Article pubs.acs.org/JPCC
Well-Organized Mesoporous TiO2 Photoelectrodes by Block Copolymer-Induced Sol−Gel Assembly for Inorganic−Organic Hybrid Perovskite Solar Cells Arpita Sarkar,†,§ Nam Joong Jeon,†,§ Jun Hong Noh, and Sang Il Seok*,†,‡ †
Division of Advanced Materials, Korea Research Institute of Chemical Technology, 141 Gajeong-Ro, Yuseong-Gu, Deajeon 305-600, Republic of Korea ‡ Department of Energy Science, Sungkyunkwan University, Suwon, Gyeonggi 440-746, Republic of Korea S Supporting Information *
ABSTRACT: A well-aligned mesoporous thin layer was formed by spin-coating followed by the burn-out of its organic components at 500 °C. This film, prepared by an inorganic sol−gel process using block copolymers as a sacrificial template, showed mesoporous (mp) and crack-free layers with various pore sizes depending on the amount of 1,3,5trimethylbenzene (TMB), which acted as a swelling agent. The mesoporous photoanodes were used in the fabrication of CH3NH3Pb(I0.9Br0.1)3 perovskite-based solar cells. The cells showed overall power conversion efficiencies of 11.7% and 12.8% with the 10 and 15 nm mp-layers, respectively. The superior performance shown in the 15 nm mp-layers may be attributed to the easier filling of CH3NH3Pb(I0.9Br0.1)3 by bigger pore sizes, and the efficiency is comparable with that of the reference cell fabricated by TiO2 paste. This template-induced self-organizing sol−gel process was shown to be a useful technique for depositing mesoporous photoanodes in the fabrication of perovskite-based solar cells. films with perpendicular mesochannels, the use of unique microphase separations of self-assembled block copolymers is the most convenient method because this method offers a unique combination of controllable nanoscale architecture. Furthermore, it has the ability to use various materials and it involves low-cost solution processing.15 Using the triblock copolymer templated route, Wu et al. reported the formation of well-designed mp-anatase TiO2 films with ordered vertical mesoporosity by a structural transformation from P63/mmc (i.e., ABABAB stacking of mesopores).16 Koh et al. reported the preparation of mp-TiO2 thin film with a vertical (R3m) mesopore structure derived from the cubic Im3̅m mesostructure by self-assembled ordering of the titania species.17 To date, the realization of ordered mp-TiO2 has intensively concentrated only on the development of mesoscopic alignment of the mesopore space using different type of pluronic block copolymers, but in all cases, pore sizes are restricted to below 10 nm. Therefore, research on perpendicular mesoporosity (e.g., P6 mmm, Fm3̅m, and Im3̅m symmetry) with large pore size (>10 nm) represents a big challenge. With the aim of proposing a simple synthetic procedure for selectively generating TiO2 films with a single phase and for
1. INTRODUCTION During the past few years there has been a great deal of interest in the use of titanium dioxide in developing new nanostructures for possible technological applications in fields such as photocatalysis,1 photochromism,2 Li-ion batteries,3 chemical and gas sensors,4 and photoanodes for quantum dot or nanocystalline-sensitized solar cells.5−7 Increased attention has also been given to the exploitation of well-organized or ordered mesoporous (mp) TiO2 films prepared by the evaporation induced self-assembly (EISA) process.8 Such well-organized or ordered films with high porosity and various mesostructures can be effective photoanodes. To define the structural entity on the nanometer scale