Ultrastable, Deformable, and Stretchable Luminescent Organic

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Applications of Polymer, Composite, and Coating Materials

Ultrastable, Deformable, and Stretchable Luminescent Organic–Inorganic Perovskite Nanocrystal-Polymer Composites for 3D Printing and White Light-Emitting Diodes Ching-Lan Tai, Wei-Li Hong, Yi-Tong Kuo, Che-Yu Chang, Mu-Chun Niu, Mahesh Karupathevar Ponnusamythevar Ochathevar, Ching-Ling Hsu, Sheng-Fu Horng, and Yu-Chiang Chao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b06248 • Publication Date (Web): 25 Jul 2019 Downloaded from pubs.acs.org on July 25, 2019

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ACS Applied Materials & Interfaces

Ultrastable, Deformable, and Stretchable Luminescent Organic–Inorganic Perovskite Nanocrystal-Polymer Composites for 3D Printing and White Light-Emitting Diodes Ching-Lan Tai,† Wei-Li Hong,‡ Yi-Tong Kuo,† Che-Yu Chang,† Mu-Chun Niu,⊥ Mahesh Karupathevar Ponnusamythevar Ochathevar,⊥ Ching-Ling Hsu,† Sheng-Fu Horng,‡ and Yu-

Chiang Chao*,⊥

†Department

of Physics, Chung Yuan Christian University, Chung-Li, Taiwan 32023,

R.O.C. ‡Institute

of Electronics Engineering, National Tsing Hua University, Hsinchu, Taiwan

300, R.O.C. ⊥Department

of Physics, National Taiwan Normal University, Taipei, Taiwan 11677,

R.O.C.

ACS Paragon Plus Environment

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KEYWORDS perovskite; nanocrystals; polymer; composites; light-emitting diodes

ABSTRACT

Organic–inorganic perovskite nanocrystals with excellent optoelectronic properties have been utilized in various applications, despite their stability issue. The perovskite materials are sensitive to environments such as polar solvents, moisture, and heat. Thus, they are not used for extrusion 3D printing, as it is usually conducted in the ambient environment and requires heating to liquefy the printed materials. In this work, 11 thermoplastic polymers conventionally used for extrusion 3D printing were investigated to test their capability as protective encapsulation materials for perovskite nanocrystals. Three of them exhibited good protective properties, and one (polycaprolactone, PCL) of these three could be blended with perovskite nanocrystals to form perovskite nanocrystal-PCL composites, which were deformable and stretchable once heated. Because of the low melting point of PCL, the perovskite nanocrystals maintained their optical properties after 3D printing, and the printed objects were still having fluorescent behavior.


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Moreover, fluorescent micrometer-sized fibers based on the perovskite nanocrystal-PCL composites could also be simply prepared using cotton candy makers. Perovskite nanocrystal-PCL composite films with different emission wavelengths were incorporated with a blue light-emitting diodes (LED) to realize white LEDs with Commission Internationale de l'Éclairage chromaticity coordinates of (0.33, 0.33).

INTRODUCTION Solution-processable organometal halide perovskite semiconductors have shown tremendous possibilities in the fields of solar cells,1-3 optical sensors,4-6 LEDs,7-9 and lasers.10-12 The intrinsic properties of perovskites, such as high absorption coefficients, long carrier diffusion lengths, defect tolerance, and flexible band-gap tuning, make them revolutionary materials for photovoltaic technology. Moreover, because of the benefits of perovskite quantum dots (QDs), which include narrow spectral line widths, high quantum yield, and composition and size-tunable band gaps, LEDs based on perovskite QDs have attracted significant attention and have shown excellent performance.13-15 However, perovskite QDs have a serious stability issue: they are prone to


