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Abstract
The novel fluorescent nanofiber membranes of CsPbX3 (FNMs/CPX, X = Cl, Br, I) with a wide photoluminescence range from 405 nm to 675 nm are fabricated by a one-step electrospinning method in this paper. Owing to the polymer cladding, these FNMs/CPX show much better thermal and humid stability compared to the common CsPbX3 particles, and the corresponding white light-emitting diode prepared by them also exhibits excellent optical properties. Without adopting any complicated processes, this method opens up a brand new way for the perovskite materials using in lighting and display fields.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Over the last several years, perovskite materials have undoubtedly been attractive stars in the material science due to their developments in solar cells [1,2]. These attentions not only bring great evolutions in the photovoltaic fields, but also promote applications of the perovskite materials in other aspects, such as photodetector, light-emitting diode (LED), photocatalysis, laser device, etc [3–10]. Among these, all-inorganic halide perovskite materials (e.g. CsPbX3, X = Cl, Br, I) have been proved to show promising prospect in applications for luminous devices, e.g. white light-emitting diodes (WLEDs) [11,12], due to its desirable optical performance, such as wide luminescence spectrum (from 400 to 700 nm), high quantum yields (over 90%), narrow full width at half maximums (FWHMs, ca. 12 - 35 nm) and so forth [13–15]. Although CsPbX3 is regarded as one of the most potential fluorescent materials, its applications in WLEDs is still a challenge: on the one hand, mixing CsPbX3 particles with different colors (i.e. different chemical compositions) together will easily lead to the cation exchanges between each other [16], on the other hand, the poor heat and water resistance can also cause their quick degradation [17], which are both adverse to the long-term stability for the WLED. Generally, coating particles with a shell, such as silica or polymers [18,19], is a common method to resolve this problem, but the tedious preparation process and low throughput production make them still far away from practical applications.
Inspired by the encapsulation method, fluorescent nanofiber membranes (FNMs) with structure of CsPbX3@polymers by electrospinning (E-spin) have attracted more attentions during the past few years [19–21]. Besides the advantages of encapsulation method, FNM is more flexible and durable in contrast with the core-shell nanoparticles [22–24], and moreover, E-spin is usually regarded as one of the simplest way for preparing large-scale nanofiber membranes (NMs), which therefore is very suitable for commercial viability [25–27]. In 2016, Li’s group prepared a monolithic polystyrene (PS) fiber membrane encapsulated with CsPbBr3 quantum dots (QDs) by E-spin for the first time [19]. Subsequently, they explored its applications in many fields, such as fluorescence resonance energy transfer device, biomolecules sensor, pH sensor and metal ions detector [19,21]. In the same year, Wang et.al. also obtained the hybrid CsPbX3@PS NMs through the same way, and assembled them into a WLED which can perform not only better optical properties but also long-term stability [20]. Although all these FNMs exhibit outstanding properties, the preparation processes are still too complicated, which are usually multistep: firstly, synthesizing CsPbX3 QDs by a chemical method, then mixing them into precursor solution, and at last forming the FNMs by E-spin. Moreover, at the present stage, preparing CsPbX3 QDs usually involves not only the high temperature and toxic solvent, but also the time-consuming purification process. Therefore, the whole progress to prepare the FNMs is very troublesome.
To solve this problem, in this paper we report a one-step method to fabricate the CsPbX3-based FNMs (FNMs/CPX) through E-spin, which is much easier in contrast with those traditional ways. Compared with the CsPbX3 particles, our FNMs/CPX show same luminescence properties, e.g. wide luminescence spectrum (from 405 to 673 nm), narrow FWHMs (from 16 to 35 nm), etc., and better stability under hostile environments including humid and hyperthermal conditions. The FNMs/CPX are subsequently integrated into the commercial blue light-emitting chip to fabricate a WLED, which also shows excellent color rendering property and durability. In consideration of the remarkable performance and relative simple preparation process, these FNMs/CPX will certainly show promising applications in many fields such as lighting and display.
