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Abstract
The organic-inorganic hybrid perovskite CH3NH3PbBr3(MAPbBr3) has been well developed in the X-ray to visible light band due to its superior optoelectronic properties, but this material is rarely studied in the infrared band. In this paper, a UV-NIR broadband optical detector based on MAPbBr3 single crystal is studied, and the response range can reach the near-infrared region. In the visible light band, the optical response of the device is mainly caused by the photoelectric effect; in the near-infrared band, the optical response of the device is mainly caused by the thermal effect. The carrier response of MAPbBr3 material under different wavelengths of light was investigated using a non-contact measurement method (optical pump terahertz (THz) probe spectroscopy). This paper also builds a set of photoelectric sensor array components, and successfully realizes the conversion of optical image signals to electrical image signals in the visible light band and infrared band. The experimental results show that MAPbBr3 crystals provide a new possibility for UV-NIR broadband photodetectors.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
In recent years, organic-inorganic hybrid perovskite (MAPbX3, where MA = CH3NH3+, and X = Cl−, Br− or I−) materials have demonstrated many advantages, including high absorption coefficients, high carrier concentrations, and long carrier diffusion lengths. These perovskites have broad application prospects for use in fields including light-emitting diodes [1–3], solar cells [4–6], and photodetectors [7–14]. When compared with other halogen MAPbX3 materials, MAPbBr3 offers a more practical band gap and practical light absorption capability than MAPbCl3, greater chemical and thermal stability than MAPbI3, and requires lower temperatures in the production process, which means that fewer materials are required for its production [14–16]. These advantages mean that MAPbBr3 represents a promising platform for further exploration. When compared with single crystals, perovskite thin film materials still inevitably contain grain boundaries and surface defects, which can lead to significant hysteresis in the current-voltage (I-V) curve and reduced carrier mobility, thus greatly reducing the optoelectronic performance of the device [17–27]. Comparison with polycrystalline perovskites shows that single-crystal perovskites have lower defect state densities, longer carrier diffusion lengths, and higher carrier mobility, and are thus ideal materials for preparation of high-performance optoelectronic devices [20,26].In addition, single crystals have more stable anti-oxidation and anti-hydrolysis capabilities, and they are also ideal materials for preparation of stable photoelectric devices for long-term operation [17,28–31].
In 2019, Xu et al. prepared an X-ray photodetector based on an MAPbBr3 single crystal [10]. In 2020, Pavao et al. produced a gamma-ray detector that showed no signs of degradation after 100 hours of irradiation with a high radiation source [32]. In 2020, Li et al. demonstrated that a photodetector on organic bulk heterojunctions composed of low-bandgap nonfullerene polymers and organic-inorganic perovskites achieved a broadband response spectrum up to 1 µm [33]. In 2020, Li et al. prepared a vertical structure detector with an MAPbI3 thin film as its main functional layer that could detect signals from the ultraviolet to the terahertz band [34].In 2021, Bouanani, et al. studied and prepared a detector based on MAPbBr3 single crystal and gallium gxide (Ga2O3) oxide heterojunction diode, which can perform alpha particle, gamma ray and neutron detection under bias conditions as low as -5 V [35]. In 2022, Li et al. prepared a broadband high-efficiency photodetector with a detection range from near-ultraviolet to near-infrared by adding Spiro-OMeTAD to perovskite [36]. Visible light photodetectors [37–40], X-ray detectors [10,41–47], γ-ray detectors [48], and α particle detectors [15,49] based on MAPbBr3 single crystals have all been manufactured and tested. However, there is little published work related to the infrared band detection capabilities of MAPbBr3 single crystals.
In this work, an ultraviolet-to-infrared (355 nm–1560 nm) broadband photodetector based on organic-inorganic hybrid perovskite (CH3NH3PbBr3, MAPbBr3) single crystals was investigated. The perovskite single crystals were prepared by the methods of inverse temperature crystallization and seed-induced crystallization [50–52]. When compared with other traditional detectors, perovskite single-crystal detectors have simple structures and require fewer fabrication processes, thus presenting a new option for fabrication of photodetectors in large quantities at low cost [11,39,53]. On this basis, a 4 × 4 pixel visible-to-near infrared light sensing imaging matrix was designed and fabricated.
