Polymer Luminous Hybrid

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Applications of Polymer, Composite, and Coating Materials

Stretchable and Ambient Stable Perovskite/Polymer Luminous Hybrid Nanofibers of Multicolor Fiber Mats and Their White LED Applications Ender Ercan, Ping-Chun Tsai, Jung-Yao Chen, Jeun Yen Lam, Li-Che Hsu, Chu-Chen Chueh, and Wen-Chang Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b05527 • Publication Date (Web): 06 Jun 2019 Downloaded from http://pubs.acs.org on June 6, 2019

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ACS Applied Materials & Interfaces

Stretchable and Ambient Stable Perovskite/Polymer Luminous Hybrid Nanofibers of Multicolor Fiber Mats and Their White LED Applications

Ender Ercan,† Ping-Chun Tsai,‡ Jung-Yao Chen,† Jeun-Yen Lam,‡ Li-Che Hsu,‡

Chu-Chen Chueh,†,§ and Wen-Chang Chen*,†,§

†Department of Chemical Engineering, National Taiwan University, Taipei 10617,

Taiwan.

‡Institute of Polymer Science and Engineering, National Taiwan University, Taipei

10617, Taiwan.

§Advanced Research Center of Green Materials Science and Technology, National

Taiwan University, Taipei, 10617, Taiwan.

*Corresponding author:  [email protected]

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ABSTRACT We report the fabrication and optical/mechanical properties of perovskite/thermoplastic polyurethane (TPU) based multicolor luminescent core-shell nanofibers and their large-scale fiber mats. One-step coaxial perovskite/TPU nanofibers had a high PLQY value exceeding 23.3% surpassing its uniaxial counterpart, due to the homogeneous distribution of perovskite NPs by the confinement of the TPU shell. The fabricated core-shell nanofibers exhibited a high mechanical endurance owing to the well elastic properties of TPU and maintained the luminescence intensity even under a 100%-stretched state after 1000-time stretching-relaxing cycles. By taking advantage of hydrophobic nature of TPU, the ambient and moisture stability of the fabricated fibers were enhanced up to one month. Besides, large area stretchable nanofibers with a dimension of 15 cm x 30 cm exhibiting various visible-light emissions were fabricated by changing the composition of perovskite NPs. Moreover, a large-scale luminescent and stretchable fiber mat was successfully fabricated by electrospinning. Furthermore, the white-light emission from the fabricated fibers and mats was achieved by incorporating orangelight emitting MEH-PPV into the TPU shell and coupling the turquoise blue-light emitting perovskite NPs in the core site. Finally, the integrity of the perovskite based TPU fibers was realized by fabricating an LED device containing the orange light emitting fibers embedded in PFO emissive layer. This work demonstrated an effective way to prepare stable and stretchable 2 ACS Paragon Plus Environment

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luminous nanofibers and integration of such nanofibers into LED devices, which could facilitate the future development of wearable electronic devices.

KEYWORDS: Perovskite, Hybrid, Luminescence, Nanofibers, Stretchable Fibers, white LED.

1. INTRODUCTION In recent years, stretchable electronics has been considered as the most important nextgeneration consumer products since they possess decent flexibility, foldability, bendability, and stretchability that can well meet the ergonomic needs. In response to this emerging demand, numerous research efforts have been dedicated and various stretchable optoelectronic devices have been developed, such as light-emitting diodes (LEDs),1-5 solar cells,6-9 field-effect transistors (FETs),10-11 and memories devices,12-16 etc. The stretchable apparatuses possess a promising potential for the future development of advanced electronics, such as smart phone, wearable electronics, and mobile electronics. To exploit efficient stretchable devices, the engineering in both material and device levels has been widely investigated. From the material aspect, owing to the superior ductility, organic semiconductors have attracted more attention than the inorganic materials that are generally highly crystalline. However, recently, organic-inorganic hybrid perovskites have become a rising star in various optoelectronic fields due to its tunable bandgap and solution-based processability, including memories,17-25 LEDs,26-28 and light-harvesting applications.29-32



