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Highly Stable and Luminescent Perovskite-Polymer Composites from a Convenient and Universal Strategy Yumeng Xin, Hongjie Zhao, and Jiuyang Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16442 • Publication Date (Web): 15 Jan 2018 Downloaded from http://pubs.acs.org on January 15, 2018

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ACS Applied Materials & Interfaces

Highly Stable and Luminescent Perovskite-Polymer Composites from a Convenient and Universal Strategy Yumeng Xin1, Hongjie Zhao2, Jiuyang Zhang1* 1

School of Chemistry and Chemical Engineering, Southeast University, Nanjing,

211189, PR China 2

School of Chemical Engineering & Technology, China University of Mining and

Technology, Xuzhou, 221116, PR China KEYWORDS: Perovskites, Polymer, Water resistant, Radical polymerization, In-situ reaction ABSTRACT: Extensive attention has been received in recent years for perovskite-polymer composites due to their combination of properties from polymers and perovskites. In this work, a convenient and universal strategy is reported to prepare cesium lead bromide (CsPbBr3) or organolead halide CH3NH3PbBr3 polymer composites. This technique integrates the formation of perovskite crystals and polymer matrix in a one-pot reaction, avoiding the tedious separation and preparation of perovskites. The method is universal for most of commercially available monomers and polymers, which has been verified in this report by poly(methyl methacrylate) (PMMA), poly(butyl methacrylate) (PBMA) and polystyrene (PS). The physical properties of the varied polymers lead to different luminescent properties and stabilities of the composites. None organic solvent is required during the preparation, indicating a green technique for the composites. Additionally, the resulted 1 ACS Paragon Plus Environment

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perovskite-polymer composites are extraordinarily stable, maintaining their quantum yield for more than one month in air. Based on the above properties, a prototype of white LED was successfully constructed with feasible color characters and narrow bandwidths.

Furthermore,

large

area

(Dimension:

10×

7

×

0.15

cm)

perovskite-polymer plates are easily prepared via the one-pot strategy, showing that the technique is ready for possible large-area optical devices. This work provides an efficient technique towards various kinds of perovskite-polymer composites for both scientific researches and future applications. ■INTRODUCTION

Perovskite has received lots of interest in the recent years due to their unique advantages in luminescent ability,1-3 narrow emission band width,4,5 and tunable emissive lifetimes6. Perovskite has been broadly applied in many devices, including photon detectors,7 solar cells8-12 and light emitting diodes (LEDs)13-15. However, the applications of perovskite are heavily hurdled by the stability of the perovskite crystals which are moisture- and air- sensitive.10,16-20 The practical use of perovskite usually required additionally protective techniques from the decomposition in daily environment. Development of the protective strategies have been successfully made in the past three years, such as protective coating,21-23 crosslinking of ligands16,18,24,25 and polymer blends9,17,19,26-28. The team of Tan group and Greenham group in 2016 has reported an effective method via coating a protective aluminum oxide around the perovskite crystals to fabricate stable and highly efficient LEDs.25 Meanwhile, in May 2017, Li group successfully utilized silica/alumina as inert shell to produce a luminescent and stable cesium lead bromide (CsPbBr3) composite.22 Crosslinking of the ligands is another kind of effective technique for protection. Francisco et al.

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reported the crosslinking of surface ligands by X-ray to enhance the stability of the perovskite.24 Grätzel group used phosphoric acid as new ligands to crosslink among perovskite crystals to build stable perovskite solar cells.18 In 2017, Sun et al. crosslinked styrenic ligand-perovskites to prepare highly stable perovskite composite for light emitting applications.16 Blending with commercially available polymers has been developed in recent years to provide the high stability for the perovskite.9,17,26-29 Compared with coating and ligand crosslinking, blending with polymers is relatively easier to handle. Besides the enhanced stability, polymers endow the perovskite with advantages in many aspects, such as convenient device fabrication, mechanical performance and enhanced luminescent properties.17,27,28 For example, recently (September 2016), the power conversion efficiency of the solar cell built on poly(methyl methacrylate) (PMMA) and perovskite has achieved as high as 22.1%.9 There have emerged a couple of reports since 2016 via utilization of different kinds of commercial polymers to enhance perovskite stability for advanced applications.17,26-29 Although fantastic applications are achieved, serious issues have been found from the method of blending polymers with perovskite. The physical blending with polymers is notorious for the heterogeneous phase aggregation.17,30-33 The large discrepancy in polarity between perovskite and polymers can lead to aggregation of the perovskite crystals, thus showing very low quantum yields.26,34,35 Another important issue is that perovskites usually should be prepared before blending.9,17,26 The preparation and storage of perovskites required careful handlings in protective conditions to optimize the property of the perovskite-polymer composite.36 The time-consuming blending process in air will inevitably result in the decomposition of perovskites by oxygen or moisture. This is more prominent when blending with high molecular weight



