Stable and Efficient Green Perovskite NanocrystalâPolysilazane Films

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Functional Inorganic Materials and Devices

Stable and Efficient Green Perovskite Nanocrystal-Polysilazane Films for White LEDs Using an Electrospray Deposition Process Hee Chang Yoon, and Young Rag Do ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b04164 • Publication Date (Web): 30 May 2019 Downloaded from http://pubs.acs.org on May 30, 2019

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ACS Applied Materials & Interfaces

Stable and Efficient Green Perovskite Nanocrystal-Polysilazane Films for White LEDs Using an Electrospray Deposition Process Hee Chang Yoon, and Young Rag Do* Department of Chemistry, Kookmin University, Seoul 136-702, Republic of Korea *

E-mail address: [email protected]

Keywords: CsPbBr3, perovskite nanocrystal, perovskite film, electrospray deposition, white LED

Abstract We successfully fabricated stable, efficient, and easy-to-use CsPbBr3 perovskite nanocrystal (PeNC)-embedded inorganic polymer film through an encapsulation step with a Si-N/Si-O-based polysilazane (PSZ) matrix via the electrospray (e-spray) deposition of a silazane (SZ) oligomer-decorated PeNC solution. To eliminate Pb2+ defect sites which are generated when the ligands are peeled from the PeNC surface, surface passivation of the Lewis acid/base adduct is possible by coupling the SZ oligomer (the donor of lone pairs) with Pb2+ sites (the acceptor of lone pairs). With the addition of the SZ oligomer, the PLQY of photodegraded CsPbBr3 PeNC was recovered and increased by 2.35-fold while the stability was improved significantly from an untreated CsPbBr3 PeNC solution. During the e-spray deposition process, SZ-treated CsPbBr3 PeNC solution droplets can react with atmospheric moisture to polymerize and form a Si-N/Si-O network encapsulant via a sol-gel reaction. The resultant CsPbBr3-PSZ films showed improved stability levels under most environmental conditions, including air storage, blue light exposure, UV exposure, thermal exposure, and

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water immersion. The optimum CsPbBr3-PSZ film-covered blue LED showed good performance capabilities, with a luminous efficacy (LE) of 85.9 lm/W and color-by-blue conversion efficiency (CE) of 60.1%. Furthermore, this easy-to-use CsPbBr3-PSZ film can be employed to realize a remote-type white-by-blue LED by combining it with red emissive K2SiF6: Mn4+ (KSF)/silicone film. The LE and CE rates of the white LED were 71.0 lm/W and 50.8%, respectively, at a correlated color temperature of 9334 K, with only an 8% drop in the LE for long-term operation of 100 hours. This result indicates that e-spray deposition is a simple fabrication process by which to create stable and efficient PeNC films from an unstable PeNC solution using a rapid sol-gel reaction between droplets and moisture from the air.

Introduction Recently, colloidal fully inorganic cesium lead halide (CsPbX3, X = Cl, Br, I, or a mixture of these) perovskite nanocrystals (PeNCs) have emerged and attracted the attention of scientists owing to their outstanding optoelectronic properties, such as their high color purity levels with narrow full-width at half-maximum (FWHM) values, high photoluminescence quantum yields (PLQYs), strong light absorption, low non-radiative recombination rates, and compositional direct-bandgap color tunability.1-3 These fascinating properties have allowed PeNCs to become a highly efficient luminophore for the use in lighting, display, and optoelectronic applications, such as light-emitting diodes (LEDs)4-6, color-by-blue photoluminescence devices6-11, and lasers12,13. Despite their excellent optical performance and applicability to practical devices, CsPbX3 PeNCs are vulnerable to normal air conditions, humidity, temperature, and even excitation light sources (UV, blue light) due to their structural defects, which originate from their low formation energy and ionic-dominant compounds.1 That is, their intrinsic instability can be an inevitable obstacle that prevents versatile PeNCs from being incorporated into optoelectronic applications to realize viable commercial-stage development. With regard to the surface chemistry, considerable advances have been made in PeNC protection strategies with the development of various methods aimed to improve the stability of PeNCs.14,15 Under air/light conditions, the primary aggravating factor related to the lead halide PeNC surface is the generation of multiple unwanted electronic carrier traps stemming from the detachment of ligands (oleic acid and oleylamine) and leading to the formation of undercoordinated Pb2+ vacancy-type defect sites. This is why Pb0 clusters flocculate on the surface


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of PeNCs and collapse their shapes.16,17 Moreover, this photochemical dissociation of the crystal facets can induce the reconstruction of other nearby PeNCs, causing aggregation (see Figure S1).18,19 Specifically, the photo-induced Pb2+-terminated surface acts as a Lewis acid.2022

To lower the non-radiative recombination centers, on the one hand, efficient surface

passivation techniques involving several chemical treatments are crucial, including (i) Lewis acid/base adducts upon the introduction of functional materials, with lone electron pairs (N-, O-, and S-) to bind with Pb2+ on the PeNC surfaces22-25; (ii) monovalent cation-assisted surface passivation (K+, Rb+, Ag+, and organic-inorganic hybridized cations) to backfill interstitial sites on PeNC surfaces, suppress the detachment of halide ions, and stabilize the lead halide PeNC surface26-29; and (iii) halide-rich surroundings to retain the self-passivation conditions against the peeling off of the ligand30,31. On the other hand, to develop a secondary protective barrier layer, an encapsulation process for PeNCs can effectively prevent exposure from external damaging factors such as oxidation, moisture, and heat via a sol-gel reaction18,22 or an embedding reaction into the functional structure32. Although synergetic protection effects through both surface passivation and encapsulation can enhance the stability of PeNCs while maintaining high PLQYs, the introduction of encapsulation techniques for use with PeNCs remains a challenging process. This is also true for surface passivation considering the proper selection of protectable raw materials against the high ionicity of PeNCs. Li et al. investigated a two-step protection process of CsPbBr3 PeNCs via the sequential steps of passivation using didodecyl dimethyl ammonium bromide (DDAB) and encapsulation of the SiO2/Al2O3 monolithic matrix with enhanced photostability.33 Pan et al. also devised a protection strategy which used O-based methacrylic acid passivation on CsPbX3 PeNC surfaces. With an acrylatebased UV-curable polyhedral oligomeric silsesquioxane (POSS) encapsulant, photo-, thermal-, and water-stability issues were ameliorated.18 Godin et al. employed a mesoporous Al2O3 matrix with an O-based passivation ligand by growing organic-inorganic PeNCs within the pores of alumina.32 As previously reported by the authors, Si-N-based silazane (SZ) oligomers effectively function as a passivation ligand for PeNC surfaces and as a raw encapsulation material which can cross-link as polysilazane (PSZ) via a sol-gel reaction with a small amount of water.34 Hence, many reports have demonstrated that these simultaneous protection pathways should be achieved by introducing organic- and/or inorganic-based functional ligands to form Lewis acid/base adducts and a cross-linking matrix. In addition to the development of simultaneously encapsulated and passivated materials/processes, stable, efficient, and easy-to-use morphologies of encapsulated PeNCs can



