Polymer Solar Cells Proc - Wiley Online Library

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Aug 8, 2016 - Y. Guo, H. Han, Prof. D. Zhao. Beijing National Laboratory for Molecular Sciences. Department of Applied Chemistry. Center for the Soft Matter ...
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A Vinylene-Bridged Perylenediimide-Based Polymeric Acceptor Enabling Efficient All-Polymer Solar Cells Processed under Ambient Conditions Yikun Guo, Yunke Li, Omar Awartani, Jingbo Zhao, Han Han, Harald Ade,* Dahui Zhao,* and He Yan* Bulk-heterojunction (BHJ) polymer solar cells (PSCs) have attracted great research interests over the past two decades due to their low cost, light weight, and mechanical flexibility.[1–4] In order to achieve a high power conversion efficiency (PCE), it is essential to form an interpenetrating bicontinuous phase separation of the electron donor and acceptor at a length scale of about 10–20 nm via simple solution processing. Historically, fullerene derivatives have been widely used as the acceptor material in PSCs due to their superior electron transport properties and ability to form a favorable BHJ morphology.[1,5] Via rational design of the donor polymer hosts, researchers have pushed the PCE of polymer/fullerene-based BHJ solar cells to over 10%.[6–12] Despite such a remarkable success, fullerene materials exhibit several drawbacks, including the high production cost, low absorption coefficient, as well as limited morphological and chemical stability.[13,14] To overcome these problems, a large number of nonfullerene small-molecule (SM) acceptors have recently been investigated,[15–19] which show great promise for achieving PCEs comparable to those reached by fullerene derivatives.[20–33] Compared to polymer:SM-acceptor-based PSCs, all-PSCs offer additional advantages, such as the higher morphological stability and better donor–acceptor compatibility which may lead to higher PCE. However, in contrast to the large variety of SM-acceptor structures available, there have been very limited choices of

Y. Guo, H. Han, Prof. D. Zhao Beijing National Laboratory for Molecular Sciences Department of Applied Chemistry Center for the Soft Matter Science and Engineering and the Key Laboratory of Polymer Chemistry and Physics of the Ministry of Education College of Chemistry Peking University Beijing 100871, China E-mail: [email protected] Y. Li, Dr. J. Zhao, Prof. H. Yan Department of Chemistry Hong Kong University of Science and Technology Clear Water Bay, Kowloon, Hong Kong, China E-mail: [email protected] Dr. O. Awartani, Prof. H. Ade Department of Physics North Carolina State University Raleigh, NC 27695, USA E-mail: [email protected]

DOI: 10.1002/adma.201602387

Adv. Mater. 2016, DOI: 10.1002/adma.201602387

polymeric acceptors that can achieve efficient all-PSCs. Most of the high-efficiency all-PSCs are based on naphthalenediimide (NDI)-based polymers,[34–43] particularly a commercially available n-type semiconducting polymer named N2200 (poly((N,N′bis(2-octyldodecyl)-1,4,5,8-naphthalenedicarboximide-2,6-diyl)alt-5,5′-(2,2′-bithiophene))).[37–43] This material is known for its impressive electron mobility, with a field-effect transistor mobility of 0.85 cm2 V−1 s−1 and a space-charge-limited-current (SCLC) mobility of 5 × 10−3 cm2 V−1 s−1,[44,45] which is comparable to that of phenyl-C61-butyric acid methyl ester (PCBM). With these properties, highly efficient all-PSCs has been achieved using N2200 as the acceptor.[37] However, the N2200 polymer exhibits serious drawbacks, as its possesses a strong tendency to aggregate, often leading to excessively large domains for BHJ blends. Due to this problem, special processing conditions such as hot processing[42] or spin-coating using low-boiling point solvents[37,41] (e.g., chloroform) are often needed to reduce the domain size and to yield efficient devices. Therefore, it is important to develop alternative polymeric acceptors that exhibit relatively high electron mobility but that does not possess the aforementioned problems of N2200. An alternative building block to construct polymeric acceptors for all-PSCs is perylenediimide (PDI).[46,47] PDI derivatives offer good absorption that is complementary to state-of-the-art lowbandgap donor polymers and also possesses reasonably good electron transport property.[18,48–50] Previously, PDI-thiophene copolymers have been demonstrated as potential candidates of acceptors for all-PSCs.[51–53] However, PDI-based polymeric acceptors generally led to significantly lower all-PSC performance than those obtained from N2200. The lower performance was mainly attributed to the nonplanar nature of the poly(PDIthiophene) backbone due to the strong steric hindrance at the bay region of the PDI unit. Twisting angle between two adjacent PDI in poly(PDI-thiophene) backbone was estimated to be about 50° to 70°,[54] which is the main factor that contributed to the low polymer crystallinity and electron mobility of the PDI polymer. This is a major disadvantage of the PDI polymeric acceptors compared to the N2200 acceptor. In addition, the bromination of the PDI monomer can yield two regio-isomers, which require repetitive column chromtography to purify and is thus costly to separate. Without separation, the isomerically impure monomer would lead to a regiorandom polymer that may not be beneficial to PSC performance.[55] In the present contribution, we design and study a new PDI polymer acceptor, that exhibits much enhanced PCE in all-PSCs relative to the previously reported poly(PDI-thiophene).[51–53]

