Direct Hot-Injection Synthesis of Lead Halide Perovskite Nanocubes in

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Functional Nanostructured Materials (including low-D carbon)

Direct Hot-Injection Synthesis of Lead Halide Perovskite Nanocubes in Acrylic Monomers for Ultrastable and Bright Nanocrystal–Polymer Composite Films Jianyu Tong, Jiajing Wu, Wei Shen, Yukang Zhang, Yao Liu, Tao Zhang, Shuming Nie, and Zhengtao Deng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20681 • Publication Date (Web): 08 Feb 2019 Downloaded from http://pubs.acs.org on February 8, 2019

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ACS Applied Materials & Interfaces

Direct Hot-Injection Synthesis of Lead Halide Perovskite Nanocubes in Acrylic Monomers for Ultrastable and Bright Nanocrystal–Polymer Composite Films Jianyu Tong†,⊥, Jiajing Wu†,⊥, Wei Shen†, Yukang Zhang†, Yao Liu†, Tao Zhang†, Shuming Nie†,‡, and Zhengtao Deng*,† †Department

of Biomedical Engineering, College of Engineering and Applied Sciences, Nanjing

University, Nanjing 210023, People’s Republic of China ‡Departments

of Bioengineering, Chemistry, Electrical and Computer Engineering, and Materials

Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL, 61801, USA *Email: [email protected]

KEYWORDS: lead halide perovskite; colloidal nanocrystals; color-conversion optical films; photostability; display backlight

ACS Paragon Plus Environment

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ABSTRACT: In recent years, lead halide perovskite nanocrystals (NCs) have attracted significant attention in both fundamental research and commercial applications due to their excellent optical and optoelectrical properties. However, the protective ligands on the surface of the perovskites NCs could be easily removed after the tedious process of centrifugation, separation, and dispersion, which greatly hampers their stability against light, heat, moisture, and oxygen and limits their practical applications. Here we report a new post-processing-free strategy (i.e., without centrifugation, separation, and dispersion process) of using an UV-polymerizable acrylic monomer of lauryl methacrylate (LMA) as the solvent to synthesize CsPbBr3 NCs, and then adding polyester polyurethane acrylates oligomer, monomer (IBOA) and initiator for directly UV polymerization to fabricate NC-polymer composite films. These films exhibited an improved photoluminescence quantum yield (85-90%) than classic NC-film (40-50%), which were processed using octadecene (ODE) as the solvent for NC synthesis and post-processed for UV polymerization. Significantly, the as-fabricated films by post-processing-free strategy exhibited excellent photostability against strong Xe lamp illumination; while the other films using classic methods were quickly photodegraded. Meanwhile, these NC-polymer composite films showed good stability against moisture and heating when aging in water at 50oC for over 200 hours. These films, along with K2SiF6:Mn4+ (KSF) phosphor emitters, were used as downconverters for blue LEDs in liquid crystal displays with a wide color gamut of 115% in the International Commission on Illumination (CIE) 1931 color space. This work provides a facile and effective strategy for the preparation of ultrastable and bright color-conversion NC films for the development of the next-generation wide color gamut displays.


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INTRODUCTION Colloidal lead halide perovskites nanocrystals (NCs) have attracted increasing interest in the past few years due to their potential applications in light emitting diodes (LEDs), lasers, solar cells, luminescent solar concentrators (LSCs), photodetectors, and display backlights.1-5 Among them, lead halide perovskite NCs have more advantages in wide color gamut and high brightness backlight displays on account of their low-cost solution processing, narrow and tunable emission spectra, and high absolute photoluminescence quantum yield (PLQY).6-9 However, perovskite NCs are quickly degraded when they suffer from light, heat, moister, and oxygen, because of their strong ionic nature and low formation energy.10-13 In recent years, many studies have been conducted on the stability of perovskite NCs including ion doping,14-18 surface ligand engineering,19-23 and inorganic material or polymer overcoating.24-28 For instance, Zeng et. al. demonstrated a strategy through Mn2+ substitution to stabilize perovskite lattices of CsPbX3 QDs.15 Ning et. al. develop a post-synthesis passivation process for CsPbI3 NCs by using a bidentate ligand to achieve stable near-unity photoluminescence quantum yield.19 Moreover, Li and co-workers embedded CsPbBr3 NCs into a SiO2/Al2O3 monolith to improve the stability.29 Polymer encapsulation was also proved to be an effective strategy to stabilize perovskite NCs. The water resistance of MAPbBr3 and CsPbBr3 perovskite NCs could be improved with the encapsulation of commercially available polymers, such as polystyrene and polyacrylic acids.26, 30 However, the stability of the lead halide perovskite NC products under all severe aging conditions is not satisfactory. Perovskite NC-polymer composite film is attractive as color-conversion optical films for blue LEDs in liquid crystal display backlight applications. The polymer substrates tend to form a