2. Experimental details
The perovskite single crystals were prepared by inverse temperature crystallization and seed-induced crystallization. Dimethylformamide (DMF) was selected as the solvent because of its good solubility with respect to the reactants (MABr, PbBr2) required in the preparation process, although the solubility of these two reactants in the solvent decreased with increasing temperature. First, the perovskite precursor solution was prepared: MABr and PbBr2 with 1 mol masses were dissolved in 1 ml of DMF, and the solution was dissolved for 2–3 h at room temperature; the solution was then filtered, and the filtered solution was sealed in an oil bath at 80°C for growth [50,52]. After approximately 2 h, orange crystalline grains appeared, and these grains continued to grow. When the grains grew to the required extent, the grains were extracted and quickly placed into a fresh perovskite precursor solution to continue to grow. When the crystal grew to the desired size, the crystal was extracted from solution, and the growth process ended.
An actual digital photograph of an MAPbBr3 bulk single crystal (6 mm × 5 mm × 2 mm) prepared by inverse temperature crystallization and seed-induced crystallization is shown in Fig. 1(a). As Fig. 1(b) shows, the surface of the MAPbBr3 single crystal is smooth and shows no obvious fluctuations, which means that it is suitable for use in the fabrication of single-crystal devices. Subsequently, to provide better characterization of the structure and the crystallinity of the MAPbBr3 perovskite single crystals, we measured the X-ray diffraction (XRD) patterns of the samples using an X-ray diffractometer. The results are shown in Fig. 1(f), in which there are only two sharp diffraction peaks in the [100] and [200] crystal planes. Comparison with the reported results in the literature shows that the perovskite single crystals prepared via this method have the preferred [100] orientation and good crystallinity. The absorption spectrum of the MAPbBr3 single crystal is shown in Fig. 1(c). The absorption edge at 574 nm indicates an energy band gap (Eg) of 2.16 eV, which is very close to the previously reported value of 2.15 eV [15].
Fig. 1. (a) Actual digital photograph of the MAPbBr3 single crystal. (b) Scanning electron microscope image of the MAPbBr3 single crystal. (c) Absorption spectrum and band gap diagram of the MAPbBr3 single crystal. (d) Schematic diagram of the MAPbBr3/Ag photodetector. All current data are measured by the Source Measure Unit (SMU). (e) Actual digital photograph of the MAPbBr3 single-crystal detector. (f) XRD pattern for MAPbBr3; the inset illustration shows an actual digital photograph of the 4 × 4 optical sensor imaging matrix.
Interdigital electrodes with a parallel structure (channel width: 100 µm; thickness: 500 nm) were deposited on the surface of the MAPbBr3 single crystal by thermal evaporation. The complete sample structure is shown in Fig. 1(d), and an actual digital photograph of the MAPbBr3/Ag single-crystal detector is shown in Fig. 1(e). The interdigital electrodes were selected to increase the effective illumination area as much as possible based on the premise of ensuring that the electrode spacing was small, and also to improve the carrier capacity of the electrodes to enable carrier capture and thus obtain higher photocurrents. The prepared MAPbBr3/Ag parallel structure perovskite single-crystal detector was then tested under continuous light irradiation at four wavelengths: 450 nm, 520 nm, 780 nm, and 1560 nm. Similarly, a 4 × 4 single-slit electrode array was deposited on the surface of an MAPbBr3 single crystal by thermal evaporation. The structure was then tested under continuous light irradiation at 520 nm and 1560 nm. The light and dark currents obtained during the experiment were all controlled via the upper computer control source meter (Keithley 2450) measurements. All measurement results were acquired in the air at room temperature.
