Highly Stable and Luminescent PerovskiteâPolymer Composites from

Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 4971−4980

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Highly Stable and Luminescent Perovskite−Polymer Composites from a Convenient and Universal Strategy Yumeng Xin,† Hongjie Zhao,‡ and Jiuyang Zhang*,† †

School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, PR China School of Chemical Engineering & Technology, China University of Mining and Technology, Xuzhou 221116, PR China

‡

S Supporting Information *

ABSTRACT: Extensive attention has been received in recent years for perovskite−polymer composites because of their combination of properties from polymers and perovskites. In this work, a convenient and universal strategy is reported to prepare cesium lead bromide or organolead halide methylammonium bromide polymer composites. This technique integrates the formation of perovskite crystals and the polymer matrix in a one-pot reaction, avoiding the tedious separation and preparation of perovskites. The method is universal for most of the commercially available monomers and polymers, which has been verified in this report using poly(methyl methacrylate), poly(butyl methacrylate), and polystyrene. The physical properties of the varied polymers lead to different luminescent properties and stabilities of the composites. No organic solvent is required during the preparation, indicating a green technique for the composites. Additionally, the resulted perovskite−polymer composites are extraordinarily stable, maintaining their quantum yield for more than 1 month in air. On the basis of the above properties, a prototype of white light-emitting diodes 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 toward various kinds of perovskite−polymer composites for both scientific research studies and future applications. KEYWORDS: perovskites, polymer, water resistant, radical polymerization, one-pot reaction

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INTRODUCTION Perovskites have received lots of interest in the recent years because of their unique advantages in luminescent ability,1−3 narrow emission bandwidth,4,5 and tunable emissive lifetimes.6 Perovskites have been broadly applied in many devices, including photon detectors,7 solar cells,8−12 and light-emitting diodes (LEDs).13−15 However, the applications of perovskites are heavily hurdled by the stability of the perovskite crystals, which are moisture- and air-sensitive.10,16−20 The practical use of perovskites usually requires additionally protective techniques from decomposition in the daily environment. Development of the protective strategies has been successfully made in the past 3 years, such as protective coating,21−23 cross-linking of ligands,16,18,24,25 and polymer blends.9,17,19,26−28 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 an inert shell to produce a luminescent and stable cesium lead bromide (CsPbBr3) composite.22 Cross-linking of the ligands is another kind of effective technique for protection. Palazon et al. reported the © 2018 American Chemical Society

cross-linking of surface ligands by X-ray to enhance the stability of the perovskite.24 Grätzel group used phosphoric acid as a new ligand to cross-link among perovskite crystals to build stable perovskite solar cells.18 In 2017, Sun et al. cross-linked styrenic ligand−perovskites to prepare highly stable perovskite composites for light-emitting applications.16 Blending with commercially available polymers has been developed in recent years to provide high stability for perovskites.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 perovskites 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 a solar cell built on poly(methyl methacrylate) (PMMA) and perovskites has achieved as high as 22.1%.9 There have emerged a couple of reports since 2016 via utilization of different kinds of Received: October 29, 2017 Accepted: January 15, 2018 Published: January 15, 2018 4971

DOI: 10.1021/acsami.7b16442 ACS Appl. Mater. Interfaces 2018, 10, 4971−4980

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ACS Applied Materials & Interfaces

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) UV- or Thermal-Polymerized Perovskite−Polymer Composites. Representative Disks (under Room and UV Light) Are Shown in the Photos

in the procedure: only a trace amount of dimethylformamide (DMF) was utilized to dissolve inorganic precursors. The asprepared perovskite−polymer composite also showed many advantages over those previously 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 1 month in air, indicating its extraordinary stability. The diverse selections of polymers could provide flexible perovskite−polymer materials. On the basis of the above advantages, a prototype of white LEDs 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 perovskites in polymer science.