during the synthesis of ordered mp-TiO2 films, coassembly of poly(ethylene oxide) (PEO)-based block copolymer, e.g., Pluronic P123 or F127 with inorganic precursors of TiO2 or preformed TiO2 nanoparticles, are often used. In addition, different approaches have been used to produce mp-thin films with accessible pores, including pore alignment with an applied magnetic field9 or modification of the substrate surfaces.10 Formation of a completely perpendicular orientation has been achieved using the ternary surfactant system11 or the microwave irradiation method,12 but the system is limited to the formation of powdery samples. The use of straight pores perpendicular to the anodic porous alumina has also emerged as an alternative route for generating perpendicularly arranged mesochannels; however, this has ultimately resulted in discontinuous mp-films.13,14 To prepare continuous © XXXX American Chemical Society
Special Issue: Michael Grätzel Festschrift Received: December 26, 2013 Revised: March 29, 2014
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Scheme 1. Preparation of Mesoporous TiO2 Film by Evaporation Induced Self-Assembly Combining Pore-Enlarging Effect of Swelling Agent, 1,3,5-Trimethyl Benzene (TMB)
2. EXPERIMENTAL SECTION Preparation of Well-Organized Mesoporous TiO2 Films. The mp-TiO2 film with 15 nm pore size was prepared by spin coating a precursor solution followed by calcination (Scheme 1). The titanium precursor solution was prepared first by mixing 0.68 g of Ti(OBu)4 with 0.286 mL of HCl. Then, 0.34 g of F127 was dissolved in 2.86 mL of EtOH and mixed with TMB; the mixture was stirred for 1 h. The weight ratio of TMB/F127 was maintained between 0.5 and 2 (0.18−0.68 g of TMB used). The titanium precursor solution was then added to the above solution and stirred for 24 h. Then, the solution was deposited on the substrate by spin coating at 2000 rpm for 60 s. For the films under aging conditions, the spin-cast films were aged at 18 °C for 3 days, whereas for the films without aging, spin-cast films were thermally aged at 200 °C for 10 min. Finally, films were calcined at 500 °C for 2 h with a heating ramp of 1 °C/min to obtain polymer-free well-organized mpTiO2 films. For comparison, TiO2 films were also prepared following the same method in the absence of TMB. For a reference cell, TiO2 paste was prepared using homemade TiO2 nanoparticles (average particle size, ca. 20 nm; crystalline phase, anatase; BET pore size, 14.8 nm). Solar Cell Fabrication. A 60 nm thick dense blocking layer of TiO2 (bl-TiO2) was deposited onto a F-doped SnO2 (FTO, Pilkington, TEC15) substrate by spray pyrolysis deposition carried out using a 20 mM titanium diisopropoxide bis(acetylacetonate) solution (Aldrich) at 450 °C to prevent direct contact between FTO and the hole-conducting layer. Wellorganized mp-TiO2 layers with pore sizes of 15 and 30 nm were deposited by spin coating at 1800 rpm for 20 s on the bl-TiO2 layered FTO. The film was then aged thermally at 50 °C for 10 min; the temperature was increased to 300 °C (with 45 min ramp), and the film was heated for 5 min. Finally, the film was calcined at 500 °C for 3 h with a heating ramp of 5 °C/min to obtain polymer-free, well-organized mp-TiO2 films. CH3NH3I was synthesized from 30 mL of hydroiodic acid (57% in water, Aldrich) by reacting 27.86 mL of methylamine (40% in methanol, Junsei Chemical Co., Ltd.) in a 250 mL roundbottomed flask at 0 °C for 2 h with stirring. The precipitates were recovered by evaporating the solutions at 50 °C for 1 h.
