English

• Single-component organic solar cells (SCOSCs) have attracted increasing attention over the past five years due to their advantages of high thermal stability, simple solution processing, and homogenous thin films toward large-area fabrication [1–5]. This is because SCOSCs use one conjugated material as the photoactive layer, distinct from physically mixed blends of electron donor and electron acceptor materials in bulk-heterojunction (BHJ) OSCs. Single-component materials in SCOSCs were called "homojunction" conjugated materials, but they have difficulty in separating excitons into free charges and do not have separate channels for electron and hole transport. Therefore, homojunction-conjugated materials exhibit power conversion efficiencies (PCEs) below 3% due to strong charge recombination in SCOSCs [6]. The efficient alternative is to combine electron donor and electron acceptor units in one conjugated material, so that the driving force for exciton separation and separate channels for charge transport can be obtained. These materials include molecular dyads, diblock conjugated polymers, and double-cable conjugated polymers [7,8]. With the development of new single-component conjugated materials and new methods to tune phase separation in thin films [9,10], the PCE of SCOSCs has dramatically increased to over 13%. However, such PCEs still lag behind BHJOSCs which have achieved PCEs of over 19%, suggesting that the PCE of SCOSCs can be improved further with increased effort devoted to materials innovation and morphology optimization [11–18].

  Double-cable conjugated polymers with conjugated polymer backbones as electron donors and aromatic side units as electron acceptors can efficiently absorb light and convert photo-excitation into free charges. These polymers can be universally designed with the consideration of conjugated backbones, acceptor side units, and also the linkers that connect the backbones and side units. For example, conjugated polymers backbones with different structures, from polythiophene derivatives [19], benzo[1,2-c: 4,5-c′]dithiophene-4,8-dione [20], isoindigo [21], and diketopyrrolopyrrole [22,23] based polymers, have been applied into double-cable polymers with tunable absorption spectra. Another efficient method is to introduce different electron acceptors into the side units, such as fullerene [24], rylene diimides [25–28], non-fused electron acceptors [29–36], and Y-series acceptors [37]. The distinct nature of these electron acceptors enables double-cable polymers to show absorption spectra from 300 nm to 900 nm, so that PCE could be significantly enhanced to 13% for based Y-series acceptors [38].

  Besides conjugated backbones and acceptor side units, the linkers between them play an important role to tune their phase separation in thin films. The length of the linkers is directly related to the distance between the backbones and acceptor side units. For example, in perylene diimide (PBI)-based double-cable polymers, the short alkyl linker C₆H₁₂ results in a small distance, and hence severe charge recombination resulting in low short-circuit current density (J_sc) and low fill factor (FF). When C₆H₁₂ was replaced by C₁₂H₂₄, charge recombination was reduced and hence the PCE was enhanced due to the larger distance between the backbone (donor) and pendant acceptor. The further increase in the length of the alkyl linkers in PBI-based double-cable polymers was found to show an unusual non-linear behavior, in which the donor/acceptor distance was observed to decrease with increasing length of the alkyl spacer before increasing again [39]. Similar behavior was also observed in naphthalene diimide (NDI)-based double-cable polymers [40]. The ability to tune the spacer length and hence the distance between donor and acceptor units in double-cable polymers also provides a convenient route to study the photoelectric conversion process [26]. Near-infrared electron acceptors usually use the strong electron-withdrawing 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile (IC) unit as the end group, which have been successfully applied into BHJOSCs. As different from PBI and NDI acceptors with face-to-face stacking structures, IC-based electron acceptors show three-dimensional networks [41–43]. Therefore, their introduction into double-cable polymers is expected to show different behavior in morphology, hence motivating this study into the linker effect on the morphology of IC-based double-cable polymers.