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Figure 1.  a) Molecular structure of acceptor PDI-V and donor PTB7-Th. b) Normalized UV–vis absorption spectra of PDI-V and PTB7-Th. c) Absorption spectrum of PDI-V thin film upon photoirradiation. d) Synthesis of PDI-V. e) The schematic device configuration used in this work and the energy levels of PDI-V and PTB7-Th.

Our main design rationale is to improve the planarity of the polymer backbone by reducing the steric hindrance near the bay region of PDI, which should increase the π–π stacking ability of the polymer backbone and thus enhancing electron transport. With such a design guideline, a new polymer, PDI-V, composed of PDI units joined by vinylene linkers is synthesized (shown in Figure 1). Density functional theory (DFT) calculations offer supportive evidence for the relatively planar conformation of PDI-V, and grazing incident wide-angle X-ray scattering (GIWAXS) further proved the structural regularity of the polymer backbone in the film state. Dove-tailed 1-hexylheptyl side chains are introduced onto the imide nitrogen atoms on PDI to ensure a good solubility for processing. Solar cell devices were fabricated based on a well-known donor polymer named PTB7-Th and PDI-V, which achieved a high efficiency of 7.57%. This value represents the best result to date among allpolymer solar cells using a PDI-polymer as the acceptor. This high efficiency was achieved from pristine polymer-blend films processed without using the additives, nor was post-annealing treatment needed. More importantly, the materials appeared fairly stable and processible under ambient conditions. The devices fabricated in ambient air with humidity of 90% without encapsulation still afford PCEs as high as 7.49%. This feature should be very attractive to the industrial large-scale processing. As illustrated in Figure 1, the new polymer acceptor PDI-V was synthesized with the Stille polymerization of dibromoperylenediimide (PDI-Br2) and trans-1,2-bis(tri-n-butylstannyl) ethylene, using Pd2(dba)3/P(o-tol)3 as the catalyst. A regioisomer mixture of 1,6- and 1,7-PDI-Br2 (at a molar ratio of ≈1:4, according to the 1H NMR spectrum)[56] obtained from the bromination reaction was used for the polymerization without 2

