Editorial Cite This: Chem. Mater. 2018, 30, 3151−3154
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MRS Fall 2017 Symposium: Organic SemiconductorsSurface, Interface, Bulk Doping, and Charge Transport
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organic photovoltaics and transistors, perovskite solar cells, thermoelectrics, electrochemical transistors, electrochromic devices, organic electronic textiles, etc. Karl Leo opened the symposium by providing a comprehensive overview of key aspects of OSCs, and their application in organic electronics, starting with a recent study on exploiting charge-transfer absorptions in near-infrared photodetectors,1 and discussing new insights into the role of disorder for charge separation in doped OSCs after initial charge transfer.2 He further highlighted the role of (deep) traps and discussed the processes of trap filling for both n- and pdoping3a recurring topic of the symposium. Nobuo Ueno discussed general aspects of gap states in OSCs, as studied by high-sensitivity photoelectron spectroscopy,4 and Frank Ortmann presented novel aspects of n-doping C60 by combining direct and (low-energy) inverse photoelectron spectroscopy with theoretical modeling. Ortmann proposed the energy difference between the electron affinity of the pristine OSC and the ionization energy of the n-doped material as the key parameter for the doping process.5 Novel strategies for n-doping OSCs and, in particular, for overcoming the intrinsic susceptibility of n-dopants to oxidation were discussed in detail by Stephen Barlow, Xin Lin, and Antoine Kahn, who all reported the successful photoactivation of cleavable airstable dimeric dopants to allow for high efficiency OLEDs.6 Environmental stability and the (unintentional) doping of OSCs by impurities were addressed by Mark Nikolka, who presented a study on organic field-effect transistors (OFETs) that not only identified the detrimental role of water incorporated into voids of the active layer but also demonstrated how to overcome this effect by substituting water with inert molecular additives.7 Henning Sirringhaus deepened this discussion and presented recent work on the interplay between microstructure and carrier mobility in conjugated polymers and the role of water as a trap in pchannel transistors. Altering the peripheral substitution pattern allows for high-mobility OFETs,8 as does the close stacking of copolymer backbones.9 In general, the microstructure of doped OSCs was a leitmotif of numerous contributions in the symposium. In this regard, Alberto Salleo and Adam Moulé discussed the prototypical system of poly(3-hexylthiophene) (P3HT) sequentially doped with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ),10,11 an approach likewise reported by Dieter Neher who employed a strong Lewis acid.12 In all cases, they reported sequential doping to be a reliable pathway for overcoming solubility issues and achieving doped polymer films of superior morphology. Notably, based on inelastic neutron scattering and theoretical modeling, Moulé proposed a structural motif for F4TCNQ-doped P3HT with dopants intercalated between the polymer chains.13 Improved dopant
he Fall Meeting of the Materials Research Society (MRS), held annually in Boston, is the prime venue for researchers from around the world to discuss the latest advances and developments in materials science and beyond. The 2017 meeting hosted awith over 200 abstracts exceptionally well attended, five day symposium dedicated to doping and charge transport of organic semiconductors (OSCs). The symposium brought together current theoretical and experimental viewpoints from physics, chemistry, and materials science, examining both the fundamental processes underlying the doping of OSCs, as well as pathways to capitalize on their potential in organic electronics. 23 invited speakers from 10 different countries (Table 1) led a stimulating Table 1. Invited Speakers Invited speaker Thomas Anthopoulos Stephen Barlow David Beljonne Lay Lay Chua Enrico Da Como Steffen Duhm Antonio Facchetti Oana Jurchescu Antoine Kahn Martijn Kemerink Bernhard Kippelen Norbert Koch Karl Leo Harsumi Mori Adam Moulé Dieter Neher Yabing Qi Alberto Salleo Rachel Segalman Henning Sirringhaus Henry Snaith Nobuo Ueno Elisabeth von Hauff
University
Country
King Abdullah University of Science & Technology (KAUST) Georgia Institute of Technology Université de Mons National University of Singapore University of Bath
Saudi Arabia
Soochow University Northwestern University
China USA
Wake Forest University Princeton University Linköping University
USA USA Sweden
Georgia Institute of Technology
USA
Humboldt Universität zu Berlin Technical University Dresden University of Tokyo University of California, Davis Potsdam University Okinawa Institute of Science and Technology Stanford University University of California, Santa Barbara
Germany Germany Japan USA Germany Japan
University of Cambridge
UK
University of Oxford Chiba University Free University of Amsterdam