The tunable bandgap of organic-inorganic hybrid perovskites could be achieved by simply tuning the composition of perovskite materials.33,34 Accompanied by this feature, the concomitant light emission of the materials can be also simply tuned and show narrow emission range, thus manifesting promising potential for the light-emitting applications in addition to the prevailing light-harvesting applications. However, the stretchability on such class of materials remains a significant challenge. Therefore, how to integrate the perovskites into the development of stretchable materials has attracted myriads of research interest in these years. It is worth noting that the dimension of perovskite materials had been proven to have a significant impact on the resultant high photoluminescence quantum yield (PLQY) through the whole visible range, similar to the quantum confinement effect observed in the nanodimensional crystals.35 For example, organic-inorganic perovskite nano-dots with a high PLQY of 83 % was reported by controlling its crystal growing condition.36 Such impressive result indicates the feasibility of using organic-perovskite hybrid to prepare stretchable luminous materials. It suggests that the luminous perovskite nanoparticles (NPs) can be embedded in a uniform polymer matrix possessing respectable stretchability. Based on this rationale, we herein are interested in developing stretchable, luminous perovskite nanofibers. On one hand, the perovskite NPs can grow in the polymer template (or matrix) by taking advantage of chemical interactions between them.37-40 On the other hand, the electrospinning technique endows the homogeneous distribution of perovskite NPs through the fabricated nanofibers as demonstrated in our recent work.41 We successfully fabricated uniform luminous perovskite/polymer electrospun nanofibers using one-step core-shell (or named as coaxial) electrospinning technique and manifested that the perovskite NPs were homogenously dispersed inside the nanofibers.41 However, achieving stretchable white light emitting fibers and the compatibility of nanofibers in LED device to tune desired emission color remain a significant challenge. Standing on this basis, we thus propose to prepare hybrid organic-


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inorganic based white light emitting stretchable nanofibers using emissive organic and stretchable polymer as shell. Moreover, a new approach of the nanofiber integrated LED device has to be developed on tuning the desired color of emission benefitting from energy transfer. In this study, we report the fabrication and optical/mechanical properties of perovskite/thermoplastic polyurethane (TPU) based multicolor luminescent core-shell nanofibers and their large-scale fiber mats. The thermoplastic polyurethane (TPU) was employed as the stretchable polymeric shell due to the rubber-like elastic properties. Besides, TPU also possesses a hydrophobic nature and is non-toxic and biocompatible, which are very suitable for integration in the smart textiles.42 We first used one-step coaxial electrospinning technique to fabricate luminescent electrospun TPU/perovskite nanofibers and compared with the nanofibers prepared by the regular uniaxial electrospinning method. Then, the perovskite nanoparticles were grown inside the fibers and the morphology were characterized by TEM. In the following, the physical properties of the prepared core-shell fibers were characterized under the ambient condition, including optical absorption, photoluminescence, and mechanical stretching. By changing the composition of perovskite NPs, stretchable nanofibers with various visible-light emissions were fabricated. Furthermore, white-light emission of the fabricated fibers and mats was achieved by incorporating orange-light emitting MEH-PPV into the TPU shell and coupling the turquoise blue-light emitting perovskite NPs in the core site. Besides, large-scale fiber mats were fabricated using a commercially available electrospinning equipment with a rotary fiber collector. Finally, orange light emitting nanofiber was incorporated into the PFO based emissive layer to perform white light emitting LED device. This work provided an effective way to prepare stable and stretchable luminous nanofibers and integration of such nanofibers into light emitting devices for flexible or wearable electronic devices.



2. RESULTS AND DISCUSSION We would like to develop stretchable perovskite nanofibers with the characteristics of uniformly luminous feature, mechanical endurance, and ambient stability. To this end, a thermoplastic polyurethane (TPU) was used as the fiber shell materials, comprising the urethane moiety in the main chain that connects to both hard and soft functional segments (Figure 1). In addition, it is non-toxic and biocompatible and possesses a hydrophobic nature. More importantly, the constituent urethane group of TPU might form certain interactions with the Pb atom or methyl ammonium (CH3+) cation of perovskite materials to modulate the associated crystal growth.43 Regarding these features, we thus selected TPU as the polymer matrix for our electrospinning process. Prior to fabricate core-shell perovskite/polymer nanofibers, a typical uniaxial electrospinning process was first conducted to prepare the nanofibers and the obtained nanofibers were named as MAPbBr3-Uni (MA+ = CH3NH3+). However, such uniaxial perovskite/polymer nanofibers exhibited a low PLQY of 2.1% at 533 nm (green-light emission), which was inferior to most values reported for MAPbBr3 NPs. This deficiency is likewise due to the non-uniform aggregated morphology of the perovskite NPs inside the electrospun fibers. Figure S1 (Supporting Information, SI) shows the scanning electron microscope (SEM) images of the fabricated uniaxial nanofibers, in which salient perovskite aggregates are clearly observed at the fiber surface. Such inhomogeneous formation of the perovskite NPs might be attributed to the rapid nucleation of perovskite crystals during the uniaxial electrospinning process.41 On one hand, the non-uniform distribution of perovskite NPs would degrade the homogeneity of the emitting light; on the other hand, the rapid crystal growth could engender the formation of defective states, which results in increased nonradiative decay of charges to yield low PLQY. Moreover, the direct contact of the perovskite aggregates with ambient air led to its degradation to cause emission instability.44