  With this background, we systematically investigate the influence of alkyl linker length on the photovoltaic performance of near-infrared double-cable polymers in this work. A series of IC-based double-cable polymers, denoted as Px (x = 8, 10, 12, 14, 16), with varying lengths of the alkyl linker ranging from C₈H₁₆ to C₁₆H₃₂, are synthesized for SCOSCs. Among these, the linker length x = 12 (P12) is found to optimize the power conversion efficiencies (PCEs) in SCOSCs. Specifically, PCEs increase from P8 (7.83%) to P10 (8.86%), reaching a peak at P12 (10.09%), and then decrease from P14 (8.05%) to P16 (7.53%). Comprehensive investigations involving optical absorption, charge transport, and morphology analysis provide valuable insights into the underlying factors driving the observed variations in PCEs. Notably, the double-cable conjugated polymer P12 exhibits a delicate balance between enhanced charge separation facilitated by the increased spacer length and reduced crystallinity resulting from longer spacers, higher charge mobilities, and well-balanced hole/electron transport characteristics, which account for the exceptional photovoltaic performance observed in SCOSCs based on P12.

  The chemical structures of the double-cable conjugated polymers Px are shown in Fig. 1a, with the detailed synthetic routes provided in Supporting information. Polymer P12 has been reported in our previous work [29], and similar procedures were used to synthesize the remaining double-cable polymers. In general, the key monomer of M11x is synthesized by a two-step bromination of the hydrogen-lithium exchange reaction/the bromination of CBr₄ at the α-position of the benzodithiophene unit and the bromination of hydrobromic acid on the methoxy group at the end of the alkyl chain. M12x monomer was obtained by Knoevenagel condensation of TPD dialdehyde with alkyl bromide. Finally, under the same reaction conditions, these double-cable conjugated polymers Pxs were obtained by Stille polymerization based on M12x and M13. All polymers can be well dissolved in common organic solvents such as chlorobenzene (CB), o-dichlorobenzene (o-DCB) and trichlorobenzene (TCB). The molecular weights of the polymers were determined by gel permeation chromatography (GPC) with TCB as the eluent at 140 ℃. The number average molecular weights (M_ns) of these polymers are in the range of 72.3-138.3 kDa (Fig. S1 and Table S1 in Supporting information), and the molar mass distributions (Đ_M) are 1.38-2.56.

  Figure 1

  

  Figure 1.  (a) The chemical structures of the double-cable conjugated polymer Pxs and their synthetic routes. (i) Stille polymerization, Pd₂(dba)₃/P(o-tolyl)₃, toluene, 115 ℃, 12 h. Normalized absorption spectra of the double-cable polymers in (b) chloroform solution and (c) thin films.

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  The absorption spectra of the double-cable polymers, donor polymer PBDB-T, and acceptor molecule TPDIC are depicted in Fig. 1b (in chloroform solution) and Fig. 1c (in thin films), while the associated data are summarized in Table 1. In solution, all polymers exhibit similar absorption spectra, displaying significant absorption within the 500-800 nm range, attributable to the presence of donor backbones and acceptor side units. Notably, a distinct red shift in wavelength is observed when transitioning from solution to thin film. For instance, P12 experiences a redshift of approximately 52 nm. Plotting the onset wavelength against different polymers in Fig. 1c reveals a red shift from P8 to P12, followed by a blue shift from P12 to P16, indicating variations in polymer aggregations and optical bandgaps (E_gs).

  Table 1

  

  Table 1.  Photophysical properties of the double-cable polymer Pxs.

  DownLoad: CSV

  SCOSCs were fabricated with an inverted device structure of ITO/ZnO/Pxs/MoO₃/Ag to examine the influence of alkyl linker length on photovoltaic properties. To ensure polymer solubility, o-DCB was chosen as the main solvent due to its high solubility with all Pxs. An additive of 4% 1,8-diiodooctane (DIO) was incorporated into polymer films based on previous research [29]. During the spin coating process, the active layer thickness was carefully controlled between 65 nm and 75 nm to optimize device performance. A detailed description of the fabrication process can be found in Supporting information. Fig. 2a illustrates the optimized current density versus bias voltage (J-V) curves for SCOSCs based on various double-cable polymers P8-P16 under simulated AM1.5G illumination. Photovoltaic parameters are summarized in Table 2 and Tables S2–S6 (Supporting information). An interesting parabolic-type trend of PCE as a function of alkyl spacer length was observed, which is similar to the trend in previous PBI-based and NDI-based double-cable polymer systems [39,40]. Specifically, the champion PCE of the Px-based SCOSCs is observed to increase from 7.83% for P8 to 8.86% for P10, reaching a peak of 10.09% for P12 and then decreasing to 8.05% for P14, and further decreasing to 7.53% for P16. Notably, SCOSCs based on P12 display the highest PCE of 10.09%, accompanied by an open-circuit voltage (V_OC) of 0.767 V, a short-circuit current density (J_SC) of 21.2 mA/cm², and a fill factor (FF) of 62.3%.

