PbS nanocrystal solar cells fabricated using microwave-assisted chemical bath deposition

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 8 0 7 e8 1 5

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PbS nanocrystal solar cells fabricated using microwave-assisted chemical bath deposition A.S. Obaid a,b,*, M.A. Mahdi a,c, Z. Hassan a, M. Bououdina d,e a

Nano-Optoelectronics Research and Technology Laboratory, School of Physics, Universiti Sains Malaysia, 11800 USM, Penang, Malaysia Department of physics college of Sciences University of Anbar P.O.box PO. (55 431), Baghdad, Iraq c Physics Department, College of Science, Basrah University, Basrah, Iraq d Nanotechnology Centre, University of Bahrain, PO Box 32038, Kingdom of Bahrain e Department of Physics, College of Science, University of Bahrain, PO Box 32038, Kingdom of Bahrain b

article info

abstract

Article history:

n-CdS/p-PbS heterojunction solar cell was fabricated using microwave-assisted Chemical

Received 16 August 2012

Bath Deposition (CBD). The CdS window layer (340 nm thickness) was deposited on

Received in revised form

ITO-glass. The PbS absorber layer (685e1250 nm thickness) with different molar concen-

11 October 2012

tration (0.02, 0.05, 0.075 and 0.1 M) was then grown on ITO/CdS to fabricate the pen junction.

Accepted 14 October 2012

X-ray diffraction analysis confirms the formation of pure and nanocrystalline CdS and PbS

Available online 27 November 2012

phases with a preferred orientation depending on molarity; (111) or (200). Scanning electron microscopy observations show a uniform surface morphology with gatherings. UVeVis

Keywords:

spectrophotometer and FTIR was used to estimate the optical properties. Optical

Nanocrystalline material

measurements gave an energy gap of 2.6 eV for CdS whereas that for PbS thin films were

Solar cell

found to vary in a narrow range 0.40e0.47 eV, depending on the molar concentration. The

PbS

photovoltaic properties under 30 mW/cm2 solar radiation including JeV characteristics, short-circuit current (Isc), open-circuit voltage (Voc), fill factor (ff), efficiency (h) of CdS/PbS

CdS

heterojunction cells have been as well examined. The results show that changing the molar concentration improved the performances of the fabricated photovoltaic cells; a high efficiency was observed at 0.1 M. However, high series resistance and poor crystallinity of PbS lead to low efficiency at lower molarity. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

Nanocrystalline inorganic materials have been widely studied for solar cell application, because of their fundamental structural, electrical and optical properties due to quantum confinement [1]. As a result, an increase in the absorption of solar radiation in near infrared region for narrow band gap such as PbS, occurs [2]. Lead sulfide (PbS) is an important binary IVeVI semiconductor material with a direct narrow

optical energy gap (0.41 eV at 300 K) and relatively large excitation Bohr radius (18 nm) [3]. This provides strong quantum confinement of electrons and holes, hence regulating the value of band gap by controlling the particle size according to the effective mass model [4]. This property of PbS makes it desirable for new applications such as solar cells [3]. A variety of methods, such as spray pyrolysis (SP), chemical vapour deposition (CVD) and thermal evaporation (TE) have been traditionally used to prepare PbS thin films, but each

* Corresponding author. Nano-Optoelectronics Research and Technology Laboratory, School of Physics, Universiti Sains Malaysia, 11800 USM, Penang, Malaysia. Tel.: þ60 174170405; fax: þ60 46579150. E-mail address: [email protected] (A.S. Obaid). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.10.046