UK Japan The Netherlands
USA Belgium Singapore UK
USA USA
interdisciplinary discussion in 7 clear-cut thematic sessions, with a further 68 contributed talks and 118 posters shown within two poster events. The thematic sessions covered both subject areas that were fundamental in nature, such as structure−property relationships, synthetic aspects, interface vs bulk doping, as well as topics with an explicit application focus, such as well-established but also emerging areas like © 2018 American Chemical Society
Published: May 22, 2018 3151
DOI: 10.1021/acs.chemmater.8b01683 Chem. Mater. 2018, 30, 3151−3154
Chemistry of Materials
Editorial
intercalation was further reported by Jan Anton Koster as well as others for thin films of fullerenes,14 naphthalenedicarboximide-bitiophene-based copolymers, 15−17 and polythiophenes,18 achieved by replacing the alkyl side chains with polar oligoethylene glycol-based substituents. In another impressive presentation, Howard Katz discussed a polythiophene with alkylthio side chains, which upon coprocessing with the dopant NOBF4 displayed a conductivity of 350 S cm−1.19 In the same vain, Rachel Segalman highlighted the role of sidechain bulkiness and, hence, steric repulsion on packing and its interplay with intermolecular interactions for liquid crystallineconjugated polymers.20 For P3HT, Bernard Kippelen reported successful p-doping via postprocess immersion of the films into solutions of polyoxometalates, where a limited interdiffusion length in the low two-digit nanometer range was determined for such bulky dopant species.21 For low-weight dopants like F4TCNQ, however, diffusion is known as a major issue, where again Moulé demonstrated that substituting one fluorine with a methyloxycarbonyl pendant group not only results in significantly reduced diffusion through P3HT, but also in increased solubility, thereby remedying another crucial problem one typically faces with this type of dopant species.22 Thermoelectricity was a further vibrant topic of the symposium. Particularly noteworthy was the work presented by Martijn Kemerink, who discussed a simple analytical model based on variable range hopping in a Gaussian disordered OSC. Kemerink’s model allows for an explanation of the power law relationship between the Seebeck coefficient and conductivity that is commonly observed for doped OSCs across a wide range of materials. Strikingly, n-type materials are predicted to have a higher potential to ultimately achieve a high figure of merit of close to unity than p-type materials.23 A further contribution to the theory of molecularly doping of OSCs by David Beljonne highlighted the contrast to the doping of inorganic semiconductors, mainly by taking into account electron−hole interaction and polaronic effects.24 Enrico Da Como discussed the locality of dopant to OSC charge transfer in copolymers25 as well as the physics of electron−phonon coupling, with Elizabeth von Hauff contributing the experimental determination thereof.26 Norbert Koch highlighted the versatility of molecular electron acceptors and donors as agents that enable tuning the charge density within and at interfaces of established and emerging electronic materials, such as inorganic and organic semiconductors, two-dimensional (2D) transition metal dichalcogenides, and perovskites to optimize interfacial energy levels for applications.27,28 Aspects of interfacial doping, of the structure, and of the energetics at OSC/electrode interfaces were also addressed by Steffen Duhm,29 who further discussed the energetics of perovskites upon illumination.30 For this materials class (covered by a different symposium), Yabing Qi presented a novel method for combining hybrid chemical vapor deposition and cation exchange, promising a route toward scale up to enable future industrial fabrication.31 Finally, on behalf of the large number of excellent presentations at the symposium, some selected contributions shall be mentioned here, which pleasantly broadened its scope. These presentations include work on electrochromic devices by Sandra Pittelli,32 rubrene homoepitaxy by Masahiro Hiramoto,33 fundamental aspects of charge-transfer crystallites by Hatsumi Mori,34 push−pull polymers by Melissa Aplan,35 submolecular resolution imaging of coupling defects in conjugated polymers by Dan Warr, and topics as intriguing as electrically conducting fabrics by Trisha Andrew.36
The wealth of topics resulted in a lively symposium that continued to attract a large audience throughout the whole duration of the MRS Fall Meeting, as evidenced by a full lecture hall during Alberto Salleo’s inspiring talk Ef fect of Doping on Microstructure, Charge Delocalization and Transport in Conjugated Polymers on the last (fifth) day of the conference (Figure 1). Vivid discussions that caused many cups of coffee to
Figure 1. Alberto Salleo speaking on the Friday morning, the fifth and last day of the conference, in a still crowded lecture hall.