  Figure 2

  

  Figure 2.  (a) Optimized J-V characteristic curves for SCOSCs based on the double-cable polymers P8-P16 under simulated AM1.5G illumination. J_SC, V_OC, and FF values (b) and EQE spectra (c) of the optimized SCOSC devices based on different polymers. The integrated current densities are also included. (d) Hole and electron mobilities of these polymers.

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  Table 2

  

  Table 2.  Photovoltaic parameters of the optimized SCOSC devices based on different double-cable polymer Pxs.

  DownLoad: CSV

  Further analysis was focused on the photovoltaic parameters, including V_OCs, J_SCs_, and FF values as a function of linker lengths (Fig. 2b). Remarkably, it was observed that the V_OCs exhibited a decreasing trend from P8 (0.794 V) to P16 (0.759 V) with an increase in linker length, which is consistent with the trend reported in previous PBI-based polymer systems [35]. The J_SCs and FF values show similar variation trends with PCEs. Specifically, J_SCs increased from 18.2 mA/cm² for P8, to 19.3 mA/cm² for P10, peaking at 21.2 mA/cm² for P12, and then decreasing to 18.2 mA/cm² for P14 and further to 18.1 mA/cm² for P16. The FF values exhibited a similar trend, rising from 54.3% for P8 to 59.3% for P10, reaching a peak of 62.3% for P12, and subsequently decreasing to 57.9% for P14 and further to 54.8% for P16. The discrepancies in J_SCs were also reflected in the external quantum efficiencies (EQEs) depicted in Fig. 2c.

  To investigate the trends in J_SCs and FF values, we conducted space charge-limit current (SCLC) measurements to examine the vertical charge transport characteristics of the Px films. The J-V curves for hole-only and electron-only devices are presented in Figs. S2 and S3 (Supporting information). The calculated hole mobilities (μ_hs) and electron mobilities (μ_es) for all films based on P8-P16 are shown in Fig. 2d and Table S1. The μ_h values increase from 2.7 × 10⁻⁴ cm² V⁻¹ s⁻¹ for P8, to 1.5 × 10⁻³ cm² V⁻¹ s⁻¹ for P10, reaching a peak of 1.8 × 10⁻³ cm² V⁻¹ s⁻¹ for P12, and then decreasing to 6.8 × 10⁻⁴ cm² V⁻¹ s⁻¹ for P14, and further still to to 4.5 × 10⁻⁴ cm² V⁻¹ s⁻¹ for P16. Similar trends are observed for μ_e values which can be attributed to differences in charge mobilities and light absorption characteristics of the photoactive layers. Additionally, the μ_h/μ_e ratios were calculated (Fig. 2d and Table S1), exhibiting a similar trend with FF and reaching the lowest value at P12 (1.82), thereby explaining the variations in FF values.

