Efficient and Stable CsPbBr3 Quantum-Dot Powders Passivated and

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Functional Inorganic Materials and Devices 3

Efficient and Stable CsPbBr Quantum-Dot Powders Passivated and Encapsulated with a Mixed Silicon Nitride and Silicon Oxide Inorganic Polymer Matrix Hee Chang Yoon, Soyoung Lee, Jae Kyu Song, Heesun Yang, and Young Rag Do ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b01014 • Publication Date (Web): 22 Mar 2018 Downloaded from http://pubs.acs.org on March 22, 2018

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ACS Applied Materials & Interfaces

Efficient

and

Stable

CsPbBr3

Quantum-Dot

Powders Passivated and Encapsulated with a Mixed Silicon Nitride and Silicon Oxide Inorganic Polymer Matrix Hee Chang Yoon†, Soyoung Lee†, Jae Kyu Song‡, Heesun Yang#, and Young Rag Do†,* †

Department of Chemistry, Kookmin University, Seoul 136-702, Republic of Korea

‡

Department of Chemistry, Kyung Hee University, Seoul 130-701, Korea

#

Department of Materials Science and Engineering, Hongik University, Seoul 121-791,

Republic of Korea *

E-mail address: [email protected]

Abstract Despite the excellent optical features of fully inorganic cesium lead halide (CsPbX3) perovskite quantum dots (PeQDs), their unstable nature has limited their use in various optoelectronic devices. To mitigate the instability issues of PeQDs, we demonstrate the roles of dual-silicon nitride and silicon oxide ligands of the polysilazane (PSZ) inorganic polymer to passivate the surface defects and form a barrier layer coated onto green CsPbBr3 QDs to maintain the high photoluminescence quantum yield (PLQY) and improve the environmental stability. The mixed SiNx/SiNxOy/SiOy passivated and encapsulated CsPbBr3/PSZ core/shell composite can be prepared by a simple hydrolysis reaction involving the addition of adding

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PSZ as a precursor and a slight amount of water into a colloidal CsPbBr3 QD solution. The degree of the moisture-induced hydrolysis reaction of PSZ can affect the compositional ratio of SiNx, SiNxOy, and SiOy liganded to the surfaces of the CsPbBr3 QDs to optimize the PLQY and the stability of CsPbBr3/PSZ core/shell composite, which shows a high PLQY (~81.7%) with improved thermal, photo, air, and humidity stability as well under coarse conditions where the performance of CsPbBr3 QDs typically deteriorate. To evaluate the suitability of the application of the CsPbBr3/PSZ powder to down-converted white-lightemitting diodes (DC-WLEDs) as the backlight of a liquid crystal display (LCD), we fabricated an on-package type of tri-color-WLED by mixing the as-synthesized green CsPbBr3/PSZ composite powder with red K2SiF6:Mn4+ phosphor powder and a polymethylmethacrylate-encapsulating binder and coating this mixed paste onto a cup-type blue LED. The fabricated WLED show high luminous efficacy of 138.6 lm/W (EQE = 51.4%) and a wide color gamut of 128% and 111% without and with color filters, respectively, at a correlated color temperature of 6762 K.

Keyword: CsPbBr3, Perovskite quantum dot, Polysilazane protection, Silicon nitride passivation, Silicon oxide encapsulation, Single-package white LED.

Introduction CsPbX3 (X = Cl, Br, I), all inorganic perovskites, are recently emerging semiconducting materials which have been applied to various optoelectronic devices, such as solar cells, photodiodes, and perovskite light-emitting diodes (PLEDs), as color converters in downconverted white LEDs (DC-WLEDs), lasers, and light-emitting electrochemical cells.1-6 In addition to the variety of applications, remarkable optical properties such as high photoluminescence quantum yields (PLQY), extremely narrow bandwidths (full width at half maxima (FWHM)) of the PL, broad tunability of the emission spectra, and short radiative lifetimes have been demonstrated.7-9 Although CsPbX3 perovskite QDs (PeQDs) are highlighted as emerging materials themselves and are used in various applications, the greatest obstacle has been their intrinsic instability against moisture, elevated temperatures,


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polar solvents, and UV light. These types of instability have hindered their practical use in most optoelectronic devices. Specifically, the types of instability of nanosized CsPbX3 PeQDs are intensified by the enhanced surface instability of the larger surface areas of nanocrystals. Importantly, it is well known that the surface properties dominate the structure, stability, and optical properties of PeQDs.10-12 Thus, it is necessary to develop surface passivation technologies using appropriate molecular ligands to stabilize the surface defects of PeQDs. To date, many researchers have made tremendous efforts to address this critical issue, as summarized in Table 1.13-21 They have developed various capping and encapsulating with two viable approaches. The first is the development of a passivation ligand molecule which enhances the optical properties and the stability of PeQDs by passivating surface defects and dangling bonds. The second is the development of encapsulating or barrier materials to protect PeQDs from moisture, oxidation, or other chemical attacks. Most recently, as a typical example of developing simultaneously both passivating and encapsulating materials, Li’s research group employed a di-dodecyl-dimethylammonium bromide (DDAB) salt as a capping ligand molecule to passivate CsPbX3 PeQDs, significantly improving the PLQY (67 80%) and slightly enhancing the photostability after a ligand exchange process.17 Regardless of the excellent research advances thus far, a process using only capping ligand molecules cannot solve the stability problem of PeQDs. Additionally, Li’s research group employed a hybrid SiO2/Al2O3 matrix using a sol-gel method without adding water or catalysts to improve the stability of PeQDs by limiting the diffusion of ions, atoms and small molecules, such as O2 and H2O. Otherwise, other groups have developed passivating materials such as (3-aminopropyl)triethoxysilane (APTES),13 polyhedral oligomeric silsesquioxane (POSS),18 NaNO3 salts,19 or TiO220 to play both capping and encapsulating roles simultaneously. Among them, APTES is one of the best materials in which NH2 can serve as a capping ligand and where the SiOC2H5 functional group can serve as a precursor for a silica matrix. However, the PLQYs of their resultant powders obtained by coating steps with bi-functional capping and encapsulating materials are not as high as that of the resultant powder obtained by a two-step coating process with capping and encapsulating materials.13 Recently, Jang’s research group synthesized thermally stable PeQD-SiO2 composites using perhydropolysilazane (PHPS) dibutyl ether solution and PeQD hexane solution by simply keeping the mixture overnight at room temperature. They have only obtained PeQD powder encapsulated with SiO2. Also, they realized green and red monochromatic down-converted LEDs and white LED using PeQD-SiO2 composites.21