cool off in the break included, to give but one example, the efficiency and stability of doping processes, where stability in particular of n-dopants remains a formidable challenge, with some progress being made. Needless to say that a lively discourse extended beyond the lecture hall to locations wellknown to returning MRS attendees such as the Lir and the Pour House, which conveniently lie across the street from the Hynes Convention Center, the conference venue. As an aside, we note that also more discrete events such as a karaoke evening, as well as the speakers’ dinner, proved to be ideal settings both for following up on science and pushing the frontiers. Overall, doping conjugated polymers and small conjugated molecules is the oldest topic in the field, which, in fact, originated in the successful doping of polyacetylene in the late 1970s. To this day, doping of conjugated organics is one of its most contemporary and dynamic areas, with the prospect of a continual increase in fundamental understanding to be directly translated into technological progress. This is reflected in the overwhelming interest in the present 5-day symposium at the 2017 MRS Fall Meeting dedicated to deliberately tuning the electrical properties of organic semiconductors to applicationspecific demands. This year’s 2018 MRS Fall Meeting will again see another symposium dedicated to organic electronics on Excitons, Electrons, and Ions in Organic Materials, which we anticipate to be of equal success. Further, the 2018 Spring Meetings of the MRS in Phoenix, as well as the European MRS (E-MRS) in Strasbourg, will feature related symposia entitled Advanced Polymer SemiconductorsKey Properties and High-Performance Electronics and Charge Transport in Organic Semiconductors: Inf luence of Processing and Doping, which impressively shows the tremendous interest that doping attracts in the organic electronics community.
Christian Müller† Ingo Salzmann‡ †
Department of Chemistry and Chemical Engineering, Chalmers University of Technology, Göteborg, Sweden 3152
DOI: 10.1021/acs.chemmater.8b01683 Chem. Mater. 2018, 30, 3151−3154
Chemistry of Materials
Editorial
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Rigidity in Charge Transport of Semi-Crystalline Polymers. J. Phys. Chem. Lett. 2017, 8 (20), 4974−4980. (11) Jacobs, I. E.; Aasen, E. W.; Oliveira, J. L.; Fonseca, T. N.; Roehling, J. D.; Li, J.; Zhang, G. W.; Augustine, M. P.; Mascal, M.; Moulé, A. J. Comparison of solution-mixed and sequentially processed P3HT: F4TCNQ films: effect of doping-induced aggregation on film morphology. J. Mater. Chem. C 2016, 4 (16), 3454−3466. (12) Pingel, P.; Arvind, M.; Kölln, L.; Steyrleuthner, R.; Kraffert, F.; Behrends, J.; Janietz, S.; Neher, D. p-Type Doping of Poly(3hexylthiophene) with the Strong Lewis Acid Tris(pentafluorophenyl)borane. Advanced Electronic Materials 2016, 2, 1600204. (13) Harrelson, T. F.; Cheng, Y. Q. Q.; Li, J.; Jacobs, I. E.; RamirezCuesta, A. J.; Faller, R.; Moule, A. J. Identifying Atomic Scale Structure in Undoped/Doped Semicrystalline P3HT Using Inelastic Neutron Scattering. Macromolecules 2017, 50 (6), 2424−2435. (14) Liu, J.; Qiu, L.; Portale, G.; Koopmans, M.; ten Brink, G.; Hummelen, J. C.; Koster, L. J. A. N-Type Organic Thermoelectrics: Improved Power Factor by Tailoring Host−Dopant Miscibility. Adv. Mater. 