be realized for use in optoelectronic devices. Given the regularity of research on popular morphologies to overcome irregular powder forms, it is necessary to realize a transparent, ultrastable, and highly efficient CsPbBr3 PeNC-PSZ emissive thin film. In this study, we develop a stable SZ oligomer-treated CsPbBr3 PeNC ink to obtain a stable and efficient CsPbBr3 PeNC/matrix film converting a versatile SZ oligomer to crosslink the PSZ polymer matrix. We then develop an electrospray (e-spray) technique to fabricate a uniform thin film via the rapid hydrolysis between tremendously large numbers of small-sized PeNC-SZ droplets in mist form with air moisture. The bulk ink of the SZ-treated PeNC solution is somewhat stable, allowing the production of large amounts of small-sized mist particles, which are somewhat reactive with regard to the moisture in the air, forming high-quality PeNC-embedded films during the e-spray deposition process. As described in Figure 1, Si-N-based SZ oligomers can serve as both Lewis acid/base adducts and as a hydrogen bonding bridge capable of the self-assembly passivation and recovery of photo-degraded colloidal CsPbBr3 PeNCs. The SZ-treated CsPbBr3 PeNCs show brighter luminescence and more stable properties than non-treated CsPbBr3 PeNCs. Furthermore, self-assembled SZ oligomers on PeNC surfaces can act as a cross-linker to form an encapsulant as a Si-O matrix, providing a sturdy protection layer and high PLQYs against harsh conditions.34,35 Another key point is the unique e-spray coating process, which not only minimizes solution waste during the deposition process but typically can also control the film thickness with uniform coverage and smooth surfaces, even on rough and curved substrates, compared to the spin-coating process (See Figure S2). With simplicity and scalability, the conventional spray system has recently been utilized in studies of PeNCs, such as in the synthesis of colloidal PeNCs36,37, the fabrication of emissive PeNC-polymer microstructures38, and as a film for PeNC photodiodes39. However, there are few studies of the formation of easy-to-use PeNC thin-film forms for use as color-by-blue PeNC emitters using the e-spray method because it is difficult to choose appropriate protective materials and e-spray conditions that do not degrade the delicate PeNCs. In this study, we develop a three-step process to fabricate stable, efficient, and easy-to-use PeNC films while maintaining the PLQYs. First, the synthesis of CsPbBr3 PeNCs with the traditional hot-injection method is done. Second, we prepare homogeneous CsPbBr3 PeNC-SZ ink by mixing a purified CsPbBr3/hexane (or toluene) solution and a commercial SZ/xylene solution without adding water or a catalyst. With a SZ treatment of the CsPbBr3 PeNC solution, it is possible to protect the PeNC surfaces from atmospheric oxygen and moisture during the spraying process. Third, we utilize the e-spray fast-hydrolysis fabrication process to obtain CsPbBr3 PeNC-PSZ emission film. In order to


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realize highly luminescent and sturdy film samples, we undertook an optimization process by adjusting the volume ratio between the CsPbBr3 PeNCs and SZ oligomer solutions and the injection volume of the CsPbBr3 PeNC-SZ solution. The obtained CsPbBr3 PeNC-PSZ films could be demonstrated as a stable, highly luminescent, and easy-to-use color converter for use in conventional air/lamp-light storage and harsh thermal, UV, and water-immersion conditions. Moreover, we fabricated remote-type laminated films with a green (G) emissive PeNC-PSZ film and a red (R) K2SiF6:Mn4+ (KSF)/silicone film to confirm the fabrication of the highquality and easy-to-use PeNC-PSZ film for use in ultra-stable, high-efficient, and wide-colorgamut white (W)-LEDs.

Experimental section 1. Materials All chemicals for this research were used as received without purification. Cesium carbonate (Cs2CO3, 99.95%), lead bromide (PbBr2, 99.999%), lead iodide (PbI2, 99.999%), 1octadecene (ODE, 90%), oleylamine (OLA, >98%), oleic acid (OA, 90%), hexane (anhydrous, 95%), and toluene (anhydrous, 99.8%) were purchased from Sigma-Aldrich. As the Cs source for the CsPbX3 PeNCs, a Cs-oleate solution was synthesized by mixing 0.32 g of Cs2CO3, 12 mL of ODE, and 1 mL of OA in a glass vial while stirring during a 160 °C heat treatment. The SZ oligomers as a PSZ precursor (Coatalent® NC, UP Chemical Inc.) were obtained at a concentration of 5 wt% in xylene as a solvent. A cover glass (20 x 20 x 0.15 mm) was used as a substrate for the PeNC film. The InGaN blue (B) LEDs (λemi = 450 nm) used here were purchased from Dongbu LED, Ltd.