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To address this issue, a core-shell coaxial electrospinning technique was employed to better control the morphology, dimension, and distribution of the perovskite NPs inside the prepared nanofibers.41 Figure 1 illustrates the coaxial electrospinning setup and the structure of the prepared core-shell nanofibers. The perovskite precursor solution was injected by the inner syringe to constitute the core flow while the precursor solution of TPU was injected by the outer syringe to form the shell flow. As listed in Table 1, the stoichiometry ratio of perovskite precursor, the concentration of TPU solution, and the core flow rate were carefully engineered to understand the structure-performance relationship of the prepared core-shell nanofibers. Figure S2a (SI) shows the SEM image of the core-shell MAPbBr3-1 nanofibers, which was prepared by using a TPU solution with a concentration of 150 mg/ml and a perovskite solution with a MABr/PbBr2 ratio of 1.5. As can be clearly seen, the MAPbBr3-1 nanofibers possess a much smoother surface than MAPbBr3-Uni, suggesting the smaller aggregation of the perovskite NPs inside the nanofibers. This result affirms the advantage of using core-shell electrospinning technique for preparing uniform polymer/perovskite nanofibers. On this basis, the influence of the stoichiometric ratio between MABr and PbBr2 is listed in Table 1 and Figure S2b (SI) presents the Tauc plots of the MAPbBr3-1 and MAPbBr3-2 nanofibers as the example. As shown, the bandgap for both nanofibers is similar, which is not surprising since the resultant bandgap is governed by the embedded perovskite NPs. However, there is a huge difference in their PLQY. The MAPbBr3-2 nanofibers exhibit a PLQY of 16.3%, outperforming the value (8.3%) of the MAPbBr3-1 nanofibers. The enhanced PLQY suggests the better crystal growth of perovskite NPs in the MAPbBr3-2 nanofibers and such discrepancy reveals that the non-trivial role on the stoichiometric ratio of the precursors plays in the electrospinning. Since one of the most important advantage of the coaxial electrospinning method is the tunable operating conditions, we next examine the influence of the core flow rate on the



morphology of the prepared nanofibers and the embedded perovskite NPs. Figure S3 (SI) shows the SEM images of the MAPbBr3-2 and MAPbBr3-3 nanofibers, which were prepared by same operating parameters except for different core flow rate (0.1 ml/h and 0.01 ml/h, respectively). As observed, the average fiber diameters for the MAPbBr3-2 and MAPbBr3-3 nanofibers are 345 ± 48 nm and 253 ± 35 nm, respectively. The higher core flow rate seems to result in a larger fiber diameter, which is possibly due to the larger amount of perovskite precursor flowing through the nozzle and the formation of larger perovskite NPs. To confirm this, a transmission electron microscopy (TEM) analysis of the nanofibers was conducted. As shown in Figure 2, the size of perovskite NPs in the MAPbBr3-2 nanofiber is estimated to be in the range of 96±12 nm and some large aggregates are clearly observed. Whereas, the NPs in the MAPbBr3-3 nanofibers are smaller with a size of ~ 52±12 nm. Notably, such difference in morphology and distribution of the perovskite NPs again leads to their distinct PLQYs. The PLQY of the MAPbBr3-3 nanofiber is further enhanced from 16.3 to 23.3%. Furthermore, its corresponding PL peak is blue-shifted from 534 to 525 nm. This result clearly manifests the size effect of the perovskite NPs, wherein the smaller NPs might afford a better quantum confinement effect. Meanwhile, the more uniform formation of NPs in the MAPbBr3-3 nanofibers reduces the non-radiative decay benefitting from quantum confinement and thus improve the overall PLQY.37,41,45-49 After studying the influence of stoichiometry ratio of perovskite precursor and the core flow rate, we next tune the concentration of the TPU solution to elucidate the role of the shell solution. The original shell concentration of 150 mg/ml was reduced to 120 mg/ml while the core flow rate was kept at 0.1 ml/h or 0.01ml/h to prepare MAPbBr3-4 and MAPbBr3-5 nanofibers. Figure S4a-b (SI) shows their corresponding SEM images, wherein a poor shell effect is clearly observed. The protruding NPs on the fiber surface suggest that a thin shell is not favorable to cover the NPs. Such poor morphology thus leads to their inferior PLQY as