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technique has shown some limitations. These methods normally require high temperatures to enable the successful formation of chalcogenides [5e7]. On the other hand, chemical solution methods, i.e. hydrothermal, solvothermal and chemical bath deposition (CBD), generally require lower temperatures to synthesize PbS thin films of a good quality [8]. CBD has become an attractive method due to several reasons, including ease of manufacturing, low cost, suitability for large scale deposition areas, the ability to deposit thin films on different substrates and ease of controlling thin film properties by controlling the deposition parameters [9]. In general, CBD for PbS preparation requires low temperature and a long processing period [8,9]. Microwave irradiation is widely used to prepare organic and inorganic materials. When a material is exposed to electromagnetic wave, it will absorb the electromagnetic energy and transform it into thermal energy [10,11]. Heat is generated from inside the material in microwave irradiation, whereas heat is transmitted from the outside to the inside in other methods. The production of internal heat reduces reaction time and energy cost, and makes new material synthesis possible [12]. Therefore, microwave assisted CBD (MACBD) synthesis is generally quite-fast, simple and energy-efficient. On the other hand, cadmium sulfide (CdS) is one of the most important IIeVI semiconductors with a direct band gap of 2.42 eV at room temperature [13]. Among the several n-type semiconductor materials, it has been observed that CdS is the most promising heterojunction partner for the well-known polycrystalline photovoltaic material as a window layer [14]. Nair et al. [15] prepared CdS as window layer for CdS/PbSe solar cell. Hernandez-Borja et al. [16] fabricated CdS/PbS solar cell with CdS window layer. The Fabrication of high efficiency and low cost solar cells is always of great importance and remains a hot topic for scientists. Moreover, up to the knowledge of the authors, no data have been reported in the literature on the fabrication of PbS/CdS heterojunction solar cells using MACBD. In the present work, structure and microstructure as well as optical properties of both CdS and PbS thin films prepared using the microwave-assisted CBD method, were investigated. The aim of this work is to fabricate good quality PbS/ CdS heterojunction solar cells. The performance of the fabricated solar cells was evaluated by measuring current densityevoltage (JeV) curves.

2.

Experimental details

2.1.

Thin films preparation

The conducting ITO/glass commercial substrates (ITO shortened for indium tin oxide), were washed with hot distilled water and then cleaned ultrasonically using diluted HCl solution for 10 min and then washed in acetone. After that, the substrates were cleaned ultrasonically by distilled water for 20 min and dried under nitrogen atmosphere. The CdS nanocrystalline thin film was deposited on the ITO/glass substrates using the microwave-assisted CBD method in an alkaline aqueous solution containing the following precursors with fixed molarity, cadmium chloride [CdCl2] (0.05 M),

ammonia acetate (0.15 M) and thiourea [CS(NH2)2] (TU, 0.04 M), which are the optimal molar concentrations for the deposition of stoichiometric CdS compound. Then ammonia solution was added to adjust the pH of 10; the total volume was 100 ml. The substrates were fixed vertically in a beaker. The beaker was then heated in a microwave oven (2.4 GHz) and the preparation temperature was fixed at 80  C for 30 min. The deposition temperature was controlled using a power supply attached to the microwave oven. After that, the samples were taken out of the solution and washed ultrasonically with distilled water for 2 min to remove any contaminant. To fabricate the CdS/PbS solar cell, the PbS nanocrystalline thin films were prepared on CdS/ITO/glass. The samples were immersed in solution containing lead nitrate [Pb(NO3)2] and TU with various molar concentrations (0.025, 0.05, 0.075, and 0.1 M). The pH of the solutions was fixed at 12 by adding sodium hydroxide (NaOH); the total volume was 100 ml. The beakers were then heated in the microwave oven (2.4 GHz) and the reaction temperature was fixed at 80  C for a deposition time of 30 min. Beyond 10 min, the solution turned dark grey indicating that the PbS was formed. The preparation of PbS thin films was retuned twice sequentially to obtain thick PbS thin films. After the deposition process was completed, the samples were removed from the solution, cleaned ultrasonically with distilled water for 5 min, and then dried in air. Aluminium (Al) metal was deposited on the surface of PbS thin films via thermal evaporation method as Ohmic back contact. The active area of the device was 1 cm2. Fig. 1 shows the schematic diagram of the fabricated cells.

2.2.