2. Preparation of the colloidal CsPbX3 PeNC The CsPbBr3 G emissive PeNC was synthesized by following a previously reported protocol.1 First, 0.138 g (0.376 mmol) of PbBr2, 10 mL of ODE, 1 mL of OA, and 1 mL of OLA were loaded into a three-necked flask and assembled to a Schlenk line. The reaction pot was heated to 120 °C under degassing and stirring. After one hour (hr) of reaction, the PbBr2 powder was completely dissolved in the reaction solvent (ODE) and the reaction pot was filled with Ar gas. The reaction temperature was then raised to 180 °C, and 0.8 mL of the as-prepared



Cs-oleate was swiftly injected into the reaction pot. The reaction pot was maintained for 5 s and cooled in an ice-water bath. The obtained crude CsPbBr3 solution was centrifuged at 12000 rpm for 5 min. The supernatant was discarded, and the sunken CsPbBr3 sediment was redissolved in hexane or toluene. To remove byproducts from the CsPbBr3 sediment, the redispersed CsPbBr3 solution was centrifuged once more in the aforementioned conditions (12000 rpm, 5 min). The R emissive CsPbBrI2 PeNC was synthesized using 0.046 g (0.125 mmol) of PbBr2 and 0.115 g (0.250 mmol) of PbI2. All other synthetic processes proceeded in the manner described above.

3. Fabrication of green CsPbBr3-PSZ film The purified CsPbBr3 PeNC solution was diluted to an optical density of 2.0 (at 510 nm) using UV-vis spectroscopy. The obtained G CsPbBr3 PeNC solutions are passivated by adding 5 wt% of a SZ oligomer after optimizing the ratio between the CsPbBr3 NCs and the SZ oligomer. The optimum CsPbBr3 PeNC-SZ solution was loaded into a 30 mL disposable syringe and mounted on a syringe pump. Likewise, the CsPbBr3 PeNC-SZ solution was utilized in the optimization experiments in a spraying solution in amounts of 4 ~ 32 mL. After setting 3 x 3 cm repetitive movements of the x- and y-axes and a stationary distance of 20 cm (z-axis) between the needle and glass substrate, the CsPbBr3 PeNC-SZ ink was sprayed onto the glass substrate at an injection rate of 0.01 mL/min under an electric field of 20.0 kV/cm. After the e-spraying process, the CsPbBr3-PSZ film obtained on glass could be used without any further processes, such as heating or solvent annealing.

4. Fabrication of remote-type green CsPbBr3-PSZ film and red KSF/silicone film R emissive KSF phosphors can be synthesized using a protocol described in our previous report.40 The obtained KSF phosphors were mixed with a silicone-based LED encapsulant (Dow Corning, OE-6636 A/B kit) with 33.3 wt% of phosphors in a binder. The KSF/silicone paste was loaded onto a spacer/glass 100 ㎛ thick and a curing process was conducted with a two-step heating treatment in an oven (60 min at 100 °C, 60 min at 150 °C). The hardened and


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flexible KSF/silicone film was then put onto the as-fabricated G CsPbBr3-PSZ film on a glass sample. This composite was placed on a BLED package and operated under an applied current of 20 mA.

5. Characterization The emission and absorbance/transmittance spectra of colloidal CsPbX3 PeNCs and CsPbX3-PSZ film can be obtained using a PL spectrophotometer with an Xe lamp (Darsa, PSI Trading) and a UV-visible spectrometer (Lambda 365, Perkin Elmer), respectively. The QYs of the colloidal CsPbX3 NC solutions can be calculated with a commercial dye (rhodamine 6G in ethanol, with a QY of 95% under an excitation source of 450 nm, Merck).41 The timeresolved PL spectra of the colloidal- and film-type CsPbBr3 PeNCs were recorded under excitation with the second harmonic of a cavity-dumped oscillator (355 nm) (Mira/PulseSwitch, 710 nm, 150 fs). A transmission electron microscope (TEM; JEM-3010, JEOL, Ltd.) was utilized to confirm the morphology and size of the CsPbBr3 PeNCs. Quantitative characterization of the elements on the PeNC surface were accomplished by means of TEM (FEI, Talos F200 X) with energy-dispersive spectrometer (EDS) signal detection. The binding energies of the compositional elements and crystal structures were surveyed by X-ray photoelectron spectroscopy (XPS; K-alpha, Thermo U. K.) and X-ray diffraction (XRD; D/MAX-2500V, Rigaku), respectively. The functional groups in the CsPbBr3 PeNCs and CsPbBr3/PSZ film were confirmed through a Fourier transform infrared (FT-IR) survey with an IR spectrophotometer (Nicolet iS50, Thermo Fisher Scientific). The electroluminescence (EL) spectra of the G emissive CsPbBr3/PSZ film and a WLED with the KSF film were measured in an integrating sphere using a spectrophotometer (Darsapro-5000, PSI).

Results and discussion Colloidal CsPbBr3 PeNCs were synthesized by a typical colloidal hot-injection method of the type commonly reported in many studies.1-5,8 As the main ligands, OA and OLA, which respectively consist of a carboxyl and an amine group based on aliphatic compounds, are attached to the cation (Cs+, Pb2+) and anion (halide ion), respectively, on the surface of the PeNC, making it possible for CsPbBr3 PeNC to dissolve in a non-polar solvent such as hexane