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 perovskites. The physical blending with polymers is notorious for its heterogeneous phase aggregation.17,30−33 The large discrepancy in polarity between perovskites 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 handling 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 highmolecular-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 A large amount of such anhydrous organic solvents will be utilized to completely dissolve polymers and to avoid the aggregation of the inorganic crystals, which is not environmentally friendly and is of high cost. Finally, because of the diverse physical properties of different polymers,39 it is necessary to investigate individual conditions for specific polymers when blending with perovskites. There has been no universal technique for most commercially available polymers. In this work, we have developed a convenient technique to prepare a perovskite−polymer composite: the formation of perovskites and a polymer network is integrated in a one-pot reaction (Scheme 1), avoiding the redundant synthesis, separation, and protection of the perovskites. Such a technique is universal for most of the 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 detail. Meanwhile, this method is a green technique without using organic solvents

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RESULTS AND DISCUSSION In 2015, Zhong group40 and Zeng group41 reported the synthesis of organic and inorganic halide perovskites, respectively, by precipitation and crystallization in organic toluene. Considering the similar structures of styrene and toluene, styrene may also be suitable as a precipitation solvent to form perovskite crystals in bulk styrene. To verify this hypothesis, a trace amount (25 μL) of the precursor solution (DMF) of lead bromide (PbBr2) and cesium bromide (CsBr) was transferred into bulk styrene (1.5 mL) under vigorous stirring. A 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 a trace amount of DMF). To form a polymer−perovskite composite, thermal initiator (2,2′-azobis(2-methylpropionitrile), AIBN) or UV-light initiator (diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide, TPO) is needed for polymerization. Meanwhile, cross-linker, divinylbenzene, is also preadded into the bulk monomers before transferring the precursor solution to improve the mechanical 4972


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Figure 1. (a) Photos under room light (left) and under UV illumination (middle) of bulk MMA after adding a trace amount of the precursor solution; the right photo is taken under UV illumination of the perovskite−polymer composite after the polymerization of MMA. (b) Illustration of a one-pot strategy to prepare perovskite−polymer composites. Photos of transparent disks (diameter: 3 cm) from the perovskite−polymer composite under ambient room light (c) and UV illumination (d).

enhanced polymerization rate, UV-light initiation was successfully applied in bulk MMA monomers to prepare the composites. The resulted rigid materials exhibited green color (Figure 1c1,d1), different from the yellow samples from thermal initiation. As shown in Figure 1c3,c5, thermal-initiated CsPbBr3−PS composites exhibit a heavier yellow color than CsPbBr3− PMMA composites. This difference was contributed by the different radical polymerization rates and polarities of MMA and styrene.42−44 The ability of MMA to form a polymer network in a short time protects the perovskites from aggregation. Meanwhile, polar CsPbBr3 nanocrystals (NCs) should have higher solubility to alleviate their aggregation in polar MMA compared with that in styrene. In sharp contrast, CsPbBr3−PMMA from UV-light initiation behaves similar to the classical green perovskite−polymer materials (Figure 1c1,d1). 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 NCs, the relatively high polarity of MMA is still unfavorable for the high stability of perovskites. A possible way to address this problem is to find another monomer with comparable polymerization rates but with less polarity than MMA. Butyl methacrylate (BMA) with long alkyl chains should meet these requirements. Following the same strategy, BMA was selected to prepare CsPbBr3− PBMA. The resulted materials from thermal and UV-light initiations showed a comparable or deeper green color (Figure 1c2,d2) than those from PMMA, indicating the enhanced