polymers, which needs very long time to dissolve the polymers. Additionally, considering the poor thermal stability of the perovskites, solvent blending of polymers and perovskites is more prevalent in the lab compared with melt blending.16,17,26,27,34 The organic solvent used for blends should be anhydrous to avoid any moisture in the composite.37,38 Large amount of such anhydrous organic solvent will be utilized to completely dissolve polymers and to avoid the aggregation of the inorganic crystals, which is not environmentally friendly and at high cost. Finally, because of the diverse physical properties of the different polymers,39 it is necessary to investigate individual conditions for specific polymers when blending with perovskite. There has been no universal technique for most commercially available polymers. Scheme 1. Illustration of one-pot strategy to prepare perovskite-polymer composites (CsPbBr3-polymer or CH3NH3PbBr3-polymer). a) Formation of perovskite crystals in bulk monomers. The photo taken under room light is for the emissive bulk styrene after adding precursors. b) The UV- or thermal- polymerized perovskite-polymer composites. Representative disks (under room and UV light) are shown in photos.


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In this work, we have developed a convenient technique to prepare perovskite-polymer composite: the formation of perovskite and polymer network is integrated in a one-pot reaction (Scheme 1), avoiding the redundant synthesis, separation and protection of the perovskites. Such technique is universal for most of commercially available monomers and polymers, such as PMMA, polystyrene (PS), and poly(butyl methacrylate) (PBMA). Composites of perovskites with different polymers are conveniently prepared and the effects of physical properties of polymers on perovskite-polymer composites are also investigated in details. Meanwhile, this method is a green technique without using organic solvents in the procedure: only tracing amount of dimethylformamide (DMF) was utilized to dissolve inorganic precursors. The as-prepared perovskite-polymer composite also showed many advantages over those previous reported perovskite composites, such as high stability, flexibility and excellent quantum yields (>60% in bulk polymers). The composite could maintain its quantum yield for more than one month in air, indicating the extraordinary stability. The diverse selections of polymers could provide flexible perovskite-polymer materials. Based on the above advantages, a prototype of white LED was successfully constructed with feasible color characters and narrow bandwidths. Finally, large area (10 × 7 × 0.15 cm) perovskite-polymer composites are conveniently prepared via the one-pot strategy, indicating that the technique is ready for possible large-area luminescent devices. We believe this strategy enables the construction of perovskite-polymer composites in a convenient and low cost avenue, which could definitely broaden the use of perovskite in polymer science.



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■RESULT AND DISCUSSION In 2015, Zhong group40 and Zeng group41 reported the synthesis of organic and inorganic halide perovskite, respectively, by precipitation and crystallization in organic toluene. Considering the similar structures between styrene and toluene, the styrene may be also suitable as a precipitation solvent to form perovskite crystals in bulk styrene. To verify the hypothesis, tracing amount (25µL) of precursor solution (DMF) of lead bromide (PbBr2) and cesium bromide (CsBr) was transferred into bulk styrene (1.5 mL) under vigorous stirring. Strong and green emissive solution was immediately observed, indicating the successful formation of CsPbBr3 crystals in styrene. This success sheds light on the one-pot strategy integrating the formation of perovskites and polymers in a single reaction without using organic solvents (except tracing amount of DMF). To

form

polymer-perovskite

(2,2'-Azobis(2-methylpropionitrile),

composite,

AIBN)

(Diphenyl(2,4,6-trimethylbenzoyl)phosphine

or

oxide,

thermal

initiators

UV-light

initiators

TPO)

is

needed

for

polymerization. Meanwhile, crosslinkers, divinylbenzene, is also pre-added into the bulk monomers before transferring precursor solution to improve the mechanical property of the composites. The bulk monomer solution was degassed by nitrogen to remove any oxygen for polymerization. Thermal initiation (70 ºC) was firstly tried to prepare perovskite-PS composites. A yellow and semi-transparent composite was finally obtained (Figure 1c5 and 1d5) after 12 hours polymerization. The crosslinker, divinylbenzene, leads the composite as hard thermosets under room temperature. Meanwhile, photo-polymerization via UV-light is a common method in polymer science. The photoinitiator, TPO, was irradiated by 365 nm UV light to initiate the polymerization. However, after 12 hours UV-exposure, the bulk styrene only turned 6 ACS Paragon Plus Environment

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into viscous liquids instead of rigid solids and yellow precipitates (aggregates of perovskite crystals) were observed in the bottom of the flask. This phenomenon was attributed by the slow radical polymerization rate of styrene,42-44 which aggregation was faster than polymerization.