or toluene. After the synthesis step, the CsPbBr3 crude solution is centrifuged to separate the CsPbBr3 PeNCs and the reaction solution and the separated CsPbBr3 NCs are redispersed in hexane or toluene solvents. Figures 2a-c show the absorption, PL emission spectra and the CIE color coordinates of the normal CsPbBr3 PeNC, SZ-treated normal CsPbBr3 PeNC, photodegraded CsPbBr3 PeNC, and SZ-treated/photo-degraded CsPbBr3 PeNC solutions. (Photodegraded samples are exposed to an indoor fluorescent light environment for one day.) When the optical properties of these four samples are compared with each other, the absorption spectra and the CIE coordinates show nearly identical characteristics. There is a slight difference in the obtained PL spectra. That is, the emission wavelength peak at 515-516 nm and the FWHMs are in the range of 18-19 nm. However, there are significant differences in the PL intensity levels among the as-synthesized, photo-degraded and SZ-treated CsPbBr3 PeNC solutions. After photo-degrading the normal CsPbBr3 PeNC solution for one day, the PL intensity of the photo-degraded CsPbBr3 solution was reduced by half compared to that of the normal sample. In particular, upon exposure to an excitation light source, UV light can break down the surface structure of the CsPbX3 PeNCs and greatly reduce their PL intensity, as the increased Pb2+ sites which undergo photo-induced ligand erosion can act as electron trap sites, causing the electrons to be non-radiatively recombined (PL quenching). When the SZ oligomer is added to the photo-degraded CsPbBr3 solution, PL recovery can be confirmed, as the PL intensity of the SZ-treated photo-degraded CsPbBr3 solution is greatly increased. The presence of dislodged ligand positions can be managed simply by adding a Si-N-based SZ oligomer solution, which acts as a self-passivating Lewis-base ligand, to the photo-degraded CsPbBr3 PeNC solution.34 The PL intensity of SZ-treated photo-degraded CsPbBr3 PeNC can be greatly increased by having the lone electron pair fill the electron trap sites via Lewis acid/base adduct passivation, preventing metallic Pb0 segregation. The TEM analysis results were also compared to determine the metallic Pb behavior of the CsPbBr3 surface. The as-prepared CsPbBr3 PeNC was measured at a size of nearly 10 ~ 11 nm. As shown in the TEM images in Figures 2d-f, metallic Pb0 clusters are formed after photo-exposure through the reduction reaction of both excited electrons and Pb2+ ions.16,17,21,30 Some amount of the Pb0 cluster remains on the surface of the SZ-treated photo-degraded CsPbBr3 PeNC, although it undergoes significant PL recovery with the addition of SZ oligomers. It can be seen that the SZ-passivation process can remove Pb2+ ions, but it does not remove the metallic Pb0 clusters completely, instead only minimizing them. In addition, we identified which of the Pb2+ ions and Pb0 clusters more actively decrease the PL efficiency due to photo-induced degradation. As previously


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reported,21,42 Pb2+ sites can act as electron trap sites which directly affect the PL efficiency, and a Pb0 cluster forms due to the reduction of Pb2+ ions. When both Pb2+ and Pb0 defects coexist in the PeNC, the defect levels of Pb2+ ions and Pb0 clusters in the band gap can lead to the formation of deep defects and shallow defects (or in the conduction band), respectively.42 Hence, the energy levels of Pb2+ ions can cause these ions to act more strongly as electron trap sites, which further decrease the PL efficiency compared to these levels of Pb0 clusters. Likewise, we can confirm the enhancement of the PL intensity when the normal CsPbBr3 solution and the SZ oligomer are mixed, as in the case of PL recovery with the degraded CsPbBr3-SZ solution. As explained above, some established Pb2+ ions can be passivated by SZ oligomers, which can be activated as a self-passivating ligand in a SZ-rich system. Therefore, it is clear that SZ-passivation suppressed the formation of Pb0 clusters compared to the nonpassivated sample (Figure 2h). This optical behavior can also demonstrate SZ-passivation effects, as found when comparing the PLQY values (see Figure 2i and Table 1). The PLQY values of the normal and photo-degraded CsPbBr3 PeNCs are 76.8%, and 40.8%, respectively, indicating that the PLQY value of the photo-degraded CsPbBr3 PeNC solution is approximately 47% lower than that of the normal CsPbBr3 solution. Figure 2i also summarizes the PLQYs of the CsPbBr3 PeNC solutions after adding the optimum volume ratio of the SZ oligomer solution (5 wt% in xylene solvent) to PeQD solutions. The highest PLQY value of the photo-degraded CsPbBr3-SZ solution was obtained by adding an amount of approximately 70 vol% relative to the SZ oligomer solution. Therefore, the mixture solution with a CsPbBr3:SZ ratio of 3:7 was regarded as the optimal passivated solution for both the normal and photo-degraded CsPbBr3 NC solutions. As a result of adding the optimum SZ oligomer solution to the CsPbBr3 PeNC solution, the PLQYs of SZ-treated normal ad photo-degraded CsPbBr3 PeNC solutions recovered to 96.8 and 96.0%, respectively. Therefore, further e-spray filming studies were carried out with the SZ-treated normal CsPbBr3 PeNC solution rather than the SZ-treated photo-degraded CsPbBr3 PeNC solution due to the highest PLQY and passivated non-degraded outcome of the as-synthesized PeNC surface. It is crucial to ensure the stability of PeNCs without a tradeoff in their PLQYs. As previously reported in our publication to obtain CsPbBr3-PSZ powders,34 the Si-N-based SZ oligomer not only serves to provide passivating Si-N-based ligands on the surfaces of CsPbBr3 PeNCs but also reacts to form a Si-O encapsulation network through a rapid sol-gel reaction with a small amount of water. This one-step synergetic effect minimizes the PL loss of CsPbBr3 PeNCs during the protection process and secures PeNC stability at the same time. For further



studies, we propose an e-spray deposition method for the fabrication of a transparent, stable, efficient, and easy-to-use G emissive CsPbBr3 PeNC-PSZ film. Figure 3a shows a schematic illustration as well as an actual photograph of the e-spray system with a 20 cm distance between the bottom metal plate and nozzle (0.33 mm) while spraying a Si-N passivated CsPbBr3-SZ solution onto a glass substrate under a 20 kV cm-1 electric field. The sprayed CsPbBr3-SZ solution reacts with the moisture in the atmosphere and is subjected to a hydrolysiscondensation reaction, leading to Si-O encapsulation around the CsPbBr3 PeNCs and forming a CsPbBr3 PeNC-embedded film matrix. The CsPbBr3-PSZ film deposited in this way has transmittance rates of 65 ~ 70% above 515 nm and of 30-50% below 515 nm, presenting a reversed curve of the absorption spectra of the CsPbBr3 solution (see Figure 3b). In Figure 3c, the TEM image of the CsPbBr3-PSZ film confirms that the CsPbBr3 PeNCs are embedded into the encapsulated matrix with the Si-O- and Si-N-based inorganic polymer. The size reduction of PeNC particles embedded in CsPbBr3-PSZ film may be a hydrolysis-induced outcome and/or an underestimation during the analysis of the TEM images caused by overlaid particles and the screening effect of the encapsulated PSZ matrix.34 Fortunately, the hydrolysisinduced size reduction does not induce a PL reduction or a blue-shift of the PL wavelength; nor does it reduce the stability under harsh conditions. As shown with regard to the optical properties in Figures 3d and e, the emission wavelength of the G emissive CsPbBr3-PSZ films is red-shifted by 3 nm compared to that of the CsPbBr3 solution (515 nm → 518 nm), while the measured FWHMs of the CsPbBr3 solution and the CsPbBr3-PSZ film were 19 nm. Given that SZ passivation on the surface of each CsPbBr3 nanoparticle is preceded by a deposition process, agglomeration among CsPbBr3 PeNCs is suppressed with minimization of the selfenergy transfer, resulting in the emission wavelength being scarcely shifted. In Figure 3f, the time-resolved PL results of the CsPbBr3 solution, CsPbBr3-SZ solution, and CsPbBr3-PSZ film show the protective effect of SZ passivation and PSZ encapsulation on the CsPbBr3 surface. The detailed parameters for the time-resolved PL decay are displayed in Table S1, and the average decay times are calculated using Equations S1 and S2. Generally, in time-resolved PL decay studies of the structures of quantum dots (QDs), the decay curves of core/shell or surface-passivated QDs can be confirmed as having longer PL lifetimes than those of core or non-passivated QDs due to the suppression of non-radiative decay. This may result in a recombination of generated excitons into radiative pathways, leading to a PL improvement.33 The result of the longer PL lifetime of the CsPbBr3-SZ solution than that of the normal CsPbBr3 solution demonstrates that the SZ oligomer treatment can passivate the surface defects of