Here, we demonstrate the suitability of another possible candidate bi-functional material, in this case polysilazane (PSZ), to passivate the surface of green (G) CsPbBr3 and form a barrier layer over the G PeQDs within a one-step coating process. As previously reported, the nitrogen-based ligand group of the PSZ matrix (-H2-SiNH-) can be used as a capping group, and the remaining silazane moiety can be reacted with water to induce a hydrolysis condensation reaction for the formation of the PSZ inorganic polymer matrix with different ratios of SiNx, SiNxOy, and SiOy as a barrier layer.22,23 In addition, we study the bi-functional passivation and encapsulation effect of a PSZ inorganic polymer coating on the surface of PeQDs to affect the PLQY and stability of G-emitting CsPbBr3 QDs by comparing the optical properties and the morphological, crystalline, and chemical properties of a CsPbBr3/PSZ core/shell nanocomposite of the moisture-induced hydrolysis temperature of the PSZ precursors. In relation to this, we found that embedding PeQDs into PSZ to be an effective means of improving the stability and maintaining the optical properties of PeQDs after the one-step hydrolysis reaction. The resultant CsPbBr3/PSZ PeQD nanocomposite powders exhibit highly efficient PLQYs and significantly improved thermal stability and photo stability levels. Moreover, we fabricated tri-color DC-WLEDs using G CsPbBr3/PSZ nanocomposite powder and red (R) K2SiF6:Mn4+ (KSF) phosphor to determine the suitability of the application of these nanocomposites into WLED backlight systems to realize a widecolor-gamut LCD display.

Experimental section Materials Cesium carbonate (Cs2CO3, 99.995%), lead bromide (PbBr2, 99.999%), 1-octadecene (ODE, 90%), oleylamine (OLA, >98%), oleic acid (OA, 90%), hexane (95%), methyl methacrylate monomer solution (99%), SiO2 powder, HF (48 wt% in H2O), KMnO4 (> 99.0%), and H2O2 (30 wt%) were purchased from Sigma-Aldrich. The photo-initiator used here to cure the poly(methyl methacrylate) was obtained from BASF Corp (Irgacure 819). The xylene solvent-based 5 wt% PSZ precursor solution was obtained without a catalyst from UP Chemical Inc. Cup-type blue LEDs were purchased from Dongbu LED, Ltd. (AL051).


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Synthesis of CsPbBr3 perovskite quantum dots To synthesize green emissive CsPbBr3 PeQDs, 5 mL of ODE and 0.188 mmol of PbBr2 powders were loaded into a three-neck flask and installed on a Schleck line linked to Ar gas and a vacuum pump. Next, this reaction flask was heated at 120 °C and simultaneously degassed for 1 hour. Subsequently, the flask was filled with highly purified Ar gas, after which 0.5 mL of OLA and 0.5 mL of OA were added to the pot. The PbBr2 white powder was dissolved in ODE solvent, with the mixture then becoming a clear solution. After 10 min of additional reaction time, the reaction pot was heated to 190 °C and the as-prepared 0.125 M ODE solvent-based Cs-oleate was injected into the pot. After 5 s of reaction time, the reaction solution turned yellowish in color, and it was cooled by an ice-water bath. To separate the CsPbBr3 QDs from the reaction solvent, the obtained crude CsPbBr3 PeQD solution was centrifuged at 12000 rpm for 5 min. After centrifugation, the sunken CsPbBr3 QD powders were re-dispersed in hexane solvent and re-centrifuged under the same condition. The supernatant CsPbBr3/hexane solution was stored in a desiccator, which can block external light and maintain a low humidity level.

Synthesis of CsPbBr3/PSZ composite powder First, the green emissive CsPbBr3/hexane solution was prepared at an optical density of 3.0 at 515 nm using a UV-vis spectrometer. The obtained CsPbBr3 solution and 5 wt% silazane monomer/Xylene solution were mixed at a corresponding 2:3 volume ratio under stirring. In addition, some distilled water of a corresponding 0.1 volume ratio was added to the reaction mixture at temperatures (20 – 140 °C). In this study, the amounts of water and the reaction temperatures were properly optimized for a high quantum yield and good stability. The mixture gradually became a gel-shaped formation with the gelation process of the SiNx/SiOy-based polysilazane. The CsPbBr3 embedded gel-shape PSZ matrix was centrifuged at 12000 rpm for 5 min, after which the supernatant (hexane and xylene) was discarded and the sunken green gel was under vacuumed for two hours. Subsequently, the hard CsPbBr3/PSZ composite powder was ground using an agate mortar. A schematic diagram of the fabrication process of the CsPbBr3/PSZ composite is displayed in Figure S1.