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degrade once exposed to moisture and oxygen under illumination.16 Therefore, surface passivation is critical to slow down their degradation and protect them from the environment. To enhance the stability of perovskite QDs, many passivation methods have been demonstrated, including the treatment of the QDs surface with ligands17-24 and the embedment of QDs into inorganic25-35 or organic36-42 hosts. Researchers have demonstrated that the stability of perovskite QDs can be improved dramatically through polymer encapsulation,36-42 even though the perovskite QDpolymer composite films are stored in water.36,38,40,41,42 These perovskite QD-polymer composite films have also been used as color converters to realize white LEDs.37-42 Extrusion 3D printing based on polymers in filament form is a versatile technology that allows the production of products without the need for machining or molding.43-45 The filaments are made from a variety of thermoplastics. The filament is loaded into the extrusion head of the 3D printer, which heats the filament to liquefy it before printing. Recently, organic roomtemperature phosphorescent luminogens46 and fluorescent QDs47 have been added as additives into 3D printing materials. However, the thermoplastics suitable as protective encapsulation materials for perovskite QDs in order to enable 3D printing have not been fully explored. In this work, 11 thermoplastics that are widely used as 3D printing filaments were tested as protective encapsulation materials for HC(NH2)2PbBr3 (FAPbBr3) and FAPb(Br0.36I0.63)3 perovskite nanocrystals. Experimental procedures for the fabrication of composites in various form and for the applications are shown in Scheme S1. Eight of these filaments could not be dissolved completely within 2 hours in the toluene solution of perovskite nanocrystals; of these eight, two, which are polylactide (PLA) and poly(methyl methacrylate) (PMMA), exhibited a good protective property for the perovskite nanocrystals that diffused into the filaments. Of the other three filaments, which could be dissolved completely within 2 hours in the toluene solution of perovskite


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nanocrystals, one, polycaprolactone (PCL), exhibited an excellent protective property for perovskite nanocrystals. The FAPbBr3-PCL composites exhibited fluorescence under UV irradiation, even in boiling water. Upon heating, the FAPbBr3-PCL composite films were deformable and stretchable. When perovskite nanocrystal-polymer composite filaments were used as 3D printing materials to fabricate some objects, only the objects printed from the perovskite nanocrystal-PCL filaments were fluorescent, because of the low melting point of PCL. This is the first time that perovskite nanocrystals have been incorporated into 3D printing materials and used to realize fluorescent objects by 3D printing. The operation temperature of the 3D printer for other filaments was too high, which degrades the perovskite nanocrystals and leads to non-fluorescent objects. Micrometer-sized fluorescent fibers can also be prepared from FAPbBr3-PCL composites by a cotton candy machine. Finally, FAPbBr3-PCL and FAPb(Br0.36I0.63)3-PCL composite films were utilized as down-converters to realize white LEDs with a blue LED as the excitation source.

RESULTS AND DISCUSSION The FAPbBr3 and FAPb(Br0.36I0.63)3 nanocrystals were prepared using a method modified from the literature.39 The detailed preparation procedures can be found in the Experimental Section. In summary, FABr, PbBr2, oleic acid, and octylamine were first prepared in dimethylformamide (DMF), and then the precursor was injected into a vigorously stirred toluene. After centrifugation at 3500 rpm for 10 min to discard the


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precipitates, the FAPbBr3 nanocrystals were obtained and used in the following experiments. The synthesis of FAPb(Br0.36I0.63)3 nanocrystals was similar to that of FAPbBr3 nanocrystals, but the precursor was prepared from FABr, FAI, PbBr2, PbI2, oleic acid, and octylamine.

The transmission electron microscopy (TEM) images of the FAPbBr3 and FAPb(Br0.36I0.63)3 nanocrystals are shown in Figures 1a, 1b and Figure S1. The highresolution high-angle annular dark field scanning TEM (HAADF-STEM) images are also shown in Figure S1b, S1f, S1h, S1l and S1n. The crystalline lattice planes of these nanocrystals can be resolved. The sizes of the FAPbBr3 and FAPb(Br0.36I0.63)3 nanocrystals were estimated to be about 5~8 nm and 7~10 nm from the bright-field TEM images, respectively. From TEM images with low magnification (Figures S1a, S1b, S1k and S1l), the nanocrystals on lacey-carbon grids shown narrow distributions in size and shape, indicating a successful centrifugation process which removes large crystals, such as nanoplatelets, nanorods and nanowires, as precipitates at the bottom of the centrifugation tube. The perovskite nanocrystals in the supernatant show an irregular