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CsPbBr3 PeNCs as a passivating ligand. Moreover, the longer PL decay time of the CsPbBr3PSZ film than that of the CsPbBr3-SZ solution explains why the CsPbBr3-SZ solution becomes a sturdy and protected CsPbBr3-PSZ film through the sol-gel reaction during the e-spray process, which can more strongly passivate and encapsulate CsPbBr3 PeNCs in the vicinity at the same time. Figure 4a shows the full XPS spectra of the CsPbBr3 PeNC, CsPbBr3-SZ PeNC, and CsPbBr3-PSZ film samples. CsPbBr3 PeNC (black line) confirmed the binding peaks of Cs, Pb, Br, C, O, and N elements as identified from the crystal components and ligands of the PeNCs.34,43 The CsPbBr3-SZ PeNC spectrum (red line) shows that the SZ-derived Si, which is confirmed at the near-unity value of 103 eV of the Si 2p binding peak, was found to be mixed with the binding peak state of CsPbBr3 PeNC (black line). For the CsPbBr3-PSZ film (blue line), the binding energies of CsPbBr3 PeNC were confirmed to have relatively low intensities of the binding peaks compared to the strong binding peaks of the Si, O, and N components. That is, considering the results of the XPS surface analysis, the surface of the CsPbBr3-PSZ film is formed with an encapsulating Si-O network and is thus analyzed as being in the Si-, O-, and N-dominant surface states. Figures 4b-g show the binding peaks of Cs 3d, Pb 4f, Br 3d, Si 2p, N 1s, and O 1s, respectively. Overall, the binding peaks of the compositional elements in the CsPbBr3-SZ and CsPbBr3-PSZ films showed higher binding energies by approximately 0.4 ~ 0.9 eV than those of the CsPbBr3 PeNC. This likely occurred because the strong bonds between the surfaces of CsPbBr3 PeNC composed of Cs+, Pb2+, and Br- and the SZ oligomer constituting the Si-N and the amine groups are dative-covalent bonds and hydrogen bonds, which act as a mantle of the stabilizer on the surfaces of the PeNCs.44-47 Note that in the results from the CsPbBr3-SZ PeNC and CsPbBr3-PSZ film samples, the intensity levels of the binding peaks of the N and O elements derived from the OA and OLA ligands were not clearly detected due to their low levels compared to those of Si, O, and N in the SZ ligand or the PSZ matrix (see Figures 4e-g). Figure 5 shows the results of XRD and FT-IR surveys conducted to confirm the crystal structures and the functional groups of the CsPbBr3 PeNC, CsPbBr3-SZ PeNC, and CsPbBr3PSZ film samples. First, in the FT-IR of the CsPbBr3 PeNC in the wavenumber range of 3600 ~ 400 cm-1, the vibration modes of the OA and OLA ligands were found to be the C-H asymmetric and symmetric stretching modes at 2922 cm-1 and 2852 cm-1, respectively (see the black line in Figure 5a). The peaks at 1466 cm-1 and 1408 cm-1 indicate the CH3- bending



mode stemming from the OLA and the COO- stretching mode derived from OA, respectively. The peak at 721 cm-1 is the characteristic peak of the -(CH2)n- bending mode resulting from the long-chain ligands.48,49 Si-O stretching and bending modes can be found at 1063 cm-1 and 436 cm-1, respectively, and the Si-N vibration modes are found near 825 cm-1 in the IR transmittance peaks.50,51 The IR result of CsPbBr3-SZ PeNC (red line) shows a broad hump peak of 1200 ~ 800 cm-1 based on the peaks of the functional groups in CsPbBr3 PeNC. This can be considered to be a mixture of Si-N and Si-O peaks, meaning that the CsPbBr3-SZ PeNC is formed by dative-covalent bonds between lone-pair electrons at the Si-N and surface of CsPbBr3-SZ and interlinked to some Si-O after a purification process, which can be exposure to an air condition. The blue line in Figure 5a clearly shows Si-N and Si-O peaks as a result of the PSZ matrix. The typical point is that the IR peak of the glass substrate used for the deposition of the CsPbBr3-PSZ film is confirmed in the wavenumber range of 500-400 cm-1 (See Figure S3a). Next, Figure 5b shows the XRD survey results measured in the two-theta (2θ) range of 10 ~ 60 º. The XRD peaks between both the CsPbBr3 (black line) and CsPbBr3SZ PeNCs (red line) coincide well with each other. The typical XRD positions of the crystalline phase in the CsPbBr3 PeNC have 2θ values of 15.19 º, 21.55 º, 30.65 º, 34.37 º, 37.77 º, and 43.89 º, corresponding to the (100), (110), (200), (210), (211), and (220) facets. In the result from the CsPbBr3-PSZ film on a glass substrate, the above-mentioned XRD peaks of CsPbBr3 PeNCs are not clearly visible due to the Si-N-O-based broad amorphous peak, except for only two facets of (110), and (200) (green arrows in Figure 5b). Moreover, the broad peak of the glass substrate was noted compared to the XRD result with only the glass substrate (See Figure S3b). By analyzing the results of the compositional binding energies, functional groups, and crystal structures, it could be demonstrated that the SZ oligomer presents evidence of passivation on the surfaces of CsPbBr3 PeNCs with further protection around the CsPbBr3 PeNCs by PSZ encapsulation. In this study, to verify the protective effect of the Si-O-N-based PSZ on CsPbBr3, stability experiments were carried out using CsPbBr3, CsPbBr3-SZ PeNC solutions, and CsPbBr3-PSZ film. Figure 6 presents the experimental results of the stability test under thermal and UV exposure. First, in order to assess the thermal stability, normal CsPbBr3 PeNC and passivated CsPbBr3-SZ PeNC dispersed in toluene as a solvent (boiling point of about 110 ° C) were prepared by loading them into vials and placing them on a 100 ° C hot plate for 12 hrs with PL measurements. In Figures S4a-c, the PL intensities of the normal CsPbBr3, CsPbBr3-SZ PeNCs, and CsPbBr3-PSZ film were decreased via the thermal exposure.