Characterisations

The film thickness was determined using an optical reflectometer (Filmetrics F20). The measured thickness of CdS thin film was around 300 nm whereas that of PbS films was found to increase from 685 to 1250 nm as the molar concentration increases from 0.025 to 0.1 M, respectively. This may be due to the increase of ions reaction with increasing molar concentration. The structure evolution of the as-prepared CdS and PbS thin films was examined by high-resolution X-ray diffraction (HR-XRD) using X’Pert Pro MRD diffractometer (PANalytical Company) system equipped with Cu-Ka-radiation wavelength

Fig. 1 e Schematic diagram for the fabricated solar cell.

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Fig. 2 e XRD patterns of PbS and CdS thin films with different molar concentration.

(l ¼ 0.15418 nm) operating at 40 kV and 30 mA. Morphology and microstructure of the thin films were investigated by scanning electron microscopy (SEM) using Jeol JSM-6460 LV microscope operating at 10 kV and attached to energy dispersive X-ray spectrometer (EDX) for elemental chemical composition determination. Optical measurements were conducted at room temperature for CdS using Shimadzu UVeVis 1800 spectrophotometer at wavelengths ranging from 300 to 1100 nm. Quartz substrate was used to measure optical properties for PbS thin films using Fourier transform infrared spectrometer (FTIR) at wavelengths ranging from 2000 to 4500 nm. Current densityeVoltage (JeV) measurements were performed in forward bias with a computer-controlled Keithley 2400 Source Meter under 30 mW/cm2 illumination from a solar simulator under 100 W xenon lamp serving as the light source, and the light intensity was calibrated using a standard silicon solar cell.

3.

Results and discussion

3.1.

Structural analysis: X-ray diffraction

Fig. 2 illustrates the evolution of X-ray diffraction patterns of the prepared CdS and PbS films with different molar concentration. The X-ray diffraction pattern of CdS thin film prepared via the microwave-assisted CBD exhibits a cubic structure with two diffraction peaks located at 2q of 26 and 52 , corresponding to (111) and (311) planes, respectively, in agreement with the standard JCPDS card No. 01-0800019. The deposited film reveals a preferred orientation along the (111) direction. On the other hand, all PbS thin films prepared via the same method with different molar concentration (0.025, 0.050, 0.075 and 0.100 M) are polycrystalline and crystallize within the cubic rock salt (NaCl) type structure. The observed diffraction peaks were indexed as (111), (200), (220), (311) and (222) planes according to the standard JCPDS card No. 00-0050592. The peaks are sharp indicating that the prepared thin films are of good crystalline structures [17]. The main features of the diffraction patterns are similar but the relative intensity of the diffraction peaks increases considerably, which is may be attributed to the increase of films’ thickness with increasing molar concentration. Thus, changing the molar concentration does not induce any phase transformation or the appearance of new phases aside from the cubic PbS phase. This indicates that no oxidation occurred during the preparation, thereby resulting in a good quality of the formed thin films [18]. The films grow preferentially along (111) or (200) directions depending on the molar concentration. The texture coefficient (TC) represents the texture of a particular plane, where the deviation from unity implies the direction of a preferential growth. Quantitative information on the preferential crystallites orientation was obtained from (hkl ) and defined by the following equation [19]: IðhklÞ I0 ðhklÞ  100% TCðhklÞ ¼ P IðhklÞ n I0 ðhklÞ

(1)

The variation of TC for (111) and (200) diffraction peaks are reported in Table 1. The highest TC value was obtained for (200) plane for the nanocrystalline PbS thin films with the molar concentration 0.05, 0.075, and 0.100 M and (111) for the molar concentration 0.025.

Table 1 e XRD data : interplanar spacing (d), Lattice constant (a), texture coefficient (TC), lattice mismatch determined for the PbS samples deposited for using different molar concentration. Molar concentration (M) Thickness (nm) d ( A) Calculated d ( A) a ( A) Calculated hkl TC% Lattice mismatch% 0.025

685

0.05

950

0.075

1050

0.1

1250

3.450 2.988 3.435 2.971 3.449 2.981 3.438 2.977

3.429 2.970 3.429 2.970 3.429 2.970 3.429 2.970

5.976 5.962 5.954 5.944

111 200 111 200 111 200 111 200

7.210 4.262 5.946 6.000 6.26 6.46 6.39 7.02

0.028 0.026 0.024 0.023

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Fig. 3 e SEM and EDX of CdS and PbS thin films: (a) 0.025 M, (b) 0.05 M, (c) 0.075 M, and (d) 0.1 M.