Synthesis of the red emissive K2SiF6:Mn4+ phosphor The KSF phosphors were synthesized efficiently by our previously reported experimental method.24 First, to obtain a clear H2SiF6 solution, SiO2 powders were dissolved in a HF solution for 2 hours at room temperature. When the reaction was finished, the KMnO4 powders were put into a reaction pot at a stoichiometry ratio. Through this reaction, the color of the reaction solution shifted from colorless to a purple hue. Then, a H2O2 solution was added dropwise before the yellow precipitation of the synthesized K2SiF6:Mn4+ was formed. After the reaction was complete, the reaction pot was left for 3 hours so that the K2SiF6:Mn4+ powder would sink naturally. The reaction solvent was discarded, and the K2SiF6:Mn4+ powders were filtered and sequentially washed sufficiently to eliminate any toxic residue. Finally, the wet K2SiF6:Mn4+ powder phosphors were dried in an 80 °C oven.

Characterization The photoluminescence (PL) and the PL excitation (PLE) spectra of the colloidal CsPbBr3 QDs, the CsPbBr3/PSZ powder, and the KSF phosphors were measured using a Xe lamp and a spectrophotometer (Darsa, PSI Trading Co., Ltd.). X-ray diffraction (XRD) patterns were analyzed with Cu Kα radiation (D-max 2500, Rigaku). The morphological images were recorded through a scanning electron microscope (SEM, JSM-7401F, JEOL Ltd.), and a transmission electron microscope (TEM, Tecnai G2 F30ST), which were operated with an energy dispersive spectrometer (EDS). The Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) spectra of the as-obtained samples were prepared through an IR spectrophotometer (Nicolet iS50, Thermo Fisher Scientific Co. Ltd.) using an attenuated total reflection (ATR) module and a monochromatic Al Kα (1486.6 eV) source under operation at 12 kV and 3 mA (K-alpha, Thermo VG), respectively. The electroluminescence spectra, CCT, and luminous flux of the CsPbBr3/PSZ and KSF mixed WLEDs were measured in an integrated sphere using a spectrophotometer (Darsapro-5000, PSI Co., Ltd.)

Results and discussion G-emitting CsPbBr3 were synthesized by a two-step heating process and a sequential quenching process, as previously reported in many publications, including ours.3,7-9 The


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detailed synthesis and purification processes and characterization of the G-emitting PeQDs are described in the Experimental section. The PeQDs/PSZ nanocomposites were simply synthesized by a hydrolysis and condensation reaction of liquid PSZ precursors and a small amount of water, as shown in Figure 1. The degree of the hydrolysis reaction depends on the amount of water, the heating temperature and the time. The liquid PSZ precursor solution and small amount of water were injected into the PeQDs hexane solution and stirred for 25 min at various temperatures for further hydrolysis. The centrifuged PeQDs/PSZ particles were dried under a vacuum to obtain PeQDs/PSZ nanocomposite powders. For the hydrolysis and condensation reaction and drying process under a vacuum, the mixed phase of SiNx, SiNxOy and SiOy surrounding the PeQDs was formed by the incomplete cross-linking reactions between the -H2Si-N-H- groups and H2O for full conversion to -O-Si-O- on the surfaces of the PeQDs. The process referred to above involving the small amount of water used to induce the crosslinking reaction with the PSZ precursors is termed a moisture-crosslinking process in the following scheme (moisture-induced hydrolysis, Figure 1). Our simple test demonstrated that it is difficult to polymerize silazane precursors without water. In other words, when silazane precursors are interlinked for polymerization, a moisture-induced hydrolysis process is crucial. Figure 2 shows the FT-IR peaks of the PeQD powders and a series of PeQD/PSZ powders at various hydrolysis temperatures (20 to 140 °C) with a small amount of moisture. As previously reported, the PSZ inorganic polymer consists of different compositional ratios of SiNx, SiNxOy and SiOy depending on the hydrolysis temperature.25,26 In the IR peaks of the PeQD/PSZ powder synthesized at 20 to 140 °C, intense absorption peaks are mainly found at 826 cm-1 for Si-N-Si, 3370 cm-1 for N-H, and 2146 cm-1 for Si-H. Moreover, the increased IR peaks of SiOy, in this case from Si-O-Si bonding, are mainly observed at 1060 cm-1 for the SiOy-based PSZ coatings. This Si-O-Si IR peak is prominently augmented with an increase in the hydrolysis temperature. The apparent absorption peaks of Si-H, N-H, Si-N-Si, and Si-OSi of PeQD/PSZ indicate that the partial hydrolysis of the PSZ precursor can produce a mixed inorganic PSZ polymer with different ratios of SiNx and SiOy at hydrolysis temperatures ranging from 20 to 140 °C. Based on the IR analysis outcomes of PSZ depending on the hydrolysis temperature, it can be considered that the composition of the PSZ powders changed from SiNx, to SiOy with an increase in the hydrolysis temperature. As shown in Figure 2a, IR measurements of the PeQD/PSZ nanocomposites show a tendency similar to the temperature trend with regard to the formation of PSZ inorganic polymer reported in ACS Paragon Plus Environment