2. Experimental details
2.1 Materials
Lead chloride (PbCl2, ≥ 99.0%), lead bromide (PbBr2, ≥ 99.0%), lead iodide (PbI2, ≥ 98.0%), cesium chloride (CsCl, ≥ 99.0%), cesium bromide (CsBr, ≥ 99.5%), cesium iodide(CsI, ≥ 99.9%), n,n-dimethylformamide (DMF, ≥ 99.5%), dimethyl sulfoxide (DMSO, ≥ 99.0%), polystyrene (PS, Mw = 104.14 g/mol) and polyvinyl alcohol (PVA, Mw = 44.05 g/mol) were all obtained from Aladdin Reagent Ltd., China and used without further purification. Copper foil, absolute alcohol (EtOH, AR) and acetone (AR) were all purchased from Sinopharm Chemical Reagent Co., Ltd.
2.2 Preparation of the FNMs/CPX
FNMs/CPX are prepared by a traditional E-spin method as shown in Fig. 1(a): a high voltage was applied between an injection pump and copper foil for a certain time, where the former one contains the precursor solution, including certain amounts of polymer, organic solvent, CsX and PbX2. For FNM/CPC (X = Cl) and FNM/CPB (X = Br), the polymer and solvent were respectively polyvinyl alcohol (PVA) and DMSO, and for FNM/CPI (X = I), the PS and DMF were chosen.
Fig. 1 (a) preparation progress of the FNMs/CPX by one-step E-spin method, (b0, c0 and d0) SEM images of the FNMs/CPX, (b1, c1 and d1) photographs of the FNMs/CPX under UV radiation (365 nm, 20W), (b2, c2 and d2) TEM images of the FNMs/CPX, (e0, e1 and e2) XRD patterns of FNMs/CPX, (e3 and e4) crystal structures of CsPbX3: (e3) cubic phase, (e4) orthorhombic phase.
The precursor solution was prepared by mixing 0.4 mmol PbX2 and 0.4 mmol CsX in 10 ml of DMSO (X = Cl, Br) or DMF (X = I) at 45 °C for 10 min. Then, 1.22 g of PVA (X = Cl, Br) or 0.90 g of PS (X = I) was added in this solution and kept stirring for 3 h to form a homogeneous sticky fluid. Subsequently, these precursor solution was partly transferred into a 10 ml syringe with a spinneret of 0.84 mm in diameter. During the spinning process, the flow rates of the precursor solution were kept respectively at 0.67 ml/h (X = Cl), 0.75 ml/h (X = Br) and 0.73 ml/h (X = I) and the high voltage was set at 15 - 19 kV. The distance between the Cu foil and the spinneret were changed from 6 cm to 15 cm, and the environment temperature and relative humidity (RH) were kept at 35 °C and 30% during the whole experiment.
3. Results and discussions
3.1 Morphologies, crystallization and phase
Figure 1(b1, c1 and d1) firstly show the photographs of these FNMs/CPX under the ultraviolet (UV) illumination (365 nm, 20 W, power density 12 mW/cm2). Visually, the FNM/CPC, FNM/CPB and FNM/CPI respectively emit blue, green and red lights when they are excited, and the even color distributions over the whole areas indicate that all the FNMs/CPX are well homogeneous. The micro morphologies of these FNMs/CPX are exhibited through the scanning electron microscope (SEM) images (Fig. 1(b0, c0 and d0)). It can be seen that the surface of the PVA nanofibers is smoothing, while the surface of PS is slightly rough. The widths of all the nanofibers are uniform, and according to the diameter distribution histogram (Fig. 2), the average widths of PVA and PS nanofibers are ca. 65 nm (FNM/CPC), 70 nm (FNM/CPB) and 1.7 μm (FNM/CPI), respectively.
