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Abstract
The flexible photodetector is viewed as a research hotspot for numerous advanced optoelectronic applications. Recent progress has manifested that lead-free layered organic-inorganic hybrid perovskites (OIHPs) are highly attractive to engineering flexible photodetectors due to the effective overlapping of several unique properties, including efficient optoelectronic characteristics, exceptional structural flexibility, and the absence of Pb toxicity to humans and the environment. The narrow spectral response of most flexible photodetectors with lead-free perovskites is still a big challenge to practical applications. In this work, we demonstrate the flexible photodetector based on a novel (to our knowledge) narrow-bandgap OIHP of (BA)2(MA)Sn2I7, with achieving a broadband response across an ultraviolet-visible-near infrared (UV-VIS-NIR) region as 365-1064 nm. The high responsivities of 28.4 and 2.0 × 10−2 A/W are obtained at 365 and 1064 nm, respectively, corresponding to detectives of 2.3 × 1010 and 1.8 × 107 Jones. This device also shows remarkable photocurrent stability after 1000 bending cycles. Our work indicates the huge application prospect of Sn-based lead-free perovskites in high-performance and eco-friendly flexible devices.
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1. Introduction
Photodetectors can convert incident lights into electrical signals, which are crucial for a variety of applications, such as optical communication, infrared imaging, and biological sensing [1–5]. Lead halide perovskites, owing to their tunable bandgaps, large optical absorption coefficients, low density of trap states, high charge carrier mobility and long carrier diffusion length, their applications in the field of photodetection have become a hot research topic in the past few decades [6–9]. Despite these excellent characteristics, the toxicity in these Pb-based perovskites has become a huge stumbling block that hinders their practical applications. On the other hand, the bandgap energies of the Pb-based perovskites are commonly larger than 1.5 eV (corresponding to the wavelength of 827 nm) [10,11], thus the responsive wavelength range of these perovskite-based photodetectors is much narrower than the commercial silicon-based detectors (the responsive wavelength is up to 1100 nm) [12,13]. In this sense, it is necessary to develop alternative classes of nontoxic lead-free perovskites (including Bi-, Sn-, Mn-, Sb-, Cu-, and Yb-based) for high-performing optoelectronic applications. Among them, Sn-based perovskites have a series of excellent qualities, such as relatively narrow bandgaps (1.2-1.4 eV), low exciton binding energies (∼18 meV), and high carrier mobilities (∼102-103 cm2 V-1 s-1) [14–16], rendering Sn a promising substitution of Pb.
Flexible photodetectors capable of bending, rolling, folding, and stretching, have become an indispensable component in numerous cutting-edge applications, such as e-eyes, UV radiation skin sensors, oximeters, and foldable displays [17–20]. In the last few years, flexible photodetectors based on lead halide perovskites have been extensively reported profited from their superior optical and electrical properties [21]. However, flexible photodetectors based on lead-free perovskites are relatively less to be studied [22–28]. For instance, Liu et al. reported a planar photodetector based on CsSnBr3 film, for which the longest responsive wavelength is 785 nm and the highest responsivity is 9.2 A/W [22]. A self-powered photodetector based on Cs2AgBiBr6 film was reported by Shuang et al., in which the absorption edge is at ∼550 nm and the highest responsivity is 0.075 A/W [23]. Qian et al. reported a photodetector based on two-dimensional (PEA)2SnI4 perovskite film, in which the longest responsive wavelength of 660 nm and the highest responsivity of 16 A/W were obtained [24]. However, the bandgap energy of most reported lead-free perovskites are still larger than 1.5 eV, so unable to be directly used for NIR light detection. In addition, the majority of these lead-free perovskites was synthesized through spin-coating or drop-casting method with the grain boundaries. Compared with their single crystalline counterparts, these films commonly suffer from larger trap state densities, shorter carrier lifetimes, and lower carrier mobilities [8,9]. In contrast, high performance flexible photodetectors could be obtained with the perovskite single crystals.