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compared to the reference MAPbBr3-2 and MAPbBr3-3 nanofibers, which were prepared using 150 mg/ml TPU solution. However, it is worth noting that the MAPbBr3-5 nanofibers prepared by a slower core flow rate still exhibits a higher PLQY (16.8%) than the MAPbBr3-4 ones (13.2%) owing to the formation of more uniform and smaller NPs as discussed earlier. We first examine the ambient stability of the fabricated core-shell nanofibers. It had been extensively reported in the literature that the organic-inorganic hybrid perovskite materials were sensitive to the moisture in the ambient atmosphere.50,51 Therefore, the hydrophobic TPU shell should enhance the moisture robustness of the perovskite NPs embedded inside the nanofibers. Figure 3 illustrates the track of PL of the MAPbBr3-3 nanofibers while stored in ambient condition (under daylight illumination) since it shows the highest PLQY among the fabricated fibers (Table 1). As shown, the maximum intensity of the corresponding PL peak is slightly reduced after 92-hour aging (Figure 3a). After that, it is remained at the same level until the end of tracking period of 31 days (Figure 3b), revealing an ignorable degradation. This result validates the confinement effect of NPs imposed by the TPU shell as observed in the TEM and SEM analyses. As mentioned earlier, the electrospinning technique is suitable for the scaled up production. A large-scale luminous fiber mat can be simply prepared for better practical applications. To realize this, a horizontal electrospinning system was employed to prepare a large-scale fiber mat. Figure 4a-b depicts the production apparatus of the horizontal electrospinning device and the real pictures of the rotary drum after collecting uniaxial nanofibers under room light and UV irradiation at a wavelength of 365 nm. The detailed operating parameters and conditions were described in the experimental section. Figure 4c shows a large-scale fiber mat with a dimension of 15 cm (width) x 30 cm (length) in made from the uniaxial nanofibers. A uniform distribution of nanofibers can be clearly observed throughout the aluminum foil substrate that covers whole rotary collector. Note that the prepared fiber mat is simply peeled off from the



aluminum foil, as shown in Figure 4d and emits homogenous green light under UV irradiation (365 nm). In addition to the uniaxial nanofibers, a large-scale fiber mat can also be made from the core-shell coaxial nanofibers as shown in Figure 4e. Besides the improved ambient stability, another important advantage of this fiber mat over the one made from uniaxial nanofibers is that it possesses a better stretchability that will be discussed in the later section. The above result reveals that the TPU might influence the crystal growth of perovskite and the associated particle formation since its constituent urethane group might form certain interactions with the Pb atom or methyl ammonium (CH3+) of perovskite materials.43 FTIR analysis indicates that peaks of the pure TPU shift when perovskite is incorporated into the TPU fibers as shown in Figure 5a. The characteristic peaks appeared at 3318 and 1702 cm-1 are assigned as stretching vibrations of the N-H and C=O groups for pure TPU fiber sample. Figure S5a (SI) shows that peak indicating the N-H stretching vibrations are shifted from 3318 cm-1 to 3326 and 3323cm-1 for the uniaxial and coaxial TPU/Perovskite, respectively. Besides, the peak appeared at 1702 cm-1, which corresponds to as the stretching vibrations of the hydrogen bounded carboxyl group (C=O) on TPU, are shifted to 1704 and 1703 cm-1 for uniaxial and coaxial TPU/Perovskite, respectively as clearly illustrated in Figure S5b (SI). It is clear to see that the trend in shifting of both peaks is proportional to the amount of perovskite incorporated into the fiber since its known that the uniaxial TPU/perovskite fibers contain a higher perovskite ratio. This result explains that perovskite has a distinct chemical interaction with urethane moieties of the TPU materials. It can also be realized by DSC study as shown in Figure 5b. As can be seen, the melting enthalpy decreases significantly after incorporating the perovskite into TPU fibers. It indicates that perovskite decreases the crystallization of the hard segment in the TPU backbone. As discussed earlier, pure TPU has the soft and hard segments where the hard segment forms the hydrogen bounding between urethane moieties at different TPU chains. Owing to perovskite has an interaction with urethane moieties in the composite


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fibers, this phenomenon reduces the hydrogen bonding between neighboring urethane groups at different chains and thus it retards the crystallization of the hard segment on the TPU materials. This outcome explains the lower melting enthalpy of melting estimated for the composite fibers. Moreover, the glass transition temperature reduces significantly due to the enhanced free volume upon incorporating perovskite into fibers. Because of the different segments that support various properties to the materials, TPU can effectively enrich the overall stretchability of the derived nanofibers. On one hand, TPU is a thermoplastic elastomeric block polymer; on the other hand, an existed plausible intermolecular interaction between TPU and NPs can prevent NP from degradation under stretched states. To probe this, we measured the Young’s modulus of the pure TPU film along with TPU/perovskite hybrid film. As shown in their corresponding stress-strain graph (Figure S5c, SI), the TPU/perovskite hybrid film showed a higher Young’s modulus than the pure TPU film in the linear region. It indicates enhanced mechanical properties of composites compared to pure TPU counterpart. The increased toughness of the hybrid system relative to the pure TPU suggests the existence of crosslinking-like effect between TPU and perovskite NPs, arising from the chemical interactions between them as proved by FTIR analysis.43 This effect is beneficial to enhance the NP confinement inside the fiber. In addition, owing to the well elastic property and hydrophobic nature of TPU, it can prevent the embedded NP from degradation induced by moisture or stretching, thus enhancing the ambient stability and mechanical strength. Another important feature of our prepared core-shell nanofibers is their tunable light emission by taking advantage of the composition-dependent tunable light emission on the embedded perovskite NPs.41 Red/orange/green/turquoise/ blue-light emitting fiber mats (under UV irradiation at 365 nm) fabricated from various luminous nanofibers are shown in Figure 6 along with the commensurate PL spectra, in which the corresponding composition of the