The transmission electron microscope (TEM) images (Fig. 1(b2, c2 and d2)) show that there are many nanoparticles in these nanofibers (marked as the red dashed box), which can be proved to be the CsPbX3 by subsequent X-ray diffraction (XRD) tests. The corresponding size distributions of the CsPbX3 nanocrystal are shown as Fig. 3, from which it can be found that the average size of CsPbCl3 and CsPbBr3 is ca. 10 - 15 nm, and the size of CsPbI3 is ca. 200 - 300 nm. According to the TEM images, the CsPbX3 are well wrapped by the PVA or PS nanofibers, which will greatly improve the stability of these FNMs/CPX as we mentioned in the introduction, and the succedent stability tests further demonstrate this fact as well. Figure 1(e0, e1 and e2) show the XRD patterns of these FNMs/CPX in 15° - 70° 2θ range: the patterns in Fig. 1(e0) and 1(e1) are extremely similar, except for some subtle migrations of the characteristic peaks, which may mainly originates from the weak crystal deformation caused by different atomic radius of Cl and Br. All the peaks in these two patterns correspond well with the standard diffraction peaks of JCPDS card No. 75-0411 (CsPbCl3) and JCPDS card No. 75-0412 (CsPbBr3), implying that the FNM/CPC and FNM/CPB are both cubic phase. Without any other noise peaks observed in these patterns, the FNM/CPC and FNM/CPB show high purity and crystallization. By contrast, the XRD intensity of the FNM/CPI is very weak, which indicates its crystallization is relatively low, and the peaks located ca. 25.77°, 27.30°, 31.49°, 37.73° respectively correspond to the (121), (122), (105) and (200) planes of the orthorhombic phase (JCPDS card No. 74-1970), demonstrating the orthorhombic crystallization of this film. According to previous reports [28–30], in comparison with the CsPb(Cl, Br)3, CsPbI3 is more unstable when lacking the sufficient surface ligands, which will result in the fast agglomeration, weak crystallization and undesired phase transformation from cubic to orthorhombic. According to the size distributions of the CsPbX3 nanocrystal, it’s known that the size of CsPbI3 is obviously larger than those of CsPb(Cl, Br)3, verifying the fact of agglomeration meanwhile further demonstrating this unstable phenomenon. Figure 1(e3) and 1(e4) exhibit the two crystal structures of the cubic and orthorhombic phase. Because the PVA and PS have no contribution to the peak intensity of the XRD, these characteristic peaks can only come from the inner nanoparticles. It can be further demonstrated by the high-resolution TEM (HRTEM) images (Fig. 4(a6, b6 and c6)), where the observed clear lattice fringes with intervals of 2.51 Å (plane (210)), 2.49 Å (plane (210)) and 2.31 Å (plane (202)) implies the crystallization of the inner particles. While, this characteristic can’t be found in the surrounded polymers (PVA or PS). Figure 4(a1-a4, b1-b4 and c1-c4) are respectively the energy dispersive spectrum (EDS) and the elements mappings of these FNMs/CPX, and the sharp peaks and homogeneous distributions of Cs, Pb and X (X = Cl, Br and I) confirm the presence of CsPbX3 in these NMs, which corresponds well to the results obtained by XRD.
Fig. 4 HRTEM images, EDS spectra and elements mappings of the FNMs/CPX: (a1) - (a6) FNM/CPC; (b1) - (b6) FNM/CPB; (c1) - (c6) FNM/CPI.
