a constant. The average lifetime (τavg) was calculated as the average of the tri-exponential decay.
Here, f is the fractional contribution of each decay component and A is a constant. The accuracy of the fitting was determined to be 1±0.2 by χ2. 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).
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.
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).
Low-noise (LN) AFM measurement: A custom LN AFM system for use at a temperature of 21.8±0.1° C. and a humidity level of 21±0.5% 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.
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
Referring to
To investigate the stability of colloidal dispersion during the HZBr resurfacing process, we measured the zeta potentials (Z) of the CsPbBr3 NCs depending on the HZBr concentration (
In addition, the dispersion of HZBr-treated CsPbBr3 NCs maintained the initial PL intensity and full-width half-maximum (FWHM) for more than one week, indicating relatively good long-term PL stability (
Cryogenic PL spectroscopic measurements were performed on HZBr-treated CsPbBr3 NCs to elucidate exciton properties and exciton-phonon interactions based on the universal thermal expansion model (
The effect of HZBr treatment on the defect density on CsPbBr3 NCs and on exciton-phonon coupling can be investigated by fitting temperature-dependent PL spectra.
where Γinh 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, ΓAC is the exciton acoustic phonon coupling coefficient, kB is the Boltzmann constant, ΓLO is the exciton-LO phonon coupling coefficient, and ℏωLO is the LO phonon energy.
The linear ΓAC component dominates when the temperature is lower than 75 K (T<75 K) due to low-energy acoustic phonons, whereas the LO phonon energy (ΓLO) component is significant when the temperature is higher than 75 K (T>75 K) dominated by Bose-Einstein statistics. The extracted values of ℏωLO and ΓLO are shown in
ℏωLO is 37.4 meV for pristine CsPbBr3 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 CsPbBr3 NCs and are consistent with the results of previous studies. Meanwhile, Two significantly decreases 97.8 meV from ΓLO=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 CsPbBr3 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 [PbBr6]4− octahedron effectively suppress the formation of permanent and transient exciton traps by reducing the exciton-LO phonon coupling on the surface of CsPbBr3 NCs.
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:
The normalized PL area as a function of temperature is shown along with the fitting curves associated with
Time-resolved PL (TRPL) decay curves were obtained to understand carrier recombination dynamics depending on the temperature (
Transmission electron microscope (TEM) images and atomic force microscope (AFM) images were obtained to investigate the surface characteristics of a film of CsPbBr3 NCs according to HZBr treatment (
Uniform PL emission and lifetime of the CsPbBr3 nanocrystalline film were confirmed from images obtained by PL and TRPL mapping (
The presence of NH2—NH3+ ions was confirmed through Fourier Transform Infrared Spectroscopy (FT-IR) (
The unique properties of CsPbBr3 NCs are closely related to the chemical reaction with the surface containing the [PbBr6]4− octahedron composed CsPbBr3 NCs, Pb2+ ions, and Br− vacancies when HZBr is added. The surface chemistry of CsPbBr3 NCs was further investigated by XPS.
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
A patterned ITO glass substrate (25 mm×25 mm) was sonicated sequentially in deionized water, acetone, and isopropyl alcohol for 15 minutes. The substrate was subjected to N2 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 μm 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 (H2O and O2 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 CsPbBr3 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 2.0×10−5 torr or higher, at rates of 0.35, 0.1, 2.0 Å/s, respectively. Finally, optical epoxy (NOA71) was used to encapsulate the device in the glove box. The active area (4 mm2) of the device is defined as the overlapping area between the ITO and Al electrodes.
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.
A green LED based on CsPbBr3 NCs was fabricated with a flat band energy level diagram for all functional layers (
In addition, a hole-only device using the HZBr-treated CsPbBr3 NCs exhibited a reduced current density due to a decrease in the trap filling voltage compared to an existing hole-only device using pristine CsPbBr3. However, there was no significant difference in current density between an existing electron-only device using pristine CsPbBr3 and an electron-only device using the HZBr-treated CsPbBr3 NCs (
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.
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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10-2023-0050712 | Apr 2023 | KR | national |
