Water-Resistant Efficient Stretchable Perovskite-Embedded Fiber

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Water-Resistant Efficient Stretchable PerovskiteEmbedded Fiber Membranes for Light-Emitting Diodes Chun Che Lin, Dai-Hua Jiang, Chi Ching Kuo, Chia-Jung Cho, Yi-Hsuan Tsai, Toshifumi Satoh, and Chaochin Su ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15989 • Publication Date (Web): 08 Jan 2018 Downloaded from http://pubs.acs.org on January 8, 2018

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ACS Applied Materials & Interfaces

Water-Resistant Efficient Stretchable Perovskite-Embedded Fiber Membranes for Light-Emitting Diodes Chun Che Lin,†,‡ Dai-Hua Jiang,†,‡ Chi-Ching Kuo,*,‡ Chia-Jung Cho,‡ Yi-Hsuan Tsai,‡ Toshifumi Satoh,§ and Chaochin Su‡ ‡ §

Institute of Organic and Polymeric Materials, National Taipei University of Technology, Taipei 106, Taiwan Faculty of Engineering, Hokkaido University, Sapporo 060-8628, Japan

Supporting Information Placeholder ABSTRACT: Cesium lead halide perovskite nanocrystals (NCs) with excellent intrinsic properties have been employed universally in optoelectronic applications but undergo hydrolysis even when exposed to atmospheric moisture. In the present study, composite CsPbX3 (X = Cl, Br, and I) perovskite NCs were encapsulated with stretchable [poly(styrene-butadiene-styrene); SBS] fibers by electrospinning to prepare water-resistant hybrid membranes as multicolor optical active layers. Brightly luminescent and colortunable hydrophobic fiber membranes (FMs) with perovskite NCs were maintained for longer than 1 h in water. A unique remote FMs packaging approach was used in high-brightness perovskite light-emitting diodes (PeLEDs) for the first time.

KEYWORDS: perovskite • SBS polymer • fiber membrane • moisture • LEDs

In the past two decades, group II-VI semiconductor nanocrystals (NCs), such as ZnO, ZnS, ZnSe, CdS, and CdSe, have attracted considerable attention because of their prospective performance in diverse fields: particularly light-emitting diodes (LEDs),1 but also solar cells,2 bioimaging,3 photodetectors,4 and field effect transistors.5 Colloidal semiconductor NCs have been developed progressively through synthetic strategies, but the preparation of quantum dots (QDs) continues to suffer from challenges including toxic chemicals, low yield, and complex procedures. Hence, novel types of semiconductor NCs with excellent electrical and optical properties that can be prepared using a reproducible, simple, and costeffective approach are highly desired. For this propose, colloidal perovskite NCs have been demonstrated as an effective material for optoelectronic applications because of their attractive properties including low cost, simple preparation, and high reproducibility.6,7 For example, lead halide perovskites MPbX3 (where M = CH3NH3+, HC(NH2)2+, and Cs+; X = Cl, Br, and I) have recently been applied in solar cells and LEDs because of their favorable intrinsic properties, including tunable emission spectra (blue → IR), broad absorption spectra (ultraviolet → visible), narrow emission linewidths (90% in solution for NCs).8–11 In addition, the electroluminescence quantum efficiency of organic-inorganic perovskite (Cs0.87MA0.13PbBr3)-based LEDs (PeLEDs) was improved to approximately 10% with a high brightness of 91,000 cd m–2.12 After Kovalenko et al. introduced all-inorganic cesium lead halide (CsPbX3, X = Cl, Br, I) perovskite NCs, many studies