3. Results and discussion
To study the photoelectric characteristics of the MAPbBr3 single-crystal detector further, we performed a series of electrical measurements of the samples, and measured their I-V characteristic curves under laser irradiation at 450 nm, 520 nm, 780 nm, and 1560 nm, as shown in Fig. 2. The dark current curves were drawn in the figure for reference. The I-V characteristic curves of the MAPbBr3 single-crystal detector under 450 nm and 520 nm light irradiation are shown in Fig. 2(a) and 2(b), respectively. These curves show that the device photocurrent changes obviously under irradiation by 450 nm and 520 nm light at different intensities. The corresponding I-V characteristic curves of the MAPbBr3 single-crystal detector under 780 nm and 1560 nm light irradiation are shown in Fig. 2(c) and 2(d), respectively. However, these curves show that the device photocurrents do not change obviously under irradiation by 780 nm and 1560 nm light at different intensities. The I-V curves obtained under 450 nm and 520 nm light irradiation showed obvious nonlinear rising behavior, as illustrated in Fig. S1(a) and Fig. S1(b) (Supplement 1). The photocurrent growth rate decreases with increasing external bias voltage. However, the difference is that the I-V characteristic curves obtained under 780 nm and 1560 nm light irradiation show linear rise behavior under application of the same external bias voltage, as illustrated in Fig. S1(c) and Fig. S1(d) (Supplement 1), respectively. This phenomenon indicates that a Schottky contact is formed on the Ag and MAPbBr3 surfaces [13].
Fig. 2. I-V characteristic curves of the MAPbBr3/Ag photodetector at (a) 450 nm, (b) 520 nm, (c) 780 nm, and (d) 1560 nm.
The excited carriers are rapidly separated under the action of the electric field and collected by the electrode, but due to the existence of the Schottky barrier, the carriers accumulate at the electrode, so that there will be built-in electric filed generated. And the built-in electric field near the electrode hinders the substantial increase of the photocurrent [16]. Obviously, the photocurrent increments in the responses of the MAPbBr3 single-crystal detector to light irradiation at 450 nm and 520 nm are significantly greater than those for the light irradiation at 780 nm and 1560 nm.
To enable better evaluation of the detection ability of the MAPbBr3 single-crystal detector for light at different wavelengths, according to the measured light and dark current, the responsivity (R), the external quantum efficiency (EQE), and the detection rate or detectivity (D*) are calculated by the following formula, which are important indicators for characterization of the photoelectric performance of the photodetector:
where ILight is the photocurrent, IDark is the dark current, PLaser is the optical power density of the incident light, and S is the effective illumination area. In addition, h is Planck's constant, e is the quantity of electric charge, c is the speed of light, and λ is the wavelength of the incident light. The light intensity has a significant effect on the photodetector’s performance; we therefore plotted curves of R, EQE, and D* versus the light intensity at different wavelengths, as shown in Fig. 3. The figures show that the R, EQE, and D values for irradiation at 450 nm, 520 nm, and 780 nm decrease with increasing light intensity, which means that the corresponding R, EQE, and D* values are at their highest at the minimum optical power density. Under 450 nm laser irradiation at 1.0 µW/cm2, the R, EQE, and D* values of the MAPbBr3 single-crystal detector were 268.37 mA/W, 73.94%, and 1.44 × 1012 Jones, as shown in Fig. 3(a), 3(b), and 3(c), respectively. Under 520 nm laser irradiation at 1.5 µW/cm2, the R, EQE, and D* values of the MAPbBr3 single-crystal detector were 230.88 mA/W, 55.04%, and 1.43 × 1012 Jones, as shown in Fig. 3(a), 3(b), and 3(c), respectively. Under 780 nm laser irradiation at 0.06 W/cm2, the R, EQE, and D* values of the MAPbBr3 single-crystal detector were 0.0024 mA/W, 0.000381%, and 1.28 × 107 Jones, as shown in Fig. 3(d), 3(e), and 3(f), respectively. It was found that the light responses of the single-crystal devices to 450 nm and 520 nm light are basically at the same level, while the light response to 780 nm was much smaller, with a difference between them of five orders of magnitude. This result is consistent with the absorption spectrum of the single crystals. The R, EQE, and D* values of the MAPbBr3 single-crystal detector in response to 1560 nm light irradiation are shown in Fig. 3(d), 3(e), and 3(f), respectively. Under 1560 nm laser irradiation at 1.38 W/cm2, the R, EQE and D* values of the MAPbBr3 single-crystal detector were 0.229 µA/W, 0.000018%, and 2.207 × 106 Jones, respectively. The figure shows that the R, EQE, and D* values increased with increasing light intensity; this result is the opposite of that observed in the visible light band. The values of R, EQE, and D* are much smaller than the corresponding visible light band values. The experimental results demonstrated that the light response mechanism of the single-crystal devices to 1560 nm irradiation is different to the mechanism for visible light. We measured the EQE of the device at light of different wavelengths, and calculated the R and D* under light of different wavelengths. The R, EQE and D* reaches a high plateau before the wavelength of 520 nm, then starts to decline sharply, and is almost zero at after 570 nm, as shown in Figs. 3(g), (h), and (i). The trend of these is consistent with the absorption spectrum of MAPbBr3 (Fig. 1(c)).Fig. 3. (a) Responsivity R, (b) external quantum efficiency (EQE), and (c) detectivity D* of the MAPbBr3/Ag photodetector under 450 nm and 520 nm light irradiation at different optical power densities. (d) R, (e) EQE, and (f) D* of the MAPbBr3/Ag photodetector under 780 nm and 1560 nm light irradiation at different optical power densities. (g) R, (h) EQE, and (i) D* of the MAPbBr3/Ag photodetector at different wavelength.