2017, 29 (36), 1701641. (15) Liu, J.; Qiu, L.; Alessandri, R.; Qiu, X. K.; Portale, G.; Dong, J. J.; Talsma, W.; Ye, G.; Sengrian, A. A.; Souza, P. C. T.; Loi, M. A.; Chiechi, R. C.; Marrink, S. J.; Hummelen, J. C.; Koster, L. J. A. Enhancing Molecular n-Type Doping of Donor-Acceptor Copolymers by Tailoring Side Chains. Adv. Mater. 2018, 30 (7), 1704630. (16) Liu, J.; Qiu, L.; Alessandri, R.; Qiu, X.; Portale, G.; Dong, J.; Talsma, W.; Ye, G.; Sengrian, A. A.; Souza, P. C. T.; Loi, M. A.; Chiechi, R. C.; Marrink, S. J.; Hummelen, J. C.; Koster, L. J. A. Enhancing Molecular n-Type Doping of Donor−Acceptor Copolymers by Tailoring Side Chains. Adv. Mater. 2018, 30 (7), 1704630. (17) Kiefer, D.; Giovannitti, A.; Sun, H.; Biskup, T.; Hofmann, A.; Koopmans, M.; Cendra, C.; Weber, S.; Anton Koster, L. J.; Olsson, E.; Rivnay, J.; Fabiano, S.; McCulloch, I.; Müller, C. Enhanced n-Doping Efficiency of a Naphthalenediimide-Based Copolymer through Polar Side Chains for Organic Thermoelectrics. ACS Energy Letters 2018, 3 (2), 278−285. (18) Kroon, R.; Kiefer, D.; Stegerer, D.; Yu, L.; Sommer, M.; Müller, C. Polar Side Chains Enhance Processability, Electrical Conductivity, and Thermal Stability of a Molecularly p-Doped Polythiophene. Adv. Mater. 2017, 29 (24), 1700930. (19) Li, H.; DeCoster, M. E.; Ireland, R. M.; Song, J.; Hopkins, P. E.; Katz, H. E. Modification of the Poly(bisdodecylquaterthiophene) Structure for High and Predominantly Nonionic Conductivity with Matched Dopants. J. Am. Chem. Soc. 2017, 139 (32), 11149−11157. (20) Bridges, C. R.; Ford, M. J.; Bazan, G. C.; Segalman, R. A. Molecular Considerations for Mesophase Interaction and Alignment of Lyotropic Liquid Crystalline Semiconducting Polymers. ACS Macro Lett. 2017, 6 (6), 619−624. (21) Kolesov, V. A.; Fuentes-Hernandez, C.; Chou, W. F.; Aizawa, N.; Larrain, F. A.; Wang, M.; Perrotta, A.; Choi, S.; Graham, S.; Bazan, G. C.; Nguyen, T. Q.; Marder, S. R.; Kippelen, B. Solution-based electrical doping of semiconducting polymer films over a limited depth. Nat. Mater. 2017, 16 (4), 474. (22) Li, J.; Koshnick, C.; Diallo, S. O.; Ackling, S.; Huang, D. M.; Jacobs, I. E.; Harrelson, T. F.; Hong, K.; Zhang, G.; Beckett, J.; Mascal, M.; Moule, A. J. Quantitative Measurements of the TemperatureDependent Microscopic and Macroscopic Dynamics of a Molecular Dopant in a Conjugated Polymer. Macromolecules 2017, 50 (14), 5476−5489. (23) Zuo, G. Z.; Li, Z. J.; Wang, E. G.; Kemerink, M. High Seebeck Coefficient and Power Factor in n-Type Organic Thermoelectrics. Advanced Electronic Materials 2018, 4 (1), 1700501. (24) Li, J.; D’Avino, G.; Pershin, A.; Jacquemin, D.; Duchemin, I.; Beljonne, D.; Blase, X. Correlated electron-hole mechanism for molecular doping in organic semiconductors. Physical Review Materials 2017, 1 (2), 025602. (25) Di Nuzzo, D.; Fontanesi, C.; Jones, R.; Allard, S.; Dumsch, I.; Scherf, U.; von Hauff, E.; Schumacher, S.; Da Como, E. How intermolecular geometrical disorder affects the molecular doping of donor-acceptor copolymers. Nat. Commun. 2015, 6, 6460.
Department of Physics, Department of Chemistry & Biochemistry, Concordia University, Montréal, Québec, Canada
AUTHOR INFORMATION
ORCID
Christian Müller: 0000-0001-7859-7909 Notes
Views expressed in this editorial are those of the authors and not necessarily the views of the ACS.