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The lattice constant of the cubic rock salt structure is given by [18]: 1=2  a ¼ d h2 þ k2 þ l2

(2)

where h, k and l are the Miller indices; and d is the interplanar distance. It is found that the calculated value of the lattice constant decreases with increasing molar concentration (Table 1), which clearly indicates that the crystallisation and stress result in lattice compression [20]. The lattice mismatch between CdS substrate and PbS thin films was calculated using the following simple equation [21]: a  a0 (3) lattice mismatch ¼ a0 where a is the lattice constant for PbS thin films, a0 the lattice constant for the substrate CdS thin film (estimated to be around 0.5811 nm). It is found that the lattice mismatch has a small value (less than 0.03%) which decreases slightly as the molar concentration increases. This may be expected to lead to the growth of PbS thin films on CdS as substrate with a good crystalline quality, which indeed was observed and consequently lead to the improvement in the optical properties as well as pen junction performances. The variation in the lattice mismatch with molar concentration is summarized in Table 1.

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diffraction results, no additional diffraction peaks belonging to impurities were observed. The EDX spectra of PbS thin films prepared at different molar concentration confirm the presence of both Pb and S elements and that the Pb:S ratio increases from 0.894 for the lower molar concentration (0.025 M) to reach a steady value around 1.057 for higher molarity (0.100 M). It is very important to note that no additional peaks attributed to impurities or contamination are observed, thus confirming the purity of the prepared thin films as confirmed previously by XRD analysis. The other peaks, shown in Fig. 3, at molar concentration (0.025, 0.050 M) are related to the Cd which came from substrate elements.

3.3.

Optical properties

The optical transmittance, reflectance and absorption spectra (Fig. 4) of the prepared CdS and PbS thin films were also investigated to evaluate the absorption coefficient (a), the forbidden energy gap (Eg) and the types of optical transitions. CdS thin film was observed to exhibit a high transmittance in the visible region and low reflectance. Fig. 4b shows the

3.2. Microstructural analysis: scanning electron microscopy Fig. 3 shows scanning electron microscopy (SEM) images for the deposited films. CdS thin film is covered completely, no pinholes or cracks can be observed. The images of nanocrystalline PbS thin films (Fig. 3) reveal that some microstructural changes occurred influenced by changing molar concentration. It is observed that at lower molar concentration (0.025 M), the thin films have a uniform surface morphology over the entire substrate. Also it can be observed the presence of some gatherings, which increases with increasing molar concentration. Energy dispersive X-ray analysis spectra of CdS and PbS thin films are reported in Fig. 3 and the corresponding quantitative chemical analysis is reported in Table 2. The EDX spectrum for the CdS thin film shows the presence of Cd and S without any contaminants, which is in agreement with X-ray

Table 2 e Energy dispersive X-ray analysis spectra of PbS thin films. Molar concentration (M) 0.025

0.05

0.075 0.1

Element

Weight%

Atomic%

Pb/S ratio

Pb S Cd Pb S Cd Pb S Pb S

80.40 13.91 5.69 80.77 13.69 5.54 86.24 13.76 87.54 12.46

44.48 49.72 5.80 45.0 49.72 5.69 49.25 50.75 52.10 49.25

0.894

0.912

0.970 1.057

Fig. 4 e (a) Transmittance/reflectance and (b) absorbance of CdS thin film.

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Fig. 5 e Absorbance of PbS thin film with different molar concentration.