Figure 1. a) Photos in room light (left) and in UV-illumination (middle) of the bulk MMA after adding tracing amount of precursor solution; the right photo is taken under UV-illumination of the perovskite-polymer composite after polymerization of the MMA. b) Illustration of one-pot strategy to prepare perovskite-polymer



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composites. Photos of transparent disks (diameter: 3 cm) from perovskite-polymer composite under ambient room light (c) and UV illumination (d). To address the issue, monomers with high radical polymerization rates are also applied in this strategy. Methacrylate monomers showed much faster polymerization kinetics compared with styrenic ones.42-44 MMA was chosen as bulk monomers to prepare perovskite-methacrylic polymer composites. Interestingly, although the much higher polarity of MMA compared with styrene may lead to the decomposition of perovskites, CsPbBr3 was still successfully synthesized in bulk MMA with strong emissive behavior of the solution (Figure 1a). A light yellow and strongly emissive (Figure 1c3 and 1d3) perovskite-PMMA composite was also easily obtained through thermal imitation via AIBN under 70 ºC for 12 hours. As expected, with much enhanced polymerization rate, UV-light initiation was successfully applied in bulk MMA monomers to prepare the composites. The resulted rigid materials exhibit green color (Figure 1c1 and 1d1), different from yellow samples from thermal initiation. As shown in Figure 1c3 and 1c5, thermal initiated CsPbBr3-PS composite have heavier yellow than it of CsPbBr3-PMMA. This difference should be contributed by the different radical polymerization rates and polarities of MMA and styrene.42-44 The ability of MMA to form polymer network in short time protects the perovskites from aggregation. Meanwhile, polar CsPbBr3 nanocrystals should have higher solubility to alleviate its aggregation in polar MMA compared with styrene. In sharp contrast, CsPbBr3-PMMA

from

UV-light

initiation

behaves

like

classical

green

perovskite-polymer materials (Figure 1c1 and 1d1). Heating in thermal initiation inevitably accelerates the aggregation and decomposition of perovskite crystals in solution, resulting in yellow materials. Although polar MMA may retard the aggregation of CsPbBr3 nanocrystals, the relatively high polarity of MMA is still 8 ACS Paragon Plus Environment

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unfavorable for the high stability of perovskites. A possible way to address this problem is to find another monomer with comparable polymerization rates but less polar than MMA. Butyl methacrylate (BMA) with long alkyl chains should meet these requirements. Following the same strategy, butyl methacrylate was selected to prepare CsPbBr3-PBMA. The resulted materials from thermal and UV-light initiation showed comparable or deeper green color (Figure 1c2 and 1d2) than those from PMMA, indicating the enhanced stability of CsPbBr3 in PBMA matrix. This conclusion was also supported by the following measured luminescent properties. Moreover,

the

organic

halide

perovskite-polymer

composite

(such

as

MAPbBr3-polymer) could be also synthesized by this strategy. After the injection of the precursor solution into the emissive bulk MMA, strong green emission was emerged immediately. It indicates that the MAPbBr3 crystals were formed in bulk MMA (Figure S1a1 and S1a2). And a green and strongly emissive MAPbBr3-PMMA composite (Figure S1b1 and S1b2) was also easily obtained through UV-light initiation via TPO under UV-light for 12 hours. This technique could be extended to blue perovskite-polymer composites (CsPbClBr2-PMMA) (Figure S2), but does not apply to red emissive samples (CsPbBr1.2I1.8 inorganic perovskite quantum dots). Although the CsPbBr1.2I1.8 perovskite quantum dots were formed in bulk monomers solution (MMA and BMA), the color of CsPbBr1.2I1.8 perovskite quantum dots is orange (Figure S2), indicating the damaged CsPbBr1.2I1.8 perovskite quantum dots possibly due to the high polarity of the monomer solution. We will further investigate how to prepare red emissive samples via this technique in future.



Figure 2. a) Normalized PL spectra of emissive bulk MMA solution and resulted CsPbBr3-PMMA composite. b) X-ray diffraction (XRD) patterns and c) transmission electron microscopy (TEM) image of CsPbBr3 crystals obtained from emissive bulk MMA solution. Peaks with stars symbols were matched with standard cubic CsPbBr3 (PDF) pattern, while rings labeled peaks were followed in patterns of orthorhombic phase. d) Cross-section scanning electron microscope (SEM) image for the resulted CsPbBr3-PMMA composite. After adding precursor, the emissive solution of bulk MMA, BMA and styrene showed narrow photoluminescence (PL) emission peaks at 520nm (Figure 2a and S3). The PL spectra of all bulk monomer solution have very narrow full width at the half-maximum (FWHM) in the range of 20 nm. The UV-vis absorption spectra of the above solutions exhibit single exciton absorption peaks ranging from 515 to 539 nm (Figure S4 and S5). The nanocrystals in bulk MMA solution were directly observed in TEM images (Figure 2c) with a size of 10-40 nm, similar as previous reports.26,41