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isomer separation. More polymer synthesis details can be found in the Experimental Section. The number-average molecular weight (Mn) of the PDI-V was 14.6 kDa with a polydispersity index (Mw/Mn) of 2.03, measured by the high-temperature gel permeation chromatography (GPC). Due to the branched alkyl side chains, PDI-V exhibits good solubility in common organic solvents, such as o-dichlorobenzene (DCB), chlorobenzene (CB), and chloroform. To test the polymer's photostability, we examined the UV– vis absorption and 1H-NMR spectra of the PDI-V polymer (Figure 1c and Figure S1, Supporting Information) and a dimer model molecule[57] (Figure S2, Supporting Information) after continuous irradiation with a 500 W mercury lamp for up to 24 h. Our results showed that both the UV−vis absorption and the 1H-NMR spectra of the PDI-V polymer remain essentially the same after 24 h irradiation in air, which proves that the PDI-V polymer exhibits good photostability in air. The UV–vis absorption spectra of PDI-V in solution and film state are shown in Figure 1b, and the photophysical data are summarized in Table S1 of the Supporting Information. The PDI-V polymer shows a relatively broad absorption spectrum ranging from 300 to 700 nm, with two peaks at 350 and 600 nm. An optical bandgap of 1.74 eV is estimated for the polymer based on the absorption onset at 714 nm. The optical bandgap of the polymer is similar in solution or in the film state. According to the reduction potential determined by the cyclic voltammetry (Figure S3, Supporting Information), the lowest unoccupied molecular orbital (LUMO) level of PDI-V is determined to be −4.03 eV, which is similar to those of PCBM and N2200 and match well with commonly used donor polymers.[6,10,37] The relatively low lying LUMO

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Figure 2.  Optimized geometry obtained using DFT calculation. The dihedral angels between adjacent PDIs are shown in the diagram.

compared to previous reported PDI-thiophene copolymers may also account for the enhanced air stability.[51,53] Using the SCLC method and a device structure of ITO/ZnO/PDI-V/Ca/ Al, the electron mobility of the PDI-V film was estimated to about 1.5 × 10−3 cm2 V−1 s−1, which is on the same order with fullerene derivatives such as PCBM. To obtain information about the backbone conformation of PDI-V, the ground-state geometry was calculated with DFT using the B3LYP functional and a 6–31G* basis set. The calculation was performed on a PDI-V oligomer analogue, which can present structural information for the polymer.[58] Here, the energy-minimized geometry of a tetramer is shown in Figure 2. The backbone conformation of the PDI-V is quite planar compared to other PDI-based structures.[54,59,60] Specifically, the dihedral angle between the adjacent PDIs ranges from 2° to 9°. The more planar structure of the polymer led to high electron mobility and polymer crystallinity, which will be discussed later in detail. To fabricate all-PSC devices incorporating PDI-V as the acceptor, a benchmark narrow-bandgap semiconducting polymer PTB7-Th[61] (see Figure 1a) possessing a complementary absorption spectrum and properly matched energy

levels was used as the electron donor. The BHJ all-PSCs were fabricated with an inverted device structure of ITO/ZnO/PTB7Th:PDI-V/V2O5/Al. The active layer was spin-cast from a solution of CB with a donor/acceptor (D/A) weight ratio of 1:1. The absorption spectrum of a blend film is shown in Figure S4 of the Supporting Information. The current density–voltage curve of a representative PTB7-Th:PDI-V cell illustrated in Figure 3a presenting a PCE of 7.57%, with Voc = 0.74 V, Jsc = 15.9 mA cm−2, and FF = 0.63 (see Table 2). The enhanced device performance relative to previously reported PDI-thiophene represents the best PCE among PDI-based polymeric acceptors.[51,53] The maximum external quantum efficiency (EQE) (Figure 3b) reaches about 71%. Processing using different D/A weight ratio or solvent additives such as 1,8-diiodooctane and 1-chloronaphthalene resulted in lower PCEs for these PTB7-Th:PDI-V all-PSCs (see Table 1, Figure S5 and S6, Supporting Information). The fact that high performance can be achieved without any additives or post-treatment present distinctive advantages for largescale production. The photoluminescence (PL) quenching of the all-PSC blends was measured to determine the percentage of excitons that can reach and dissociate at the donor/acceptor interface. Blending

Figure 3. a,b) J–V curve (a) and EQE spectrum (b) of all-PSC with D/A weight ratio of 1:1 under the illumination of AM 1.5G, 100 mW cm−2. c) PL spectrum of PTB7-Th:PDI-V blend film (gray triangles) compared with those of PTB7-Th (black squares) and PDI-V (black triangles) films. The inset shows the full PL spectrum and the main graph is the enlarged view of low intensity zone. The excitation wavelength was 633 nm for the PL experiment.