meeting the pore-size requirements, the present work reports an easy method of preparing mp-anatase TiO2 films. The films contain (i) enhanced pore size and (ii) an ordered mesospace throughout the entire film thickness. The films reported herein consist of large mesopores (10−15 nm) with ordered cubic networks achieved by using a pluronic copolymer F127 and an apolar solvent, 1,3,5-trimethylbenzene (Mesitylene, TMB), which acted as a swelling agent (Scheme 1). To the best of our knowledge the use of a swelling agent combined with EISA to produce well-defined mp-versions of these oxide films with cubic symmetry has not yet been reported. In particular, introduction of the swelling agent for expanding the size of the template micelles serves as an alternative strategy for enlarging the pore size of TiO2 films up to 15 nm. Recently, solar cells based on methyammonium lead halide perovskites as promising light harvesters have been reported by several groups, including ours.18−24 A typical perovskite-based solar cell contains an mp-photoanode in which perovskite lightharvesting materials as well as hole-transporting materials (HTMs) are deposited. Even though vertically aligned nanostructural electrodes were used as photoanodes for the perovskite-based solar cells, the performance was lower than that of cells fabricated from mp-photoanodes.25 The mpphotoanodes were prepared from a paste containing TiO2 nanoparticles to obtain uniform and crack-free thin mpstructures with high surface areas. The procedure for preparing the pastes is highly complex and thus may prohibit the advancement of cost-effective practical perovskite solar cells. In addition, we reported that the cells fabricated from x = 0−0.2 in MAPb(I1−xBrx)3 exhibit an average of more than 10% in conversion efficiency under full sun (100 mW cm−2) of AM 1.5 G radiation, with the relatively higher stability. Therefore, in this study, we have demonstrated the use of these wellorganized mp-TiO2 films as an alternative photovoltaic architecture combined with CH3NH3Pb(I0.9Br0.1)3 into inorganic−organic hybrid systems and shown that the system exhibits highly efficient photovoltaic performance (>10%). B
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The products were dissolved in ethanol, recrystallized using diethyl ether, and finally dried at 60 °C in a vacuum oven for 24 h. The CH3NH3Pb(I0.9Br0.1)3 solution (∼40 wt %) was prepared by reacting the appropriate amount of synthesized CH3NH3I powder and PbI2 (Aldrich) and PbBr2 (Aldrich) in γbutyrolactone at 60 °C for 12 h. The CH3NH3Pb(I0.9Br0.1)3 solution was then coated onto the mp-TiO2/bl-TiO2/FTO substrate by consecutively carrying out spin coating at 2000 rpm for 60 s and at 3000 rpm for 60 s; the substrate was then dried on a hot plate at 100 °C for 10 min. A solution of poly(triarylamine) (15 mg, PTAA, EM index, Mw = 17 500 g/ mol) in toluene (1.5 mL) was mixed with 15 μL of a solution of lithium bistrifluoromethanesulfonimidate (170 mg) in acetonitrile (1 mL) and 7.5 μL of 4-tert-butylpyridine (TBP); the misture was spin-coated on CH3NH3Pb(I0.9Br0.1)3 at 3000 rpm for 30 s. Finally, a Au counter electrode was deposited by thermal evaporation. Characterization Techniques. TiO2 films were characterized by field emission scanning electron microscope (FESEM; Tescan Mira 3 LMU FEG) and wide-angle X-ray powder diffraction (XRD; Rigaku D/Max II X-ray dffractometer) before they were used for device fabrications. The J−V curves were measured using a solar simulator (Newport, Oriel Class A, 91195A) with a source meter (Keithley 2420) at 100 mA cm−2, illumination AM 1.5G, and a calibrated Si-reference cell certificated by NREL. The J−V curves for all devices were measured by masking the active area with a metal mask of area 0.096 cm2.
Figure 1. (a) Top view of mesoporous TiO2 film prepared with 1,3,5trimethylbenzene. (b) 6-fold arrangement of mesopores. (c, d) Models of Im3m ̅ mesostructure with ⟨111⟩ direction oriented perpendicular to substrate.
3. RESULTS AND DISCUSSION The introduction of a swelling agent to expand the size of template micelles serves as an alternative strategy for enlarging the pore size of TiO2 films.26 The mp-TiO2 film was prepared by spin coating the precursor solution on glass and FTO coated glass substrates. The precursor solution was prepared by mixing a triblock copolymer Pluronic F127 (EO106PO70EO106), TMB, titanium tetra n-butoxide, hydrochloric acid, and ethanol under stirring. After the coating, the as-prepared thin films were aged at room temperature (18−20 °C) for 3 days and finally calcined (see Experimental Section). Films were also prepared without aging to investigate the surface properties. The mp-TiO2 prepared using 0.18 g of TMB (where the TMB/copolymer ratio was 0.5) showed a 6-fold arrangement of periodically arranged arrays of uniform cages (10−15 nm) throughout the top surfaces (Figure 1a,b). The 6-fold arrangement is generally obtained when viewed along the ⟨111⟩ plane of the mp-films with body centered cubic (Im3̅m) symmetry. This can be understood from the model of the top views of oriented Im3̅m (Figure 1c,d). The small-angle X-ray diffraction (SAXRD) pattern (Figure 2a) of the sample synthesized using 0.18 g of TMB (where the TMB/copolymer ratio was 0.5) exhibited two well-resolved diffraction peaks, which were indexed to the (110) and (200) characteristic diffractions of the cubic Im3m ̅ mesostructure, with a cell parameter of 20.2 nm. The absence of the ⟨111⟩ peak in SAXRD and the presence of the 6-fold arrangement on the top view in SEM analyses together indicated that the ⟨111⟩ plane was perpendicular to the substrate.27 The cell parameter calculated from the SEM images was 19 nm, which is in good agreement with the SAXRD pattern. The wide-angle XRD pattern indicated the formation of the crystalline anatase TiO2 (Figure S1 in Supporting Information) phase after calcination.