embedded perovskite NPs are also indicated. As seen, the emitting color covering the whole visible-light region is achieved by simply tuning the halide composition of the perovskite NPs. With these encouraging results, we next carefully examine the stretchability of the fabricated luminous fibers to understand the effectiveness of using TPU as the elastic polymer shell. Fiber mats derived from various luminous nanofibers were made for stretchability test. As shown in Figure 7a, the fiber mat prepared from the green-light emitting MAPbBr3-3 nanofibers is stretched under different strain up to 100 %. As can be clearly seen, the mat is maintained as one piece without showing any torn under stretched states. Besides, a uniform emission is clearly observed under the UV irradiation at 365 nm during stretching, suggesting the well stretchability of the fabricated core-shell nanofibers. This should be also attributed to the uniform distribution of perovskite nanoparticles, as shown by fluorescence image of MAPbBr3-3 single nanofiber (Figure S6, SI). The corresponding PL spectrum of the fiber mat under different stretched states is shown in Figure 7b, wherein an intense emission peak at ~530 nm is clearly observed. However, during the stretching state, there is a clear reduction of the emission. It could be attributed to the shrunk width of fiber mat during stretching, which results in the reduced emitting area for measurement.

Figure 7c shows a correlation between

the area under PL curve and the width of the fiber mat, in which a linear decreasing trend is clearly observed for the stretching from 0 to 100 %. The stretchability test of other luminous fiber mats including red/orange/turquoise/violet-light emitting is presented in Figure S7 (SI), which exhibits a similar decreasing trend in emission. However, a uniform emission (under UV irradiation at 365 nm) during stretching is similarly observed. It is worth noting that the violetlight emitting nanofibers shows a weaker emission and lower PLQY compared to other nanofibers. It could be due to the less solubility of Cl-based compounds than others, which might produce a poor quality and a lower density of the embedded NPs.


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We further investigated the PLQY change under different stretched states and the MAPb(ClxBr1-x)3 nanofibers was employed for testing. The detailed illustration of a complete stretching-relaxing cycle is depicted in the top of Figure S8 (SI), and the PL spectrum and PLQY after 100-time stretching-relaxing cycles under different stretching conditions as indicated are recorded in the bottom. Note that the PL intensity of the MAPb(ClxBr1-x)3 nanofibers is slightly increased after the cycling tests of stretching-relaxing, which is probably resulted from the reduced thickness of fiber mat. Impressively, the corresponding PLQY of the mat shows a negligible change after 100-time stretching-relaxing cycling test, manifesting its well stretchability. Besides, MAPbBr3-3 nanofiber mat is stretched up to 100 % and the PL emission is maintained at the same magnitude of order with a slight decrease after 1000 stretching relaxing cycles, as shown in Figure S9 (SI). It is not only because of the execellent elastic property of the TUP shell but also due to the well preservation of the perovskite NPs inside the TPU matrix induced by equivalent crosslinking effect as discussed earlier. After obtaining these various luminous core-shell nanofibers, we thus consider the feasibility of fabricating white-light emitting nanofibers. As known, a white-light emission can be achieved by the integration of red, green, and blue emitting components or emitting species through the visible light region. Such multi-material blend can possess Forster or Dexter energy transfer between each component to realize white-light emission. We recently fabricated a white-light emitting polymer/perovskite nanofibers by modulating the energy transfer between the polymer matrix and the perovskite NPs.41 However, the overall light-emitting efficiency needs to be improved. To achieve this goal, we rationally blend the orange-light emitting polymer, MEH-PPV, into the TPU shell and coupling the use of turquoise-light emitting MAPb(ClxBr1-x)3 NPs. MEH-PPV was chosen not only because of its emission wavelength range but also due to its suitable absorption spectrum to enable possible energy transfer with the employed NPs.52 To



better clarify the energy transfer between MEH-PPV and MAPb(ClxBr1-x)3 NPs, a uniaxial TPU/MEH-PPV nanofiber was first prepared.