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Table 1.  The photovoltaic parameters of PTB7-Th:PDI-V-based inverted all-PSCs processed in different conditions in nitrogen atmosphere. Shown in the brackets are the highest PCE. Presented were average data from 10 cells. D/A ratio

Voc [V]

Jsc [mA cm−2]

FF

PCE [%]

1:1

0.74 ± 0.01

15.8 ± 0.3

0.63 ± 0.01

7.3 ± 0.2 (7.57)

1:1.5

0.75 ± 0.01

14.9 ± 0.3

0.61 ± 0.03

6.8 ± 0.4 (7.00)

1.5:1

0.73 ± 0.01

15.1 ± 0.4

0.63 ± 0.03

6.9 ± 0.2 (7.11)

No additives, annealed under 100 °C for 5 min

1:1

0.75 ± 0.01

14.5 ± 0.3

0.64 ± 0.02

6.9 ± 0.2 (7.09)

1% DIO, no post-treatment

1:1

0.74 ± 0.01

13.3 ± 0.4

0.65 ± 0.02

6.5 ± 0.2 (6.71)

1% CN, no post-treatment

1:1

0.75 ± 0.01

12.5 ± 0.3

0.66 ± 0.02

6.1 ± 0.3 (6.38)

Processing conditions No additives or post-treatment

PDI-V with PTB7-Th results in fluorescence quenching of over 95%, indicating highly efficient photoinduced charge transfer between PTB7-Th and PDI-V in the blend film (see Figure 3c and Figure S7, Supporting Information). The bulk charge transport properties of the PTB7-Th:PDI-V blend film was also investigated using the SCLC method. The hole mobility was measured with a device structure of ITO/V2O5/PTB7-Th:PDIV/V2O5/Al, and the electron mobility was measured with ITO/ ZnO/PTB7-Th:PDI-V/Ca/Al. The hole and electron mobilities were calculated to be 1.3 × 10−3 and 4.2 × 10−4 cm2 V−1 s−1, respectively, corresponding to a reasonably balanced charge transport (μh/μe = 3.1). The effective fluorescence quenching, high carrier mobility and balanced carrier transport in the active layer may account for the high PCEs achieved in PTB7Th:PDI-V solar cells. To explore the potential suitability of PDI-V in large-scale manufacturing, device fabrication in ambient conditions was investigated. The active layers were spin-coated in the air with a humidity of 90% without encapsulation. As shown in Table 2 and Figure S8 of the Supporting Information, the PCE of the PTB7-Th:PDI-V device can still reach 7.49%, which represents only 1% decrease when compared to devices made in nitrogen atmosphere with encapsulation. The loss of PCE mainly originated from a decrease in the Jsc. The PCE is comparable to those of fullerene derivatives under similar conditions in previous reports[62,63] and from our own experiment (see Table S2 and Figure S8, Supporting Information). According to the above results, it can be concluded that the photovoltaic performance of the PTB7-Th:PDI-V device is relatively insensitive to the oxygen and water levels in air. To understand the microstructure and morphology of PDI-V and the blend film, GIWAXS was employed to characterize the molecular packing in a neat PDI-V film (see Figure 4). From the 2D GIWAXS pattern, a lamellar (100) peak is observed in both the in-plane (IP) and out-of-plane (OOP) directions. As Figure 4a shows, the (100) peak is at a different q in the IP and OOP directions, which is also apparent from the oval-like (100) ring rather than a circular ring, a feature typically associated Table 2.  The photovoltaic parameters of PTB7-Th:PDI-V-based inverted all-PSCs processed in ambient air with 90% humidity without encapsulation. Average data from 10 cells are presented. Processing conditions 90% humidity

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Voc [V]

Jsc [mA cm−2]

FF

PCE [%]