Figure 2. (a) Small angle X-ray diffraction and (b) high-resolution transmission electron microscopy image (inset: fast Fourier transformation pattern of selected area) of mesoporous TiO2 film prepared with 1,3,5-trimethylbenzene.
The transmission electron microscopy (TEM) image of the powder samples scratched from the substrate (Figure 2b) further supported the porosity and crystallinity of the TiO2 film. The TEM image showed the typical ordered mesostructure of the material, where pores were hexagonally surrounded by anatase nanocrystallites of ca. 10−13 nm in diameter. In addition, the bright spots in the fast Fourier transform (FFT) pattern in Figure 2b (inset) also indicated that the pores were hexagonally surrounded by anatase nanocrystallites. Selected area electron diffraction (SAED) pattern (Figure S2a in Supporting Information) showed the crystalline nature of the pore walls. The lattice fringes (Figure S2b in Supporting Information) and the diffraction rings in SAED were consistent with the anatase TiO2 phase. Figure S2c shows the FFT pattern that was collected from the lattices of a single anatase nanocrystal. However, a completely different scenario was observed for the mp-TiO2 film prepared in the absence of TMB. The obtained TiO2 film exhibited high-density crystalline TiO2 anatase nanopillars surrounded by the inverse mesospace as was observed by Wu et al.16 instead of the differences that exist in the form of inorganic sources, surfactants, and synthetic processes. We found that the periodically distributed arrays of titania pillars were generated after performing calcination at 400 °C for 2 h, which lead to the formation of pores (of size 6−7 nm), as shown by the SEM images (Figure 3a,b). This porosity is regarded as the “inverse mesospace” of a 2D hexagonal C
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the synthesis of TiO2 films which finally produced an ordered cubic network of mesopores. To investigate the effect of several experimental conditions on the surface morphology, we prepared films with varying parameters by our fabrication technique: (i) films prepared without aging, which possessed different pore symmetry or geometry and, more importantly, the connectivity of the pores and the TiO2 matrix and (ii) films with varying swelling agent/ copolymer (w/w) ratio prepared by using the same experimental technique that controls the nature of the microphase morphology. Systematic variation of these parameters was used to determine their effect on the ensuing film morphology, which finally helped develop the unique synthetic procedures required to prepare mp-TiO2 films with different mesospace ordering. The SEM images of as-coated films prepared in the presence of 0.18 g of TMB when directly calcined at 500 °C for 2 h without aging for 3 d (Figure S4a in Supporting Information) clearly showed the organized mesopores. However, a low-angle XRD pattern (Figure S4b in Supporting Information) did not show distinguished maxima corresponding to cubic or hexagonal mesophases, while aging at 18 °C for 3 d followed by calcination at 500 °C for 2 h induced the cubic mesophase formation. This can be explained by the improvement in the degree of ordering of the pores by aging under controlled conditions of temperature and humidity. The films produced by only spin-coating followed by calcination (without aging) generally lack such a high degree of mesoscopic ordering, as indicated by the small-angle XRD pattern of Figure 2a because the faster solvent evaporation in the spin-coating method makes the film material more viscous, which in turn makes the movement of the titania and surfactant species too sluggish to achieve a high degree of ordering in a short period. Therefore, in spite of the formation of a selfassembled structure by spin-coating, the degree of ordering cannot be as high as that in the case of aged films. During the aging step, the liquid-crystal-like film materials can attain the most thermodynamically stable ordering and orientation under the given conditions by the movement of titania particles and surfactant molecules.29 This resulted in the ordering of our films with the largest exposed surface of the micelles facing the interfaces, and the ⟨111⟩ orientation appeared to be the most thermodynamically favored orientation for the Im3m mesostructured film. Figure 4 shows the top-view SEM images of TiO2 films synthesized