investigating them have been published.11 CsPbX3 QDs were proven to possess bright and tunable photoluminescence (PL) characteristics, a flexible composition, and controllable morphology when prepared through a straightforward and efficient solution-based synthesis approach. However, unstable perovskite NCs are sensitive to moisture and oxygen, which can be ascribed to surface traps and metal oxidation (Pb2+ → Pb4+) induced by surface dangling bonds or unsaturated atoms.13,14 Regarding the water instability, it is likely due to ionic nature of perovskites. A rapidly increasing number of studies have focused on the composition and synthetic methods of colloidal perovskite NCs with the objective of improving their photostability and thermostability in potential applications.15,16 For example, oxide shell growth, ligand exchange processes,17–20 and encapsulation with silica,21 polymers,22 and glass matrices23 have been studied for their potential for enhancing the stability of perovskite QDs, but none have exhibited long-term stability in practical high-performance devices. Electrospinning (ES) has emerged as a new technique for producing various functional fibers because it has the advantages of flexible morphology tuning and high-throughput continuous production.24–27 Moreover, it is a straightforward, versatile, and inexpensive technique for producing nanometerscale fibers and encapsulating other composites, such as QDs, to assemble various functional nanofibers. The high surface-tovolume ratio of ES nanofibers has motivated extensive studies on their sensory applications. In the past, various fluorescent sensor-based ES polymer nanofibers prepared from multifunctional conjugated copolymers for sensing pH levels,24,25 temperatures,26 NO gases,27 and metal ions24,25 have been success-

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fully prepared by our group. Recently, Wang et al.23 used polystyrene blending with a CsPbBr3 perovskite QD-based ES fiber membrane as an ultrasensitive fluorescent sensor in an aqueous medium to detect rhodamine dyes. However, no studies have investigated the application of perovskite NCs in hydrophobic [poly(styrene-butadiene-styrene); SBS] stretchable fiber membranes (Pe@SBS FMs) to high-brightness PeLEDs through a unique remote FMs packaging approach. We embedded CsPbX3 (X = Cl, Br, and I) perovskite NCs into SBS fibers through electrospinning. By using this attractive strategy, we can improve the photostability of Pe@SBS FMs in atmospheric environments. Highly efficient white LEDs (WLEDs) were fabricated by combining blue LED chips (450 nm) and green and red flexible Pe@SBS FMs. The present study demonstrated that the switchable Pe@SBS FMsWLEDs, which are based on full-color elastomer nanofibers, have great potential for use in light-emitting displays. The reported approach is a promising avenue for encapsulating perovskite QDs into polymeric FMs to enable stable optical properties for this new class of QD. The flexible all-inorganic CsPbX3 (X = Cl, Br, I)@SBS FMs using a straightforward electrospinning method were fabricated. As we known, the solvent with higher dielectric constant is good for the process of electrospinning, but good solvent for both of perovskite QDs and SBS is also very important. Thus, the choice of solvent is key role for perovskite QDs and polymers in our electrospun process. Figure S1 shows that stable QDs and SBS can be dissolved successfully in dichloromethane (DCM) with high dielectric constant (9.1). The properties of QDs and SBS must be considered carefully including surface ligands. First, the stock solution of colloidal CsPbX3 QDs in toluene was mixed with pre-prepared SBS in DCM solution, as shown in Figure 1a,b. This approach features a mixing process without anti-phase or precipitate precursors. SBS is easily dissolved in DCM solution, which would also not destroy the morphology of CsPbX3 NCs.28 The mixed solutions with SBS/DCM (4 mL, SBS concentration of approximately 0.15 mg/mL) and CsPbX3/toluene (100 µL, CsPbX3 NCs concentration of approximately 10 mg/mL) are shown under ultraviolet (UV)-light excitation in Figure 1c. Second, the mixed solution was inserted into a metallic needle using syringe pumps with a feed rate of 0.8–1.0 mL h−1. The tip of the metallic needle was connected to a high-voltage power supply that was set at 10.2 kV during electrospinning (Figure 1d). The fabrication processes for different colored Pe@SBS FMs were implemented by adjusting halide components as shown in Figure 1a–d. The optical image of the blue [CsPb(Cl0.5Br0.5)3 NCs], green [CsPb(Br0.8I0.2)3 NCs], yellow [CsPb(Br0.6I0.4)3 NCs], and red [CsPb(Br0.4I0.6)3 NCs] @SBS composite FMs with tailored size 2 × 2 cm was excited using UV light (Figure 1e). The PLQYs in the flexible FMs were extremely high at 10.8%, 23%, 14.6%, and 12.2% for the blue, green, yellow, and red Pe@SBS composite fiber membranes, respectively. Perfectly circular 0.8-µm thick Pe@SBS FMs that were fabricated through the electrospinning method are displayed in Figure S2 of the Supporting Information. The exact locations of perovskite QDs in the SBS nanofibers were analysed using fluorescence microscopic images, as shown in Figure 1f–i. The results indicate individual red yellow green blue (RYGB) perovskite QDs fluoresce in homogenous distributions. No interaction and exchange effect of the various multihalide perovskite QDs was observed in the single-nozzle electrospinning equipment. Figure S3 (Supporting Information) presents the confocal microscopy images of blue