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coherent barrier layer around the perovskite NCs, protecting them from water and oxygen of the surrounding environment. Recently, several strategies for preparing of lead halide perovskite NCpolymer composite films used in the backlight of LCD have been developed. One classical method is the synthesis of lead halide perovskite NCs in octadecene (ODE) by hot-injection method, and then to centrifuge, separate, and re-disperse the perovskite NCs for making NC-polymer composite

films.31-36

For

instance,

Wong

et.al.

reported

a

perovskite-initiated

photopolymerization of the perovskite NCs-monomer mixture, the resulting perovskite NCpolymer composites exhibit excellent stability against moisture.36 However, such technique often leads to the lower PLQY and poorer photostability of resulting NC-polymer composite films, which is mainly caused by insufficient passivation due to excessive removal of surface ligands in the process of separation and purification.11 In addition, the mixing of perovskite NCs with polymers often leads to unacceptable agglomeration of NCs due to the large difference in polarity between perovskite NCs and polymers.31, 37 Moreover, perovskite NC-polymer films were also fabricated by solvent evaporation strategy.24, 27, 38-40 For example, Dong et. al. developed an in situ polymer swelling/de-swelling strategy to fabricated MAPbBr3-PS and MAPbBr3-polycarbonate composite films with PLQY of 48% that were able to survive boiling water treatment for 30 minutes with decay in PLQY of less than 15%.24 While thermal treatment would accelerate the aggregation of NCs before the formation of the optical polymer film, leading to defects that inevitably decreasing the PLQYs. Meanwhile, the in situ photo-polymerization strategy with bulk monomers was also developed to make perovskite NC-polymer composite films.41-42 Moreover, it was reported that the polymerization of bulk monomers into polymer is time-consuming (i.e. 10 hours or more), which increases the chemical damage of NCs by radicals during the polymerization process, resulting in the significantly decrease of PLQY from NC solution to the solid NC-polymer


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films.43 Therefore, it is essential to explore a facile yet effective strategy to prepare metal halide perovskite NC-polymer films with bright emission and good stability under all harsh aging conditions, especially under the condition of photostability aging test. In this study, we employed lauryl methacrylate (LMA) as a solvent, replacing the commonly used octadecene (ODE), to synthesis CsPbBr3 NCs by a modified hot-injection method. Generally, the colloidal lead halide perovskite NCs are synthesized by hot-injection method4, 44-48 or solvent-induced reprecipitation technique49-51. For the former, ODE is a classic high boiling point solvent, and need to be removed by centrifugation before future applications. For the latter, N, N-dimethylformamide (DMF) is also required to be removed by heat treatment or by centrifugation before further usage. Here, we choose to replace ODE with LMA as a solvent for four main reasons: (1) the high boiling point (322.7°C at 760 mmHg) of LMA is suitable for the synthesis of lead halide perovskite NCs, which is usually under 200oC; (2) LMA is a hydrophobic long-chain alkyl monomer, which not only affords a high colloidal stability for the perovskite NCs, but also enhances the moister resistance of the NC-polymer films; (3) methacrylic group allows for radical polymerization as well as crosslinking between two adjacent NCs, so the polymerization performance of the NC-polymer films is improved. (4) LMA was photopolymerized with other suitable monomer, oligomer and initiators within 5 minutes, much less than previous method (10 hours), resulting in NC-polymer composite films with high PLQY and improved stability.


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Scheme 1. Schematic diagram of the classic strategy and our new strategy for the CsPbBr3 NCpolymer composite films fabrication process.