The absorption and PL spectra of the prepared

hybrid nanofibers are shown in Figure 8a, in which an intense absorption below 600 nm and a broad emission across 450-700 nm are observed. As discussed earlier, the MAPb(ClxBr1-x)3 nanofibers emitted turquoise light with a PL peak located at ~ 465 nm, which well overlaps the absorption spectrum of the TPU/MEH-PVV nanofibers as portrayed in Figure 8a. This suggests that efficient energy transfer between is possible and favorable to achieve white-light emission. We therefore blended proper amount of MEH-PPV into the TPU shell solution to prepare the nanofibers and the detailed preparation conditions are described in the experimental section. As shown in the Commission internationale de l’éclairage chromaticity (CIE) coordinates (Figure 8b), the collected core-shell nanofibers emitted a light with a coordinate of (0.313, 0.321) located in the white-light region. Moreover, a fiber mat made from these nanofibers exhibits a PLQY of ~ 2.5%, which is only slightly lower than its parent MAPb(ClxBr1-x)3 nanofibers, revealing the effectiveness of this approach to prepare white-light emitting nanofibers. The stretchability of this fiber mat is shown in Figure 8c, in which a uniform whitelight emitting (under UV irradiation at 365 nm) is clearly observed even under stretched states. The corresponding PL intensity of the mat was slightly decreased under a 100% stretched state (Figure 8d), similar to other various luminous nanofibers discussed above (Figure 7 and S7 (SI)). These above results clearly manifest the successful fabrication of stretchable white-light emitting nanofibers, highlight the advantage of using one-step coaxial electrospinning method to prepare stretchable luminous nanofibers. Finally, we have integrated our perovskite nanoparticle embedded TPU fibers into a lightemitting diode (LED) to fabricate white light emitting diode (W-LED). Conventionally, polymer based LED technology offers thinner, lighter and enhanced physical properties such


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as more flexibility to the device. However, it possible to enhance the luminous efficiency (Lmax) and color-rendering properties upon incorporating quantum dot based materials into LED device through the attributed energy transfer similar to the one achieved in our [MAPb(ClxBr1x)3-TPU/MEH-PPV] coaxial nanofiber. Figure S10a (SI) illustrates the device structure of PFO

based conventional LED device consist of glass substrate / ITO/PEDOT:PSS /PFO/Ca/Al. The fabricated LED exhibits a clear blue light in the wavelength between 400-550 nm corresponding to the CIE index of (0.187-0,1881) and its reaching around 97.9cd/m-2 of maximum luminescence LMAX, as shown in Figure S10b-d (SI). To fabricate W-LED, MAPb(BrxI1-x)3 nanofibers were collected underneath emissive PFO layer. Rest of the components of the device was kept similar to one that we utilized in the LED case for comparison. Studied W-LED device architecture is illustrated in Figure 9a. The white light color emission, as indicated by (0.2645, 0.3526) of CIE coordinate in Figure 9b and 8d, is suggested to be achieved by the down conversion between blue light and orange light. The JV-L curve indicates that the maximum achievable luminescence is reached close to the one that is similar to the LED device and it is comparable with the previous perovskite based white light emitting devices in literature.52 This result indicates the potential of perovskite based TPU nanofibers to be utilized for color rendering in LED devices and it is easy to integrate feature on such kind of light emitting circuit.

3. CONCLUSION We have demonstrated light emitting perovskite based fibers using electrospinning. Uniaxial electrospun perovskite fibers were performed low PLQY resulted by the aggregation of perovskite nanoparticles on the fiber surface. However, the core-shell electrospun perovskite fibers exhibited better PL properties, that exceeding PLQY of 23.3%, owing to small sized perovskite nanoparticles growth and fiber morphology benefitting from particle confinement



imposed by the TPU shell.

Increase in stoichiometric ratio of MABr/PbBr2 in perovskite

precursor enhanced PL characteristics, attributed to the better crystal growth of perovskite. Also, decreasing in core flow rate led to the better fiber morphology and the formation of smaller nanoparticles, which further enhanced PL properties due to the quantum confinement effect. More importantly, collected core shell fibers were maintained their emission over 1 month by the hydrophobic nature of the TPU shell. Besides, scaled up fiber mats were fabricated to demonstrate practical applications in wearable electronics. Light emission from core-shell fibers were controlled by simply changing halide composition and mixing the ratio between PbX2 and MAX where X stands for Cl, Br and I atoms. By taking advantage of the elastic properties of the TPU shell matrix, different-color emitting fibers were performed a high flexibility as well as superior stretchability up to 100% while remaining the light emission intensity after 1000 times of stretching-relaxing cycles. Consequently, the stretchable white light emitting MAPb(ClxBr1-x)3 core/ MEH-PPV/TPU shell fibers were obtained via one step coaxial electrospinning with the PLQY of 2.5% and sustained its white color emission during stretching test up to 100 %.