3.2 Formation mechanism and optical properties
According to the above description, the FNM/CPC, FNM/CPB and FNM/CPI are visually blue, green and red respectively under UV illumination, and for further analysing this phenomenon, the photoluminescence (PL) spectra of the FNMs/CPX are tested. The solid lines in Fig. 5(a) show that the PL peaks for these FNMs/CPX are ca. 405 nm (blue), 514 nm (green) and 673 nm (red), respectively, and the corresponding FWHMs can be measured as 16 nm, 23 nm and 35 nm (from Cl to I). The absorption peaks of them (dashed lines) locate at ca. 399 nm, 501 nm and 658 nm respectively, and among them it’s observed that the intensity of FNM/CPI is obviously weaker than those of the other two, which may mainly originate from the lower crystallization of FNM/CPI. By changing the ratio of Cl, Br and I in FNMs/CPX, we successfully obtain almost all the PL peaks between 405 nm - 673 nm, which are partly shown in Fig. 5(b) (more spectra can be found in Fig. 6). To our knowledge, the known PL ranges for CsPbX3 nanoparticles usually change from 400 nm to 700 nm, and many of them can only alter from 410 to 640 nm (Table 1). Moreover, although the size and shape of CsPbX3 in FNMs are not uniform and regular, the FWHMs of these FNMs/CPX are relatively narrow, which is at the same level with some CsPbX3 QDs. Table 1 shows the comparison of PL scope and FWHMs between our FNMs/CPX and other reported samples. These wide selectivity for colors and narrow PL FWHMs will provide the possibility for these FNMs/CPX to get high color rendering index (CRI) and broad color gamut, which are very critical to improve the properties of the LED based on them.
Fig. 5 (a) normalized PL (solid line) and absorption (dashed line) spectra of FNMs/CPX, (b) partial PL spectra of FNMs/CPX between 405 nm - 673 nm, (c) reaction mechanism of the FNMs/CPX.
Table 1. Comparisons of PL ranges and FWHMs between the FNMs/CPX and other known samples.
In fact, the reaction mechanism for preparing the FNMs/CPX is simple, and for better stating this phenomenon we divide the whole process into three parts, which are marked as I, II and III in Fig. 5(c). In part I, the Cs+, Pb2+, and X- ions are uniformly dispersed in a mixed solution, containing PVA (PS) and DMSO (DMF), which performs as a precursor. From the corresponding dashed box, it can be observed that these solutions are all translucid and without fluorescence no matter under fluorescent lamp (left one) or UV lights (right one), implying that there are no CsPbX3 formed. In part II, these solutions are ejected from the syringe and form a Taylor cone under the influence of high voltage [23]. During the stretching of the filament, the DMSO (DMF) volatilizes quickly accompanying with the reduction of the fiber size, which leads to the decline of solubility for CsPbX3 and PVA (PS), and subsequently the PVA (PS) will solidify and the CsPbX3 will be separated out (part III). Essentially, this process is the same as obtaining salt from water: getting solid crystals by changing the solubility. In previous reports, both the macro crystal and micro QDs of CsPbX3 can be obtained through the similar ways [11,36,37], but it’s the first time to use this methods to synthesize the FNMs/CPX.
3.3 Stability
Stability is a very important criterion when evaluate the practical feasibility of fluorescent material, thus the common stability of these FNMs/CPX under natural environment (RT: 20 °C - 30 °C, RH: 25% - 32%) are firstly tested, and the corresponding results are shown in Fig. 7(a0-a3). Benefiting from the packaging of PVA and PS, it can be observed that the PL intensities of all FNMs/CPX are almost unchanged after one week except for slight fluctuations, and the photographs of them under UV radiation (insets in Fig. 7(a0)) look basically the same as well. When increasing the environment temperature from 20 °C to 100 °C, the PL intensities (Fig. 7(b0) solid lines) respectively decline 25% (FNM/CPC), 30% (FNM/CPB) and 40% (FNM/CPI), and then when the temperature cools down to room temperature, all the intensities will nearly recover to those before the heat treatment (Fig. 7(b0) dashed lines). This reversible thermal stability can be attributed to the restricted