RESULTS AND DISCUSSION The fabrication process of NC-polymer films containing CsPbBr3 NCs with and without post-processing was shown in Scheme 1. As compared to the previous work, we added hydrophobic polyester polyurethane acrylates oligomer into to NCs solution to reduce the polymerization time, which decreased damages from UV light, as well as damages by radicals during the polymerization process. The addition of IBOA can balance the polarity of oligomer and LMA. On this basis, high quality green CsPbBr3 NC-polymer composite films were successfully fabricated within 5 min via photopolymerization. The absolute PLQY (85-90%) of the assynthesized films was higher than that of the previously reported NCs polymer composite films, while maintaining excellent photostability as well as water and heat resistance. Furthermore, the liquid crystal display (LCD) devices based on CsPbBr3 NCs polymer composite film exhibit a


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wide color gamut. Therefore, we believe that this work paves the way for production and application of perovskite NCs to improve the performance of current liquid crystal display technologies.

Figure 1. (a-c) TEM image, HRTEM image, and size distribution of the as-prepared CsPbBr3 NCs. (d) XRD pattern of CsPbBr3 NCs showed that the spectrum is in good agreement with orthorhombic perovskite CsPbBr3. (e) UV–vis absorption (black line), PL emission (red line) and PL excitation (blue line) spectra of a typical CsPbBr3 NCs solution. Inset is the optical image of CsPbBr3 solution under UV light (λ = 365 nm). CsPbBr3 NCs were prepared by a modified hot-injection method, and their morphology, size distribution histogram, crystal structure, and optical properties are shown in Figure 1. In this reaction system, LMA acts as a hydrophobic long-chain alkyl solvent instead of ODE for the growth of NCs. According to FTIR spectra (Figure S1), it can be observed that there are no


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significant differences between the growth of NCs in LMA and ODE, suggesting that their growth mechanisms were similar. Transmission electron microscopy (TEM) image in Figure 1a revealed a cubic shape of as-synthesized CsPbBr3 NCs with an average size of 9.8 nm, the size distribution was shown in Figure 1c. High-resolution TEM (HRTEM) confirmed that it has an orthorhombic crystalline structure exposing (110) crystal lattice plane (Figure 1b). The X-ray diffraction (XRD) pattern of the as-prepared nanocubes is shown in Fig. 1d. The product can be indexed to orthorhombic CsPbBr3phase (ICSD # 01-072-7929, a = 0.8207 nm, b = 0.8255 nm, c = 1.1759 nm), which is consistent with previous report. 48, 52-53X-ray photoelectron spectroscopy (XPS) was used to analyze the elemental composition of as-synthesized NCs (Figure S2), the elements Cs 3d, Pb 4f, and Br 3d can be detected, the Cs: Pb: Br atomic ratio obtained from the XPS peak areas of survey spectra was close to 1:1:3. As shown in the inset of Figure 1e, the CsPbBr3 crude solution dispersed in toluene exhibited bright green fluorescence under ultraviolet (UV) light irradiation. The UV-Vis absorption and PL spectra of CsPbBr3 NCs dispersed in toluene are shown in Figure 1e. The PL emission peak is located at 520 nm with PLQY of 87%. The PL spectra have a narrow full width at the half maximum (FWHM) of 20 nm, which exhibited good monochromaticity. The Stokes shift is only 10 nm, indicating that the PL emission mainly arises from the exciton recombination. The blue light conversion NC-polymer composite films based on CsPbBr3 NCs synthesized in LMA monomer were fabricated by photopolymerization. Since oligomers have a good affinity for LMA, the addition of oligomer into the CsPbBr3 NCs NCs synthesized from LMA monomers can form a homogeneous mixture. Thus, uniform optical films were obtained via photopolymerization, as shown in inset of Figure 2a (film 1). Cross-sectional scanning electron microscopy (SEM) image of film 1 was exhibited in Figure S3a. The Figure 2a exhibited the UV–


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vis absorption and PL emission spectra of as-fabricated film 1, the emission peak of the film shifted to a slightly longer wavelength compared to the solution-based samples (Figure 1e), which is due to fluorescence resonance energy transfer (FRET) or self-absorption related red shift of photoluminescence.53 Remarkably, the PLQY of film 1 did not change significantly after photopolymerization, reaching 85-90%. Such PLQY is comparable or higher than previously reported perovskite polymer films.