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sphere shape, which is consistent with previous report.48 It has been demonstrated that the organometallic lead halide quantum dots usually show an irregular sphere shape, and only inorganic perovskite quantum dots show perfect cubic shape.48 Besides, we noticed that, after finding a relatively larger nanocrystals with cubic-like shape when TEM is at low magnification, the shape and orientation of the nanocrystals changed with time when we zoom in trying to get a closer look at the nanocrystals. This situation is even worse when the magnification is extremely high. For instance, Figure S1c shows a cubic-like FAPbBr3 nanocrystals with relatively larger size, the shape and orientation of the nanocrystals changed in just few seconds as shown in Figure S1d. This result indicates that perovskite nanocrystals are sensitive to electron beam irradiation.

The absorption and photoluminescence (PL) spectra of the FAPbBr3 and FAPb(Br0.36I0.63)3 nanocrystals are shown in Figure 1c and 1d, respectively. The photographs of these solutions under UV light are shown as insets. The shape of absorption spectra is clear and clean like previous reports.49 The band gaps of FAPbBr3 and FAPb(Br0.36I0.63)3 nanocrystals were roughly estimated to be 2.31 eV and 1.83 eV


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from the absorption spectra, respectively (Figure S2). The PL spectra of these nanocrystals in solution showed an emission peak at 531 and 669 nm for FAPbBr3 and FAPb(Br0.36I0.63)3 nanocrystals, respectively, which agrees with the band gap estimated from the absorption spectra. Such a property of band-gap tuning by modulating the halide composition is similar to that reported in the previous studies in which MASnX3, MAPbX3, CsSnX3, and CsPbX3 material systems were used.50,51 The time-resolved PL spectra of the FAPbBr3 and FAPb(Br0.36I0.63)3 nanocrystals are shown in Figure 1e. Both PL decay curves of FAPbBr3 and FAPb(Br0.36I0.63)3 nanocrystals could be described by biexponential fitting. A short lifetime (1) of 6.2 ns (85.4%) and long lifetime (2) of 57.9 ns (14.6%) were obtained for FAPbBr3 nanocrystals, while 1 was 7.6 ns (83.4%) and 2 was 74.8 ns (16.6%) for FAPb(Br0.36I0.63)3 nanocrystals. The fast decay component, 1, was attributed to the non-radiative decay at the grain boundary, while the slow decay component, 2, was attributed to the radiative decay.52,53 As shown in Figure 1f, the X-ray diffraction (XRD) patterns of the FAPbBr3 and FAPb(Br0.36I0.63)3 nanocrystals were generally the same. The prominent diffraction peaks were at 14.7°, 20.8°, 29.7°, 33.4°, 36.6°, 42.6°, and 45.3° for FAPbBr3 nanocrystals, while those of FAPb(Br0.36I0.63)3


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nanocrystals were at 14.15°, 28.8°, 32.05°, 41.2°, and 43.95°. The peaks of FAPbBr3 nanocrystals match well with those reported in previous studies,54-58 and most of the previous studies indicate these peaks from the cubic phase of FAPbBr3.55,57,58 With the addition with larger I ions, a shift to lower diffraction angles was observed, indicating an increase in the lattice constant.55,57,58 Neither a diffraction peak at 12.2° of δ- FAPbBr3 impurity nor a peak at 18.6° of PbBr2 impurity was observed, indicating a high purity of these nanocrystals.54

To understand the capability of polymeric matrix to protect perovskite nanocrystals, the FAPbBr3 and FAPb(Br0.36I0.63)3 nanocrystals in toluene were blended with 11 important polymer materials: PLA, polycarbonates (PC), polyethylene terephthalate glycol-modified (PETG), PMMA, poly(vinyl chloride) (PVC), low-density polyethylene (LDPE), polyamide (PA), polypropylene (PP), acrylonitrile butadiene styrene (ABS), highimpact polystyrene (HIPS), and PCL. These polymer materials in filament form are commercially available for 3D printing. They are transparent or white under daylight, and most showed a very dim blue light under UV irradiation, which might have come from the


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optical brightener, as shown in Figure S3. The PL spectra of these polymer filaments are shown in Figure S4 as reference. Some basic characteristic properties of the thermoplastic polymer filaments are shown in Table 1.