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On the one hand, the QY of the normal CsPbBr3 was decreased by 79.4% after 8 hrs of thermal exposure compared to that of the pristine CsPbBr3 PeNC (72.8% to 15.0%). After 12 hrs, there were no dispersed PeNCs owing to aggregation among the CsPbBr3 PeNCs.18,19 The emission wavelengths were shifted from 515 nm to 521 nm, while the FWHM values rarely changed, at 18 ~ 19 nm. The CsPbBr3-SZ PeNC decreased by 74.3% (96.5% to 24.8%) after 12 hrs, indicating that the CsPbBr3-SZ PeNCs are more stable than the normal CsPbBr3 PeNC without any passivating treatment. The CsPbBr3-PSZ film which is passivated and encapsulated with the Si-N-O PSZ matrix shows a decrease of only 34.4% (63.7% to 41.8%) after 150 hrs (see the comparison in Figure 6a). After 12 hrs of thermal exposure in CsPbBr3-SZ, the solution becomes slightly opaque due to certain dispersed particles which can be formed by some cohesive PeNCs and PSZ particles from polymerization among the remaining SZ oligomer moieties. Therefore, similar to the optical measurement results from the thermal stability test of the normal CsPbBr3, the emission wavelengths became slightly red-shifted from 514 nm to 519 nm and the FWHM values were somewhat broadened with a longer thermal exposure time. In addition, UV exposure tests of the CsPbBr3, CsPbBr3-SZ PeNCs, and CsPbBr3-PSZ film samples were carried out to confirm the photostability of these samples (See Figure 6b). The CsPbBr3 and CsPbBr3-SZ PeNCs solutions were exposed to UV light (5 W cm-2) for 150 hrs with the PL measurements performed at intervals of 24 hrs. In Figures S5, the PL intensities of the normal CsPbBr3, CsPbBr3-SZ PeNC solutions and the CsPbBr3-PSZ film were decreased via UV exposure. The normal CsPbBr3 PeNCs were attenuated by 61% in QY compared to that before UV exposure and were completely agglomerated by UV exposure after 5 hrs. The CsPbBr3-SZ PeNCs were shown to be relatively stable under UV exposure, with a decrease in the QY to 23% of the initial QY over 120 hrs (See Figure 6b). Meanwhile, the CsPbBr3-PSZ film was decreased to 87% compare to the initial QY. These stability tests demonstrate that only a SZ treatment in the CsPbBr3 PeNC solution and cross-linking to the PSZ matrix can help to improve the stability of PeNCs. A G emissive CsPbBr3-PSZ film sample was fabricated on a glass substrate using the asprepared CsPbBr3-SZ solution according to the e-spray method described above. As shown in Figure S6, optimization experiments were carried out to determine the optimal amount of CsPbBr3-SZ solution to spray. It was found that it is best when 4 ~ 12 mL of the CsPbBr3-SZ solution is sprayed considered the conversion efficiency (CE). In this experiment, the CsPbBr3PSZ film produced by spraying 12 mL of the CsPbBr3-SZ solution was used in further lighting application studies. As previously reported,52 the e-spray method enables the preparation of



highly concentrated PeNC-embedded films because the PeNC-PSZ film coating is formed as quickly as possible using highly dispersed colloidal solutions in which the agglomeration of PeNCs can be minimized by the rapid deposition of the highly dispersed PeNC solution. As explained above, the higher the transmittance, as shown in Figure 3b, the TEM image in Figure 3c, and the longer PL the decay time in Figure 3f confirm the highly dispersive PeNCloaded PSZ polymer films. The optimum thickness of the PeNC-PSZ films was approximately ~52.5 μm, as shown in the optical microscope image presented in Figure S7a. This figure clearly shows that this G emissive film has a remote-type color-by-blue LED structure with a BLED as an excitation source. The obtained spectrum of the CsPbBr3-PSZ/BLED consists of two portions of B and G wavelengths which result from the BLED and the G CsPbBr3-PSZ film emitted by the B excitation source (see Figure S7b). The luminous efficacy (LE), external quantum efficiency (EQE; EQE = LE/luminous efficacy of radiation)53, and CE were found to be 85.9 lm/W, 38.7%, and 60.1%, respectively. In addition, to evaluate its suitability for use in LED applications, stability tests were conducted under harsh conditions, in this case air storage, B light from a BLED, UV exposure, thermal exposure, and water immersion. After exposing the as-prepared CsPbBr3-PSZ film samples to these harsh conditions for 150 hrs, the relative LEs were found to have decreased to 0.900 for air storage, 0.901 for B light, 0.851 for UV, 0.639 for heating, and 0.297 for water (See Figure S7c). These results indicate excellent stability for lighting applications of PeNCs compared to the earlier results shown in Table S2. The photo-induced degradation of the CsPbBr3 PeNC is greatly reduced after the PSZ treatment, which results in surface passivation by Lewis acid/base adducts and encapsulation by Si-O-N polymerization. In addition, good thermal stability is ensured by the heat-resistant PSZ matrix around the CsPbBr3 PeNCs. The fabrication process of the stable, efficient, and easy-to-use CsPbBr3-PSZ films provides the possibility of forming PeNC films by means of the e-spray deposition of a PeNCs-SZ solution for application to remote-type color-by-blue LEDs. In Figure S8, to assess its potential use as a G source in the backlighting or sub-pixels of displays, the realization and characterization process of a G monochromatic LED were carried out. We fabricated the monochromatic G emissive CsPbBr3-PSZ LED with the help of a light-recycling filter (LRF) and a long-wavelength pass dichroic filter (LPDF) (see Figure S8a).54,55 The amount of CsPbBr3-SZ solution used for the deposition of the CsPbBr3-PSZ film was set to 32 mL to increase the green portion, neglecting the small drop in the CE. The CsPbBr3-SZ solution is sprayed onto the LRF instead of the glass substrate, and the LPDF is placed on the obtained CsPbBr3-PSZ film. In Figure S8b, the transmittances of the LRF and