Figure 2. (a) UV–vis absorption (red line), PL emission (blue line) spectra of a CsPbBr3 NCpolymer film (name as “film 1”). The Inset is optical image of the same CsPbBr3 NC-polymer film under UV light (λ = 365 nm). (b) PLQY and (c) Time-resolved PL decays spectra of the CsPbBr3 polymer films with different processing methods (Note: film 1: LMA without post-processing; film 2: ODE with post-processing; film 3: ODE with post-processing and adding OA; film 4: ODE with post-processing and adding OAm; film 5: ODE with post-processing and adding OA and OAm; film 6: ODE without post-processing). We then compared the optical properties of the control films (film 2−film 6) with various treatments to CsPbBr3 NCs that were synthesized using ODE as the solvent, shown in Figure 2b. The PLQY of as-fabricated film 2 based on purified QDs was only 54%, which may be caused by the loss of ligand and the increase of surface trap states. Therefore, we try to add 50 μL oleic acid


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(film 2), oleylamine (film 3) or a mixture of OLA/OA (volume ratio=1:1, film4) into purified CsPbBr3 NCs solution, respectively. Their UV-vis absorption and PL emission spectra are shown in Figure S4 and S5, both the absorption and emission peak are slightly shifted to longer wavelengths after post-processing (film 2~5, compared with film 6). The PLQYs are shown in Figure 2b, it can be observed that the addition of OA did not change much. The PLQY of film 3 and film 4 were improved from 54% to 71% and 72%, respectively, indicating oleylamine ligands were able to increase quantum yield of CsPbBr3 perovskite owing to the enhanced binding of the carboxylic acid.11 The limited PLQY also illustrated that the addition of ligands cannot completely recover the optical performance of purified NCs. Then we also fabricated film 6 based on the stock of CsPbBr3 perovskite NC without purification, its PLQY is 80%. This is because the use of ODE without reactive groups would hinder photo-polymerization and increase the pinhole defects, which cannot well protect NCs from photo-oxidation. Considering surface ligands are crucial for the properties of NCs, Fourier Transform Infrared Spectroscopy (FTIR) was examined, as shown in Figure S6. It can be seen that supplement ligands can only partly recover the surface ligands of NCs, thus retaining excess ligands (both acid and amine) in the synthesis of perovskite NCs in LMA solvent is necessary for obtaining the best protection to perovskite NCs. In fact, the PLQY of the perovskite film (film 1) is indeed higher due to the presence of excessive ligands.11 In order to further understand the mechanism of PLQY change, the PL decay kinetics of these films were investigated, as shown in Figure 2c. The PL decay curves were fitted with triexponential decay functions, 6, 54-55and the average PL lifetimes (τavg) were shown in Table S1. It was observed that film 1 have longer average PL lifetime (47.41 ns) than other films: film 2 (12.84 ns), film 3 (11.48 ns), film 4 (20.05 ns), film 5 (23.14 ns), film 6 (36.91 ns). It can also be seen from Table S1 that the short-lifetime components of CsPbBr3 NCs polymer composite film


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decreased after adding ligands into film 2, suggesting that the deep surface hole traps decreased, which is consistent with previous literatures.54, 56-57 For films fabricated by stock solution, shortlifetime component of film 1 (17%) is less than that of film 6 (18%), indicating that film 1 have few deep surface hole traps. In addition, we also studied the recombination dynamics of these perovskite-polymer composite films, the radiative (kr) and nonradiative (knr) decay rates were obtained based on the PLQYs and the average lifetimes τavg (Table S2).58-59 Notably, among all the films, the nonradiative recombination rate knr of Film 1 is the lowest (0.0029 ns-1) and the kr/knr is the highest (6.14), indicating that our strategy can minimize the nonradiative pathways. Depending on the smaller short-lifetime component, longer τavg and larger kr/knr, it is reasonable to infer that the film 1 possess fewer trap states.

Figure 3. (a) Photostability curves indicated by the PLQYs of the films and (b) corresponding optical images of different CsPbBr3 polymer films under different illumination time (Note: film 1: LMA without post-processing; film 2: ODE with post-processing; film 3: ODE with postprocessing and adding OA; film 4: ODE with post-processing and adding OAm; film 5: ODE with post-processing and adding OA and OAm; film 6: ODE without post-processing).