previous papers.26 These IR figures indicate that the decreased compositional ratio of SiNx to SiOy can reduce the density of N-H ligands for passivation on the surfaces of the PeQDs with the increase of the hydrolysis temperature to form SiOy-based PeQD/PSZ nanocomposite. These figures also indicate that the IR peaks of C-H and COO- group are negligible at ~2900 cm-1 and ~1448 cm-1,27 respectively, owing to the screening effect of the thick PSZ coatings or the detachment of the OLA and OA ligands from the PeQD surfaces during the moistureinduced hydrolysis reaction (detailed in Figure 2b). Furthermore, the XPS measurements were conducted to analyze the binding status of the PeQD/PSZ components as shown in Figure 3. The full XPS spectrums of the PeQD and PeQD/PSZ powder are also summarized in Figure S2. The XPS spectrum of PeQD/PSZ shows that the most of the strong XPS peaks from the PeQD/PSZ composite powder coincide with the mixed peaks of the PSZ powder and PeQD powder.28-30 In Figures 3a and b, Si and N binding peaks of PeQD/PSZ powders are shown to be indexed at peak positions corresponding to each composition reported in the database. To detail the binding characteristics of PeQD/PSZ depending on the hydrolysis temperature (20 ~ 140 °C), peak deconvolution was performed mainly on the Si and N binding peak. As shown in Figure 3a, the XPS peak of Si 2p is confirmed to be located around 103 eV. When this Si 2p peak is deconvoluted at 101.7 eV for the Si3N4 peak and 103.5 eV for the SiO2 peak, the relative intensity of the Si3N4 peak decreases with an increase in the hydrolysis temperature. As shown in the ratio graph in Figure 3c, the spectral sum ratio of the Si3N4-to-SiO2 peaks decrease as the hydrolysis temperature increases. These figures also show that the Si3N4 peak decreases dramatically at 60°C and that only the binding peak of SiO2 appears in the PeQD/PSZ powder obtained above 120°C. A dramatical change of the compositional phase of the PSZ coated onto the PeQDs appears from the Si3N4 to the SiO2 phase above 120 °C. However, the temperature dependence of the N 1s peak at around 398 eV, which corresponds to O-Si-N bonding, indicates that the compositional phase of the PSZ coated onto the PeQDs gradually changes from silicon nitride to silicon oxide through the formation of silicon oxynitride. As shown in Figure 3b, the N 1s single peak consisted of both NSi3 and NSi2O peaks, as confirmed by the spectrum deconvolution results. Both NSi3 and NSi2O peaks are observed at 398.0 eV and 399.9 eV, respectively. Hence, the single N 1s peak can be convoluted from both 1N 1s peaks of the Si3N4 and SiNxOy phases. According to the ratio graph in Figure 3d, the spectral sum ratio of the NSi3/NSi2O peaks decreases gradually as the hydrolysis temperature increases, even if the hydrolysis temperature exceeds 60°C. As a ACS Paragon Plus Environment

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result of analyzing the Si and N XPS signals, we confirmed that the SiNx phase is converted to the SiOy phase through the formation of the SiNxOy phase in the PeQD/PSZ nanocomposite as the hydrolysis temperature increases. CsPbBr3 PeQDs were also analyzed to identify the binding energy of the PeQD components (See Figure S2). Most of the Cs 3d, Pb 4f, and Br 3d peaks of the CsPbBr3 PeQDs contribute negligibly to the XPS peaks of PeQD/PSZ owing to the screening effect of the thick PSZ passivated and encapsulated shell layer. Based on the overall IR and XPS results, the PeQD surface can be passivated and encapsulated with different ratios of SiNx, SiNxOy, and SiOy depending on the hydrolysis temperature. The crystal phase and powder morphology of the CsPbBr3 PeQD/PSZ core/shell composites were clearly analyzed by X-ray powder diffraction (XRD), FE-SEM and FETEM measurements. The XRD patterns of the G PeQDs indicate that the CsPbBr3 PeQDs powder is indexed as the cubic phase, even in the PeQDs/PSZ nanocomposites overall (See Figure 4). They also indicate that the broad bands ranging from ~15 to 35° originated from the mixed amorphous phases of SiNx, SiNxOy and SiOy. The EDX data summarized in Figure S3 and Table S1 indicate that the compositional elements of PSZ in the PeQD/PSZ core/shell nanocomposites changed from nitrogen to oxygen with an increase in the hydrolysis temperature. That is, the composition of SiOy in CsPbBr3 increased significantly, whereas the composition of SiNx decreased considerably while that of SiNxOy decreased more slowly as the temperature increased. This trend was found to be identical to the above-mentioned FT-IR and XPS measurements. As shown in Figure 5, the FE-TEM images of the PeQD/PSZ core/shell composites synthesized at 20 and 140 °C (40 °C interval) indicate that round CsPbBr3 PeQDs are obtained in the PSZ inorganic polymer instead of the original cubic-shaped CsPbBr3 PeQDs. These images also indicate that the PeQDs were well dispersed in the PSZ inorganic polymer matrix. The typical sizes of the CsPbBr3 QDs in the PSZ inorganic matrix are approximately 8 – 9 nm when prepared at 20 to 140°C, slightly smaller than those of the as-prepared PeQDs prepared in the solution before the PSZ treatment. They also indicate that the inorganic polymer wrapped the PeQDs effectively. The slight change in the shape and decrease in the size suggest that the surface elements of PeQDs can diffuse into the matrix and that the -NHfunctional group can be liganded to Pb sites after the detachment of the OLA from the PeQD surfaces during the hydrolysis reaction to form the PSZ coating on the PeQDs. In addition,