In this work, we demonstrated a flexible photodetector based on the single crystal of (BA)2(MA)Sn2I7 (abbreviate to BMSI, where BA = n-butylammonium). The as-fabricated device exhibit broadband photodetection capacity, which was able to detect light ranging from UV to NIR band (i.e., 365-1064 nm), with the maximum responsivity and detectivity of 28.4 A/W and 2.3 × 1010 Jones at 365 nm, respectively, as well as 2.0 × 10−2 A/W and 1.8 × 107 Jones at 1064 nm, respectively. Furthermore, the photodetector showed excellent flexibility and robustness with no obvious attenuation of photocurrent after bending 1000 cycles. These superior photodetection performances suggest that BMSI single crystal could be a promising candidate for broadband flexible optoelectronic devices.
2. Results and discussions
Based on our former works, we designed a novel lead-free perovskite structure of BMSI, which might possess superior optoelectronic properties [29]. The crystal structure of BMSI is shown in Fig. 1(a). Its inorganic frameworks composed of the corner-connected SnI64- octahedra are anchored to the two bilayers of organic cations of BA+ along c-axis by strong N-H···I hydrogen bonds, with the protonated cations of MA+ residing in the voids of perovskite skeletons by moderate hydrogen bonding interactions, while the two neighboring organic layers are linked together by weak van der Waals forces. These features make the bulk crystals easily cleaved into microsheets through mechanical exfoliation. The bulk crystal of BMSI with centimeter scale was obtained from the aqueous solution by a temperature-cooling method (the inset of Fig. 1(a)), of which the phase purity was verified by the powder X-ray diffraction (PXRD, Fig. 1(b)). The transmission electron microscope (TEM) image (Fig. 1(c)) and the corresponding selected area electron diffraction (SAED) patterns of the exfoliated microsheets (the inset of Fig. 1(c)) manifest the two-dimensional layered structure with good stretchability and single crystalline feature of BMSI, respectively. The absorption and photoluminescence (PL) spectra were utilized to reveal the optical bandgap. An obvious absorption edge and a sharp PL peak at ∼1000 nm indicate the optical bandgap of BMSI is 1.24 eV (Fig. 1(d)). To the best of our knowledge, this bandgap is narrower than those of the typical lead-free perovskites, such as CsSnBr3, CsAgBiBr3, (PEA)2SnI4, and Cs3Bi2I9 [22–24,30]. These merits motivate us to exploit the broadband-responsive photodetectors based on the single crystal of BMSI. Figure 1(e) shows the Raman spectra of the bulk and two microsheets with the different thickness (D = 1.5 µm and 60 nm, respectively, corresponding to the layer number of 714 and 28) of BMSI. The atomic force microscopy (AFM) images and the corresponding height profiles of these two microsheets are given in Fig. S1 in Supplement 1. It can be seen that the Raman characteristic peaks of BMSI do not manifest strong thickness dependence. We ascribe this phenomenon to the unique crystal structure of layered OIHPs. Along the staking axis direction, the alternative arranged organic and inorganic layers are highly resistive and conductive, respectively. The large conductivity difference weakens the interlayer interaction between adjacent inorganic layers, so that the dependence of optical properties on the layer number for BMSI is weak. The sharp Raman peak at 114 cm-1 corresponding to the librations of MA+ cations, as well as two broad peaks at 208 and 289 cm-1 corresponding to the torsional modes of MA+ cations can be clearly observed under 532 nm excitation [31,32]. The PL peaks for the bulk and the two microsheets with different thickness of BMSI are all centered at ∼1000 nm (Fig. S2 in Supplement 1), further testify that the optical properties of BMSI are nearly independent on the layer number. The stability is a crucial issue for the applications of hybrid OIHPs. Here the Raman and the PL spectra of the as-exfoliated microsheet (D = 60 nm) and this microsheet stored in the vacuum drying oven for 1 month are compared. As shown in Fig. S3 in Supplement 1, the peak positions of both Raman and PL spectra do not manifest obvious shift, except for small attenuation of peak strength, verifying the long-term stability of BMSI. To further test the robustness of BMSI, the Raman and the PL spectra of the microsheet (D = 1.5 µm) before and after etching treatment (50 ccm Ar, 100 W power) are compared. It can be observed from Fig. S4 in Supplement 1 that the characteristic peaks of Raman and PL spectra are almost unchanged, which proves that the microstructure of BMSI are not destroyed after etching process.