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It is well known that light, heat, water, and oxygen can trigger and accelerate the degradation of perovskite NCs due to their ionic-crystal nature and dynamic coverage of the surface ligands.11 The photostability is the key performance index of perovskite polymer film in the practical application of photovoltaic and optoelectronic devices. The photostability of CsPbBr3 NCs-polymer composite films was tested in a photochemical reactor with a work current of 10 A. As shown in Figure 3a, the PLQY of film 2, film 3 and film 4 rapidly degraded and their photoluminescence completely disappeared after 60 h aging. The absolute PLQY of film 5 and film 6 dropped to 38% and 46% after illumination for 100 h, respectively. However, for the relatively insensitive light film 1 was relatively insensitive to light, and about 82% of absolute PLQY still remained after it has been exposed to the high intensity of illumination for 90 h under the ambient condition without the protection of inert gas, indicating its superior photostability compared with the conventional CsPbBr3-ODE polymer films. In addition, to measure moisture and heat stability, the film 1 was directly immersed in water and heated to 50oC. We found that under this condition, the PLQY and peak shape of PL spectra of film 1 was nearly no change as shown in Figure S7 and S8. To test the thermal stability of film 1, the film without gas protection was placed in drying oven at 60, 80, and 100 oC for 300 min, respectively. As shown in Figure S9, after heating at 60 oC for 300 min, the normalized PLQYs of films changed little. After heating at 80 and 100 oC for 5hours, the PLQY of film 1 decreased to 81.2% and 48.3%, respectively. These results further suggested that the choice of LMA as the solvent helps to form a denser polymer matrix, thereby preventing oxygen and moisture from reaching the NCs. The high PLQY and photostability of CsPbBr3 NCs polymer composite film have motivated us to investigate their potential applications in the light emitting device. As illustrated in Figure 4a, the CsPbBr3 NC-polymer composite film and red-emissive phosphor K2SiF6:Mn4+


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(KSF) with UV-cured adhesive on a blue LED were successively superimposed to produce a wide color gamut LCD backlight with a CIE color temperature of 9622 K. The luminous efficiency and chromaticity coordinates (CIE) of the LCD backlight were measured to be 135 lm·W-1, x=0.293, y=0.304, respectively. As shown in Figure 4b, a bright green photoluminescence emission peak was observed at 520 nm, which corresponded with the PL emission peak. The CIE chromaticity coordinates of the blue-chip, green CsPbBr3 NCs polymer composite film, and red KSF are exhibited in Figure 4c. An exceedingly wide color gamut of 115% relative to the National Television System Committee (NTSC) standard in CIE 1931 can be obtained by integrating our green-emitting CsPbBr3 NCs polymer composite film on a KSF/blue chip. We have continuously illuminated the devices at room temperature in the ambient conditions for 1000 hours, and the luminous efficiency and color gamut of the backlight were 135±10 lm·W-1, 115±0.6%, respectively, indicating the stability of the devices very high. It is worth to note that the commercially available TV displays are usually have a narrower color gamut of around 105% in the same color space with CdSe-based quantum dots.


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Figure 4. (a) Schematic illustration of the configuration of the wide color gamut LCD backlight with a CsPbBr3 NC-polymer composite film. (b) Emission spectrum of the constructed LCD using the backlight unit. (c) The color coordinates (gray dot) of the obtained LCD in CIE diagram with a wide color gamut of about 115% in the International Commission on Illumination (CIE) 1931 color space.