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Typical scattering signals in X-ray diffraction (XRD) (Figure 2b) experiments are observed for CsPbBr3 crystals obtained from emissive bulk MMA solution. It is interesting that two types of CsPbBr3 (cubic and orthorhombic) crystals were formed in the MMA solution. The patterns of cubic (peaks at 22.4, 25.5, 30.3, 34.2 and 37.6)26,28,41 and orthorhombic (peaks at 27.1 and 28.2)45,46 phases are consistent with previous reports (Figure 2b). However, it is difficult for us at current stage to know how to regulate the ratio of the two phases during the polymerization and we will explore this topic in future. The same patterns were observed for the CsPbBr3 (or MAPbBr3) perovskite crystals obtained in bulk MMA solution and toluene (Figure S6 and S7), indicating that no MMA monomer incorporated into the crystals. Additionally, the absence of the signals of MMA monomer in the 1H NMR spectra of the perovskites (Figure S8) again confirms that MMA will not alter the structure of CsPbBr3 (or MAPbBr3) crystals. After polymerization, these crystals were well dispersed in polymer matrix as indicated by cross-section scanning electron microscope (SEM) image (Figure 2d). The CsPbBr3-PMMA, -PBMA and -PS composites showed photoluminescence (PL) emission peaks at 521 nm with narrow FWHM width, similar as their bulk solutions. Meanwhile, peaks at 723.9 eV and 737.8 eV (Cs 3d), 137.8 eV and 142.5 eV (Pb 4f), 67.6 eV(Br 3d) in X-ray photoelectron spectroscopy (Figure S9, S10 and S11) of the perovskite-PMMA composite clearly indicated the existence of Cs, Pb and Br elements, respectively.24 PL emission peaks of the composites were nearly the same as their corresponding solution with only 1 nm shift toward high wavelength (Figure 2a and S3) with very narrow FWHM values at 20 nm. Sharp contrast in photoluminescence quantum yields (PLQYs) was observed between composites prepared from thermal polymerization and UV polymerization. Thermal polymerized yellow CsPbBr3-polymer composites



have very low PLQYs at 7.3 % (PMMA), 9.2 % (PBMA) and 0.7 % (PS) (Table S2). As discussed above, heating during the thermal polymerization accelerates the aggregation of nanocrystals before the formation of polymer matrix. However, green CsPbBr3-polymer composites from UV polymerization dramatically improved the PLQYs of the bulk materials, reaching as high as 54.6 % (PMMA) and 62.2 % (PBMA) (Table S2) (As mentioned above, solid perovskite-PS composite was not able to be obtained from bulk styrene solution due to the low polymerization rate.). Such PLQYs were comparable or higher than many previously reported solid perovskite films (See Table S9),17,26,27,47,48 lying a solid foundation to conveniently fabricate luminescent devices. We should mention that increasing the loading content of perovskite crystals in polymers will also result in a decrease of PLQY of the composites due to the large sizes (10 ~ 150 nm) of perovskites by aggregation. Besides, the free radicals generated by TPO could have a mild interaction with perovskites in solution. One control experiment by mixing UV-radical initiators (TPO) with perovskites under UV-light (365 nm) for 1 h indicates that radicals lead a slightly decrease (7.5%) of the PLQYs in CsPbBr3 quantum dots (Table S3).

Figure 3. a) Time-resolved PL decays and fitting curves of emissive bulk MMA solution and the resulted CsPbBr3-PMMA composite. b) Time-resolved PL decays 12 ACS Paragon Plus Environment

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and fitting curves of emissive bulk styrene solutions and the resulted CsPbBr3-PS composite. c) Time-dependent PL intensity of CsPbBr3-PMMA and CsPbBr3-PBMA composites in air. d) Normalized PL spectra of CsPbBr3-PBMA composite before and after immersing in water for 30 days. e) Photos of CsPbBr3-PBMA composites in water taken under ambient room light and f) under UV-illumination. The average PL life time (τ1, τ2) before and after polymerization were also measured (Figure 3a, 3b, S14 and Table S4-S6) by time-resolved PL spectra. τ1 is attributed to the intrinsic recombination while τ2 comes from surface state recombination. Different monomers will have strong influence on PL life time (τ1, τ2) of the emissive monomer solution. Varied PL life time ((3.0 ns, 16.0 ns), (3.9 ns, 25.6 ns) and (5.2 ns, 37.9 ns)) were found for MMA, BMA and styrene solution, respectively. The resulted perovskite-polymer composite via thermal- or photopolymerization exhibited different trends in terms of PL life time. Figure 3a and 3b show that photo-polymerized materials have longer τ2 values than the original monomer solution, indicating the possibly decreased surface defects after polymerization.16 Additionally, the Auger lifetimes extracted from ultrafast transient absorption (TA) spectroscopy (Figure S15 and S16) were 701 ps and 763 ps for CsPbBr3-PMMA and CsPbBr3 perovskite in toluene, respectively, indicating similar Auger recombination for materials. On the other hand, thermal polymerized perovskite-PMMA composite has similar PL life time as original MMA solution. However, after thermal polymerization, the PL life time of styrene solution sharply decreased from (5.2 ns, 37.9 ns) to (4.5 ns, 17.8 ns) (Figure 3b), indicating the thermal destruction of the perovskite crystals. Air stability and water resistance are significantly important characters of perovskite composites for their practical applications as the photo voltaic and 13 ACS Paragon Plus Environment