Also, the emission color of stretchable fibers could be

successfully manipulated for green (MAPbBr3-1), violet (MAPbCl3-1), red (MAPbI3), turquoise blue (MAPb(ClxBr1-x)3), orange (MAPb(BrxI1-x)3), yellowish (TPU/MEH-PPV) and white emission (Pero-TPU/MEH-PPV). Finally, orange light emitting (MAPb(BrxI1-x)3) nanofibers were integrated underneath the PFO film for the white light emission of W-LED. These results demonstrated that perovskite based TPU electrospun fiber have potential applications in stretchable wearable cutting edge electronic devices.

4. EXPERIMENTAL SECTION 4.1 Materials: Polyether-based thermoplastic polyurethane (TPU), named as Elastollan 1185 A, with density of 1.2 g/cm3 and shore hardness of 86 was received from BASF. The lead


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halide compounds with ≥98% purity (PbX2, X = Cl, Br, I) were purchased from Alfa Aesar. N, N-Dimethylformamide (DMF, anhydrous ≥99%), dimethyl sulfoxide (DMSO, anhydrous, ≥ 99%), methylammonium halide (MAX, X = Cl, Br, I), Poly[2-methoxy-5-(2-ethylhexyloxy)1,4-phenylenevinylene]

[(MEH-PPV)

(Mn:

40,000-70,000)],

and

poly(9,9-di-n-

octylfluorenyl-2,7-diyl [(PFO) (Mn: >10,000)] were purchased from Sigma-Aldrich. All chemicals were used as received without any further purification.

4.2 Precursor Solution Preparation: To prepare the precursor solutions of MAPbBr3, MAPbI3, and MAPb(Brx-I1-x)3, PbX2 and MAX were dissolved in DMF. Nevertheless, due to the low solubility of Cl-based precursor solutions, MACl was mixed with PbBr2 and PbCl2 in a mixed solvent of DMSO/DMF at a volume ratio of 1/1 to prepare the precursor solutions of MAPb(Clx-Br1-x)3 and MAPbCl3. The perovskite precursor was prepared by mixing PbX2 and MAX at desired compositions. To prepare the precursor solution of polymer, TPU was dissolved in a mixed solvent of DMF/THF (v/v: 2/1) with a concentration of 150 mg/ml. For the uniaxial electrospinning process, both prepared TPU and perovskite solutions were directly mixed to give a final mixing ratio of perovskite to TPU as 10%. Note that, for preparing the white-light emitting fibers, 4.5 mg of MEH-PPV was added into the TPU precursor solution. To fabricate LED devices, PFO precursor was prepared in chlorobenzene to yield final concentration of 14mg/ml.

4.3 Preparation of Regular Electrospun Fiber: The coaxial (core-shell) electrospinning process was performed by separately feeding the core and shell solutions to fabricate the coreshell fibers, similar to that reported in our previous work.41 Detailed schematic illustration of the coaxial (core-shell) electrospinning setup is presented in Figure 1. As shown, two syringes connected to a coaxial spinneret with a diameter of 0.90 mm (for the inner one) and 1.25 mm



(for the outer one) were used to feed solutions by two syringe pumps (KD Scientific Model 100, USA). The flow rate of the shell precursor was kept at 1 ml/h while the rate of the core solution was controlled in the range of 0.01-0.1 mL/h. To prepare the core-shell electrospun fibers, for which the perovskite NPs were confined in the core site by the outer polymer shell, the tip of the core needle was connected to a high-voltage power supply (chargemaster CH30P SIMCO, USA) and a voltage was set at 16.6 kV. The nonwoven nanofibers were collected vertically with a fixed working distance (between the tip of the needle and the collector) of 20 cm. The whole electrospinning process was operated at room temperature (25°C) with a relative humidity (RH) of 50%. For the regular uniaxial electrospinning process, the TPU/perovskite blending solution was supplied to a single needle with an inner diameter of 0.84 mm by one syringe pump (KD Scientific Model 100, USA) at a flow rate of 1ml/h and operated under a voltage of 16.6 kV. Similar to the coaxial electrospinning process, the nanofibers were collected vertically with a working distance if 20 cm and proceeded in an ambient condition (25°C and a RH of 50%).