The FAPbBr3 and FAPb(Br0.36I0.63)3 nanocrystals in toluene were added into vials containing these polymer materials, as shown in Figure S5. After 2 hours, ABS, HIPS, and PCL are completely dissolved, while the others were intact and still in filament form. The undissolved materials were taken out and dried at 50°C for 1 hour, as shown in Figures S6 and S7. The dissolved polymer materials were put on glass substrates and dried at 50°C for 1 hour, as shown in Figures S8 and S9.

For the undissolved materials, the filaments were swollen to some extent in the solution, indicating the nanocrystals diffused into the filaments. After drying, the nanocrystals can be trapped inside the filaments. Filaments allowing more swelling can provide better protection, as the nanocrystals can diffuse deeper. As can be seen in Figures S6 and S10, FAPbBr3-PVC and FAPbBr3-PA showed a dim green light, which decayed in a short time, indicating they cannot provide sufficient protection for green


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nanocrystals, while FAPbBr3-PP showed a very dim green light, but light lasted for a longer time. On the contrary, FAPbBr3-PLA, FAPbBr3-PC, FAPbBr3-PETG, FAPbBr3PMMA, and FAPbBr3-LDPE showed a bright green light even after one month storage in ambient environment. Since FAPbBr3 nanocrystals are stable, it is not easy to determine which polymer materials are better for protection. Therefore, the properties of the filaments embedded with FAPb(Br0.36I0.63)3 nanocrystals, which are less stable than FAPbBr3 nanocrystals, were investigated, as shown in Figure S7 and S11. Apparently, FAPb(Br0.36I0.63)3-PLA and FAPb(Br0.36I0.63)3-PMMA filaments still emitted a red light even after one month; therefore, these two are better materials for protective encapsulation.

As for the dissolved materials, all FAPbBr3-polymer composite films provided sufficient protection for a long time, as shown in Figure 2a-2h, S8 and S10. However, FAPb(Br0.36I0.63)3-PCL composite film exhibited the longest lifetime, as shown in Figure 2i–2p, S9, and S11. In addition to PCL filaments, PCL granules were also used as another polymer source. The resulting perovskite nanocrystal-PCL composite films also showed similar stability performance. All the optical properties mentioned above are summarized


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in Table 2 and Table 3.The XRD pattern of the FAPbBr3-PCL composites showed two more XRD peaks at 21.3° and 24.2°, belonging to the (110) and (200) reflections of PCL, respectively,60 without affecting the original peaks of the FAPbBr3 nanocrystals (Figure S12a,b). Therefore, PCL is another good protection polymer. The better protective capability of the perovskite nanocrystal-polymer composites is attributed to the property of PLA, PMMA, and PCL barrier to oxygen and water. Besides, the differential scanning calorimetry was used to understand the influence of the incorporation of the perovskite nanocrystals into the PCL on the glass transition temperature (Tg). Figure S12c showed that the Tg of the PCL was about -62oC, while the Tg of the FAPbBr3-PCL composite film was about -55oC. The changes in Tg is common when two materials are blended.61-64

To understand how the PCL protects the embedded nanocrystals, the dynamics of PL intensity of the FAPbBr3-PCL composite film and bare FAPbBr3 nanocrystals were monitored in different gaseous environments for comparison. The sample was put in a chamber with a gas inlet, a gas outlet, and a pumping system. The PL intensity as a function of time for the sample in air, vacuum, nitrogen, and oxygen was recorded in sequence as shown in Figure 3. The bare FAPbBr3 nanocrystal film was prepared by drop casting and vacuum drying, and the PL variation of bare FAPbBr3 nanocrystals is shown in Figure 3a. Before evacuating the chamber, the bare FAPbBr3 nanocrystal film showed a high PL intensity in air. The PL intensity decreased dramatically when