  We then studied the morphology of the polymer thin films via atomic force microscopy (AFM) and grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements. As depicted in Fig. 3, the height images of AFM for all the polymers show smooth surfaces with similar root-mean-square roughness (RMS) values in the range of 0.89-1.66 nm, revealing excellent film-forming ability. GIWAXS measurements were further performed to investigate the evolution in the molecular packing and crystallinity of these polymer films with the 2D GIWAXS patterns (Fig. 4a) and the corresponding 1D out-of-plane (OOP, Fig. S4 in Supporting information) and in-plane (IP, Fig. 4b) plots. The appearance of (h00) peaks in the IP direction and (010) peaks in the OOP direction indicates that these polymers exhibit a preferentially face-on orientation, in which the polymer backbones are parallelly aligned to the substrates. As shown in Fig. S4 and Table S7 (Supporting information), all Px polymers possess a close π-π distance of 3.6 Å and similar intensity of (010) peaks, indicating that all Pxs possess a similar degree of π-π stacking mode [29]. Another interesting observation is that the d-spacing values of the side-chain stacking periodicity (based on the (300) peaks which are the strongest and most reliable to fit) of the polymers have a significant linear increase from P8 to P16 (Fig. 4c), which is consistent with previous NDI-based double-cable polymers but different with previous PBI-based double-cable polymers [39,40]. To examine differences in the degree of crystallinity in these polymers, we calculated the coherence length (CL) values corresponding along the side-chain stacking direction based on the (300) peaks of these Px polymers, which are shown in Fig. 4d and summarized in Table S7. The CL values in general show a downward trend with increasing alkyl linker length, suggesting that polymers with increasing length of alkyl linkers have lower crystallinity (i.e., smaller crystallite size or increased crystalline disorder). P12 may then represent a balance between improved charge separation facilitated by increased spacer length and increased crystalline disorder due to longer spacers.

  Figure 3

  

  Figure 3.  AFM height images for all Px thin films.

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  Figure 4

  

  Figure 4.  2D GIWAXS patterns (a) and the corresponding 1D IP plots (b) of the double-cable conjugated polymers. (c) d-spacings of the 300 peaks for different polymers. (d) Calculated CL values based on the (300) peaks versus different Px polymers.

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  In summary, we systematically investigated the influence of alkyl linker lengths on the photovoltaic performance of near-infrared double-cable polymers in SCOSCs. Five near-infrared double-cable polymers with varying alkyl linker lengths from C₈H₁₆ to C₁₆H₃₂ were synthesized. The investigation revealed a parabolic-type distribution in PCEs, where the P12-based SCOSC shows the optimal PCE of 10.09%, and PCEs increase from P8 to P12 and then decline from P12 to P16. Comprehensive analyses including optical absorption, charge transfer, and morphology provided valuable insights into the underlying factors driving these variations. It was found that the double-cable conjugated polymer P12 exhibited a delicate balance between enhanced charge separation facilitated by the increased spacer length and increased crystalline disorder resulting from longer spacers, higher charge mobilities, and well-balanced hole/electron transport characteristics, accounting for the exceptional photovoltaic performance observed in P12-based SCOSC devices. This work highlights the significant impact of alkyl linker length on the photovoltaic performance of double-cable conjugated polymer-based SCOSCs. It also provides valuable insights and guidance for the design of new double-cable conjugated polymers.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  Acknowledgments

  This study is jointly supported by the Beijing Natural Science Foundation (Nos. 2212045 and JQ21006) and the National Natural Science Foundation of China (Nos. 21905158, 52073016 and 92163128). This work was further supported by the Fundamental Research Funds for the Central Universities (Nos. buctrc202111, buctrc201828, and XK1802-2), the Opening Foundation of State Key Laboratory of Organic-Inorganic Composites of Beijing University of Chemical Technology (No. oic-202201006) and Jiangxi Provincial Department of Science and Technology (Nos. 20202ACBL213004, 20212BCJ23035, jxsq2019102004). This work was performed in part on the SAXS/WAXS beamline at the Australian Synchrotron, part of ANSTO.

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2023.109164.

  
  
• 1. [1]

     W. Lai, C. Li, J. Zhang, et al., Chem. Mater. 29 (2017) 7073–7077. doi: 10.1021/acs.chemmater.7b02534

  2. [2]

     Y. He, T. Heumuller, W. Lai, et al., Adv. Energy Mater. 9 (2019) 1900409. doi: 10.1002/aenm.201900409

  3. [3]

     J. Roncali, I. Grosu, Adv. Sci. 6 (2019) 1801026. doi: 10.1002/advs.201801026

  4. [4]

     S. Liang, X. Jiang, C. Xiao, et al., Acc. Chem. Res. 54 (2021) 2227–2237. doi: 10.1021/acs.accounts.1c00070

  5. [5]

     S. Liang, W. Li, L. Ding, J. Semicond. 44 (2023) 030201. doi: 10.1088/1674-4926/44/3/030201

  6. [6]