property of the composites. The bulk monomer solution was degassed by nitrogen to remove any oxygen for polymerization. Thermal initiation (70 °C) was first tried to prepare perovskite−PS composites. A yellow and semitransparent composite was finally obtained (Figure 1c5,d5) after 12 h of polymerization. The cross-linker, divinylbenzene, turns the composite into a hard thermoset under room temperature. Meanwhile, photopolymerization 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 h of UV exposure, bulk styrene only turned into a viscous liquid instead of a rigid solid and yellow precipitates (aggregates of perovskite crystals) were observed in the bottom of the flask. This phenomenon was attributed to the slow radical polymerization rate of styrene,42−44 in which aggregation was faster than polymerization. To address this 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 Methyl methacrylate (MMA) was chosen as a bulk monomer to prepare perovskite−methacrylic polymer composites. Interestingly, although the much higher polarity of MMA compared with that of 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,d3) perovskite−PMMA composite was also easily obtained through thermal initiation via AIBN under 70 °C for 12 h. As expected, with a much 4973


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Figure 2. (a) Normalized photoluminescence (PL) spectra of the emissive bulk MMA solution and the resulted CsPbBr3−PMMA composite. (b) Xray diffraction (XRD) patterns and (c) transmission electron microscopy (TEM) image of CsPbBr3 crystals obtained from the emissive bulk MMA solution. Peaks with star symbols were matched with the standard cubic CsPbBr3 (PDF) pattern, whereas ring-labeled peaks were followed in the patterns of the orthorhombic phase. (d) Cross-sectional scanning electron microscopy (SEM) image for the resulted CsPbBr3−PMMA composite.

stability of CsPbBr3 in the 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 also be synthesized by this strategy. After the injection of the precursor solution into the emissive bulk MMA, strong green emission was observed immediately. This indicates that the MAPbBr3 crystals were formed in bulk MMA (Figure S1a1,a2). Also, a green and strongly emissive MAPbBr3−PMMA composite (Figure S1b1,b2) was easily obtained through the UV-light initiation via TPO under UV light for 12 h. 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 monomer solutions (MMA and BMA), the CsPbBr1.2I1.8 perovskite quantum dots exhibited orange color (Figure S2), indicating damaged CsPbBr1.2I1.8 perovskite quantum dots that are 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. After adding the precursor, the emissive solution of bulk MMA, BMA, and styrene showed narrow PL emission peaks at 520 nm (Figures 2a and S3). The PL spectra of all bulk monomer solutions have a very narrow full width at halfmaximum (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 (Figures S4 and S5). The NCs in the bulk MMA solution were directly observed in TEM images (Figure 2c) with a size of 10−40 nm, similar to previous reports.26,41 Typical scattering signals in XRD (Figure 2b) experiments are observed for CsPbBr3

crystals obtained from the 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 the previous reports (Figure 2b). However, it is difficult for us at the current stage to know how to regulate the ratio of the two phases during polymerization, and we will explore this topic in future. The same patterns were observed for the CsPbBr3 (or MAPbBr3) perovskite crystals obtained in the bulk MMA solution and toluene (Figures S6 and S7), indicating that no MMA monomer is 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 the CsPbBr3 (or MAPbBr3) crystals. After polymerization, these crystals were well-dispersed in the polymer matrix, as indicated by the cross-sectional SEM image (Figure 2d). The CsPbBr3−PMMA, CsPbBr3−PBMA, and CsPbBr3−PS composites showed PL emission peaks at 521 nm with narrow fwhm, similar to their bulk solutions. Meanwhile, the peaks at 723.9 and 737.8 eV (Cs 3d), 137.8 and 142.5 eV (Pb 4f), and 67.6 eV (Br 3d) in X-ray photoelectron spectroscopy (XPS) (Figures S9−S11) of the perovskite−PMMA composite clearly indicated the existence of Cs, Pb, and Br elements, respectively.24 The PL emission peaks of the composites were nearly the same as their corresponding solutions with only 1 nm shift toward high wavelength (Figures 2a and S3) with very narrow fwhm values at 20 nm. A sharp contrast in PL quantum yields (PLQYs) was observed between the composites prepared from thermal polymerization and UV polymerization. Thermal-polymerized yellow CsPbBr3−polymer composites have very low PLQYs at 7.3% (PMMA), 9.2% 4974