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the chamber was pumped down. The PL intensity remained in the same level when the chamber was filled with nitrogen. When the chamber was purged using oxygen, the PL intensity increased dramatically to the level as in air. The chamber was again purged with nitrogen, with oxygen, evacuated, and the vacuum was broken, and the resulting PL levels were similar to the previous ones. The PL peak wavelength was the same throughout the processes, and the spectra when the sample was in oxygen and nitrogen atmosphere are shown in Figure S13. This dynamic behavior corresponds with those of previous reports,65,66 in which it is attributed to the physisorbed gas molecules and the resulting surface state density. The behavior has been further explained recently by considering the formation of superoxide species, which remove the shallow states, leading to radiative-dominant recombination.67,68 On the contrary, the PL intensity of the FAPbBr3-PCL composites remained in the same level in various environments as shown in Figure 3b. The initial decrease in PL was probably due to the outgassing of moisture and oxygen from the composite, which is expected to be a slow process. When the physisorbed gas molecules detached from the FAPbBr3 nanocrystals and diffused outward, the occurrence of the shallow states leading to the non-radiative recombination and the decrease in PL intensity. When comparing the decreasing rate of PL intensity of the bare FAPbBr3 nanocrystal film, the rate of decrease in PL intensity of the FAPbBr3-PCL composites is quite slow which is suitable to be assigned as the diffusion process. Similarly, after the chamber was refilled with ambient air, the PL intensity of the composites remained at the same level. If we focus on the final stage in which the materials were exposed to the air, comparing the abrupt increase in PL intensity of the bare FAPbBr3 nanocrystal film, the increase in PL of the FAPbBr3-PCL composites is quite slow. All these results indicate the PCL could serve as a good barrier layer for moisture and oxygen. The FAPbBr3-PCL composite film was further tested in hot water environment. The composite film remained luminescent for 10 min


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after the boiling water was poured into a beaker containing the composite film (movie S1). Since the melting point of PCL is as low as 56°C–64°C, the composite film softened in hot water and became deformable and stretchable and thus can be molded into another structure. In contrast, the luminescence of FAPbBr3 solution disappeared in 3 seconds after the solution contacted with the hot water. When the FAPbBr3-PCL composite film was put in boiling water, its luminescence decreased considerably after 5 min (movie S2). The fluorescence decay of the composites is attributed to the degradation of perovskite nanocrystals. This is because when the composite film was kept at a high temperature for a long time, moisture severely penetrated into the softened polymer chain, leading to the degradation of the perovskite nanocrystals in a short time. Since the perovskite nanocrystal-PCL composites could be easily deformed due to the low melting point of PCL, millimeter-sized filaments for 3D printing and colored down-conversion films for white LEDs were obtained to demonstrate the possible applications of these composites. For 3D printing, the FAPbBr3-PCL composite film needed to be reformed into millimetersized filaments with a diameter suitable for the 3D printer. Due to the lack of an extruder for polymeric materials, the FAPbBr3-PCL composite film was reformed into a fluorescent filament by molding. A mold with a trench of suitable diameter was prepared in advance using a 3D printer (Figure S14a). A very thin layer of silicon oil was coated on the mold surface for better demolding. The heated and softened composites were put into the trench and allowed to cool (Figure S14b,c). The filament was ready for use after it was pulled out and polished (Figure S14d). Perovskite nanocrystals with different colors could also be incorporated with PCL and then processed into filaments with the same procedures (Figure S14e, Figure 4a). Different filaments could also be simply connected into one by heating locally (Figure S14f,g). The filament was then loaded into a 3D printing pen, and a vertical line was drawn (Figure 4b). The video showing the printing of the


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vertical line was shown in movie S3. The photo of the vertical line was taken with a UV light behind (Figure 4b). The temperature of the heater in the 3D printing pen was set to be 90°C, and no significant degradation in the luminescence of the composite was observed. This result demonstrates that the fluorescent filaments prepared from FAPbBr3-PCL composites are suitable for 3D printing. More complex objects can also be prepared by 3D printing as shown in Figure S15. The FAPbBr3-PCL composite films were first prepared and cut into small pieces (Figure S15a) which were loaded into a container of a 3D printer. The container was heated electrically and the softened composites were extruded to prepare 3D printed words (Figure S15b,c). The digital images of the printed words were taken under daylight and UV irradiation (Figure S15c,d,e) to demonstrate the luminescence behavior of the printed objects. In addition to millimeter-sized filaments, micrometer-sized fluorescent fibers based on FAPbBr3-PCL filaments could also be prepared using a cotton candy maker. The FAPbBr3-PCL composites were loaded into a bowl enclosed in the spinning head of a cotton candy maker. Heaters at the bottom of the bowl melted the composites, which were then squeezed out through small holes on the bowl by centrifugal force and was solidified in air. The diameter of the FAPbBr3-PCL fibers was about 7 m, as shown in Figure 4c,d. A schematic diagram of the micrometer-sized fluorescent fibers was added in the upper right of Figure 4b to represent the position of the fiber in the photo clearly. The distribution of the FAPbBr3 nanocrystals in PCL fibers seemed be nonuniform. Most of the nanocrystals were located near the surface of the fibers. The fluorescence lifetime was long, and no significant decay in fluorescence was observed even after two months. As for perovskite nanocrystal-PLA filaments, which also showed a good gas barrier property in ambient condition, the distribution of the FAPbBr3 in a FAPbBr3-PLA filament was