     G. Chamberlain, Sol. Cells 8 (1983) 47–83. doi: 10.1016/0379-6787(83)90039-X

  7. [7]

     D. Zhou, X. Cheng, H. Xu, et al., J. Mater. Chem. A 4 (2016) 18478–18489. doi: 10.1039/C6TA08049A

  8. [8]

     D. Xia, S. Zhou, W.L. Tan, et al., Aggregate 4 (2022) e279.

  9. [9]

     G. Zhang, Q. Chen, Z. Zhang, et al., Angew. Chem. Int. Ed. 62 (2023) 202216304. doi: 10.1002/anie.202216304

  10. [10]

      C. Zhao, Z. Zhang, F. Han, et al., Angew. Chem. Int. Ed. 60 (2021) 8526–8531. doi: 10.1002/anie.202100755

  11. [11]

      W. Gao, F. Qi, Z. Peng, et al., Adv. Mater. 34 (2022) e2202089. doi: 10.1002/adma.202202089

  12. [12]

      Y. Wei, Z. Chen, G. Lu, et al., Adv. Mater. 34 (2022) e2204718. doi: 10.1002/adma.202204718

  13. [13]

      G.Y. Ding, T.Y. Chen, M.T. Wang, et al., Nano-Micro Lett. 15 (2023) 14. doi: 10.1007/s40820-022-00981-8

  14. [14]

      G. Ding, T. Chen, M. Wang, et al., Nano-Micro Lett. 15 (2023) 92. doi: 10.1007/s40820-023-01057-x

  15. [15]

      J. Fu, P.W.K. Fong, H. Liu, et al., Nat. Commun. 14 (2023) 1760. doi: 10.1038/s41467-023-37526-5

  16. [16]

      C. Han, J. Wang, S. Zhang, et al., Adv. Mater. 35 (2023) e2208986. doi: 10.1002/adma.202208986

  17. [17]

      J. Wang, Y. Wang, P. Bi, et al., Adv. Mater. 35 (2023) e2301583. doi: 10.1002/adma.202301583

  18. [18]

      L. Zhu, M. Zhang, J. Xu, et al., Nat. Mater. 21 (2022) 656–663. doi: 10.1038/s41563-022-01244-y

  19. [19]

      A. Kiriy, V. Senkovskyy, M. Sommer, Macromol. Rapid Commun. 32 (2011) 1503–1517. doi: 10.1002/marc.201100316

  20. [20]

      D. Wang, Z.F. Yang, F. Liu, et al., Chin. Chem. Lett. 33 (2022) 466–469. doi: 10.1016/j.cclet.2021.06.042

  21. [21]

      C.S. Yu, Y.H. Xu, C. Li, et al., Chin. J. Chem. 36 (2018) 515–518. doi: 10.1002/cjoc.201800009

  22. [22]

      C.W. Zhao, Y.T. Guo, Y.F. Zhang, et al., J. Mater. Chem. A 7 (2019) 10174–10199. doi: 10.1039/C9TA01976F

  23. [23]

      C. Li, C. Wang, Y.T. Guo, et al., J. Mater. Chem. C 7 (2019) 3802–3810. doi: 10.1039/C9TC00377K

  24. [24]

      B.Q. Liu, Y.H. Xu, F. Liu, et al., Chin. J. Polym. Sci. 40 (2022) 898–904. doi: 10.1007/s10118-022-2732-2

  25. [25]

      H.S. Fang, D.D. Xia, C.W. Zhao, et al., Dyes Pigments 203 (2022) 110355. doi: 10.1016/j.dyepig.2022.110355

  26. [26]

      S. Liang, J. Wang, Y. Ouyang, et al., Macromolecules 55 (2022) 2517–2523. doi: 10.1021/acs.macromol.1c02593

  27. [27]

      W. Lai, S. Karuthedath, C. Xiao, et al., Chin. Chem. Lett. 35 (2024) 108287. doi: 10.1016/j.cclet.2023.108287

  28. [28]

      C. Wang, D. Xia, F. Yang, et al., ACS Appl. Polym. Mater. 3 (2021) 4645–4650. doi: 10.1021/acsapm.1c00743

  29. [29]