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Figure 3. (a) Time-resolved PL decays and fitting curves of the emissive bulk MMA solution and the resulted CsPbBr3−PMMA composite. (b) Time-resolved PL decays 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 composites 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.

transient absorption (TA) spectroscopy (Figures S15 and S16) were 701 and 763 ps for CsPbBr3−PMMA and CsPbBr3 perovskites in toluene, respectively, indicating similar Auger recombination for the materials. On the other hand, the thermal-polymerized perovskite−PMMA composite has a PL lifetime similar to the original MMA solution. However, after thermal polymerization, the PL lifetime of the styrene solution sharply decreased from (5.2, 37.9 ns) to (4.5, 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 photovoltaic and optoelectronic devices. The instability of CsPbBr3 NCs in air and moisture has been broadly reported. Embedding the inorganic NCs 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 shows that the PL intensity of CsPbBr3− PMMA and CsPbBr3−PBMA could remain as high as 70 and 78% in air, respectively, for 1 month, showing the excellent stability in air. PBMA with long alkyl chains protects the structure of CsPbBr3 perovskite crystals and retains the size, crystallinity, and optical properties of the CsPbBr3 perovskite, leading to better luminescent behaviors and higher stability than PMMA (Table S2 and Figures 3c and S17). While under water, the PL intensity of CsPbBr3−PMMA and CsPbBr3− PBMA could remain 54 and 56%, respectively, for 48 h (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 the PL spectra (Figure 3d) was observed, indicating high stability. Figure 3e,f shows the photos of the strongly emissive CsPbBr3−PBMA composite after immersing in water for 1 month. The sample still possesses the emissive properties. Thermal stability is another important character of perovskite

(PBMA), and 0.7% (PS) (Table S2). As discussed above, heating during thermal polymerization accelerates the aggregation of NCs before the formation of the 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, a solid perovskite− PS composite was not able to be obtained from the bulk styrene solution because of 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 because of 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 to a slight decrease (7.5%) of the PLQYs in the CsPbBr3 quantum dots (Table S3). The average PL lifetime (τ1, τ2) before and after polymerization was also measured (Figures 3a,b and S14 and Tables S4−S6) by time-resolved PL spectra. τ1 is attributed to the intrinsic recombination, whereas τ2 comes from the surfacestate recombination. Different monomers will have a strong influence on the PL lifetime (τ1, τ2) of the emissive monomer solution. Varied PL lifetimes ((3.0, 16.0 ns), (3.9, 25.6 ns), and (5.2, 37.9 ns)) were found for MMA, BMA, and styrene solutions, respectively. The resulted perovskite−polymer composite via thermal polymerization or photopolymerization exhibited different trends in terms of the PL lifetime. Figure 3a,b shows that photopolymerized 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 4975


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Figure 4. (a) Schematic diagram of the configuration of the prototype LED device. (b) Photos of the CsPbBr3−PMMA composite film (left) and K2SiF6−Mn4+ (KSF) composite film (middle) under UV illumination. The right photograph is for the white LED. (c) Emission spectrum of the constructed white LED. (d) Color coordinates (gray dot) of the obtained white LED in Commission Internationale de l’Elcairage (CIE) diagram.

Figure 5. (a) Representative engineering stress vs strain curves for the resulted CsPbBr3−PMMA, CsPbBr3−PBMA, and CsPbBr3−PS composites. (b) 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 of the CsPbBr3−PMMA rectangular plate (10 × 7 × 0.15 cm) under UV illumination.