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investigated, and then the FAPbBr3-PLA filament was tested for 3D printing. A confocal laserscanning fluorescence microscope was used to investigate the distribution of the FAPbBr3 in FAPbBr3-PLA filament. As shown in the microscopic image (Figure S16a), there was a green area near the surface of the FAPbBr3-PLA filament, indicating the presence of FAPbBr3 nanocrystals near the surface. The cross-sectional SEM image of the FAPbBr3-PLA filament reveals a bright region near the surface (Figure S16b). When the confocal image overlaps with the SEM image, it is apparent that the green area of the confocal image corresponds precisely to bright region in the SEM image (Figure S16c). The filaments swelled when the filament was submerged in the perovskite nanocrystals/ toluene solution, indicating the perovskite nanocrystals diffused into the filament and were trapped and protected by the filament. The penetration depth of the FAPbBr3 nanocrystals in PLA filament was estimated to be within a few micrometers, which is consistent with the result of a previous report.40 However, because of the high melting point of the PLA filaments (150°C–160°C), the operation temperature of the 3D printing pen for the FAPbBr3-PLA filaments was as high as 190°C, which led to the degradation of perovskite nanocrystals and nonfluorescent printed objects (Figure S16d,e). The melting point of PMMA is about 160°C, which is similar to that of PLA; therefore, the printed object is also expected to be non-fluorescent. Hence, both FAPbBr3-PLA and FAPbBr3-PMMA filaments are not suitable for 3D printing. To further demonstrate the possible application of PCL-based composite films, FAPbBr3PCL and FAPb(Br0.36I0.63)3-PCL composite films were utilized as down-converters for white LEDs. Rather than pursuing a high-performance white LED, we focused more on the stability issue. Therefore, PCL-based composite films were conformally stacked on a blue LED chip without considering the heat from the LED chip, as shown in Figure 5a. Moreover, in addition to using a Br-rich nanocrystal with slightly better thermal stability and 630 nm emission, an


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FAPb(Br0.36I0.63)3-PCL composite film was used, which showed 669 nm emission and inferior thermal stability. The photograph taken when the LED was turned on is shown in Figure 5b. The spectra and the Commission Internationale de l'Éclairage (CIE) chromaticity coordinates at various driving voltages are shown in Figure 5c and 5d, respectively. The CIE coordinates changed from (0.35, 0.33) to (0.32, 0.34) when the bias voltage was increased from 4 V to 7 V. The CIE coordinates closest to the ideal pure white light emission coordinates (0.33, 0.33) were obtained at about 5 V. The spectra showed three peaks, belonging to the blue LED, FAPbBr3 nanocrystals, and FAPb(Br0.36I0.63)3 nanocrystals. When the spectra were normalized to the intensity of the blue LED, the intensity of the FAPbBr3 peak was the same, while the intensity of the FAPb(Br0.36I0.63)3 peak decreased with increasing driving voltages. The decrease in electroluminescence intensity is attributed to the degradation of FAPb(Br0.36I0.63)3 nanocrystals at high temperature and under blue irradiation,69,70 which further indicates the importance of the thermal stability of the red emissive component. Because of the good protective capability of the PLA and PMMA, we also try to use them for white LEDs purpose. Since PLA and PMMA filaments cannot be easily dissolved in a solvent within a short time, the PLA and PMMA filaments are taken in a beaker that are continuously stirred at 200oC until filaments completely softened in it. The hot blend of PLA/PMMA was then poured in a flat surfaced Petri dish to form the PLA/PMMA blend film. Similar method was used to prepare the perovskite nanocrystal-PLA and perovskite nanocrystal-PMMA filaments, the PLA/PMMA blend films were then submerged in the perovskite nanocrystals toluene solution. In the solution, the PLA/PMMA blend film swelled and the perovskite nanocrystals diffused into the blend film. FAPbBr3-PLA/PMMA and FAPb(Br0.36I0.63)3-PLA/PMMA composite films were realized for the demonstration of white LEDs with similar device structure as shown in Figure 5a.