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ACKNOWLEDGMENTS We are indebted to the Center of Advance Materials and Interfaces and the Center for Organic Photonic and Electronics, both at the Georgia Institute of Technology, Applied Materials Inc., Flexterra Inc., 1-Material Inc., Millipore Sigma, and to Guangzhou China-Ray Optoelectronic Materials Co. Ltd. for their generous support. We further gratefully acknowledge support by the RSC journal Materials Horizons, as well as the ACS journal Chemistry of Materials.
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REFERENCES
(1) Siegmund, B.; Mischok, A.; Benduhn, J.; Zeika, O.; Ullbrich, S.; Nehm, F.; Bohm, M.; Spoltore, D.; Frob, H.; Korner, C.; Leo, K.; Vandewal, K. Organic narrowband near-infrared photodetectors based on intermolecular charge-transfer absorption. Nat. Commun. 2017, 8, 15421. (2) Tietze, M. L.; Benduhn, J.; Pahner, P.; Nell, B.; Schwarze, M.; Kleemann, H.; Krammer, M.; Zojer, K.; Vandewal, K.; Leo, K. Elementary steps in electrical doping of organic semiconductors. Nat. Commun. 2018, 9 (1), 1182. (3) Tietze, M. L.; Pahner, P.; Schmidt, K.; Leo, K.; Luessem, B. Doped Organic Semiconductors: Trap-Filling, Impurity Saturation, and Reserve Regimes. Adv. Funct. Mater. 2015, 25 (18), 2701−2707. (4) Yang, J. P.; Bussolotti, F.; Kera, S.; Ueno, N. Origin and role of gap states in organic semiconductor studied by UPS: as the nature of organic molecular crystals. J. Phys. D: Appl. Phys. 2017, 50 (42), 423002. (5) Gaul, C.; Hutsch, S.; Schwarze, M.; Schellhammer, K. S.; Bussolotti, F.; Kera, S.; Cuniberti, G.; Leo, K.; Ortmann, F. Insight into doping efficiency of organic semiconductors from the analysis of the density of states in n-doped C60 and ZnPc. Nat. Mater. 2018, 17, 439. (6) Lin, X.; Wegner, B.; Lee, K. M.; Fusella, M. A.; Zhang, F. Y.; Moudgil, K.; Rand, B. P.; Barlow, S.; Marder, S. R.; Koch, N.; Kahn, A. Beating the thermodynamic limit with photo-activation of n-doping in organic semiconductors. Nat. Mater. 2017, 16 (12), 1209. (7) Nikolka, M.; Nasrallah, I.; Rose, B.; Ravva, M. K.; Broch, K.; Sadhanala, A.; Harkin, D.; Charmet, J.; Hurhangee, M.; Brown, A.; Illig, S.; Too, P.; Jongman, J.; McCulloch, I.; Bredas, J. L.; Sirringhaus, H. High operational and environmental stability of high-mobility conjugated polymer field-effect transistors through the use of molecular additives. Nat. Mater. 2017, 16 (3), 356. (8) Chen, H.; Hurhangee, M.; Nikolka, M.; Zhang, W. M.; Kirkus, M.; Neophytou, M.; Cryer, S. J.; Harkin, D.; Hayoz, P.; Abdi-Jalebi, M.; McNeill, C. R.; Sirringhaus, H.; McCulloch, I. Dithiopheneindenofluorene (TIF) Semiconducting Polymers with Very High Mobility in Field-Effect Transistors. Adv. Mater. 2017, 29 (36), 1702523. (9) Chaudhari, S. R.; Griffin, J. M.; Broch, K.; Lesage, A.; Lemaur, V.; Dudenko, D.; Olivier, Y.; Sirringhaus, H.; Emsley, L.; Grey, C. P. Donor-acceptor stacking arrangements in bulk and thin-film highmobility conjugated polymers characterized using molecular modelling and MAS and surface-enhanced solid-state NMR spectroscopy. Chemical Science 2017, 8 (4), 3126−3136. (10) Chew, A. R.; Ghosh, R.; Shang, Z. R.; Spano, F. C.; Salleo, A. Sequential Doping Reveals the Importance of Amorphous Chain 3153