0.74 ± 0.01

15.3 ± 0.3

0.64 ± 0.01

7.3 ± 0.3 (7.49)

with an equal spacing in the lamellar direction in both IP and OOP orientations. The lamellar spacing distances for the PDI-V polymer film in the IP and OOP directions are 23 and 16.5 Å, respectively. It is worth noting that a second (100) peak appears at q = 0.36 Å−1 which indicates that there might be a polymorph in the in-plane direction with a d-spacing of 17.6 Å (Figure S9 of the Supporting Information shows the fit details of the polymorph peaks in the IP orientation). The difference in IP and OOP d-spacing could be due to strong interdigitation between the side chains in the OOP, or flexibility in the side chains themselves. The origin of the differences in this interdigitation in the IP and OOP are presently not clear and would need to be investigated further. Another strong feature that appears in the GIWAXS 2-D pattern is a well-defined (001)-like peak at q = 0.77 Å−1 that corresponds to the polymer backbone. The observed texture indicates that the backbone direction is predominantly in the plane of the film. The observed q corresponds to a spacing distance of 8.2 Å, which is slightly smaller than the DFT predicted backbone spacing of 9.1 Å. The DFT simulations used are not capable of simulating a large number of polymer units. Lastly, a weak π–π (010) peak is observed at q = 1.55 Å−1 in the OOP direction corresponding to a d-spacing of 4 Å, mostly overlapping an amorphous band at q = 1.42 Å−1, which suggests that the film does not strongly favor a face-on or edge-on orientation. Given the strong IP (001) peak, the orientation distribution is likely 2D rather than 3D. Qualitatively, the relative intensities and width suggest that the ordering along the backbone is quite high. Furthermore, GIWAXS data from the blend film are shown in Figure 4c. It is clearly observed that the 2D pattern of the blend looks very similar to that of the neat polymer of PDI-V, which indicates that the morphology of the blend is mostly dominated by the PDI-V rather than PTB7-Th. It is shown that the (001) peak is slightly suppressed in the blend compared to that of the neat PDI-V. The morphology of the PTB7-Th:PDI-V blend films processed under different conditions was also examined by the atomic force microscopy (AFM) and transmission electron microscopy (TEM) (see Figure S10 and S11, Supporting Information). The AFM height image of the pristine blend film exhibits a smooth and uniform surface morphology with a root-mean-square (RMS) roughness of 0.67 nm. The AFM phase image and TEM image show that the domain size of the pristine PTB7-Th:PDI-V blend film is about 10–20 nm, which is similar to that of high-performance N2200-based BHJ blends processed from chlolorform.[30] The small domain size of the PTB7-Th:PDI-V blend can be achieved from simple

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Communication Figure 4.  a,c) GIWAXS pattern of: neat PDI-V film (a) and PTB7-Th:PDI-V blend (c) without any additives and post-treatment. b,d) 1D profiles in-plane (red) and out-of-plane (black) of PDI-V (b) and PTB7-Th:PDI-V (d) spin coated on a Si/SiO2 substrate.

processing condition with commonly used solvent instead of hot processing[38] or spin-coating using low-boiling point solvents,[33,37] which is a very attractive property for application to the industrial processing. In summary, we report a novel nonfullerene polymer acceptor, PDI-V, consisting of alternating PDI and vinylene units. The PDI-V polymer exhibits a relatively planar polymer backbone and thus high electron mobility. Inverted structure all-PSCs were fabricated based on PTB7-Th:PDI-V and a high PCE of 7.57% can be achieved without using any processing additives or post-treatment. This is among the highest value reported for all-PSCs and the best achieved thus far for PDI-based polymers. Furthermore, the device processed in ambient air without encapsulation can still reach a high PCE of 7.49%, which is a significant economic advantage from an industrial processing perspective. The high performance of the PDI-V polymer can be attributed to the better coplanarity of the PDI-V unit than PDI-thiophene. As a result, the PDI-V polymer exhibits high crystallinity and thus electron mobility, which leads to higher fill factors and PCEs. This study provides an alternative polymeric acceptor that can achieve high performance and that is also easily processible in ambient conditions.