ion diffusion by the polymer protecting layers. Keeping the temperature unchanged (25 °C) and increasing the humidity from 25% to 50%, it can be seen that the decline of PL intensities for FNM/CPC and FNM/CPB are more obvious than that of FNM/CPI (Fig. 7(b2)). This is mainly due to that the PVA is water-soluble, and conversely, the PS is water-insoluble. To enhance the water resistance of these films, 1H, 1H, 2H, 2H-heptadecafluorodecyltrimethoxy-silane (C13H13F17O3Si) is used to improve the hydrophobic properties of them (more details can be found in the appendices). Figure 7(c0-c5) show the water contact angle (WCA) images of these FNMs/CPX before and after modified, from which one can see that the water resistance of all FNMs/CPX is improved, and the corresponding WCAs of them are 148° (FNM/CPC), 149° (FNM/CPB) and 153° (FNM/CPI), respectively, which means that they are all highly hydrophobic. When testing the water resistance of these modified films again, we found that the PL intensities are almost unchanged when the humidity increase from 25% to 50% (Fig. 7(b3)). Figure 7(b1) shows the photostability of these FNMs under continuous UV radiation (365 nm, 20 W, power density ≈12 mW/cm2), it can be observed that the PL intensities only decrease 7% - 10% after 120 h. By contrast, the PL intensities of the bare CsPbX3 particles decline ca. 55% - 70% under the same thermal stability, water resistance and photostability tests according to previous reports [16,20]. Thus, it’s reasonable to believe that the whole stabilities of these FNMs/CPX are quite outstanding in every respect. For estimating the repeatability of the measurements between samples, the ratios of PL intensities for FNMs/CPX after and before stability test, respectively collected from 6 different batches samples, are shown as Fig. 8, from which it can be observed that these measurements are well repeated.
Fig. 7 (a0) the time-dependence PL spectra of the FNMs/CPX and the insets are corresponding photographs of them under UV radiation, (a1, a2 and a3) time-dependence relative PL intensity for FNM/CPC (405 nm), FNM/CPB (514 nm) and FNM/CPI (673 nm), respectively; (b0) relative PL intensity for FNMs/CPX under different temperatures (solid line) and recover after be heated (dashed lines), (b1) relative PL intensity change for FNMs/CPX under UV irradiation, (b2) and (b3) are respectively the relative PL intensity for FNMs/CPX under different humidity before and after hydrophobic modification; (c0-c5) are the corresponding WCAs of them.
Fig. 8 The ratios of PL intensities for FNMs/CPX after and before stability test, respectively collected from 6 different batches samples: (a) after 7 days, (b) after heated at 100 °C, (c) recovered after heated at 100 °C, (d) after exposed to UV for 120 h, (e) after treated at RH of 50%, (f) modified FNMs/CPX after treated at RH of 50%.
3.4 Application in WLED
Outstanding PL performance and stability of these FNMs/CPX endow them a well application prospect, and in this paper we selected WLED as an example to verify our hypothesis. As shown in the schematic (Fig. 9(a)), to prepare a WLED, the well-proportioned FNM/CPI and FNM/CPB are placed on the surface of a InGaN chip (more details can be found in appendices), which cannot only provide the blue light (465 nm) for the WLED, but also serve as the excitation source for the FNM/CPI and FNM/CPB. The photograph of this WLED is exhibited in Fig. 9(b1) (the right one), and after applying a voltage of 3.0 V and current of 60 mA on it, the dazzling white light is observed (Fig. 9(b2)). The spectrum of this white light is shown in Fig. 9(b) (spectra of the WLED under different currents can be found in Fig. 10 and the corresponding Commission Internationale de l'Eclairage (CIE) coordinate can be calculated to (0.34, 0.32), which is almost equal to the standard white color value of (0.33, 0.33).
Fig. 9 (a) the schematic of the prepared WLED, (b) the EL spectrum of the WLED prepared by FNM/CPB and FNM/CPI; the inset (b1) is the photograph of the blue LED (InGaN chip, the left one) and the prepared WLED (the right one); the inset (b2) is WLED drived by a voltage of 3.0 V, (c) CIE color coordinates corresponding to the FNMs/CPX and NTSC standard.