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(26) Anderson, M.; Ramanan, C.; Fontanesi, C.; Frick, A.; Surana, S.; Cheyns, D.; Furno, M.; Keller, T.; Allard, S.; Scherf, U.; Beljonne, D.; D’Avino, G.; von Hauff, E.; Da Como, E. Displacement of polarons by vibrational modes in doped conjugated polymers. Physical Review Materials 2017, 1 (5), 055604. (27) Timpel, M.; Li, H.; Nardi, M. V.; Wegner, B.; Frisch, J.; Hotchkiss, P. J.; Marder, S. R.; Barlow, S.; Brédas, J. L.; Koch, N. Electrode Work Function Engineering with Phosphonic Acid Monolayers and Molecular Acceptors: Charge Redistribution Mechanisms. Adv. Funct. Mater. 2018, 28 (8), 1704438. (28) Salzmann, I.; Heimel, G.; Oehzelt, M.; Winkler, S.; Koch, N. Molecular Electrical Doping of Organic Semiconductors: Fundamental Mechanisms and Emerging Dopant Design Rules. Acc. Chem. Res. 2016, 49 (3), 370−378. (29) Hofmann, O. T.; Glowatzki, H.; Burker, C.; Rangger, G. M.; Broker, B.; Niederhausen, J.; Hosokai, T.; Salzmann, I.; Blum, R. P.; Rieger, R.; Vollmer, A.; Rajput, P.; Gerlach, A.; Mullen, K.; Schreiber, F.; Zojer, E.; Koch, N.; Duhm, S. Orientation-Dependent WorkFunction Modification Using Substituted Pyrene-Based Acceptors. J. Phys. Chem. C 2017, 121 (44), 24657−24668. (30) Zu, F. S.; Amsalem, P.; Salzmann, I.; Wang, R. B.; Ralaiarisoa, M.; Kowarik, S.; Duhm, S.; Koch, N. Impact of White Light Illumination on the Electronic and Chemical Structures of Mixed Halide and Single Crystal Perovskites. Adv. Opt. Mater. 2017, 5 (9), 1700139. (31) Jiang, Y.; Leyden, M. R.; Qiu, L. B.; Wang, S. H.; Ono, L. K.; Wu, Z. F.; Juarez-Perez, E. J.; Qi, Y. B. Combination of Hybrid CVD and Cation Exchange for Upscaling Cs-Substituted Mixed Cation Perovskite Solar Cells with High Efficiency and Stability. Adv. Funct. Mater. 2018, 28 (1), 1703835. (32) Pittelli, S. L.; Shen, D. E.; Ö sterholm, A. M.; Reynolds, J. R. Chemical Oxidation of Polymer Electrodes for Redox Active Devices: Stabilization through Interfacial Interactions. ACS Appl. Mater. Interfaces 2018, 10 (1), 970−978. (33) Ohashi, C.; Izawa, S.; Shinmura, Y.; Kikuchi, M.; Watase, S.; Izaki, M.; Naito, H.; Hiramoto, M. Hall Effect in Bulk-Doped Organic Single Crystals. Adv. Mater. 2017, 29 (23), 1605619. (34) Higashino, T.; Ueda, A.; Yoshida, J.; Mori, H. Improved stability of a metallic state in benzothienobenzothiophene-based molecular conductors: an effective increase of dimensionality with hydrogen bonds. Chem. Commun. (Cambridge, U. K.) 2017, 53 (24), 3426−3429. (35) Lee, Y.; Aplan, M. P.; Seibers, Z. D.; Kilbey, S. M.; Wang, Q.; Gomez, E. D. Tuning the synthesis of fully conjugated block copolymers to minimize architectural heterogeneity. J. Mater. Chem. A 2017, 5 (38), 20412−20421. (36) Zhang, L.; Fairbanks, M.; Andrew, T. L. Rugged Textile Electrodes for Wearable Devices Obtained by Vapor Coating Off-theShelf, Plain-Woven Fabrics. Adv. Funct. Mater. 2017, 27 (24), 1700415.
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DOI: 10.1021/acs.chemmater.8b01683 Chem. Mater. 2018, 30, 3151−3154