optoelectronic devices. The instability of CsPbBr3 nanocrystals in air and moisture has been broadly reported. Embedding inorganic nanocrystals in polymers could effectively protect them from decomposition by air and moisture. The synthesized perovskite-polymer composites in this work exhibit outstanding stability in air and excellent water resistance. Figure 3c showed that the PL intensity of CsPbBr3-PMMA and CsPbBr3-PBMA could remain as high as 70% and 78% in air, respectively, for one month, showing the excellent stability in air. PBMA with long alkyl chains protects the structure of CsPbBr3 perovskite crystals and retain the size, crystallinity and optical properties of CsPbBr3 perovskite, leading better luminescent behaviors and higher stability than PMMA ones (Table S2, Figure 3c, Figure S17). While under water, the PL intensity of CsPbBr3-PMMA and CsPbBr3-PBMA could remain 54% and 56% for 48 hours (see Figure S17 for details), comparable with other reported moisture-stable perovskite-polymer composites.16 Moreover, after immersing in water for 30 days, no change in peak shape of PL spectra (Figure 3d) was observed, indicating high stability. Figure 3e and 3f showed the photos of the strongly emissive CsPbBr3-PBMA composite after immersing in water for one month. Samples still possesses the emissive properties. Thermal stability is another important character of perovskite composites for their practical applications. The PLQYs of the resulted CsPbBr3-PMMA composite and MAPbBr3-PMMA composite was reduced from 62.4% to 51.6% and from 49.8% to 46.5% after keeping at 80 ºC for 24 h in nitrogen atmosphere (Table S7). Compared with the resulted CsPbBr3-PMMA composite, the resulted MAPbBr3-PMMA composite have better thermal stability.


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Figure 4. a) Schematic diagram of the configuration of the prototype LED device. b) Photos of CsPbBr3-PMMA composite film (left) and KSF composite film (middle) under UV-illumination. The right photograph is for white LED. c) Emission spectrum of the constructed white LED. d) The color coordinates (gray dot) of obtained white LED in CIE diagram. Based on the above advantages, a prototype LED device was successfully fabricated by green emissive CsPbBr3-polymer composites. Figure 4a showed the construction of the white LED by stacking CsPbBr3-polymer films and red emissive rare earth phosphor (K2SiF6:Mn4+ (KSF)) composite on a commercial blue LED (454 nm). KSF are well-developed red-emitting phosphors with narrow FWHM (50 nm). The red emissive KSF composite film was fabricated by curving the mixture of KSF powder and UV cured adhesive between two slices of quartz plates. Figure 4b and Figure S18 showed the successful fabrication of the WLED by the prepared CsPbBr3-polymer composites, which was confirmed by the corresponding electroluminescence (EL) spectrum in Figure 4c. As shown in the CIE diagram of



Figure 4d and Figure S19, the color coordinate value of obtained white LED is (0.324, 0.321), very close to the standard coordinates of white light. The color temperature (CCT) of the white LED was achieved at 5955.42 K. The narrow FWHM (20 nm) and high PLQYs of the prepared CsPbBr3-PMMA composite in this work showing purer green color are vital for the potential application in the field of wide-color gamut display devices.