the PeQD and PSZ mixture solution became more opaque due to the formation of the PeQD/PSZ composite with an increase in the hydrolysis time from 0 to 25 min, as shown in Figure S4. After 25 min in the reaction solution, the degradation of the PeQDs proceeds with the vanishing of the green color due to the rapid release of N2 and NH3 gas. Although the shape and size of the PeQDs change slightly, the unchanged peak position and shape of the XRD patterns of the PeQDs and the broad amorphous peaks of PSZ indirectly indicate that that the hydrolysis reaction which formed the PSZ inorganic polymer matrix maintained the crystal quality of the PeQDs before and after the PSZ encapsulation process.7,31 The FE-SEM image and EDX mapping results are shown in Figure S5a-d. The FE-SEM images indicate that the PeQD/PSZ powders, obtained at several hydrolysis temperatures, are non-uniform in terms of their size and shape owing to the moisture cross-linking process of PSZ hydrolysis and the mechanical grinding process. An EDX mapping analysis was also conducted to confirm the uniform distribution of the PeQDs in the PSZ matrix. As shown in the elemental mapping images of CsPbBr3/PSZ, Br was found to be distributed similarly to particles throughout the crystals. Otherwise, spots of the Si, N, and O elements were shown to be uniformly distributed throughout the composite crystals. Therefore, all XRD, FE-SEM, FETEM and element composition and mapping analysis results here confirm that the PeQDs/PSZ composite powders were successfully obtained in powder form, in which the PeQDs were uniformly passivated and encapsulated by the PSZ inorganic polymer matrix. Here, the optimum hydrolysis conditions were determined by analyzing the relative PL intensity and stability levels of the PeQDs/PSZ core/shell nanocomposite powders. Figure S6 shows a series of the PL and PLE spectra of the PeQD/PSZ nanocomposites with an increase in the hydrolysis temperature. The ignorable-changed peak positions and spectral shapes of the PL and PLE indicate that the origin of the PL and PLE does not change after the moistureinduced hydrolysis reaction in the temperature range of 20 to 140°C. As demonstrated by the variation of the relative PL intensity depending on the hydrolysis temperature in Figure 6a, the relative PL intensity reaches the maximum value when the PeQD/PSZ powder is obtained at a hydrolysis temperature of 60 °C. PL lifetime measurements indicate that the lifetime trend of the PeQD/PSZ nanocomposites is well correlated with the PL intensity trend (see Figure 6b). The longest lifetime is that of the PeQDs of the PeQD/PSZ composite obtained at 60 °C with the highest intensity level. The only exception is from the PeQDs in the PeQD/PSZ composite obtained at 140 °C. As is well known, the PL intensity is not dependent only on the PL lifetime. Possibly, some other factors contribute to the PL intensity ACS Paragon Plus Environment

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of the PeQDs in the PeQD/PSZ nanocomposite. However, these PL lifetime measurements indirectly confirmed that the maximum PL intensity is obtained from the PeQD/PSZ composite at a moisture-induced hydrolysis temperature of 60 °C. Although the PL intensity trend is affected by complex factors, such as the degree of ligand detachment, the N/O ratio of passivation ligands, thermal decomposition, and thermal quenching, for instance, the nitrogen-rich phase can passivate the metal ions of the PeQDs while maintaining high PL properties. Nonetheless, similar to the moisture reactivity capability of N-based ligand elements reported in a previous publication,32,33 the hydrolysis potential of the remaining Nbased functional groups in the SiNx and SiNxOy phases can prevent the moisture and polar molecules from destroying the PeQDs, further enhancing the moisture stability of the PeQDs/PSZ nanocomposites. Therefore, the remaining N-based functional group of PeQDs/PSZ nanocomposites obtained at a hydrolysis temperature of 60 °C can passivate the metal ions of the PeQDs, maintaining the high PL properties and enhancing the stability against environmental attackers. Figure 7 shows and Table S2 summarizes the optical properties of the as-prepared G CsPbBr3 and their PeQD/PSZ nanocomposite powders. As shown in Figure 7, the absorption spectra, PL emission spectra and CIE color coordinates of pristine PeQDs and the optimum PeQD/PSZ indicate that the peak wavelength and CIE color coordinates are slightly shifted to the red wavelength and that the FWHMs are slightly widened due to the agglomeration of PeQDs during the wrapping of the PeQDs by the PSZ inorganic matrix. Nonetheless, the optimum PeQD/PSZ nanocomposite powders hydrolyzed at 60 oC have a narrow green emission peak at 522 nm with a FWHM of 23 and a PLQY of 81.7%. These PLQYs of the G PeQD/PSZ nanocomposite powders are superior or comparable to those of most PeQD powder samples in most previous reports (see Table 1). Instability against thermal, water (or polar solvent), oxygen and UV irradiation is the most critical issue associated with PeQD/PSZ nanocomposite powders intended for use in practical devices around the world. As summarized in Table 1, many researchers are currently developing various coating methods to improve the stability of PeQDs against harmful elements in the environment by means of passivating and/or encapsulating treatments, such as those which rely on organic ligands, inorganic ceramics and polymer coatings. Here, the stability of the optimum PeQD/PSZ nanocomposite powder prepared in this study was compared to that of dried pristine CsPbBr3 powders as a reference sample. The colloidal CsPbBr3 QDs were spin-coated onto glass and dried under a vacuum without any further protection. The PeQD powders and the PeQD/PSZ nanocomposite powders were illuminated ACS Paragon Plus Environment