Fig. 1. Crystal structure and physical properties of BMSI single crystal. (a) Two-dimensional structure viewed along the b-axis. Inset: photograph of the single crystal. (b) Experimental and calculated PXRD patterns. (c) TEM image of the exfoliated microsheets. Inset: the corresponding SAED patterns. (d) Absorption and PL spectra of the single crystal. (e) Raman spectra of the bulk and two microsheets with the different thickness.
The device structure of the BMSI-based photodetector is schematically illustrated in Fig. 2(a). The mechanically exfoliated sheet with micrometer scale was directionally transferred to the interdigital electrodes (5 nm Cr/ 50 nm Au, finger spacing is 9 µm) on a polyethylene terephthalate (PET) substrate, with light incident from the back of the device. The photograph of the whole device is shown in Fig. 2(b), with the microscopic photograph of the electrodes and the channel material shown in the inset of Fig. 2(b). The length and width of the channel are 9.7 and 25.7 µm, respectively, corresponding to an active area (A) of 2.5 × 10−6 cm2. The thickness of the channel material is ∼60 nm, corresponding to 28 layers. To determine the responsive wavelength range of this photodetector, the photocurrent was measured at varied wavelengths under the light intensity of 27.5 W/cm2 and the bias voltage of 1 V. As depicted in Fig. 2(c), this photodetector displays a broadband response ranging from ultraviolet (λ = 365 nm) to near-infrared region (λ = 1064 nm). The wavelength-dependent photocurrent manifests good agreement with the absorption spectrum shown in Fig. 1(d), indicating good spectral selectivity of this photodetector.
Fig. 2. The device structure and photoresponse of the BMSI-based photodetector (D = 60 nm). (a) Schematic device structure of the photodetector. (b) Optical photograph of the device. Inset: microscopic photograph of the photodetector. (c) Photocurrent dependence on light wavelength measured at the light intensity of 27.5 W/cm2 and 1 V bias voltage. (d, g) I-V curves under the different light intensities at λ = 365 and 1064 nm, respectively. (e, h) Logarithmic plot of the photocurrent dependence on light intensity under the bias voltage of 1 V at λ = 365 and 1064 nm, respectively. The solid black dots are experimental data, and the solid red line is the linear fit curve. (f, i) Responsivity and detectivity dependence on light intensity under the bias voltage of 1 V at λ = 365 and 1064 nm, respectively.
To further study the photoresponse characteristics of the BMSI-based photodetector, we measured its current-voltage (I-V) curves in the dark and under the different light intensities at two characteristic illumination wavelengths (λ = 365 and 1064 nm), as shown in Fig. 2(d) and (g), respectively. In both cases, the current increases with increasing light intensity due to the creation of photo-generated carriers, and the photoresponse is stronger at λ = 365 nm profited from the larger absorption coefficient. At the illumination wavelength of 365 nm, even the relatively weak light intensity (P = 0.94 W/cm2) and the low bias voltage (Vds = 1 V) are applied, this photodetector shows a significant photocurrent on/off (Ilight/Idark) ratio of 4.3. The slightly nonlinear and asymmetrical I-V curves indicate non-Ohmic contact between the BMSI microsheet and the gold electrodes, probably due to the Fermi-level pinning effect [33,34]. Furthermore, the photocurrent (Iphoton = Ilight - Idark) dependence on light intensity can be fitted with the power-law relation: Iphoton α Pθ, where θ is an exponent related to the photocurrent generation mechanism. At λ = 365 nm, the θ value is determined to be 0.67 (Fig. 2(e)). This fractional exponent (0.5 < θ < 1) suggests that, on one hand, the photo-generated carriers originate from conventional one-photon linear absorption process; on the other hand, trapping states within the channel material also play an important role [35,36]. However, at λ = 1064 nm, the exponent θ is derived to be 1.44 (1 < θ < 2) (Fig. 2(h)). In combination with the illumination wavelength of 1064 nm being below the bandgap of BMSI (λ = 1000 nm), and the high incident light intensity (P > 15 W/cm2), we deduce that the photo-generated carriers originate from two-photon nonlinear absorption process [37,38]. Moreover, the light intensity-dependent responsivity (R) and detectivity (D*) under the fixed bias voltage of 1 V are utilized to evaluate the performance of this photodetector. Here R and D* can be expressed as $R = ({{I_{\textrm{light}}} - {I_{\textrm{dark}}}} )/({PA} )$ and ${D^\ast } = R/{({2q{I_{\textrm{dark}}}/A} )^{1/2}}$, respectively, where q is the elementary charge. At λ = 365 nm, both R and D* decrease with increasing light intensity, and the highest values are 28.4 A/W and 2.3 × 1010 Jones, respectively, at P = 0.17 W/cm2 (Fig. 2(f)). These figures are on a par with that of many photodetectors based on low-dimensional materials [22–27,39–43], and comparison of the photodetection performance of this device with other representative works is summarized in Table 1. On the contrary, at λ = 1064 nm, both R and D* increases with increasing light intensity (Fig. 2(i)), further indicating the two-photon absorption process dominates the creation of photo-generated carriers. Because the transition probability of electrons from valence band to conduction band in the nonlinear two-photon case is much lower than that in the linear one-photon case, both R and D* measured at λ = 1064 nm are three orders of magnitude lower than those at λ = 365 nm, and the highest values are estimated to be 2.0 × 10−2 A/W and 1.8 × 107 Jones, respectively (Fig. 2(i)).