Experimental Section Synthesis of Polymer PDI-V: A Schlenk tube charged with N,N′bis(1-hexylheptyl)-1,7/1,6-dibromo-3,4,9,10-perylenediimide (158 mg,

Adv. Mater. 2016, DOI: 10.1002/adma.201602387

0.17 mmol), trans-1,2-bis(tri-n-butylstannyl)ethylene (105 mg, 0.17 mmol), Pd2(dba)3 (8 mg, 0.009 mmol), and P(o-tol)3 (11 mg, 0.035 mmol) was evacuated and refilled with nitrogen three times, and then toluene (3 mL) was added using a syringe under nitrogen atmosphere. The reaction mixture was sealed under nitrogen atmosphere and stirred at 110 °C for 96 h. Upon cooling to room temperature, a solution of potassium fluoride (1 g) in water (3 mL) was added. This mixture was stirred at room temperature for 6 h before it was extracted with chloroform (20 mL × 3). Organic layers were combined, washed with water (30 mL × 3), dried over anhydrous sodium sulfate, and concentrated on a rotary evaporator. The residue was taken with a small amount of chloroform and precipitated in acetone. The obtained deep purple solid was purified by a size exclusion chromatography column filled with BioBeads S-X1 Beads, and then concentrated on a rotary evaporator. The solid residue was redissolved in chloroform (2 mL) and added slowly to methanol (50 mL). The precipitates were collected by filtration, washed with methanol, and dried in vacuum, leading to a deep purple solid as the product (101 mg, yield 75%). 1H NMR (400 MHz, CDCl3, ppm, δ): 9.14-8.74 (m, 6H), 8.38-7.28 (m, 2H), 5.24 (m, 2H), 2.29 (m, 4H), 1.88 (m, 4H), 1.33-1.25 (m, 32H), 0.84 (m, 12H). GPC: Mn = 14.6 kDa, Mw = 29.6 kDa, PDI = 2.03. Elemental Anal.: Calcd. For (C55H62N2O4)n: C, 80.17; H, 8.02; N, 3.60; Found: C, 78.48; H, 7.91; N, 3.44. Fabrication and Characterization of All-PSCs: Prepatterned ITO-coated glass with a sheet resistance of ≈15 Ω sq−1 was used as the substrate. It was cleaned by sequential sonications in soap deionized water, deionized water, acetone, and isopropanol for 15 min at each step. After UV/ozone treatment for 60 min, a ZnO electron transport layer was prepared by spin-coating at 5000 rpm from a ZnO precursor solution (diethyl zinc). Active layer solutions (D/A ratio 1:1) were prepared in CB (polymer concentration: 9 mg mL−1). To completely dissolve the polymer, the active layer solution should be stirred on hotplate at

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110 °C for at least 3 h. Active layers were spin-coated from the cooled polymer solution on substrate in a N2 glovebox or in ambient air with a controlled humidity at 1700 rpm to obtain thicknesses of ≈120 nm. The PTB7-Th:PDI-V blend films were then transferred to the vacuum chamber of a thermal evaporator inside the same glovebox. At a vacuum level of 3 × 10−6 Torr, a thin layer (20 nm) of V2O5 was deposited as the anode interlayer, followed by deposition of 100 nm of Al as the top electrode. All cells were encapsulated using epoxy inside the glovebox. Device J–V characteristics was measured under AM1.5G (100 mW cm−2) using a Newport solar simulator. The light intensity was calibrated using a standard Si diode (with KG5 filter, purchased from PV Measurement) to bring spectral mismatch to unity. J–V characteristics were recorded using a Keithley 2400 source meter unit. Typical cells have devices area of 5.9 mm2, which is defined by a metal mask with an aperture aligned with the device area. EQEs were characterized using a Newport EQE system equipped with a standard Si diode. Monochromatic light was generated from a Newport 300W lamp source. GIWAXS Characterization: GIWAXS measurements were performed at beamline 7.3.3 at the Advanced Light Source (ALS)[64] at the Lawrence Berkeley National Lab. The samples were measured in a Helium environment to minimize air scattering using 10 keV energy X-rays. The scattered X-rays were detected using a Dectric Pilatus 2M photon counting detector.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements Y.G. and Y.L. contributed equally to this work. The work at PKU was supported by the National Natural Science Foundation of China (No. 51473003); the X-ray characterization by North Caroline State University (NCSU) was supported by UNC-GA Research Opportunity Initiative grant. The work was partially supported by the National Basic Research Program of China (973 Program; 2013CB834705), HK JEBN Limited (Hong Kong) and the Hong Kong Research Grants Council (T23–407/13-N, N_HKUST623/13). Received: May 4, 2016 Revised: June 30, 2016 Published online:

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