Fig. 10 The EL spectra of the WLED operated with different forward-bias currents, and the table below shows the corresponding optical power under these currents.
Keeping the WLED working for 12 h, a large change is found between 500 nm - 700 nm in the spectrum (Fig. 11(a)), which mainly originates from the thermal- and light-induced degradation for the FNM/CPI and FNM/CPB. However, if we stop the WLED work and cool it down to room temperature, the spectrum will recover when a voltage is applied on it again (Fig. 11(a)). Compared to the irreversible cation exchanges occurred between different bare CsPbX3 particles, FNMs/CPX can make the corresponding WLED recycling (Fig. 11(b)), greatly improving the durability of the device to a certain extent. Partial PL peaks obtained in this paper between 405 nm - 673 nm are transformed into the corresponding CIE coordinates which are all marked as the black dots in Fig. 9(c). These dots have the vast majority of CIE coordinates surrounded, in other words, implying that all the color inside this area can be theoretically obtained by mixing the different proportioned FNMs/CPX. What’s more, the scope of the NTSC standard (white dashed line) are all covered in this area as well, further demonstrating the great potential of these FNMs/CPX in lighting and display devices.
4. Conclusions
In summary, the novel FNMs/CPX with a wide PL range from 405 nm to 675 nm are fabricated by a one-step E-spin method for the first time in this paper. Due to the protection of the polymers, these FNMs/CPX exhibit remarkable stability under various harsh environments, including high temperature, heavy humidity and long-time UV illumination, which greatly improves the durability of the corresponding WLED. Although there are still some defects for these FNMs/CPX, e.g. the WLED is not very stable after a long running, in consideration of the simple preparation method and better stability in contrast with those of the CsPbX3 particles, this paper undoubtedly opens up a brand new way for the perovskite materials using in lighting and display fields.
Appendices
Stability test: The drying oven (DHG - 9076, temperature range: 25 - 300 °C) was used in the thermal stability test, which was obtained from Shanghai Jinghong Experimental Equipment Co., Ltd. The constant temperature and humidity test chamber (SPX-50B, temperature range: 5 - 50 °C, humidity range: 20 - 90% RH) was used in humidity stability test and the ultraviolet analyser with light of 365 nm (power = 20 W, power density 12 mW/cm2) was used in photostability test, and both of them were purchased from the Shanghai Lichen Technologies Co., Ltd.
Modification of the heptadecafluorodecyltrimethoxy-silane: Firstly, 150 µL of the 1H, 1H, 2H, 2H-heptadecafluorodecyltrimethoxy-silane (C13H13F17O3Si) and 3 mL of the ethyl alcohol were added into a beaker, followed by ultrasonic treatment. Subsequently, the beaker and the FNMs/CPX were put into a sealed box and heated in a drying oven at 85 °C for 4 h. Then, the samples were placed into a fume cupboard and cooled to room temperature naturally. After that, the highly hydrophobic FNMs/CPX was obtained.
Preparation of WLED: The InGaN chip with a dismountable semicircle protective shell was obtained from the Shenzhen Mingqiang Electronics co., LTD. In preparing the WLED, the FNM/CPI (0.25 mm thickness) and FNM/CPB (0.1 mm thickness) were respectively tailored to a small wafer, which can fit well to the spherical surface of the protective shell. The FNM/CPI was firstly putted closely to the inner surface of the protective shell, and then the FNM/CPB clinged to the FNM/CPI.
Funding
We sincerely thank the National Natural Science Foundation of China (No. 21701102 and No. 11747076), Natural Science Foundation of Shandong Province (No. ZR2017BA018 and No. ZR2017BA004), Shandong Provincial Science and Technology Project (NO. J17KZ002) and China Postdoctoral Science Foundation (No. 2017M612322, No. 2016M600550 and No. 2017M622251) for financial support.
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