To investigate the dependence of photodetection performance on the layer number of channel material, the I-V curves of another photodetector based on a thicker microsheet of BMSI (A = 7.3 × 10−6 cm2; D = 15 µm, corresponding to 714 layers) were measured in the dark and under the illumination of 365 nm light. As shown in Fig. S5 in Supplement 1, although the photocurrent of the current device is larger than that of the former device due to the larger lateral size of the channel material, the photodetection performance of these two devices are similar. For example, the highest R and D* of this device are 33.1 A/W and 3.2 × 1010 Jones, respectively, at P = 0.17 W/cm2. Furthermore, the R and D* of four devices with the different thickness of channel material (varied from 0.06 to 15 µm) were compared. Seen from Fig. S6 in Supplement 1, the R and D* of these four devices are very close, and do not show thickness dependency. We consider that this behavior is closely related to the unique crystal structure of layered OIHPs, as mentioned before. The highly resistive organic layers block the photo-generated carriers to transport across layers. On the other hand, for the planar-type photodetector, the electric field of source-drain electrodes as well as carrier transport is along the in-plane direction. In this condition, only the few layers close to the electrodes contribute to the creation of photocurrent, leading to the weak dependence of photoresponse on the layer number of perovskite microsheet.
Table 1. Comparison of the photodetection performance between the BMSI-based photodetector and other typical photodetectors
To investigate the dependence of photodetection performance on the microstructure of device, the photocurrent response of four devices with the different channel length (L = 4, 9, 15, and 20 µm, respectively) were characterized. As shown in Fig. S7 in Supplement 1, both R and D* decreases with increasing the channel length. We infer that, as the channel length increases, the carrier transit time increases accordingly, leading to the worse collection efficiency of photo-generated carriers, as well as poorer photodetection performance. In this sense, optimization of the electrode structure can result in better photoresponse of a photodetector. In addition, we are trying to fabricate micro-/nano- patterns on the perovskite microsheets using the plasma etching method. We consider that this might be a good approach to further increase the photodetection performance of the perovskite-based photodetector.
To further compare the photoresponse performance at different incident wavelengths, the highest R and D* measured at each wavelength variation with incident wavelength (Vds = 1 V) are displayed in Fig. 3(a) (the I-V curves measured in the dark and under the different light intensities at varied incident wavelength are given in Fig. S8 in Supplement 1). Due to the smaller absorption coefficient at longer wavelength, both R and D* decrease with increasing light wavelength, and the highest R and D* is obtained at λ = 365 nm. As the noise in this device mainly comes from the shot noise created by the dark current, the noise current can be expressed as ${i_N} = \sqrt {2q{I_D}B} $, thus the noise equivalent power (NEP) can be calculated as $\textrm{NEP} = {i_\textrm{N}}/R$. Based on the above calculations, the dependence of NEP on illumination wavelength can be obtained. As shown in Fig. S9 in Supplement 1, the NEP is lower than 107 pWHz-1/2 in the wavelength range of 365-1064 nm, which suggests that this device has the ability to detect light signal as weak as 107 pW. Moreover, the responsive wavelength range of the BMSI-based photodetector are compared with other representative works. As shown in Fig. 3(b), the responsive wavelength range of this photodetector is broader than a number of lead-free perovskite-based photodetectors [22–27], and is comparable with some photodetectors based on 2D material and/or their heterostructures [39–43], verifying the broadband detection ability of the BMSI-based photodetector.