corresponding electroluminescence spectrum in Figure 4c. As shown in the CIE diagram of Figures 4d and S19, the color coordinate value of the obtained white LED is (0.324, 0.321), very close to the standard coordinates of white light. The color temperature 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 their potential application in the field of widecolor-gamut display devices. 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 shows the tensile tests of the perovskite−polymer composites. CsPbBr 3 −PMMA and CsPbBr3−PS exhibited typical rigid resins with high moduli (320 MPa) and tensile stress (380 MPa) but low breaking strains (∼15%) because of the high glass-transition temperature (Tg) of PMMA (Tg: 105 °C) and PS (Tg: 100 °C). These rigid

composites for their practical applications. The PLQYs of the resulted CsPbBr3−PMMA composite and MAPbBr3−PMMA composite were reduced from 62.4 to 51.6% and from 49.8 to 46.5%, respectively, after being maintained at 80 °C for 24 h in a nitrogen atmosphere (Table S7). Compared with the resulted CsPbBr3−PMMA composite, the resulted MAPbBr3−PMMA composite has better thermal stability. On the basis of the above advantages, a prototype LED device was successfully fabricated by green emissive CsPbBr3− polymer composites. Figure 4a shows the construction of the white LED by stacking CsPbBr3−polymer films and red emissive rare-earth phosphor (KSF) composites on a commercial blue LED (454 nm). KSFs are well-developed red-emitting phosphors with narrow fwhm (50 nm). The red emissive KSF composite film was fabricated by curving the mixture of the KSF powder and UV-cured adhesive between two slices of quartz plates. Figures 4b and S18 show the successful fabrication of the white LED by the prepared CsPbBr3−polymer composites, which was confirmed by the 4976


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purchased from Shanghai DiBai Chemicals. CsBr (99.5%) and 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, DMF was purchased from Sinopharm Chemical Reagent, and hydrobromic acid (48% in water) was purchased from Maya Reagent, and all the liquid reagents were purified through alumina oxide columns before using. Blue LED (454 nm, 300 mA, 22−38 V, 10 W) was purchased from Xiaodan Company. The PL spectra of the emissive bulk monomer and the resulted CsPbBr3−polymer composites were collected with a FluoroMax-4 spectrometer (Horiba, Japan) at room temperature, and the electrochemiluminescence emission spectra were obtained using the same spectrometer by additional coupling with a CHI400E electrochemical workstation (CHI, USA). UV−vis spectra were recorded with a spectrometer (Cary 100, Agilent, Singapore) equipped with a diffuse reflectance accessory, and BaSO4 was used as the reference sample (100% reflectance). TEM images were recorded on an FEI Tecnai G2 T20. SEM images were recorded by an FEI Nova NanoSEM 450. The lifetimes of PL were detected by a Nikon Ni-U Microfluorescence Lifetime system (Confotec MR200, SOL, Belarus) with a 374 nm picosecond laser. XPS was performed using a Thermo ESCALAB 250XI (American Thermo Fisher Scientific). XRD was measured with an Ultima IV. The 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 conducted on a TG 209F1 apparatus with a heating rate of 10 °C min−1 under a 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 the fs-resolved TA experiment, the pump−probe delay was enabled by a translation stage. An optical parametric amplifier (Opera 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 monochromator (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 the ns TA spectroscopy, we used a frequency-doubled subnanosecond 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 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 CsPbBr3 Perovskite 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). Oleylamine (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 the solution with MMA (1.5 mL), EDMA (28 μL), and AIBN (2.45 mg) or TPO (5.19 mg) under stirring. A strong green emission was observed immediately after the injection. All of the experimental operations were performed at room temperature.

composites are feasible for engineering materials with the requirement for high mechanical stress and moduli. Meanwhile, if BMA monomers are selected to form the composite, the low Tg (20 °C) 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 CsPbBr3− PBMA before and after stretching under UV illumination (Figure 5b). The stretched samples were obtained as indicated in 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 usually potentially suitable for 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 a 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 a thickness of 1.5 mm was successfully and conveniently prepared without using any organic solvents (only a trace amount of the precursor DMF), indicating that this strategy is very promising for future industrial applications.