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CsPb(Cl0.5Br0.5)3, green CsPb(Br0.8I0.2)3, yellow CsPb(Br0.6I0.4)3, and red CsPb(Br0.4I0.6)3 QDs, which can be observed at different sites of the SBS nanofibers with a high spatial resolution.

Figure 1. (a–d) Schematic of flexible CsPbX3 (X = Cl, Br, I)@SBS FM fabrication process. (e) The optical image of blue, green, yellow, and red composite FMs under UV-light (365 nm) excitation. (f–i) Fluorescence microscopic images of versatile CsPbX3 fibers with the single-nozzle electrospinning equipment. The phase structure of CsPbX3 (X = Cl, Br, I)/SBS composite FMs was characterized by applying scanning electron microscopy (SEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM) characterizations. The effectively balanced CsPb(Br0.8I0.2)3@SBS fibers had an average diameter of approximately 800 nm and were several millimeters long (Figure 2a). The morphology of the Pe@SBS fibers in other component was similar to that of CsPb(Br0.8I0.2)3@SBS fibers. Therefore, the size of the SBS fibers can be controlled favorably using the electrospinning strategy, even though various perovskite NCs are embedded in these fibers. The CsPb(Br0.8I0.2)3 QDs (red circles) are identifiable near the surface of the SBS fiber, as shown in Figure 2b. The SEM image and elemental analysis of individual RYGB Pe@SBS FMs are provided in Figure S4 (Supporting Information). The results provide only the position and the composition of perovskite NCs near the surface of the SBS fibers, but the distribution of perovskite NCs in the fibers was unobtainable. Additionally, QDs are only rarely observed on the surface of the SBS fibers because no detectable CsPbX3 (X = Cl, Br, I) elements were observed in our X-ray photoelectron spectroscopy (XPS) measurements (Figure S5). Figure 2c shows the XRD patterns of the CsPb(Br0.8I0.2)3@SBS FMs, pure SBS, CsPb(Br0.8I0.2)3free NCs, and CsPbBr3-perovskite standard JCPDS. The formation of green CsPb(Br0.8I0.2)3-free NCs agrees sufficiently with the cubic CsPbBr3 (space group: Pm-3m No. 221) although few iodides exist in the compound. Furthermore, in the case of the as-fabricated composite CsPb(Br0.8I0.2)3@SBS FMs, we observed the broad diffraction peaks from the SBS (2θ = approximately 19°) and CsPb(Br0.8I0.2)3 NCs (2θ = approximately 30° and 40°). This indicated that perovskite QDs were successfully embedded into the SBS fibers. To understand the internal situation of the polymeric SBS fibers, we investigated the precise location and crystal structure of perovskite NCs through TEM. Figure 2d shows the TEM image of the SBS fiber with a 500-nm diameter and the embedded CsPb(Br0.8I0.2)3 QDs inside the fiber. In Figure 2e, the assynthesized CsPb(Br0.8I0.2)3 QDs are characterized using highresolution TEM (HRTEM); they exhibit cubic morphology