Figure 5. a) Representative engineering stress versus strain curves for the resulted CsPbBr3-PMMA, CsPbBr3-PBMA and CsPbBr3-PS composite. b) The photos of the resulted CsPbBr3-PBMA composite before and after stretching under UV-illumination. c). Photos are taken under UV-illumination of the CsPbBr3-PBMA rubbery cylinders. d). Photos in UV-illumination of the CsPbBr3-PMMA rectangular plate (10 × 7 × 0.15 cm). As mentioned above, the one-pot strategy is a convenient and universal technique to prepare perovskite-polymer composites. This technique enables the selection of versatile polymers, which endows the composites with extra advantages for broad applications. Figure 5a is the tensile tests of the perovskite-polymer composites. CsPbBr3-PMMA and CsPbBr3-PS exhibited like typical rigid resins with high moduli 16 ACS Paragon Plus Environment

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(320 MPa), tensile stress (380 MPa) but low breaking strains (~15%) due to the high glass transition temperature (Tg) of the PMMA (Tg: 105 ºC) and PS (Tg: 100 ºC). These rigid composites are feasible for engineering materials with requirement for high mechanical stress and moduli. Meanwhile, if BMA monomers are selected to form the composite, the low Tg (20 ºC) of PBMA endows the flexibility for CsPbBr3-PBMA with low tensile stress (1.1 MPa) but high strain (170%) (Figure 5a). The stretching of the composite will not alter the emissive properties as indicated in the photo of the CsPbBr3-PBMA before and after stretching under UV-illumination (Figure 5b). The stretched samples were obtained as indicated by Figure S20. Photos in Figure 5c are images of CsPbBr3-PBMA rubbery cylinders, which can be managed under room temperature for desired shapes. The flexible composite is potentially usually suitable for the daily and technology applications, such as flexible luminescent devices, optical films and advanced textiles. Furthermore, the convenience of the strategy is a very prominent advantage for perovskite-polymer composites over previously reported methods. Such facile technique provided an opportunity to prepare highly luminescent devices in large dimensions. As shown in Figure 5d, a green and strongly emissive 10× 7 cm rectangular CsPbBr3-PMMA plate with thickness of 1.5 mm was successfully and conveniently prepared without using any organic solvents (only tracing amount of precursor DMF), indicating that this strategy is very promising for future industrial applications.

■CONCLUSION

In summary, a convenient and universal technique was reported to prepare inorganic halide perovskite-polymer composites. This technique integrates the formation of 17 ACS Paragon Plus Environment


perovskite crystals and polymer matrix in a one-pot reaction, avoiding the tedious separation and preparation of perovskites. This strategy enables the selection of diverse commercially available monomers for the composites. CsPbBr3-PMMA, CsPbBr3-PBMA and CsPbBr3-PS composites were successfully obtained. The effects of different monomers on the final composites were discussed in details. The different behaviors of composites from thermal and photo-polymerization were also clearly observed and investigated. Perovskite-polymer composites from such convenient method were highly luminescent with excellent performance in PL spectra and high PLQYs (>60%) for bulk materials. Meanwhile, these composites are highly stable in air and resistant towards water. A white LED device was successfully prepared based on the green emissive perovskite-polymer composites with red emissive rare earth phosphors (K2SiF6:Mn4+ (KSF)). Additionally, the diverse selections of monomers endowed the composites with controlled mechanical properties, such as rigidity and flexibility. Importantly, this technique also enables the preparation of large area devices in a facile way. A large and strongly emissive plate of CsPbBr3-PMMA was successfully prepared without using organic solvents (only tracing amount of precursor DMF), indicating a promising method in future luminescent applications. We believe this strategy could lead the construction of perovskite-polymer composites in a convenient and low-cost avenue, which will definitely broaden the use of perovskite in polymer science. ■EXPERIMENTAL SECTION

Materials and Instrumentation Lead bromide (PbBr2) (99%), butyl methacrylate (BMA) (99%), styrene (≥99.5%), 2,2'-Azobis(2-methylpropionitrile) (AIBN) (98%) and methylamine solution (33 wt %


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in

methyl

alcohol)

were

purchased

from

Aladdin,

diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO) (≥98%) was purchased from Shanghai DiBai chemicals. cesium bromide (CsBr) (99.5%) and methyl methacrylate (MMA) (99%)

were purchased from Macklin, Oleic Acid (≥90%) was purchased

from Adams-beta, Oleylamine (90%) was purchased from Meryer, ethylene dimethacrylate (EDMA) (99%) and divinylbenzene (80%) were purchased from Energy Chemical, dimethyl formamide (DMF) was purchased from Sinopharm Chemical Reagent, hydrobromic acid (48 % in water) was purchased from Maya Reagent, and all the liquid reagents purified through alumina oxide columns before using. Blue LED (454 nm, 300 mA, 22 ~ 38V, 10 W) was purchased from Xiaodan Company. Photoluminescence (PL) spectra of emissive bulk monomer and the resulted CsPbBr3-polymer composites were collected with a FluoroMax-4 spectrometer (Horiba, Japan) at room temperature, and the electrochemluminescence (ECL) emission spectra were obtained at the same spectrometer by additional coupling with a CHI400E electrochemical workstation (CHI, USA). Ultraviolet and visible absorption (UV-vis) spectra were recorded with from Cary 100 (Agilent, Singapore) with a diffuse reflectance accessory and BaSO4 was used as the reference sample (100% reflectance). Transmission electron microscopy (TEM) was performed on FEI Tecnai G2 T20. Scanning electron microscope (SEM) images were recorded by FEI Nova Nano SEM 450. The lifetimes of PL were detected by Nikon Ni-U Microfluoresce Lifetime system (confotec MR200, SOL, Belarus) with a 374 nm picoseconds lasers. X-ray photoelectron spectroscopy (XPS) was performed using Thermo ESCALAB 250XI (American Thermo Fisher Scientific). X-ray diffraction (XRD) was measured with Ultima IV. Emission spectrum and CIE diagram were