with different swelling agent/copolymer (w/w) ratios, ranging from 0.5 to 2. The films were aged before calcination at 500 °C for 2 h. The pore size increased in the presence of TMB compared to that of films prepared without TMB, and the surface pore morphology was also affected by the amount of TMB. As the amount of TMB increased, the average pore sizes remained similar; however, for different TMB/ copolymer ratios, the shape of micelles began to degrade; finally, when TMB/F127 = 2 (w/w) was used, the films produced a unique morphology accompanied by the loss of nanoscale periodicity. The thicknesses of these films as a function of the amount of TMB were also determined from their cross-sectional SEM images (Figure S5 in Supporting Information). Finally, we selected films prepared with a TMB/ copolymer ratio of 1 that showed 15 nm mesopores and films prepared without TMB that showed 10 nm mesopores for solar cell devices. We fabricated and compared the photovoltaic performance of the perovskite-based solar cells fabricated by sol−gel films of
Figure 3. (a) Scanning electron microscopy top view and (b) crosssectional view of mesoporous TiO2 film prepared without 1,3,5trimethylbenzene.
structure with “mesochannels” (of size 6−7 nm) running perpendicular to the substrate. The wide-angle XRD pattern of the calcined film proved the formation of the anatase phase (Figure S3a), which was also supported by the SAED pattern (Figure S3b).The low-angle XRD pattern (Figure S3c in Supporting Information) showed the appearance of two small peaks (for example, two XRD peaks at 2θ = 0.98° and 1.86° with the corresponding d-spacings of 8.9 and 4.7 nm, respectively) for the as-synthesized film, corresponding to the ⟨100⟩ and ⟨200⟩ planes. After calcination at 200 °C, a sharp peak appeared that corresponded to the ⟨100⟩ plane of hexagonal symmetry, indicating that the periodic organization of the 3D hexagonal (P63/mmc) mp-structure was retained after heating at 200 °C. After calcination at 400 °C, a transformation of the inverse mesospace occurred. This was believed to be induced by the large contraction of the film in the perpendicular direction (along the c axis), as was seen in the SEM image. On the basis of the above observations, we proposed that large contraction of the film caused the merging of pores along the c axis of the initial 3D hexagonal structure, leading to the inverse mesospace in the final structure. A highresolution TEM image also showed the presence of mesopores connected to neighboring mesopores (Figure S3d in Supporting Information). Eventually, under the same experimental conditions in the absence of TMB, the inverse mesospace evolved as the final mesostructure, whereas in the presence of TMB, a well-ordered body-centered cubic mesostructure was generated. Therefore, the presence of TMB played a vital role not only in increasing the pore size but also in directing the mesophase of the TiO2 films. In this work, we for the first time combined the use of TMB as a swelling agent with the EISA method to prepare triblock copolymer template mp-TiO2 films. TMB is an apolar additive associated with the hydrophobic part of the surfactant micelle and is thus able to increase the size of the hydrophobic core, resulting ultimately in an increase in the pore size. In the presence of TMB with a polymer-to-TMB mass ratio of 0.5, we found that the mesophases changed to 3D cubic Im3m from the 3D hexagonal P63/mmc mesophase (as obtained in the absence of TMB) under the same experimental conditions. This transition would be governed by the requirement to sufficiently cover all the volume of TMB with a fixed amount of polymer. It was previously noticed by Lettow et al.28 in the preparation of mp-silica powder that at polymerto-TMB mass ratio of 0.5, TMB swelled out the PPO chains until saturation and then pure oil cores were formed at the center of the micelles. To cover the oil droplets with the minimum amount of polymer, the micelles become more spherical. Here, we also observed a similar phenomenon induced by the addition of TMB to the triblock copolymer in D
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TiO2 paste (η = 12.7%) and those reported recently by our group using an electrode with TiO2 nanoparticles.22 Accordingly, mesoporous TiO2 sol−gel film prepared with TMB can be applied to photoanodes for the fabrication of efficient perovskite solar cells.