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Table 1. The comparison of CsPbBr3 NCs polymer composite film with other green perovskite polymer films. Samples MAPbBr3/PS CsPbBr3@PS fibers MAPbBr3/V18 MAPbBr3/PVDF CsPbBr3/PMMA MAPbCl0.36Br2.64/PBMA CsPbBr3/PMMA CsPbBr3/Ergo CsPbBr3/EVA CsPbBr3/LMA

PL peak (nm) 532 513 514 525 521 521 527 520 527 520

PLQY in films 48% 48% 56% 94.6% 62.2% 39.2% 45% 38% 40.5% 85-90%

Stability Water/heat water/thermal thermal Light/water air/water air/water Humidity of 70% air/water air/water light/water/thermal

Color gamut >100% ― ― 121% ― ― 105% 134% 92% 115%

Ref. 24 27 39 40 41 42 60 61 62 This work

In order to evaluate the performance of the as-fabricated CsPbBr3 NCs polymer composite films, we summarized the relevant figures-of-merit (FOM) reported in the literature for other green perovskite polymer films in Table 1. The PLQY of CsPbBr3 NCs polymer composite film is lower than that of MAPbBr3/PVDF film,40 but is much higher than those of MAPbCl0.36Br2.64/PBMA,42 MAPbBr3/V18,39 MAPbBr3/PS,24 CsPbBr3/PMMA,41,

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CsPbBr3/Ergo,61 CsPbBr3@PS fibers27

and CsPbBr3/EVA.62 Furthermore, CsPbBr3 NCs polymer composite film has wide color gamut due to its narrow FWHM. The combination of highly PLQY (85-90%) and wide color gamut (115%) make CsPbBr3 a promising material for display with room for improvement through further device engineering.

CONCLUSIONS To sum up, we have demonstrated that CsPbBr3 perovskite NCs were prepared with a polymerizable acrylic monomer lauryl methacrylate (LMA) as solvent by an improved hotinjection process. Then, the CsPbBr3 perovskite NC-LMA solution was directly combined with ACS Paragon Plus Environment

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oligomers and initiators through ultraviolet polymerization to prepare the high-quality NCpolymer composite films. Compared with traditional methods, perovskite NCs prepared with polymerizable acrylic monomer do not need tedious centrifugation, separated and dispersion. These NC-polymer films exhibit remarkable optical performance, such as PLQY as high as 8590% and FWHM as narrow as 20 nm. In addition, the as-synthesized perovskite NC-polymer composite films exhibit outstanding photostability under xenon lamp illumination, which is not available in the past. The prototype LCD device was constructed using the green emissive perovskite NC-polymer composite films and red emissive phosphors, KSF, as down-converters for blue LEDs in liquid crystal displays. Its color gamut was 115% in the International Commission on Illumination (CIE) 1931 color space, which was better than the commercially available CdSe quantum dots-based displays on the market, with a typical value of 105%. By a suitable choice of raw chemical sources and synthetic parameters, it is reasonable to predict that the current synthesis method for lead halide perovskite NCs prepared in acrylic monomers and the procedures for NCpolymer composite films can be extended to the preparation of other high-performance and stable perovskite NC-based light conversion films of LCD backlight.


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EXPERIMETAL METHODS Materials. Cesium carbonate (Cs2CO3, 99.9%, Aladdin), Oleic acid (OA, 90%, Aladdin, AR), oleylamine (OAm, 80-90%, Aladdin), Lauryl methacrylate (LMA, 90%, Aladdin), lead bromide (PbBr2, 99.9%, Aladdin), toluene (99.9%, Sinopharm Chemical Reagent Co., Ltd., China), 2,2Dimethoxy-2-phenylacetophenone (DMPA, 99%, Aladdin), Polyester polyurethane acrylates oligomer (45wt%, supplied by Thunway New Materials Technology Co., Ltd., Nanjing, China), isobornyl acrylate (IBOA, >85%, Nippon Shokubai Co., Ltd, Japan) were purchased and used without further purification. Synthesis of the Cesium-Oleate Precursor. Cesium carbonate (3.9 g), oleic acid (12 mL) and lauryl methacrylate (48 mL) were added to a 100 mL three-neck round-button flask and degassed for 10 min and then heated to 120oC under vigorous stirring and vacuum conditions for 60 min. Afterward, the mixture was heated to 150oC under nitrogen until the solution became clear. Csoleate was heated to 120°C before use because it often precipitates when cooled to RT. Synthesis of CsPbBr3 Nanocrystals in LMA. Oleic acid (10 mL), oleylamine (10 mL) and lauryl methacrylate (15 mL) were loaded into a 100 mL three-neck round-button flask and dried under vacuum for 30 min at 120oC to remove the moisture from the raw materials. Under nitrogen, 1.468 g of PbBr2 were added. The resulting mixture was heated to 150oC until the PbBr2 dissolved completely. The temperature was then elevated to180oC and 4 mL of as-prepared Cs-oleate precursor (0.4 M) was swiftly injected under vigorous stirring. After 10s, the reaction mixture was transferred into an ice bath. For fabrication of CsPbBr3 NCs in ODE, 15 mL lauryl methacrylate was substituted by octadecene of 15 mL and added into the flask.