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LPDF can serve to improve the external light efficiency and to block the B light.54,55 The emission wavelength of the monochromatic G PeNC film is slightly red-shifted compared to that of the CsPbBr3-SZ solution. The spectra of BLED, the G monochromatic CsPbBr3-PSZ film, and the R KSF film with narrower FWHM values can realize a wide color gamut of 120% in comparison with the National Television System Committee (NTSC) standard (see Figures S8c and d). This G emissive monochromatic CsPbBr3-PSZ LED shows a LE of 134.2 lm/W, an EQE of 25.1%, and a CE of 51.4%, which can be considered as acceptable levels for G backlighting or sub-pixels (Figure S8e). Moreover, an R emissive CsPbBrI2-PSZ film was prepared using the same method used to prepare the G emissive film. Figure S9 summarizes the optical properties of the remote-type CsPbBrI2-PSZ film covering the BLED. The LE, EQE, and CE were confirmed to be 10 lm/W, 19.7%, and 24.0%, respectively. Its stability was confirmed as it retained its luminescence even when immersed in boiling water for 3 min (Video S1). However, compared to the optical performances and stability levels of the R KSF film, those of the R CsPbBrI2-PSZ film should be significantly improved to match the performances of R KSF films. For a closer examination of WLEDs, we used an R KSF film as an R emissive candidate to study the optical performance capabilities of remote-type white-byblue LEDs including G CsPbBr3-PSZ film and R KSF film. As an R emissive phosphor with high color purity, the KSF film chosen for use here was prepared with as-synthesized KSF powder and a thermal-curable silicone binder. The remote-type WLED was fabricated by combining G CsPbBr3-PSZ and R emissive KSF films, and they were operated at an applied current of 20 mA. Figure S9 shows the optical properties of the obtained KSF film operating on a BLED with a LE of 45.6 lm/W and a CE of 62.0%. The spectrum and color coordinates of the CsPbBr3-PSZ/KSF-based WLED are shown in Figures 7b and c, and in Table 1. The LE, EQE, and CE of the WLED were found to be 71.0 lm/W, 27.2%, and 50.8%, respectively, with a correlated color temperature of 9334 K. The detailed performance capabilities of the fabricated CsPbBr3-PSZ/KSF silicone-based WLED were determined by analyzing the spectra with the current dependence as a function of the applied current from 10 to 200 mA and while confirming the spectra from a long-term operation stability test lasting for 100 hrs. In Figures 7a and b, the spectra obtained with current dependence show a constant increase in the intensity up to a current of 200 mA, with color coordinate shifts from (0.256, 0.304) to (0.268, 0.279). Figure 7c shows the LE and EQE with the applied currents. At 200 mA, the LE and EQE values were found to be 56.6 lm/W and 23.1%, respectively, indicating high stability under a high current when compared to the values



of 71.0 lm/W and 27.2% at an operating current of 20 mA. The spectra and color coordinates according to the long-term operation stability test are shown in Figures 7d and e, respectively. At a continuous operation time of 100 hrs, the LE and the correlated color temperature (CCT) values were found to be 67.0 lm/W and 10200 K, respectively (see Figure 7f). Compared to the results before the operation, the LE is decreased to 92.5% of the relative value and the CCT increases from 9334 K to 10200 K due to the slight decrease of the G portion in the spectra. As a result of the high operation current of 200 mA and the long-term operation time of 100 hrs, the G CsPbBr3-PSZ film-based white-by-blue LED was demonstrated to be feasible and suitable in backlighting and display applications owing to the synergetic protection effect of PSZ. Therefore, the multiple benefits of PSZ-based PeNC film simultaneously promote passivation and encapsulation and the easy-to-use film form that can be readily fabricated by fast hydrolysis between SZ-treated PeNC mist droplets and air moisture during the e-spray deposition process, making the CsPbBr3 PeNC film efficient and stable.

Conclusion In this study, we successfully synthesized G emissive CsPbBr3 PeNC via a facile colloidal hotinjection method and improved the stability and PL efficiency of CsPbBr3 PeNC via a SZ oligomer treatment. SZ oligomer as a Lewis-base ligand can passivate the ligand-detached surface of CsPbBr3 PeNC when exposed to Pb2+ as Lewis acid sites. The Lewis acid/base adducts can protect the surface of the PeNC to prevent Pb0 segregation and the production of additional Pb2+ sites. During this effort, the SZ-treated CsPbBr3 PeNC increased the PLQYs of photo-degraded CsPbBr3 PeNC by 2.35-fold (40.8% → 96.0%) and secured photo and thermal stability through a self-assembly passivation process. Furthermore, we fabricated G emissive CsPbBr3-PSZ film by utilizing a facile e-spray method. During the e-spray process, the assynthesized CsPbBr3-SZ solution is polymerized from Si-N-based SZ oligomers to a Si-N/SiO cross-linking matrix via a rapid sol-gel reaction. The Si-N/Si-O network can encapsulate the periphery of the CsPbBr3 PeNC. The efficient CsPbBr3-PSZ film showed feasible optical properties, with a LE value of 85.9 lm/W, EQE of 38.7%, and CE of 60.1%. Moreover, after a stability test lasting for 150 hrs under harsh conditions, the LE levels of the CsPbBr3-PSZ film were decreased to 0.801, 0.901, 0.851, 0.639, and 0.296 when the CsPbBr3-PSZ film was exposed to air storage, B light exposure, UV exposure, heating, and water immersion