measured on THEORLABS LED-Driver-LEDE1B. Tensile tests were performed on a SANS CMT4503 tensile tester. The displacement-speed of the tensile tester was controlled at 5.0 mm min−1. The reported results represented an average over at least two measurements on two identical samples. Thermogravimetric analysis (TGA) was measured on a TG 209F1 apparatus with a heating rate of 10 ºC min−1 under nitrogen atmosphere with a protective flow rate of 10 mL min−1 and a purge flow rate of 50 mL min−1. A Ti:sapphire regenerative amplifier (Libra, Coherent Inc.) was employed for TA spectroscopy measurements. For fs-resolved TA experiment, the pump-probe delay was enabled by a translation stage. An optical parametric amplifier (Oper A Solo, Coherent Inc.) pumped by the regenerative amplifier was used to generate the pump beam with tunable wavelengths. The probe beam is a broadband supercontinuum light source. The visible and infrared probe sources are generated by focusing a small portion of the femtosecond laser beam onto a sapphire plate or a YAG plate, respectively. The chirp of the supercontinuum probe was corrected with an error to be less than 100 fs over the whole spectral range. The TA signal was then analyzed by a high speed silicon/InGaAs CCD for visible/infrared detection with a monochromater (Acton 2358, Princeton Instrument) at 1 kHz enabled by a custom-built control board from Entwicklungsbuero Stresing. The angle between the polarized pump and probe beams was set at the magic angle. For ns TA spectroscopy, we used a frequency-doubled sub-nanosecond laser (Picolo AOT MOPA, Inno Las) at 400 nm to excite the samples. The laser was synchronized to the probe pulse with a desired delay by an electronic delay generator (SRS DG645, Stanford Research System). To extract auger lifetime, it was assumed that all the multiexcitons were decayed at 1500 ps. The amplitude of decay at 1500 ps was then normalized to the same value for all pump intensities. After normalization, the high-pump-intensity


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trace

whose

maximum

amplitude

was

twice

larger

than

that

of

the

lowest-pump-intensity trace was chosen for lifetime fitting. Exponential decay function with two decay times was used for fitting. Synthesis Synthesis of Cesium lead bromide perovskite nanocrystals (CsPbBr3 NCs) in emissive bulk MMA solution. In a universal synthesis of CsPbBr3 , PbBr2 (14.7 mg, 0.04 mmol) and CsBr (8.5 mg, 0.04 mmol) were dissolved in DMF (1 mL). Oleyl amine (50 µL) and Oleic acid (75 µL) were added to stabilize the precursor solution in sequence. Then, 50 µL of the precursor solution was quickly added into the mixture of solution with MMA (1.5 mL), EDMA (28 µL) and AIBN (2.45 mg) or TPO (5.19 mg) under stirring. Strong green emission was emerged immediately after the injection. All of experimental operations were performed at room temperature. Synthesis of Cesium lead bromide perovskite nanocrystals (CsPbBr3 NCs) in emissive BMA solution. In a universal synthesis of CsPbBr3 , PbBr2 (14.7 mg, 0.04 mmol) and CsBr (8.5 mg, 0.04 mmol) were dissolved in DMF (1 mL). Oleyl amine (50 µL) and Oleic acid (75 µL) were added to stabilize the precursor solution in sequence. Then, 50 µL of the precursor solution was quickly added into the mixture of solution with BMA (1.5 mL), EDMA (18 µL) and AIBN (1.55 mg) or TPO (3.57 mg) under stirring. Strong green emission was emerged immediately after the injection. All of experimental operations were performed at room temperature. Synthesis of Cesium lead bromide perovskite nanocrystals (CsPbBr3 NCs) in emissive bulk styrene solution. In a universal synthesis of CsPbBr3, PbBr2 (14.7 mg, 0.04 mmol) and CsBr (8.5 mg, 0.04 mmol) were dissolved in DMF (1 mL). Oleyl amine (50 µL) and Oleic acid (75 µL) were added to stabilize the precursor solution in sequence. Then, 50 µL of the precursor solution was quickly added into the mixture of