Fig. 7 e Energy gap of PbS thin films estimated using (ahn)2 vs hn curve.

variation of CdS thin film absorbance, which has a low absorption from 500 to 1100 nm, thus making CdS a good candidate as a transparent window in solar cells. Fig. 5 shows the optical properties of nanocrystalline PbS thin films, as molar concentration changes (0.025, 0.05, 0.075, and 0.1 M). It can be seen that all films have high absorption value (shifts slightly) towards shorter wavelength side (2000 nm). The absorption for thin films with molar concentrations 0.075 and 0.1 M is much higher than that for lower molar concentration. Then the absorption decreases for all samples with increasing wavelength. This shift shows a decrease in energy band gap due to the increase in the grain size with increasing molar concentration. This observation indicates that the prepared films are good absorber layer and as such, can be considered as potential materials for solar cell applications due to the low transmittance compared materials with strong absorption. The absorption coefficient a can be calculated using the following relationship [22]:

where t is the film thickness and A is the absorption. The absorption coefficient was found greater than 105 cml, suggesting that both PbS and CdS have a direct band gap that increases sharply below a certain wavelength [23]. The energy gap (Eg) of the thin films was determined using the Tauc formula, given by Ref. [20], as shown in Equation (5):

a ¼ 2:303 

A t

(4)

ahv ¼ A ðhv  EgÞ

m

(5)

where A* is a constant, hv is the incident photon energy and the parameter m depends on the transmission type and equals (1/2) for the direct transmission allowed. Eg was determined by taking the extrapolation of the linear portion of a plot of (ahn)2 against hn. The determined energy gap for CdS thin film was found to be 2.6 eV (see Fig. 6), whereas that for PbS thin films was found to vary in a very narrow range 0.40e0.47 eV, depending on the molar concentration (see Fig. 7) as reported in Table 3.

3.4.

Solar cell characteristics

The (JeV) characteristics of typical solar cells fabricated using MACBD are shown in Fig. 8 under illumination condition of 30 mW/cm2. In the present study, the solar cells of ITO/CdS/ PbS/Al structure was determined to have an open-circuit voltage (Voc) of 160e358 mV, a short-circuit current density (Jsc) of 1.98e5.50 mA/cm2. The results revealed that, as the molar concentration increases, the open-circuit increases which may be due to the decreased energy band gap [24]. The

Table 3 e Energy gap of PbS thin films with different molar concentration. Molar concentration (M)

Fig. 6 e Energy gap of CdS thin film estimated using (ahn)2 vs hn curve.

0.025 0.05 0.075 0.1

Eg (eV) 0.52 0.51 0.44 0.42

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Table 4 e (JeV) characteristics for the fabricated solar cells. Molar concentration (M) 0.025 0.05 0.075 0.1

Vmax (mV) 90 120 154 150

Imax (mA/cm2) 0.86 1.38 2.24 2.86

160 338 340 358

maximum voltage (Vmax) and maximum current (Jmax) are summarized in (Table 4). Resistive effects in solar cells reduce the efficiency of the solar cell by dissipating power in the resistances [25]. The power of photovoltaic devices dissipates through the resistance of the contacts and leakages at the edge of devices. This dissipation namely series resistance (RS) and shunt resistance (Rsh), depend on the geometry of the solar cell as well as at the operating point of the solar cell [26]. In the high field regime, the series resistance (RS) dominates and can be determined from the relationship reported by Ref. [27]: 

dI dV

 ¼ I¼0

1 RS

(6)

In the low field regime, the shunt resistance (Rsh) dominates and can be determined from Ref. [27]: 

dI dV

 V¼0

1 ¼ Rsh

Isc (mA/cm2)

Voc (mV)

The (JeV) characteristics are significantly dependent on the series and shunt resistances. An ideal solar cell should have a series resistance close to zero and shunt resistance close to infinity. A low series resistance means that high currents will flow through the cell at low applied voltages and is due to contact resistance and bulk resistance of the photoactive material. A large shunt resistance results if there are shorts or leaks of photocurrent in the device. From Equations (6) and (7), the cells characteristics were summarized in Table 4.The results show that RS varies from (1.2  102 to 0.15  102 U) at different molar concentration (0.025e0.100 M). The higher resistance may be responsible for decreasing the quality of the cell at lower molar concentration [28].

Fig. 8 e (JeV) characteristics of the fabricated solar cells.