Fig. 3. (a) The highest responsivity and detectivity at each wavelength variation with light wavelength measured under 1 V bias voltage. (b) Comparison of the responsive wavelength range between the BMSI-based photodetector and other representative works.
Polarization-sensitive photodetection is an important application for a photoelectric material of anisotropic crystal structure. Motivated by the polar crystal structure of BMSI (orthorhombic system, mm2 point group), the photocurrent response under the polarized light illumination (λ = 365 nm and P = 0.94 W/cm2) and the bias voltage of 1 V was performed for the BMSI-based photodetector. The experimental setup is schematically plotted in Fig. 4(a). A half-wave plate in combination of a linear polarizer was utilized to alter the polarization of illumination light. The longer side of the exfoliated microsheet corresponds to the a-axis direction, which is parallel to the electric field of the source-drain electrodes. As shown in Fig. 4(b), the angle-resolved photocurrent demonstrate a cosine function relationship in a period of π. Moreover, the photocurrent reaches maximal (Imax) when the polarization direction parallels to the a-axis of BMSI, while the photocurrent reaches minimal (Imin) when the polarization direction parallels to the b-axis of BMSI. The Imax/Imin value is ∼1.3, and the photocurrent anisotropy ratio of this photodetector is derived to be ∼0.13 according to the expression of $({{I_{\textrm{max}}} - {I_{\textrm{min}}}} )/({{I_{\textrm{max}}} + {I_{\textrm{min}}}} )$. This anisotropy ratio is comparable with some typical materials, such as Bi2Te3 (0.23) and GeAs2 (0.33) [44,45]. As is depicted in Fig. 4(c), the synergistic effects of the distortion of SnI64- octahedra and the highly orientation of MA+ cations lead to anisotropic transition probability thereby anisotropic light absorption along different crystallographic direction. To verify this hypothesis, the polarized light absorption measurement was performed for the microsheet of BMSI. Seen from Fig. 4(d), the experimental rule of the polarized light absorption is consistent with that of the polarized photocurrent, which testify that the anisotropy of photocurrent stems from the anisotropic light absorption.
Fig. 4. Polarization-sensitive photodetection of the BMSI-based photodetector. (a) Schematic illustration of the experimental setup. (b) Polar plot of the angle-resolved photocurrent measured under 1 V bias voltage at λ = 365 nm. (c) In-plane crystal structure of BMSI. (d) Polar plot of the angle-resolved normalized absorbance measured at λ = 365 nm.
Temporal photoresponse is a crucial performance parameter for a photodetector. As shown in Fig. 5(a) and (c), the current-time (I-t) curves of the BMSI-based photodetector were measured under periodic on-off laser illumination at λ = 365 and 1064 nm, respectively. In both cases, this photodetector exhibits stable and reproducible current on-off switching behavior. Furthermore, the rise time (τrise, the time required to increase the photocurrent from 10% to 90% of the peak value) and decay time (τdecay, the time required to decrease the photocurrent from 90% to 10% of the peak value) of this photodetector can be extracted from the single on-off switching cycle. As shown in Fig. 5(b) and (d), τrise is estimated to be 0.7 and 0.6 s at λ = 365 and 1064 nm, respectively; while τdecay is estimated to be 1.2 and 1.3 s at λ = 365 and 1064 nm, respectively. This response time is slower than a number of Pb-based perovskite photodetectors, but is comparable with some lead-free perovskite photodetectors [22–27]. The slow response time may be related to the trap states caused by Sn vacancies or surface impurities, and self-trapped states caused by electron-phonon coupling [24,46–48]. There is room for improvement in device performance, for example, passivating the crystal surface with weakly polar solvent to eliminate the surface impurities, and constructing two-dimensional p-n junctions with other low-dimensional materials to facilitate the separation and transportation of photo-generated electron-hole pairs [49–51].
Fig. 5. Temporal measurements of the photocurrent for the BMSI-based photodetector. (a, c) On-off switching cycles of photocurrent measured under 1 V bias voltage at λ = 365 and 1064 nm, respectively. (b, d) The rise and decay times extracted from the single on-off switching cycle at λ = 365 and 1064 nm, respectively.