solution with styrene (1.5 mL), Divinylbenzene (18 µL) and AIBN (2.14 mg) or TPO (4.73 mg) under stirring. Strong green emission was emerged immediately after the injection. All of experimental operations were performed at room temperature. Fabrication of the CsPbBr3-polymer (PMMA, PBMA and PS) disks by thermal polymerization. Emissive bulk monomer (MMA, BMA and styrene) (0.5 mL) were placed into a flat-bottomed glass tube with a diameter of 20.0 mm and filled with nitrogen. The tube was heated to 75 ºC and kept at this temperature for 12 h. After being cooled to room temperature, disk samples were obtained. Fabrication of the CsPbBr3-PBMA rubbery cylinder by thermal polymerization. Emissive bulk BMA (4 mL) were placed into a test tube with a diameter of 8 mm and filled with nitrogen. The tube was heated to 75 ºC and kept at this temperature for 12 h. After being cooled to room temperature, a cylindrical sample was obtained. Fabrication of the CsPbBr3-polymer (PMMA, PBMA and PS) disks by UV-polymerization. CsPbBr3 PNCs/MMA composites (0.5 mL) were placed into a flat-bottomed glass tube with a diameter of 20.0 mm and filled with nitrogen. The tube was placed under the UV light (wavelength: 365 nm) for 12 h. After being cooled to room temperature, disk samples were obtained. Synthesis of methylammonium bromide (CH3NH3Br). 24 mL of methylamine solution (33 wt % in methyl alcohol) was diluted with 100 mL of absolute methyl alcohol in a 250 mL round-bottom flask. Under constant stirring, 8 mL hydrobromic acid (48 wt % in water) was added to the flask. After stirring for 2 h at room temperature, the solvent was evaporated. The obtained white solid was washed with dry diethyl ether and dried under vacuum (60 ºC, 6 h) for future use. Synthesis of methylammonium lead bromide nanoparticles (MAPbBr3 NPs) in emissive bulk MMA solution. CH3NH3Br (9 mg, 0.08 mmol) and PbBr2 (36.7 mg, 0.1


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mmol) were dissolved in DMF (5 mL). Oleyl amine (10 µL) and Oleic acid (250 µL) were added to stabilize the precursor solution in sequence. Then, 20 µL of the precursor solution was quickly added into the mixture of solution with MMA (1.5 mL), EDMA (28 µL) and TPO (5.19 mg) under stirring. Strong green emission was emerged immediately after the injection. All of experimental operations were performed at room temperature. Fabrication of the MAPbBr3 NPs/PMMA disks by UV-polymerization. MAPbBr3 NPs/MMA composites (0.5 mL) were placed into a flat-bottomed glass tube with a diameter of 20.0 mm and filled with nitrogen. The tube was placed under the UV light (wavelength: 365 nm) for 12 h. After being cooled to room temperature, disk samples were obtained. ■ASSOCIATED CONTENT

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Photos of MAPbBr3, CsPbClBr2 and CsPbBr1.2I1.8 in solution and polymer composite (S1 and S2); PL spectra of CsPbBr3 in bulk monomer solution and polymer composite (S3); absorption spectra of CsPbBr3 in bulk monomer solution and polymer composite (S4 and S5); XRD patterns of CsPbBr3 and MAPbBr3 crystals (S6 and S7); 1H NMR spectra of CsPbBr3 and MAPbBr3 perovskite crystals (S8); XPS patterns of CsPbBr3 perovskite crystals (S9, S10 and S11); SEM image for the resulted CsPbBr3-polymer composite (S12); TGA curves of CsPbBr3-polymer composite (S13); time-resolved PL decays curves of CsPbBr3 in bulk BMA and PBMA composite (S14); TA spectra of CsPbBr3 in



toluene and PMMA composite (S15 and S16); time-dependent PL intensity of CsPbBr3-polymer composites in water (S17); photos of blue LED and white LED (S18); CIE diagram of white LED (S19); photos of the resulted CsPbBr3-PBMA composite before and after stretching (S20), PLQYs of the resulted CsPbBr3-polymer composite (Table S1, S2, S3, S7 and S8); triexponential fitting results of emissive bulk monomer solution and the resulted CsPbBr3-polymer composite (Table S4, S5 and S6); comparison of this work with other current correlative research (Table S9) ■AUTHOR INFORMATION

Corresponding Authors *E-mail: [email protected] Notes The authors declare no competing financial interest. ■ACKNOWLEDGEMENTS

Funding for this work was provided by the National Natural Science Foundation of China (Grant No. 21504013 and 21774020) and the start-up in Southeast University (No. 1107047110).

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Highly Stable and Luminescent Perovskite ... - ACS Publications

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