      S. Liang, B. Liu, S. Karuthedath, et al., Angew. Chem. Int. Ed. 61 (2022) 202209316. doi: 10.1002/anie.202209316

  30. [30]

      B. Liu, S. Liang, S. Karuthedath, et al., Macromolecules 56 (2023) 1154–1164. doi: 10.1021/acs.macromol.2c02184

  31. [31]

      B.Q. Liu, S.J. Liang, S. Karuthedath, et al., J. Mater. Chem. A 11 (2023) 12236–12244. doi: 10.1039/D3TA01501G

  32. [32]

      X.Y. Xu, H.B. Wu, S.J. Liang, et al., Acta Phys. Chim. Sin. 38 (2022) 2201039.

  33. [33]

      Z. Zhang, J. Wang, Z. Hu, et al., Chin. Chem. Lett. 34 (2023) 108527. doi: 10.1016/j.cclet.2023.108527

  34. [34]

      S. Pang, X. Zhou, S. Zhang, et al., ACS Appl. Mater. Interfaces 12 (2020) 16531–16540. doi: 10.1021/acsami.0c01850

  35. [35]

      B.Q. Wu, Y. Zhang, S.Z. Tian, et al., Sol. RRL 6 (2022) 2101034. doi: 10.1002/solr.202101034

  36. [36]

      C.N. Gannett, L. Melecio-Zambrano, M.J. Theibault, et al., Mater. Rep. Energy 1 (2021) 100008. doi: 10.1016/j.matre.2021.01.003

  37. [37]

      S. Li, C.Z. Li, M. Shi, H. Chen, ACS Energy Lett. 5 (2020) 1554–1567. doi: 10.1021/acsenergylett.0c00537

  38. [38]

      S. Liang, C. Xiao, C. Xie, et al., Adv. Mater. 35 (2023) e2300629. doi: 10.1002/adma.202300629

  39. [39]

      G. Feng, W. Tan, S. Karuthedath, et al., Angew. Chem. Int. Ed. 60 (2021) 25499–25507. doi: 10.1002/anie.202111192

  40. [40]

      Z. Hu, C. Xiao, W. Tan, et al., Macromolecules 55 (2022) 5188–5196. doi: 10.1021/acs.macromol.2c00444

  41. [41]

      L. Yuan, S. Liang, C. Xiao, et al., Chem. Asian J. 16 (2021) 4171–4178. doi: 10.1002/asia.202101147

  42. [42]

      C. Xiao, C. Li, F. Liu, et al., J. Mater. Chem. C 8 (2020) 5370–5374. doi: 10.1039/D0TC00587H

  43. [43]

      D. Meng, J.L. Yang, C. Xiao, et al., Proc. Natl. Acad. Sci. U. S. A. 117 (2020) 20397–20403. doi: 10.1073/pnas.2010733117

• 

  Figure 1  (a) The chemical structures of the double-cable conjugated polymer Pxs and their synthetic routes. (i) Stille polymerization, Pd₂(dba)₃/P(o-tolyl)₃, toluene, 115 ℃, 12 h. Normalized absorption spectra of the double-cable polymers in (b) chloroform solution and (c) thin films.

  下载: 全尺寸图片 幻灯片
  

  Figure 2  (a) Optimized J-V characteristic curves for SCOSCs based on the double-cable polymers P8-P16 under simulated AM1.5G illumination. J_SC, V_OC, and FF values (b) and EQE spectra (c) of the optimized SCOSC devices based on different polymers. The integrated current densities are also included. (d) Hole and electron mobilities of these polymers.

  下载: 全尺寸图片 幻灯片
  

  Figure 3  AFM height images for all Px thin films.

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  Figure 4  2D GIWAXS patterns (a) and the corresponding 1D IP plots (b) of the double-cable conjugated polymers. (c) d-spacings of the 300 peaks for different polymers. (d) Calculated CL values based on the (300) peaks versus different Px polymers.

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  Table 1.  Photophysical properties of the double-cable polymer Pxs.

  下载: 导出CSV

  Table 2.  Photovoltaic parameters of the optimized SCOSC devices based on different double-cable polymer Pxs.

  下载: 导出CSV
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