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CONCLUSIONS In summary, a convenient and universal technique was reported to prepare inorganic halide perovskite−polymer composites. This technique integrates the formation of perovskite crystals and a 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 detail. The different behaviors of the composites prepared from thermal polymerization and photopolymerization were also clearly observed and investigated. Perovskite−polymer composites from such a convenient method were highly luminescent with excellent performance in the PL spectra and high PLQYs (>60%) for bulk materials. Meanwhile, these composites are highly stable in air and resistant toward water. A white LED device was successfully prepared based on the green emissive perovskite−polymer composites with red emissive rare-earth phosphors (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 a trace amount of the precursor DMF), indicating a promising method in future luminescent applications. We believe this strategy could lead to the construction of perovskite−polymer composites in a convenient and low-cost avenue, which will definitely broaden the use of perovskites in polymer science.

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EXPERIMENTAL SECTION

Materials and Instrumentation. PbBr2 (99%), BMA (99%), styrene (≥99.5%), AIBN (98%), and methylamine solution (33 wt % in methyl alcohol) were purchased from Aladdin. TPO (≥98%) was 4977


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ACS Applied Materials & Interfaces Synthesis of CsPbBr3 Perovskite 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). Oleylamine (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 the solution with BMA (1.5 mL), EDMA (18 μL), and AIBN (1.55 mg) or TPO (3.57 mg) under stirring. A strong green emission was observed immediately after the injection. All of the experimental operations were performed at room temperature. Synthesis of CsPbBr3 Perovskite 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). Oleylamine (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 the solution with styrene (1.5 mL), divinylbenzene (18 μL), and AIBN (2.14 mg) or TPO (4.73 mg) under stirring. A strong green emission was observed immediately after the injection. All of the experimental operations were performed at room temperature. Fabrication of the CsPbBr3−Polymer (PMMA, PBMA, and PS) Disks by Thermal Polymerization. Emissive bulk monomers (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 °C and maintained 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) was placed into a test tube with a diameter of 8 mm and filled with nitrogen. The tube was heated to 75 °C and maintained 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 perovskite NCs/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). Methylamine solution (24 mL, 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 of 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 °C, 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 mmol) were dissolved in DMF (5 mL). Oleylamine (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 the solution with MMA (1.5 mL), EDMA (28 μL), and TPO (5.19 mg) under stirring. A strong green emission was observed immediately after the injection. All of the 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.

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solution and polymer composites; XRD patterns of CsPbBr3 and MAPbBr3 crystals; 1H NMR spectra of CsPbBr3 and MAPbBr3 perovskite crystals; XPS patterns of CsPbBr3 perovskite crystals; SEM image for the resulted CsPbBr3−polymer composite; TGA curves of the CsPbBr3−polymer composite; time-resolved PL decay curves of CsPbBr3 in the bulk BMA and PBMA composites; TA spectra of CsPbBr3 in toluene and PMMA composites; time-dependent PL intensity of CsPbBr3−polymer composites in water; photos of blue and white LEDs; CIE diagram of white LED; photos of the resulted CsPbBr3−PBMA composite before and after stretching; PLQYs of the resulted CsPbBr3−polymer composite; triexponential fitting results of the emissive bulk monomer solution and the resulted CsPbBr3− polymer composite; and comparison of this work with other current correlative research studies (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jiuyang Zhang: 0000-0002-9501-5381 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We appreciated the great help from Prof. Zhengtao Deng and Prof. Wei Shen from Nanjing University on the PL measurement. Funding for this work was provided by the National Natural Science Foundation of China (grant nos. 21504013 and 21774020) and the startup in Southeast University (no. 1107047110).

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b16442. Photos of MAPbBr3, CsPbClBr2, and CsPbBr1.2I1.8 in solution and polymer composites; PL spectra of CsPbBr3 in the bulk monomer solution and polymer composites; absorption spectra of CsPbBr3 in the bulk monomer 4978


Research Article

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