4. CONCLUSIONS We have demonstrated a supramolecular template route for preparing periodic mp-TiO2 through the simple control of synthetic variable (swelling agent/F127) concentration with controlled semiconductor surface morphology on the 10−15 nm-length scale. The pore size variation is important for different applications, e.g., for the design of better porous catalysts, for large molecular separation, and for hosting quantum size objects that could also benefit from the fine control of mesopore size. In this work, a simple methodology using a swelling agent was extended for the first time to the synthesis of periodic mp-TiO2 with a cage-like mesostructure. The systematic study was performed by careful adjustment of the polymer-to-swelling agent weight ratio to systematically vary the material’s porous structure. The mesostructured layers were used as photoanodes to fabricate perovskite-based solar cells. Energy conversion efficiencies of 11.7% and 12.8% were measured for devices with 10 and 15 nm mp layers, respectively. We believe that pore-size-tuned mp-layers can be used as photoanodes for fabricating efficient and cost-effective practical perovskite-based solar cells.
Figure 4. Top view of TiO2 film prepared with aging (a) without 1,3,5-trimethylbenzene (TMB) and with TMB in a TMB/F127 ratio of (b) 0.5, (c) 1, and (d) 2.
two different pore sizes (10 and 15 nm) with PTAA as HTM. For comparison, the reference cell with conventional TiO2 paste were also fabricated. In all solar cell configurations, we have chosen gold as a top electrode because its work function is close to the highest occupied molecular orbital of PTAA. Here, we used a similar thickness of mp-TiO2, approximately 200 nm, by controlling the coating conditions. Figure 5 shows the
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ASSOCIATED CONTENT
S Supporting Information *
Wide angle X-ray diffraction of mesoporous TiO2 film prepared with 1,3,5- trimethylbenzene (Figure S1); selected area electron diffraction pattern of mesoporous TiO2 film, lattice fringes, and fast Fourier transformation (FFT) pattern obtained from the lattice of a single TiO2 nanocrystallite (Figure S2); high-angle X-ray diffraction pattern, SAED pattern, small-angle X-ray diffraction patterns, and high-resolution transmission electron microscopy image of mesoporous TiO2 film prepared without 1,3,5-trimethylbenzene (TMB) (Figure S3); scanning electron microscopy top view and small-angle X-ray diffraction of TiO2 film prepared without aging in the presence of 1,3,5trimethylbenzene (Figure S4); cross-sectional views of TiO2 film prepared with aging in the absence and presence of TMB (Figure S5). This material is available free of charge via the Internet at http://pubs.acs.org.
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Figure 5. J−V curves for mp-TiO2/perovskite/poly(triarylamine)/Au with ∼10 and ∼15 nm pore sizes as a photoelectrode, measured under 100 mA cm−2 illumination AM 1.5G condition. (J−V curve, current density−voltage curve; mp, mesoporous; Voc, open-circuit voltage; Jsc, short-circuit current density; FF, fill factor; η, overall conversion efficiency).
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (
[email protected]). Author Contributions §
A.S and N.J.J. contributed equally to this work
Notes
current density−voltage (J−V) curves for mp-TiO2/MAPb(I0.9Br0.3)3/Au using the electrodes with sol−gel films of two different pore sizes and conventional TiO2 paste. The photovoltaic parameters for these devices are summarized in the inset of Figure 5. As can be seen from the figure and table, the cell fabricated with the 15 nm mp layer is superior to that of the 10 nm layer in pore size. The device with the 15 nm mp layer affords an open-circuit voltage (Voc) of 1.04 V, a shortcircuit current density (Jsc) of 18.8 mA cm−2, a fill factor (FF) of 66.0%, and an overall conversion efficiency (η) of 12.8%. These values are similar to those of the cell fabricated with our
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study was supported by the Global Research Laboratory (GRL) Program and the Global Frontier R&D Program on Center for Multiscale Energy System funded by the National Research Foundation under the Ministry of Education, Science and Technology of Korea and by a grant from the KRICT 2020 Program for Future Technology of the Korea Research Institute of Chemical Technology (KRICT), Republic of Korea. E
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