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The EL spectra of the device at various driving voltages are shown in Figure S17. The intensity of emission peaks from both FAPbBr3-PLA/PMMA and FAPb(Br0.36I0.63)3-PLA/PMMA composite films decreased with increasing the driving voltage. This is attributed to the instability of the perovskite nanocrystals which are located near the outer surface of the PLA/PMMA composite film, and could be easily susceptible to the environment. In the previous demonstration, perovskite nanocrystals were blended with PCL and the nanocrystals could be completely protected by PCL that is the reason perovskite nanocrystals-PCL composites possesses higher stability. This results further demonstrated that the distribution of perovskite nanocrystals into the polymer matrix play a key role in the white LEDs application. Recently, inkjet printing technique was also used to demonstrate the patterned perovskite nanocomposites.71,72 The viscosity of the ink can be tuned with the addition of polymeric material for a better control of the quality of the printed objects. Many objects with high resolution were realized for various possible applications.72 Comparing with these works, 3D printing is much more easier for the construction of large 3D objects in which the resolution in a small area is not a critical issue. Besides, 3D printing process doesn’t need organic solvent to prepare the inks. However, heating is required for the extrusion process of the 3D printing which might be degraded the perovskite nanomaterials.


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CONCLUSIONS In summary, 11 thermoplastic polymers were explored as protective encapsulation materials for perovskite nanocrystals. Polycaprolactone exhibited excellent protective properties, and the perovskite nanocrystal-PCL composites could be processed into fluorescent objects and micrometer-sized fluorescent fibers via extrusion 3D printing and using a cotton candy maker. Moreover, a white LED with CIE coordinates of (0.33, 0.33) was realized by incorporating perovskite nanocrystal-PCL composite films with different emission wavelengths.

EXPERIMENTAL SECTION Materials Dimethylformamide, PbBr2, and PbI2 were purchased from Sigma Aldrich. HC(NH2)2Br (FABr) and HC(NH2)2Br (FAI) were purchased from Lumtec. The thermoplastic filaments were purchased from Extek, Voltivo, Kebixing, Min-Yau, and Shuangli Adhesive Tape Co. Synthesis of HC(NH2)2PbBr3 (FAPbBr3) First, 19.98 mg of FABr and 73.4 mg of PbBr2 were dissolved in 5 mL of DMF and heated at 75°C overnight. The precursor solution was prepared by adding 100 L of oleic acid and 4 L of octylamine into 1 mL of DMF solution containing FABr and PbBr2. Then, 1 mL of the precursor solution was injected into 15 mL of toluene under vigorous stirring. After centrifugation at 3300