4.4 Preparation of Large Scale Electrospun Fibers: The large-scale fiber mats were fabricated by using a commercial FALCO FES-COL nanofiber electrospinning setup that operated in horizontal direction. The solution was fed into a needle with an inner diameter of 0.84 mm by one syringe pump at a flow rate of 1ml/h and operated under a voltage of 27.4 kV where tip to rotary collector distance is 15 cm for the uniaxial large scale fiber mats named as Uniaxial TPU/Pero. To fabricate large scale core-shell fiber mat named as Coaxial TPU/Pero, two syringes connected to respective needles with a diameter of 0.64 mm (for the inner one) and 1.27 mm (for the outer one) were used to feed solutions fabricated by using same commercial FALCO FES-COL nanofiber electrospinning setup. The flow rate of the shell precursor was kept at 1,2


Page 18 of 39

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ml/h while the rate of the core solution was controlled at 0.3 mL/h. The elecrospinning device was operated under a voltage of 23.1 kV where tip to rotary collector distance is 15 cm. The other experimental and ambient parameters that are not specified are the same as previous electrospinning methods.

4.5 Preparation of LED Device: The PFO based LED devices was fabricated on the ITO/Glass substrate. ITO substrates were pre-cleaned sequentially using deionized water, acetone, and IPA, followed by ozone treatment for 20 min. After cleaning the substrate, a PEDOT:PSS layer (45–50 nm) was spin-coated onto the ITO glass at 3000 rpm for 60 s and annealed at 130 °C for 15 min. 14 mg/ml PFO solution in chlorobenzene was spin-coated at 1000 rpm for 60 s in glovebox. Afterwards, calcium (Ca) and aluminum (Al) was thermally deposited onto the PFO active layer with thickness of 15nm and 100nm, respectively. The active are of fabricated LED devices are 0.2 × 0.2 cm2. To fabricate W-LED, orange light emitting (MAPb(BrxI1-x)3) aligned nanofibers were collected on the PEDOT:PSS spin-coated ITO/Glass substrates. Afterwards, PFO solution was spin-coated in glovebox atmosphere and rest of the process was followed similar to PFO based LED device.

4.6 Characterization: Morphological analysis of the perovskite NPs within the prepared nanofiber was characterized using transmission electron microscopy (TEM, FEI Tecnai G2 T20). The diameter and morphology of the prepared nanofibers were characterized using scanning electron microscope (Jeol JSM-6510) after the samples were sputtered with platinum. The optical absorption spectra of the prepared nanofibers were recorded using Hitachi U-4100 UV-visible spectrophotometer and their photoluminescence (PL) emissions were measured by Horiba Fluorolog-3 spectrometer system. Fluorescence optical microscope images were



collected by a confocal laser microscope (Leica LCS SP5). Fourier-transform infrared (FT-IR) spectra of fibers were recorded by a PerkinElmer Spectrum-Two Fourier Transform Infrared (FT-IR) Spectrometer instrument using the transmission technique in the range from 500 to 4000 cm-1. Differential scanning calorimetry (DSC) analysis was carried out on TA Instruments DSC 25. Stress-strain diagram was obtained by Shimadzu EZ-SX Texture Analyzer. W-LED and LED device performances including current-voltage, luminescence curve, EQE, and EL spectra were recorded by a spectrophotometer (PR-670) coupled with Keithley 2400. All the device measurements were conducted in ambient air at room temperature.

ASSOCIATED CONTENT Conflicts of interest There are no conflicts to declare.

Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org. SEM images. FTIR spectra. Fluorescence microscopic images. Photoluminescence spectra. LED I-V curves.

ACKNOWLEDGMENTS The authors appreciate the financial supports by “Advanced Research Center for Green Materials Science and Technology” from the Featured Area Research Center Program within the framework of the Higher Education Sprout Project by the Ministry


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of Education (108L9006) and the Ministry of Science and Technology in Taiwan (MOST 107-3017-F-002-001 and MOST 108-2636-E-194-001-).

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Table 1. Preparation conditions, parameters, and characteristics of the fabricated nanofibers. Core Shell PL MAX: Flow PLQY Sample (Polymer) Peak (X,Y) PbX2 [%] Rate Concentration Position (ml/h) MAPbBr3-Unia

1.5:1

150mg/ml

N/A

533nm

2.1

(0.2058,0.7454)

MAPbBr3-1b

1.5:1

150mg/ml

0.1

534nm

8.3

(0.2024,0.7447)

MAPbBr3-2b

3:1

150mg/ml

0.1

534nm

16.3

(0.1879,0.7327)

MAPbBr3-3b

3:1

150mg/ml

0.01

525nm

23.3

(0.1637,0.7366)

MAPbBr3-4b

3:1

120mg/ml

0.1

536nm

13.2

(0.1540,0.7305)


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MAPbBr3-5b

3:1

120mg/ml

0.01

532nm

16.8

(0.1618,0.7280)

MAPbCl3-1b

1.5:1

150mg/ml

0.01

405nm

Polymer Luminous Hybrid

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