The device stability is of vital importance for the practical application of the OIHP-based photodetectors. For this reason, both operational and long-term stability was performed for the BMSI-based photodetector. Under the light illumination (λ = 365 nm, P = 0.17 W/cm2) for as long as 90 min in ambient condition (20°C temperature and 60% relative humidity), the photocurrent of this device can maintain 82% of its original value (Fig. 6(a)). Besides, the photocurrent of the device stored in the vacuum drying oven for one month can still demonstrate good on-off cycles, and can keep 73% of its original value (Fig. 6(b)). We deduce that, these superior stability of the device is closely related to the unique crystal structure of BMSI. To some extent, the hydrophilic organic layers can prevent the penetration of moisture and oxygen into the inorganic layers. The device stability may be further improved by encapsulating the BMSI microsheet with PMMA film or h-BN microsheet.
Fig. 6. The operational and long-term stability measurements for the BMSI-based photodetector. (a) The photocurrent variation with time under the light illumination (λ = 365 nm and P = 0.17 W/cm2) and 1 V bias voltage. (b) On-off switching cycles of photocurrent measured under 1 V bias voltage for the as-fabricated device and this device stored in the vacuum drying oven for 1 month (λ = 365 nm and P = 0.17 W/cm2).
The flexibility of photodetectors is also of great importance in practical applications. To test its flexibility and robustness, the BMSI-based photodetector fixed in a vernier caliper was bent at the different angles to measure its photocurrent (Fig. 7(a)). As shown in Fig. 7(b) (λ = 365 nm) and Fig. S10 in Supplement 1 (λ = 532 and 1064 nm), the I-V curves measured at the three different bending angles (corresponding to state I, III, and V, respectively, shown in Fig. 7(a)) exhibit great similarity. Furthermore, this photodetector shows stable photocurrent response at different illumination wavelengths (λ = 365, 532, and 1064 nm) and varied bending angles (corresponding to state I, II, III, IV, and V, respectively, shown in Fig. 7(a)), almost invariable with time and bending angles (Fig. 7(c)). Finally, the photocurrent signals were recorded at a fixed bending angle (corresponding to state III shown in Fig. 7(a)) after bending the device for different cycles, and no obvious degradation of photocurrent is observed even after 1000 bending cycles at the different illumination wavelengths (Fig. 7(d)). These excellent performances indicate the potential applications of the BMSI-based flexible photodetector in foldable optoelectronic devices.
Fig. 7. Flexible photodetector based on BMSI microsheet. (a) Photographs of the photodetector bent at the different angles. (b) I-V curves of the photodetector at the different bending angles illuminated by the 365 nm light (P = 0.54 W/cm2). (c) I-t curves of the photodetector with light illumination (λ = 365, 532, and 1064 nm) bent at the different angles. (d) Photocurrent variation after bending the photodetector for different cycles at a fixed angle (corresponding to state III).
Taking all the advantages of BMSI into consideration, especially broadband response, high responsivity, superior flexibility, as well as its nontoxic nature, we consider that the photodetectors based on lead-free layered OIHPs could become commercialized, as long as the stability of this kind of perovskites can be further improved. In fact, as far as we know, numerous scientists are trying their best to increase the stability of hybrid perovskites in recent years. Based on the above reasons, we have the confidence in commercialization of the lead-free hybrid perovskite-based photodetectors.
3. Conclusions
In summary, the flexible photodetector based on a narrow bandgap OIHP single crystal of (BA)2(MA)Sn2I7 was demonstrated, with superior performance including spectral response across UV-VIS-NIR band (λ = 365-1064 nm), high responsivity and detectivity (28.4 A/W and 2.3 × 1010 Jones at 365 nm, respectively; 2.0 × 10−2 A/W and 1.8 × 107 Jones at 1064 nm, respectively), as well as remarkable photocurrent stability after 1000 bending cycles. This work may open up a new avenue for environmentally-friendly peroviskites in flexible optoelectronic device applications.
Funding
National Key Research and Development Program of China (2019YFA0705000); National Natural Science Foundation of China (11774161, 12022403); Natural Science Foundation of Jiangsu Province (BK20192001); Natural Science Foundation for Excellent Young Scholars of Jiangsu Province (BK20190071).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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