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rpm for 10 min to discard the precipitates, a colloidal solution of FAPbBr3 nanocrystals was obtained. Synthesis of HC(NH2)2Pb(Br0.36I0.63)3 (FAPb(Br0.36I0.63)3) First, 5 mL of DMF containing 19.98 mg of FABr and 73.4 mg of PbBr2 and another 5 mL of DMF containing 27.5 mg of FAI and 92.2 mg of PbI2 were prepared and heated at 75°C overnight. These two solutions were blended with 4:7 volume ratio, and 225 L of oleic acid and 9 L of octylamine were added to obtain the precursor solution. Then, 0.7 mL of the precursor was injected into 15 mL of toluene under vigorous stirring. After centrifugation at 1600 rpm for 5 min to discard the precipitates, a colloidal solution of FAPb(Br0.36I0.63)3 nanocrystals was obtained. Preparation of perovskite nanocrystal-PCL composites Eleven commercial available thermoplastic filaments were loaded into vials, and the as-prepared colloidal solution of perovskite nanocrystals was distributed into different vials. The materials were submerged in toluene solvent for 2 hours. Eight materials that were insoluble in the colloidal solution were taken out of the colloidal solution for drying. The filaments were swollen in the colloidal solution, indicating the perovskite nanocrystals could still get into the filaments. The other three materials, which were soluble in the colloidal solution, were casted on a glass substrate and dried at 50°C for 1 hour to form perovskite nanocrystal-polymer composites. Preparation of millimeter-sized filaments for 3D printing The perovskite nanocrystal-PCL composites were heated to soften the composites, and then the composites were loaded into a mold to prepare the filaments for a 3D printer pen, A3, Kebixing Inc. Preparation of micrometer-sized filaments for 3D printing


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The perovskite nanocrystal-PCL composites were loaded into the extruder head of a cotton candy maker, NostalgiaTM, model number PCM305. Heaters were used to liquefy the composites, and then the composites were squeezed out through small holes on the extruder head while spinning. Characterizations Transmission electron microscopy (TEM) images of the perovskite nanocrystals were taken using a Philips Tecnai F30 field-emission gun transmission microscope. X-ray diffractometry data were obtained using a D8 Discover X-Ray diffraction (XRD) system. The UV−VIS absorbance was measured using a microspectrometer (SD1200-LS-HA, Stream-Optics Co.). Confocal laserscanning fluorescence microscope images were taken using ZEISS LSM 880. The PL spectra were recorded using a spectrometer with a 405 nm laser as an excitation source. Time-resolved PL spectra were measured using EasyLife™, HORIBA.


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Figure 1. Characteristics of perovskite nanocrystals. TEM images of (a) FAPbBr3 and (b) FAPb(Br0.36I0.63)3 nanocrystals. Absorption and PL spectra of (c) FAPbBr3 and (d) FAPb(Br0.36I0.63)3 nanocrystals. (e) Time-resolved PL curves and (f) XRD patterns of FAPbBr3 and FAPb(Br0.36I0.63)3 nanocrystals.


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Figure 2. Images of the (a–h) FAPbBr3 nanocrystal-polymer composite films and (i–p) FAPb(Br0.36I0.63)3 nanocrystal-polymer composite films under UV irradiation. The polymer


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materials used in the composites are (a),(e),(i),(m) ABS; (b),(f),(j),(n) HIPS; (c),(g),(k),(o) PCL filament; and (d),(h),(l),(p) PCL granules.

Figure 3. Variation in PL intensity of (a) bare FAPbBr3 and (b) FAPbBr3-PCL composite film in different environments.


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Figure 4. (a) Image of the red, green, and blue filaments prepared from perovskite nanocrystalPCL composites under UV irradiation. (b) Image of the 3D-printed vertical green object under UV irradiation. Image of the micrometer-sized fluorescent fibers under (c) daylight and (d) UV irradiation.


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Figure 5. (a) Schematic diagram and (b) photograph of the white LED. (c) Emission spectra of the white LED. The emission spectra from blue LED, FAPbBr3-PCL composite film, and FAPb(Br0.36I0.63)3-PCL composite film are also shown for comparison. (d) CIE coordinates of the white LED.


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ASSOCIATED CONTENT Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI: Movie S1 showing the composite film remained luminescent for 10 min after the boiling water was poured into a beaker containing the composite film. Movie S2 showing the composite film was put in boiling water. Its luminescence decreased considerably after 5 min. Movie S3 showing the 3D printing of the composites. Characterizations, additional TEM images, absorption and photoluminescence spectra, photographs and XRD patterns.

AUTHOR INFORMATION Corresponding Author

*[email protected]

Notes


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The authors declare no competing financial interest.

ACKNOWLEDGMENT This work is supported by the Ministry of Science and Technology of Taiwan under grant number MOST 106-2112-M-003-016-MY3. Authors acknowledge Instrumentation Center at NTU for TEM measurements.

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Ultrastable, Deformable, and Stretchable Luminescent Organic

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