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ACS Applied Materials & Interfaces with an average side length of 12.3 ± 0.6 nm and a d-spacing of 0.58 nm based on the (100) plane. The particle size of the embedded CsPb(Br0.8I0.2)3 QDs is similar to that of the original CsPb(Br0.8I0.2)3-free QDs, as illustrated in Figure S6 (Supporting Information). The fast Fourier transformation pattern (Figure 2f) was measured according to the HRTEM measurement shown in Figure 2e, confirming that CsPb(Br0.8I0.2)3 NCs are single crystals.

composite FMs decreases dramatically within 10 min because of the different medium. Initially the PL intensity of FMs are measured in air, but the PL value are lower after the FMs dipping in water for 10 min. It indicates that the perovskite NCs are influenced by refractive index of medium and damaged by moisture. When all the FMs were maintained in water after the initial 10 min, the CsPb(Br0.8I0.2)3@SBS FMs (green curve; Figure 3c) exhibited greater photostability in the aqueous solution without a shift in the position of the emission peaks (green curve; Figure 3d). However, a lower PL intensity was observed in the other Pe@SBS FMs because mixed-phase compounds were easily degraded by moisture, as shown in Figure 3c. No distinct peak shift (Figure 3d) was observed for waterimmersed CsPb(Clx, Bry, Iz)3@SBS-composite FMs. Therefore, no size change or aggregation occurred for perovskite NCs in SBS nanofibers that were immersed in aqueous solution. The brightly luminescent CsPb(Clx, Bry, Iz)3@SBS FMs without protective layers (e.g., silicone resin) were maintained for over 1 h in water, as shown in Figure 3e and Figure S8 (Supporting Information). The result indicates that the enhanced water resistance of the CsPb(Clx, Bry, Iz)3 NCs was due to the channel-like hydrophobic SBS fibers. The average contact angle of water on the CsPb(Br0.8I0.2)3@SBS FMs was 105.8° (i.e., it is hydrophobic), as shown in Figure S9 (Supporting Information).

Figure 2. (a,b) SEM images of the SBS fibers with perovskite NCs. [red circles in Fig. 2b indicate the locations of CsPb(Br0.8I0.2)3 QDs]. (c) XRD patterns of CsPb(Br0.8I0.2)3@SBS FMs compared with pure SBS, CsPb(Br0.8I0.2)3 free NCs and CsPbBr3 perovskite standard JCPDS. (d) TEM image of the SBS fibers with embedded CsPb(Br0.8I0.2)3 QDs. (e) HRTEM image of the typical CsPb(Br0.8I0.2)3 QDs [red square in Fig. 2d]. (f) Corresponding fast Fourier transform (FFT) image of (e). On the basis of this analysis, the perovskite NCs were encapsulated successfully into the SBS fibers. Moreover, the fluorescence microscopic image of CsPb(Br0.8I0.2)3 NCs with the highest brightness was observed in the middle of the SBS fiber, as shown in Figure 3a. This indicates that luminescent NCs primarily locate in the centre of the SBS fiber, but that the residual nanoparticles can be observed in other regions of the SBS fiber. The photostability, durability, and tenacity of an emissive layer are critical factors for high efficiency light– emitting devices (FLEDs). We investigated the mechanical properties of CsPb(Br0.8I0.2)3@SBS FMs, and Figure 3b shows the white-emitting Pe@SBS FMs before and after stretching. The integrated RGB composite FMs exhibited outstanding elongation and compatibility with the encapsulating perovskite NCs (170% strain). The extended high-brightness of the Pe@SBS FMs can be attributed to the excellent stiffness of the SBS elastomeric matrix, which protects the perovskite NCs. Even if inorganic perovskite NCs possess many favorable intrinsic properties, their photostability decreases drastically in humid conditions and upon contact with water. The unprotected NCs were almost completely degraded within 10 s after immersion in an aqueous solution, as shown in Figure S7 (Supporting Information). To obstructing moisture, the luminescent nanoparticles can be embedded into the polymeric fibers through the one-step electrospinning strategy. The water resistance of the CsPb(Clx, Bry, Iz)3@SBS-composite FMs was demonstrated by placing FMs in an aqueous solution for 1 h (Figure 3e and Figure S8 of the Supporting Information). The relative PL intensity (Figure 3c) of CsPb(Clx, Bry, Iz)3@SBS-