by a UV lamp under ambient conditions at 20 °C and at an average RH of 60%. Figure 8a shows the change in the relative PL intensity with the change in the irradiation time up to 70 min, used as a measure of their stability against light and moisture. For dried G PeQD powders synthesized by a hot-injection method, the PL intensity drops rapidly and dramatically within 10 hrs and the luminescence decreases to 20% after 20 hrs compared to the initial dried PeQD powder. This weak stability of the pristine PeQD samples is due to the chemical nature of the inorganic CsPbBr3 PeQDs, which are sensitive to humidity and oxygen. The instability of inorganic PeQDs has been well reported in numerous stability studies performed on thin films, powders and solutions. Surprisingly, the CsPbBr3/PSZ nanocomposite powders prepared by the moisture-induced hydrolysis/crosslinking method used to prepare the PSZ inorganic polymer showed superior UV stability compared to that of uncoated PeQDs. After 70 hrs of UV irradiation, they retained more than 60% of the initial PL intensity levels of the G PeQD/PSZ composites (Figure 8a). To meet the additional requirements of actual devices applied in LED backlighting systems, the thermal and storage stability levels of the G CsPbBr3/PSZ G nanocomposite powders were also tested by measuring the attenuation of the PL intensity, as shown in Figures 8b-d. The storage and thermal stability capabilities of the G PeQD/PSZ composite powders were significantly enhanced as compared to those of uncoated pure PeQDs. We carried out storage-stability tests at room temperature and RH 60% for a comparison of the storage limits of the PeQDs. These results demonstrated that the PL intensity levels of the G PeQD/PSZ composite powders remained unchanged for the period of a month. Otherwise, the pristine CsPbBr3 PeQDs decreased sharply to no emission within 10 days. After appropriate passivation and encapsulation steps, the storage-stability capabilities of the CsPbBr3 in a cabinet-storage environment were shown to be improved greatly, indicating an enhancement of their suitability for real-world applications. With regard to the temperature-stability tests, the PL intensity was measured at 60 °C, 100 °C, and 140 °C for 300 min under RH 60% ambient conditions. As previously reported in many publications, this outcome confirmed that the PL intensity of pure PeQDs decreased considerably with an increase in the temperature. As shown in Figure 8c, the intensities of the pure PeQDs decreased to less than 5% after a heat treatment lasting 300 min compared to the initial state at a temperature exceeding 60 °C. Fortunately, the PL intensity of the PeQD/PSZ composite powders did not show as steep a slump relative to that of the pure PeQDs. After a heat treatment lasting 300 min, the intensity levels of the PeQD/PSZ samples were reduced by approximately 9.8%, ACS Paragon Plus Environment

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47.9%, and 66.3% when the PeQD/PSZs were heated to 60 °C, 100 °C, and 140 °C, respectively, compared to the intensity of the initial PeQD/PSZ. In addition, to investigate the solvent-stability capabilities of our PeQD/PSZ composite powders, we compared the solvent-storage stability in six common solvents with an increase in the polarity, i.e., hexane, toluene, ethanol, IPA, methanol, and water. Figure S7 shows luminescent images of both pure and encapsulated PeQD samples taken immediately and a few hours after mixing with the solvents. The solvent-treated G PeQD/PSZ powders in the non-polar solvents (hexane and toluene) showed no changes in the luminescence for 70 hrs. Similarly, the luminescent emissions of the PeQD/PSZ with polar solvents (ethanol, IPA, acetone) scarcely changed. For the water-storage G PeQD/PSZ composite, it was confirmed that the luminescence was maintained well for 10 hrs, although it was lower than those of other solvents. As a result, our PeQD/PSZ composite powders showed much better resistance toward all solvents as compared to the pure PeQDs, as they retained strong green luminescent emission. In fact, among the six solvents, we analyzed the PL intensity levels of encapsulated PeQD/PSZs after mixing with the aforementioned six polar and non-polar solvents. Figure S7 shows the variations of the PL solvent-stability of the pure PeQDs and the encapsulated PeQD/PSZs with the storage time. The PL intensity of the pure CsPbBr3 PeQDs quenched immediately after mixing PeQD nanocrystals with three solvents and their emission outcomes were wholly degraded immediately when exposed to polar solvents including a small amount of water. This rapid degradation of the PL intensity in a polar solvent is in good agreement with the rapid quenching of the PL intensity of as-prepared PeQDs in previous reports.34 Otherwise, our PeQD/PSZ nanocomposite powders exhibited stable PL intensity after more than 70 hrs in polar solvents, also indicating that the PL can endure even afterward. Moreover, the G PeQD/PSZ nanocomposite powders showed improved stability toward water compared to the non-encapsulated green PeQDs. These solvent-stability results also indicate that PeQDs encapsulated by the PSZ inorganic polymer matrix have a much slower structural relaxation time of the perovskite and provide a barrier layer to slow down the diffusion of the polar solvents into the core compared to the small-molecule surfactant-coated PeQDs of pure PeQDs. According to the results of the various stability results presented above, encapsulation of G CsPbBr3 QDs with the PSZ inorganic polymer matrix is an effective approach to enhance the photo, thermal, moisture, storage and solvent stability levels. It can be speculated