The time-resolved photoresponse characteristics of the MAPbBr3 single-crystal detector to light of different wavelengths are analyzed and studied. Figure 4(a)–(d) show the single crystal device's time-resolved photoresponse curves (I-t) to 450 nm, 520 nm, 780 nm, and 1560 nm irradiation, respectively, under a 1 V bias voltage. The photocurrents excited by the 450 nm, 520 nm, and 780 nm light switch rapidly between the high and low levels when the incident light is switched on and off repeatedly. Figure 4(d) shows the I-t curve of the MAPbBr3 single-crystal detector under 1560 nm light irradiation. The light response of the device under 1560 nm excitation is a slow process. This phenomenon illustrates the conclusion mentioned earlier that the MAPbBr3 single-crystal detector has different response mechanisms for 1560 nm light and visible light; this will be discussed in greater detail later.
Fig. 4. Time-resolved photoresponse (I-t) curves of the MAPbBr3/Ag photodetector for (a) 450 nm, (b) 520 nm, and (c) 780 nm light under 1 V bias. (d) I-t curve under repeated switching (T = 120 s) for 1560 nm light. (e) Rise time and fall time characteristics for 450 nm continuous light irradiation. (f) Rise time and fall time characteristics for 1560 nm continuous light irradiation.
The rise time (ton) and the fall time (toff) are defined as the time required for the photocurrent to rise from 10% to 90% and the time for the photocurrent to fall from 90% to 10%, respectively. During the light switching period, the normalized photocurrent increments for the MAPbBr3 single-crystal detector during continuous light irradiation at 450 nm with a switching frequency of 200 Hz are shown in Fig. 4(e). Analysis of the rising and falling edges shows that the ton value of the detector is 213.0 µs and the toff value is 936.0 µs. This response rate result for visible light detection can be basically kept within an order of magnitude compared with other literatures [15]. These results show that there is an obvious tail at the falling edge, which indicates that the associated defect has participated in the entire light response process. The carriers that are excited by the light will initially be captured by the traps, and photocurrent will reach a maximum after all the defects are saturated. When the incident light is then turned off, the carriers captured by these traps are released slowly, and this results in a distinct tail occurring at the falling edge. The normalized photocurrent increments for the MAPbBr3 single-crystal detector during continuous light irradiation at 1560 nm. Analysis of the rising and falling edges shows that the ton value of the detector is 10.8 s and the toff value is 13.9 s. Additionally, the I-t curve of the MAPbBr3/Ag photodetector under excitation by 355 nm pulsed light was also measured, with results as shown in Fig. S2 (Supplement 1). These results demonstrated that the device was responsive to ultraviolet light at 355 nm.