0.24 0.22 0.20 0.21

m% 0.257 0.54 1.05 1.37

RS U 1.2 1 0.32 0.15

Rsh U 2

 10  102  102  102

1.49  1.3  3 5

102 103 103 103

The shunt resistance varies from (1.49  102 to 5  103 U) at different molar concentration (0.025e0.100 M). This large value indicates that shorts or leakages of photocurrent are minimal in these solar cells. The series and shunt resistance value are summarized in Table 4. The fill factor (FF) was calculated from the following relationship [22]: FF ¼

ðJ  VÞmax Jsc  Voc

(8)

This lower value of the FF may be a sign of slightly more efficient recombination of the electronehole pairs in the active layer. Similar FF were reported recently by HernandezBorja et al. [16]. The energy conversion efficiency (h) can therefore, be calculated using the equation [22]: h¼

(7)

1.98 2.2 4.64 5.5

FF

FF  Jsc  Voc Plight

(9)

The efficiency of the solar cells was found to be 0.257, 0.54, 1.05 and 1.37% for the molar concentrations of 0.025, 0.050, 0.075, and 0.100 M, respectively. Fig. 9 shows the dependence of cell performance on the molar concentration of the

Fig. 9 e Dependence of the fabricated solar cell performance (m, Jsc, and Voc) with molar concentration.

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deposited MACBD-PbS absorber layer. The results obtained in this work show that the efficiency of the cells increases as the molar concentration increases, where the highest value was found for the sample with 0.1 M molar concentration having a lower energy gap, while the lower efficiency value was found for the cell with 0.025 M. It is believed that the results obtained in this study can be associated to various parameters: when the molar concentration increases, the energy gap will decrease, which led to the increase of electron hole pair [24]. The performance of the fabricated cells can be attributed to the effect of back-contact parameter; it was made over the entire surface. This offers many advantages like protecting the absorber layer from any external effects and in the same time making it as reflective mirror to avoid transmittance. This implies that no void, pinhole or cracks were found on the CdS thin films, which may cause localized ITO/PbS junctions with inferior device parameters (Voc and fill factor), or transmission loss. For the solar cells produced by our baseline process, the optimal molar concentration was found to be 0.1 M, corresponding to an Eg of 0.42 eV and a maximum solar cell efficiency of 1.37%. The results obtained in this study show a better solar cells performance thereby are more promising than some solar cells fabricated using different compounds and similar method as reported in the literature. Moreno-Garcia et al. [29] fabricated Bi2S3/PbS solar cells with CdS and ZnO as window layer using chemical bath deposition. The authors reported that Voc varies from 130 to 310 mV and Jsc from 0.5 to 5.0 mA/cm2 with a very low conversion efficiency in the range 0.1e0.4% under 1000 W/m2 solar radiation. Moreover, the same group [30] synthesized Bi2S3/PbS solar cells with a short circuit current density (Jsc) of 6 mA/cm2 and an open circuit voltage (Voc) of 280 mV, and again report a low conversion efficiency of 0.5% under 1000 W/m2 of solar radiation. Barote et al. [27] fabricated solar cell using PbS film/ polysulfide. The solar conversion efficiency (h) and FF were found to be 0.041% and 36.8%, respectively, using 250 W/m2 solar radiations.

4.

Conclusion

In the present study, a simple technique was used to fabricate ITO/CdS/PbS solar cell using microwave assisted CBD method. XRD analysis revealed that all the prepared thin films were polycrystalline with a low lattice mismatch value, which means that PbS thin films have good crystalline quality hence leading to an improvement in the optical properties and pen junctions. SEM analysis of CdS thin film shows that there are no holes or cracks thereby eliminating any transmission loss. For PbS absorber layer, SEM observations reveal that the thin films are influenced by changing the molar concentration, gatherings have been noticed, which increase with increasing molar concentration. The conversion efficiency of the fabricated solar cells under 300 W/m2 solar radiation was found to increase from 0.257 up to 1.37%, as molar concentration increases. This improvement can be associated with several factors, but mostly attributed to the enhancement of crystallinity. It is found that the molar concentration has a significant effect on solar efficiency.

Acknowledgements The authors gratefully acknowledge the financial support from a ERGS grant and Universiti Sains Malaysia.

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