that the improved performance capabilities have two causes: their efficient passivation by unreacted PSZ moieties in the hydrolyzed PSZ coating against humidity and oxygen, and their hermetic protection by the barrier of the PSZ inorganic ceramic layer against moisture and polar solvent attacks. Therefore, the PeQD/PSZ nanocomposite powders offer favorable protection against thermal-induced damage, and storage-induced damage, and exposure to polar solvents, while also offering good photostability, making them suitable for real-world applications of down-converted white-light-emitting diodes (DC-WLEDs). To fabricate DC-WLED backlighting applications using G PeQD/PSZ powder, it is essentially to synthesize and characterize R-emitting KSF phosphor powder with a very narrow FWHM of 9 nm and an efficient QY of 58%. In Figure S7, the PL and PLE spectra of KSF show that the R-emitting KSF is not excited by light in the green ~ yellow wavelength range (510 ~ 580), demonstrating its suitability for RGB tri-color LED package applications. Based on the XRD and SEM-EDS results, it is clear that R-emitting KSF powders were successfully synthesized for further study (Figure S8). The color gamut of a selected triangle of a B LED, the G PeQD/PSZ solution, and the R KSF phosphor encompasses 129% of the NTSC standard. Otherwise, the figure indicates that a selected triangle of a B LED and PeQD/PSZ nanocomposite powders can reproduce ~128% of the NTSC RGB color gamut due to the slightly enlarged PL FWHM given the enhanced polydispersity the PeQDs by the agglomerated PeQDs during the PSZ hydrolysis step. These minor red-shifts of both GR colors from powder samples indicate that the PeQD agglomeration and energy exchange between neighboring PeQDs are not as considerable in the PeQD/PSZ binder package of tricolor white LEDs. Of course, the color gamut defined by the B LED, the G PeQD/PSZ nanocomposite powder and the R KSF phosphor powder is large enough to reproduce any colors on TV as defined by the NTSC regulation system. To verify the suitability of the G PeQD/PSZ nanocomposite powders in actual DC-WLED applications, white LEDs are fabricated by dispensing the G PeQD/PSZ composite and R KSF phosphor mixed paste with photocured polymethylmethacrylate (PMMA) monomer onto 455 nm blue LED chips. Figure 9a shows the relative electroluminescence (EL) spectra of the G CsPbBr3/PSZ and an R KSF phosphor powder paste coated onto a 455 nm blue LED as a function of the ratio of the green and red nanocomposite in the PMMA binder under a driving current of 60 mA. The transmittance spectra of the commercial R, G, and B color filters of a LCD display are shown in Figure 9b. Any colors of a full-color LCD display can


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be obtained within the color gamut area by varying the ratio of each transmitted RGB light from DC-WLED backlights passing through RGB color filters. The RGB spectrum and color gamut area are calculated with white light from color-filtered, tri-color, single-package WLEDs. Figures 9c and d display the filtered RGB spectra from the PeQD-based DCWLEDs and the calculated CIE color coordinates from the tri-color DC-WLEDs with a G CsPbBr3/PSZ composite, the R KSF phosphor powder, and a blue LED. The white and RGB color coordinates of tri-color, single-package DC-WLEDs correspond to (0.308, 0.328), (0.685, 0.303), (0.137, 0.718), and (0.144, 0.072) after filtration of tri-color single package WLEDs with color filters. The color gamut areas of the unfiltered and filtered RGB triangles of the tri-color DC-WLEDs covered 129% and 111%, respectively, of the area defined by the National Television System Committee (NTSC) in the CIE 1931 color space. As shown in Table 2, the luminous efficacy values (LE) of W and RGB-filtered LEDs were calculated to be ~138.6 and 30.3, 68.7, and 11.5 lm/W, respectively, at 60 mA. The calculated R:G:B ratio of the LE of the filtered RGB LEDs (2.97:7.23:1.00) in this study can be considered to be appropriate for use in a backlighting system for a full-color TFT-LCD. This color gamut of the R KSF phosphor powder and the G PeQD/PSZ nanocomposite powder mixed paste coated onto a blue LED is wider than that (~100) of commercialized InP-based green and red QD-based tri-color white LEDs. Moreover, this color gamut area very nearly meets the area requirement defined by Rec. 2020, which will be a future standard color gamut system of colors in TV displays. Furthermore, the EL performance of the 6500 K single-package DCWLED was determined by current dependence measurement with an applied current from 20 mA (2.56 V) to 120 mA (2.75 V). The changes in the EL spectra and CIE color coordinate are displayed as a function of the applied current. In Figure 10, the intensities of the spectra increase with an increase of applied current. The values of LE and CCT at 120 mA are 113.6 lm/W and 6140 K, respectively, whereas those at 20 mA are 153.9 lm/W and 6667 K. These results are acceptable for the highly stable green CsPbBr3 PeQD/PSZ composite to realize high-performance lighting devices as a color converter.

Conclusion In this study, we successfully synthesized G-emissive CsPbBr3 PeQDs through a colloidal hot-injection method and studied the dual passivation and encapsulation of PeQDs using a PSZ inorganic polymer matrix with SiNx, SiNxOy and SiOy. Using a moisture crosslinking



method, the G CsPbBr3/PSZ composite could be obtained by adding a silazane precursor and a small amount of water into the CsPbBr3 PeQD solution. In addition, the SiNx, SiNxOy and SiOy surface protection on PeQDs can be selectively applied by manipulating the moistureinduced hydrolysis temperature. With XPS, FT-IR, XRD and TEM analyses, we confirmed that the PeQD surface was effectively passivated and encapsulated by the mixed inorganic matrix of the SiNx, SiNxOy and SiOy. Therefore, the obtained G PeQD/PSZ powder showed a high efficiency rate of 81.7% without severe degradation of the PeQDs in a room-temperature and air atmosphere. In order to confirm the stability of the obtained PeQD/PSZ nanocomposite powder, a high-temperature stability test, a UV exposure test, a roomtemperature storage test and various solvent tests were carried out. In the high-temperature stability test, it was confirmed that the efficiency levels of the PeQD/PSZ were maintained at approximately 90.2%, 52.1%, and 33.7% at 60 oC, 100 oC, and 140 oC, respectively, compared to initial efficiency of the PeQD/PSZ composite. Moreover, compared to pristine PeQD powder not protected by PSZ, it was found that the thermal stability of the PeQD/PSZ composite is significantly improved in various thermal atmospheres. In addition, the UV exposure test confirmed that the PL efficiency was maintained at 60% after nearly 100 hours of exposure compared to the initial efficiency rate. Moreover, the efficiency did not change substantially at room temperature for 60 days. These improved stability levels indicate that the silazane-derived SiNx/SiNxOy/SiOy passivation and encapsulation method can effectively maintain the PLQY of PeQD/PSZ powder samples and protect the surfaces of PeQDs against UV, high temperatures, moisture, and exposure to a polar solvent. Owing to the improved optical stability of the PeQD/PSZ powder through the PSZ passivation and encapsulation process, the obtained optical properties of the G PeQD/PSZ with a high PLQY, a narrow FWHM and high-color purity are considered to be suitable for application to the backlighting of LCD display devices. Hence, we realized on-chip single-package DC-WLEDs using an optimized G PeQD/PSZ composite and R KSF phosphor powder. This RGB tri-color DCWLED showed high luminous efficacy of 138.6 lm/W, an EQE rate of 51.4%, and a high color gamut of correspondingly 128 and 111% without and with RGB color filters compared to the NTSC standard at 6500 K.