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Isolation and purification of CsPbBr3 NCs in ODE: The crude solution was centrifuged at 8500 rpm for 10 min firstly. After that, the supernatant containing unreacted precursors were discarded. The precipitates were collected and washed twice with toluene to fabricate the films. Fabrication of the CsPbBr3 NC-Polymer Composite Films. The cooled CsPbBr3 perovskite NCs in LMA crude solution (200 mg), 2.5 g UV adhesive containing a certain amount of polyester polyurethane acrylates oligomer, monomer (IBOA) and initiator (DMPA) were first stirred and mixed fully, then coated onto a transparent PET substrate using a doctor-blade technique, finally a bright green emission polymer film with thickness of 160±15 µm was obtained by UV light polymerization for 5 min. The control films were processed using a similar process except addition of 50 μL oleic acid (film 2), oleylamine (film 3) or a mixture of OLA/OA (volume ratio=1:1, film 4) into UV adhesive. Fabrication of LCD backlights for Liquid Crystal Displays. As shown in Figure 4a, we use a configure of “on-surface” green perovskite NC-polymer film and “on-chip” red KSF excited by blue LEDs. The red emissive phosphor K2SiF6:Mn4+ (KSF) powder were mixed with a UV-cured adhesive, and placed on a blue-chip, this is due to the KSF’ high quantum efficiency and good stability especially under high temperature. Then, the green-emitting CsPbBr3 NC-polymer composite film was placed on the top of the light guide plate, and a diffusing film was placed above. Finally, a liquid crystal module was placed on the top of the films for next-step encapsulation. Characterization Details. Transmission electron microscopy (TEM) imaging was performed by a FEI Tecnai G2 F20 electron microscope operating at 200 kV. Scanning electron microscopy (SEM) was performed on a Hitachi S-3400N electron microscope operating at 20.0 kV. X-ray powder diffraction (XRD) was carried out by a Bruker AXS D8 X-ray diffractometer equipped


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with monochromatized Cu Kα radiation (λ = 1.5418 Å). Ultraviolet and visible absorption (UV−vis) spectra were measured by a Shimadzu UV-3600 plus spectrophotometer equipped with an integrating sphere under ambient conditions. The photoluminescence (PL) spectra were carried out with a Horiba PTI QuantaMaster 400 steady-state fluorescence system or with a homemade fiber fluorimeter system from Thorlabs operating under ambient conditions. The lifetimes of PL were detected by a Nikon Ni-U Microfluorescence Lifetime System (confotec MR200, SOL, Belarus) with a 375 nm picosecond laser and a time-correlated single-photon counting system at room temperature. X-ray photoelectron spectroscopy (XPS) measurements were performed using an achromatic Al Kα source (1486.6 eV) and a double pass cylindrical mirror analyzer (ULVACPHI 5000 Versa Probe). Fourier transform infrared spectroscopy (FTIR) was performed using a FTIR-650 spectrometer (Tianjin Gangdong). The light stability test of CsPbBr3 polymer films was performed under irradiation by a xenon lamp with a working current of 10 Ampere. The color gamut of the prototype LCD containing the green perovskite NC-polymer films is measured using a Konica Minolta Display Color analyzer (model: CA 310).


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ASSOCIATED CONTENT The PLQY and the fitting results of PL decay curves, additional UV and PL emission spectra of the control films, water stability characterization of the films and related PL spectra. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ⊥These authors contributed equally. ACKNOWLEDGMENT This work was supported by National Natural Science Foundation of China (Grant No. 51502130), Natural Science Foundation of Jiangsu Province (Grant No. BK20150581), Thousand Talents Program for Young Researchers, Shuangchuang Program of Jiangsu Province, and Jiangsu Key Laboratory for Nano Technology.


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Direct Hot-Injection Synthesis of Lead Halide Perovskite Nanocubes in

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