Figure 5(a)–(c) show the time-resolved photoresponse characteristic curves of the MAPbBr3 single-crystal detector to 450 nm, 520 nm, and 780 nm light irradiation, respectively, under a 0 V bias. We infer that the MAPbBr3 single-crystal detector will generate a built-in electric field under irradiation at 450 nm, 520 nm, and 780 nm that drives the carriers toward the electrode to generate a photocurrent. Therefore, the device can be used as a self-powered UV-visible photodetector. Similarly, the photoresponse of the MAPbBr3 single-crystal detector to 1560 nm light under 0 V bias was also tested, but the experimental result showed that this detector does not respond to 1560 nm light under 0 V bias. From all the experimental results above, we infer that the physical mechanisms behind the photoresponse behavior of the MAPbBr3 single-crystal detector with respect to 450 nm, 520 nm, and 780 nm light should be similar. The MAPbBr3 single crystal generates carriers (electron-hole pairs) under irradiation by 450 nm and 520 nm light. Under application of an external bias, these carriers will then converge on the electrodes to generate the photocurrent. As shown in Fig. 1(c), the MAPbBr3 single crystal shows little or no absorption at 780 nm, and the energy carried by a single photon at a wavelength of 780 nm is not sufficient to excite the carriers. However, as shown in Fig. 2(c) and Fig. 4(c), the MAPbBr3 single-crystal detector responds to 780 nm light under both 1 V and 0 V bias conditions, although the response is weak. MAPbBr3 single crystal exhibits two-photon absorption phenomenon under 800 nm light irradiation [54–56]. Through testing, we found that MAPbBr3 single crystal does have two-photon photoluminescence phenomenon under strong 780 nm light irradiation, as shown in Fig. S3 (Supplement 1), the insert is bright MAPbBr3 single crystal under strong 780 nm excitations. This two-photon absorption effect is much smaller than the ordinary photoelectric effect, but this effect requires strong incident light (108 W/cm2) to be detected. In this work, the photocurrent was observed only at the light intensity of 70 mW/cm2, excluding the possibility of the two-photon effect, so the response under irradiation at 780 nm is more likely to be caused by sub-gap absorption [57,58]. A schematic diagram of the energy levels of the MAPbBr3 single-crystal detector is shown in Fig. 5(d), which illustrates the mechanism of carrier excitation by light at different wavelengths. Excitation by light at less than the cut-off wavelength is a common photoelectric effect, where a pair of carriers is excited by a single photon, while light above the cutoff wavelength is excited by sub-bandgap absorption.
Fig. 5. Time-resolved photoresponse characteristic curves of MAPbBr3 single-crystal detector to (a) 450 nm, (b) 520 nm, and (c) 780 nm light under 0 V bias. (d) Energy levels of the MAPbBr3 single-crystal detector under different illumination conditions. (e) Photocurrent and temperature curves of the device versus time under an external bias of 1 V when irradiated with 1560 nm light (1.9 W/cm2). (f) Resistance increments and temperature changes of the MAPbBr3 single-crystal detector under 1560 nm light of various intensities. The insert illustration is that the linear relationship between lnRT and 1/T under different intensities 1560 nm laser excitation.
The photoresponse of the MAPbBr3 single-crystal detector to the near-infrared 1560 nm light is a nontransient and long relaxation process that may be caused by the bolometric effect [34]. Therefore, to increase our understanding of the role of the bolometric effect, a series of experiments was conducted. First, the infrared thermometer was used to measure the surface temperature of MAPbBr3 single crystals under excitation at 1560 nm with different light intensities. The temperature of the MAPbBr3 single-crystal device under 1560 nm (1.9 W/cm2) light irradiation for 60 s and the temperature under no light conditions were measured. The temperature of the sample increased to 24.9°C from 23.7°C (room temperature) under 1560 nm light irradiation. For comparison with the case where the photocurrent is generated by excitation at 1560 nm, the temperature changes in the MAPbBr3 single-crystal detector were measured when the incident 1560 nm light is switched on and off repeatedly (T = 120 s), with results as shown in Fig. 5(e); these results show that the temperature change trend is consistent with that of the photocurrent in the optical switching cycle. The temperature change and resistance change of the MAPbBr3 single-crystal detector under different intensities of 1560 nm light irradiation were tested, the trend of the two changes is the same. It can be inferred that, under irradiation by near-infrared 1560 nm light, the temperature of the MAPbBr3 single-crystal detector increases, which causes the device resistance to change. These phenomena indicate that the photocurrent generated by the MAPbBr3 single-crystal detector in the near-infrared range is mainly the result of thermal effect. The thermal effect is depended on the change in device resistance caused by the heat of the incident photons. Based on the thermal effect, the formula for resistance change with temperature is [34]:
where RT is the resistance, A and B are constants, and T is temperature. Through equation transformation, a linear relationship between lnRT and 1/T can be obtained: It can be seen from Fig. 5(f) insert illustration is that the experimental data and the fitted data are in good agreement.To verify the response mechanism of the MAPbBr3 single-crystal detector to visible light and near-infrared light, the fast carrier response of the perovskite was investigated at different wavelengths via optical-pump terahertz (THz) probe spectroscopy [59,60]. Figure 6(a) shows a schematic diagram of the optical path and the principle of the optical-pump terahertz probe spectroscopy technique, and the enlarged area in the figure shows an image of the sample under test. The sample’s carrier concentration affects the terahertz transmittance through the sample, with a higher carrier concentration corresponding to lower transmittance. The terahertz transmission time-domain and frequency-domain graphs that were obtained from the testing are used to characterize the terahertz transmission. The relationship between the optical pumping time delay and the terahertz peak is illustrated in Fig. 6(b). The figure shows the changes in the terahertz signal of MAPbBr3 under excitation at different wavelengths pump light, and the terahertz peak decreases significantly under 500 nm light excitation. The terahertz peak of MAPbBr3 decreased slightly under 800 nm light excitation. To provide a better comparison of the transmittance of the sample with respect to terahertz signals under different illumination conditions, the modulation factor (MF) concept is introduced; the MF represents the change in the transmitted power of the sample with respect to the terahertz signal under the action of the pump light. The formula for the MF is defined as follows:
Fig. 6. (a) Schematic diagram of the optical-pump terahertz probe spectroscopy setup. (b) Temporal evolution of photo-induced changes in the terahertz signal peak values for MAPbBr3. (c) THz transmission amplitude and (d) transmission power through MAPbBr3 with and without the 500 nm pump light. (e) THz transmission amplitude and (f) transmission power through MAPbBr3 under irradiation by 800 nm pump light and without light irradiation.
Because of the broadband detection capability of the MAPbBr3 single-crystal photodetector, the visible to near-infrared broad-spectrum imaging application of the MAPbBr3 single-crystal photodetector was explored further. On the surface of the 8 mm × 8 mm × 3 mm MAPbBr3 single crystal, a distance of 100 µm parallel single slit devices separated by 4 × 4 arrayed were integrated on the MAPbBr3 surface substrates. Labels were arranged evenly on the surface of the single crystal to provide an imaging system with a total of 16 pixels. Metal masks with the hollow English letters “C”, “N”, and “U” were then placed between the laser light source and the photodetector. The 4 × 4 single-crystal light sensor matrix image is shown in Fig. 7. The red and green areas represent pixels with a photoresponse, i.e., “1”; and the white parts represent pixels without a photoresponse, i.e., “0”. It is emphasized here that because the MAPbBr3 single-crystal device has a high response to 520 nm light and a low response to 1560 nm light, the current increment can be used for imaging.
Fig. 7. (a) Schematic projection imaging. Imaging results for “C”, “N”, and “U” under (b) 520 nm and (c) 1560 nm laser irradiation.
4. Conclusions
An ultraviolet-to-infrared (355 nm–1560 nm) broadband photodetector with a potential imaging application based on organic-inorganic hybrid MAPbBr3 perovskite single crystals was investigated. The results verified that the photocurrent generation mechanism for the single crystals in the visible light band is the photoelectric effect. The mechanism at incident light wavelengths of less than the cut-off wavelength of 574 nm is single-photon absorption, while that at wavelengths greater than 574 nm is sub-bandgap trap state absorption. The photoresponse mechanism in the farther near-infrared region is the thermal effect. The fast carrier response of the perovskite was investigated at different wavelengths using optical-pump terahertz probe spectroscopy. The experimental results demonstrated that the detector also offer a possible new broadband imaging application by generating images of the letters “C”, “N”, and “U” for detection of visible-infrared light.
Funding
National Natural Science Foundation of China (62175168, 61505125); Nature Science Foundation of Beijing Municipality (4202013); High-level Teachers in Beijing Municipal Universities in the Period of 13th Five-year Plan.
Acknowledgment
This research was supported by National Natural Science Foundation of China (Grant Nos. 62175168 and 61505125), Nature Science Foundation of Beijing Municipality (Grant No. 4202013) and High-level Teachers in Beijing Municipal Universities in the Period of 13th Five–year Plan.
Disclosures
The authors declare no conflict of interest.
Data availability
All data included in this study are available upon request by contact with the corresponding author.
Supplemental document
See Supplement 1 for supporting content.
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