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conditions, respectively. Hence, we confirmed that the self-assembly SZ oligomer passivation and robust PSZ protection could properly provide synergetic protection to inherently unstable CsPbBr3 PeNCs. The results of time-resolved PL decay, TEM, FT-IR, XPS, and XRD surveys provided evidence of the synergetic protection. To confirm the feasibility of the easy-to-use CsPbBr3-PSZ film in display applications, a remote-type WLED was realized by combining a BLED, G emissive CsPbBr3-PSZ film, and R emissive KSF/silicone film. This white-by-blue LED showed a CCT of 9334 K, LE of 71.0 lm/W, EQE of 27.2%, and CE of 50.8% at an applied current of 20 mA. Moreover, a long-term operation test demonstrated that the fabricated WLED showed high stability under continuous B light. After an operation time of 100 hrs, the LE and CE of the CsPbBr3-PSZ-based WLED were found to have decreased to 92.5% and 93.3% of the original values, respectively. Therefore, the combination of a simple e-spray deposition process and an appropriate film matrix and precursors proved the suitability of fabricating efficient, stable, and easy-to-use PeNC-embedded inorganic PSZ films for use as

color-converter

components

in


display

devices.


ASSOCIATED CONTENTS Supporting Information HAADF images of a metallic Pb0 cluster, optimization results of the fabrication of the CsPbBr3PSZ film, stability test of the CsPbBr3-SZ solution, optical properties of the CsPbB3-PSZ film by other coating methods, i.e., the KSF film and CsPbBrI2-PSZ film. XRD and FT-IR results of bare glass substrates 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.

ACKNOWLEDGMENT This work was supported by a grant from the National Research Foundation of Korea (NRF) funded by the Korean government (Ministry of Science and ICT) (No. 2015M3D1A1069709, No. 2016R1A5A1012966, and No. 2017R1A2B2007575).


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Figure Captions

Figure 1. Schematic illumination of the two interaction modes between the Si-N-based silazane oligomer and the surface of CsPbBr3 PeNC as Lewis acid/base adducts and a Br-H hydrogen bonding bridge. A schematic and actual PeNC-PSZ film images are also shown in this figure.


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Figure 2. (a) Absorbance, (b) photoluminescence spectra, and (c) CIE color coordinates of normal (black line), SZ-treated normal (red line), degraded (blue line), and SZ-treated degraded (green) CsPbBr3 PeNC solutions. TEM images of (d) normal CsPbBr3, (e) photo-degraded CsPbBr3, (f) SZ-treated photo-degraded CsPbBr3, (g) SZ-treated normal CsPbBr3, and (h) UVexposed SZ-treated normal CsPbBr3 (UV light exposure lasted for 30 minutes). Insets in Figures (d) – (h) are actual luminescent images of each CsPbBr3 PeNC solution sample and the insets in Figures (d), (e), and (g) are high-magnification images. (i) Plot of the photoluminescence quantum yields as a function of the addition of SZ to the CsPbBr3 solution.



Figure 3. (a) Schematic figure and an actual image of the electrospray system. (b) Transmittance of the CsPbBr3-PSZ film, with the insets showing actual images of normal and luminescent CsPbBr3-PSZ film samples. (c) TEM images of the CsPbBr3-PSZ film; the inset shows high-magnification images. (d) Photoluminescence/absorbance (PL/Abs) spectra of the CsPbBr3-SZ and CsPbBr3-PSZ film samples. (e) CIE color coordinates of the CsPbBr3-SZ solution and the CsPbBr3-PSZ film. (f) The photoluminescence decay profiles of normal and SZ-treated CsPbBr3 solutions, and the CsPbBr3-PSZ film.


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Figure 4. XPS surveys of (a) the overall result, and partial (b) Cs, (c) Pb, (d) Br, (e) Si, (f) N, and (g) O component peaks of normal (black line), SZ-treated CsPbBr3 PeNC (red line), and CsPbBr3-PSZ film samples (blue line). The green and cyan lines indicate deconvoluted curves in each elemental XPS curve.

Figure 5. (a) FT-IR and (b) XRD surveys of normal, SZ-treated CsPbBr3 PeNC and CsPbBr3PSZ film samples. Insets are actual images of purified normal and SZ-treated CsPbBr3 powders and CsPbBr3-PSZ film.



Figure 6. Relative quantum yields of normal, SZ-treated CsPbBr3 PeNCs, and CsPbBr3-PSZ film after 150 hrs of (a) thermal and (b) UV stability


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Figure 7. (a) Spectra, (b) CIE color coordinates, and (c) graphs of the luminous efficacies (lm/W) and external quantum efficacies as a function of the applied current in the range of 10 ~ 200 mA. (d) Spectra, (e) CIE color coordinates, and (f) graphs of the luminous efficacies, external quantum efficacies, and conversion efficiencies for an applied time of 100 hrs at 20 mA. The inset in (d) shows a schematic illustration of the WLED.



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Table 1. Optical properties of the CsPbBr3 solution (normal, SZ-treated normal, degraded, and degraded and SZ-treated), and a remote-type film LED (green converted, and WLED)

Sample

CIE x

CIE y

Green peak wavelength

FWHM

Normal

0.085

0.767

515 nm

19 nm

76.8%

SZ-treated

0.084

0.760

516 nm

18 nm

96.8%

Degraded

0.083

0.762

516 nm

18 nm

40.8%

SZ-treated

0.082

0.765

515 nm

19 nm

96.0%

Green only

0.145

0.216

519 nm

85.9 lm/W

38.7%

60.1%

WLED with KSF film

0.258

0.302

521 nm

71.0 lm/W

27.2%

50.8%

LE

EQE

QY/CE

CsPbBr3 Solution

CsPbBr3 PSZ film


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Stable and Efficient Green Perovskite NanocrystalâPolysilazane Films

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Stable and Efficient Green Perovskite NanocrystalâPolysilazane Films