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ASSOCIATED CONTENT Supporting Information The schematic diagram of fabrication, XPS, EDX, PL/PLE, and the solvent stability test of the PeQD/PSZ composite, actual images of the PeQD/PSZ mixture solution depending on reaction times, and optical properties of red-emissive KSF phosphors are available via the ACS Publications website at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

Phone: +82-2-910-4893.

Fax: +82-2-910-4415.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (Ministry of Science and ICT) (No. 2015M3D1A1069709, No. 2016R1A5A1012966, and No.2017R1A2B2007575). This work was partially supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No. 2015R1A6A1A03031833).



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Figure/Table captions

Figure 1. Schematic illustrations of the sol-gel process (moisture-induced hydrolysis) used to obtain the PeQD/PSZ composites.

Figure 2. (a) FT-IR spectra of a CsPbBr3 green PeQD and PeQD/PSZ composite prepared at reaction temperatures ranging from 20 °C to 140 °C. (b) Highly magnified FT-IR data of PeQD/PSZ samples in a wavenumber range of 4000 cm-1 to 1250 cm-1.


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Figure 3. Partial XPS survey results of (a) Si and (b) N component peaks depending on the reaction temperature (20 °C to 140 °C). In the Si XPS spectra (a), Si3N4 (blue) and SiO2 (red) peaks are deconvoluted from the original Si spectrum. In the N XPS spectra (b), NSi3 (blue) and NSi2O (red) peaks are deconvoluted from the original N spectrum. (c) Compositional ratio from the deconvoluted Si3N4 (blue) and SiO2 (red) spectra and Si3N4/SiO2 ratio (black) in the partial Si XPS data (a). In the same manner, (d) the compositional ratio of the deconvoluted NSi3 (blue) and NSi2O (red) spectra and NSi3/NSi2O ratio (black) in the partial N XPS data (b) at all reaction temperatures (20 °C to 140 °C) are shown.



Figure 4. XRD data of CsPbBr3 PeQD and PeQD/PSZ composites obtained at reaction temperatures ranging from 20 °C to 140 °C.

Figure 5. (a) Low-magnification and (b) High-magnification HR-TEM images of CsPbBr3 PeQD and PeQD/PSZ composites obtained at reaction temperatures of 20 °C, 60 °C, 100 °C, and 140 °C. White bars in (a) and (b) indicate 20 nm and 5 nm size, respectively. In highmagnification images, the (002) lattice fringes of CsPbBr3 PeQD are shown with 0.29 nm interplanar spacing.


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Figure 6. (a) Relative PL intensity levels of PeQD/PSZ samples at reaction temperatures ranging from 20 °C to 140 °C, and (b) the PL decay profiles of PeQD/PSZ samples at reaction temperatures of 40 °C to 120 °C. Inset shows the PL lifetimes as a function of reaction temperature.

Figure 7. (a) Normalized PL (solid-line) and absorbance (dash-line) spectra of colloidal CsPbBr3 PeQDs. (b) Normalized PL intensity of an optimum green PeQD/PSZ composite obtained at 60 °C. Insets show actual images of the PeQD/PSZ powder with/without UV light. (c) CIE color coordinates of the colloidal CsPbBr3 PeQD and PeQD/PSZ composite.



Figure 8. The stability test outcomes of a dried PeQD and PeQD/PSZ composite under (a) UV light for more than 100 hours and (b) after 60 days of storage in ambient conditions. The thermal stability after operating the (c) dried PeQD and (d) PeQD/PSZ composite at applied heating temperatures of 60 °C, 100 °C, and 140 °C for more than 300 minutes.


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Figure 9. (a) The white DC-LED spectrum of a green PeQD/PSZ composite and the red KSF phosphor mixed LED package at an applied current of 60 mA. The inset shows an actual image of the white DC-LED package and the emission from an integrated sphere. (b) The transmittances of the red, green, and blue color filters. (c) Normalized EL spectra, and (d) CIE coordinates of red, green, and blue emission filtered through a RGB color filter, from the prepared white single-package LED. The insets provide schematic illustrations of white and colored LEDs corresponding to each color coordinate.



Figure 10. (a) The spectra, (b) CIE color coordinates, and (c) LE (lm/W) and CCT (K) of green PeQD/PSZ and red KSF based single-package DC-WLED as a function of the applied current (20 mA to 120 mA). The blue arrow in (a) indicates the increase of spectra intensities with the increase of applied current. The blue arrow in (b) indicates the change of color coordinates with the increase of applied current.


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Table 1. Summary of recent passivation studies of CsPbX3 peQDs for the enhancement of the stability.



Table 2. The optical properties of a green RGB color-filtered single-package white DC-LED and the corresponding R KSF phosphor and G CsPbX3/PSZ QD powder mixed and coated single-package white DC-LED.


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TOC


Efficient and Stable CsPbBr3 Quantum-Dot Powders Passivated and

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