Figure 3. (a) Schematic of CsPb(Br0.8I0.2)3 QD distribution in the SBS fibers. (b) Stress–strain test for white-emitting FMs (integrated RGB composite FMs) under UV-light (365 nm) excitation. (c,d) Relative PL intensity and the wavelength of CsPb(Clx, Bry, Iz)3@SBS composite FMs after immersion in an aqueous solution. [x + y + z = 1] (e) Images of the CsPb(Br0.8I0.2)3@SBS fiber membrane under UV-light (365 nm) excitation when immersed in an aqueous solution. Thermal stability is vital for phosphors applied to efficient WLEDs. A unique remote phosphor packaging approach was used to reduce the temperature sensitivity of the emissive materials by separating the LED chip.29 This strategy improves the luminous performance of lighting devices because of the lower power reduction at high temperatures. Because of the aforementioned reasons, we first fabricated the prototype WLED device by covering the blue chip (λmax = 450 nm) with green and red Pe@SBS FMs (Figure 4a). Furthermore, Figure S10 (Supporting Information) shows the RYGB PeLED devices that can be used in LCD backlights. The optimized devices produce warm white light emissions (Figure 4b) with a luminous efficacy of up to 9 lm/W and a broad color gamut of

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105% of the NTSC standard under an applied current of 10 mA. The CIE color coordinates and correlated color temperatures (CCT) of warm WLEDs can be adjusted by using different voltages, as shown in Figure 4c and Table 1. Additionally, the normal WLEDs and cool WLED devices were successfully fabricated based on the density of green and red Pe@SBS FMs under blue LED excitation (Figure 4d,e). Figure S11 (Supporting Information) shows the CIE diagrams of the obtained normal WLEDs and the cool WLED emission spectra at various applied voltages. The insets in Figure 4 show the WLED devices with different CCTs. The results indicate that the novel remote FMs packaging approach was successfully used in PeLEDs and overcame the thermal stability challenge for the first time.

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NCs into polymeric SBS FMs. The as-prepared flexible lightemitting FMs are resistant to moisture, which extends the lifetime of light emitters under ambient air conditions. The protected CsPbX3 (X = Cl, Br, I) perovskite QDs in hydrophobic FMs continued to emit bright fluorescence for over 1 h when immersed in water. Additionally, the WLEDs with various CCTs were easily fabricated by integrating green and red Pe@FMs and blue chips. The reported strategy is promising for exploring other perovskite QDs embedded into polymeric FMs to increase the photostability of light emitters for applications to lighting and backlight displays.

■ ASSOCIATED CONTENT Supporting Information. Experimental methods, supporting figures are available free of charge on the ACS Publications website at DOI: 10.1021/acsami.

■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

ORCID Chun Che Lin: 0000-0002-8432-9393

Notes The authors declare no competing financial interest. † C. C. Lin and D. -H. Jiang contributed equally to this work.

■ ACKNOWLEDGMENTS Figure 4. (a) Schematic for the configuration of the prototype WLED device. (b) Emission spectra of the warm WLED using green and red Pe@SBS FMs under blue chip (450 nm) excitation. The inset shows the warm WLED device. (c) CIE color coordinates (red circles) of the white emission spectra (b) with various applied voltages. (d,e) Emission spectra of WLED and cool WLED using green and red Pe@SBS FMs under blue chip (450 nm) excitation. The insets show the WLED devices with different correlated color temperature under an applied current of 10 mA.

This work was supported by the Ministry of Science and Technology of Taiwan (Contracts no. MOST 105-2221-E-027-134- and MOST 104-2113-M-027-007-MY3).

In summary, we report a novel electrospinning strategy for successfully incorporating CsPbX3 (X = Cl, Br, I) perovskite Table 1. Optical properties of various WLEDs fabricated using green and red Pe@SBS FMs with blue LED (λmax = 450 nm) excitation under the operating current of 10 mA. White

Warm White

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X

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CCT (K)

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