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    Enhancing the Backbone Coplanarity of n-Type Copolymers for Higher Electron Mobility and Stability in Organic Electrochemical Transistors

    Journal: Chem. Mat., Volume 34, OCT 11, page 8593–8602

    Authors: Maria, Iuliana P.; Griggs, Sophie; Rashid, Reem B.; Paulsen, Bryan D.; Surgailis, Jokubas; Thorley, Karl; Le, Vianna N.; Harrison, George T.; Combe, Craig; Hallani, Rawad; Giovannitti, Alexander; Paterson, Alexandra F.; Inal, Sahika; Rivnay, Jonathan; McCulloch, Iain

    Organizations: European Union [952911, 862474, 101007084]; EPSRC [EP/T026219/1, EP/W017091/1]; KAUST Office of Sponsored Research (OSR) [OSR-2019-CRG8-4086]; TomKat Center for Sustainable Energy at Stanford University; National Science Foundation [NSF DMR-1751308]; DOE Office of Science by Argonne National Laboratory [DE-AC02-06CH11357]; Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource [NSF ECCS-1542205]; Materials Research Science and Engineering Center [NSF DMR1720139]; State of Illinois; Northwestern University; International Institute for Nanotechnology (IIN); Keck Foundation; State of Illinois, through the IIN

    Electron-transporting (n-type) conjugated polymers have recently been applied in numerous electrochemical applications, where both ion and electron transport are required. Despite continuous efforts to improve their performance and stability, n-type conjugated polymers with mixed conduction still lag behind their hole-transporting (p-type) counterparts, limiting the functions of electro-chemical devices. In this work, we investigate the effect of enhanced backbone coplanarity on the electrochemical activity and mixed ionic-electronic conduction properties of n-type polymers during operation in aqueous media. Through substitution of the widely employed electron-deficient naphthalene diimide (NDI) unit for the core-extended naphthodithiophene diimide (NDTI) units, the resulting polymer shows a more planar backbone with closer packing, leading to an increase in the electron mobility in organic electrochemical transistors (OECTs) by more than two orders of magnitude. The NDTI-based polymer shows a deep-lying lowest unoccupied molecular orbital level, enabling operation of the OECT closer to 0 V vs Ag/AgCl, where fewer parasitic reactions with molecular oxygen occur. Enhancing the backbone coplanarity also leads to a lower affinity toward water uptake during cycling, resulting in improved stability during continuous electrochemical charging and ON-OFF switching relative to the NDI derivative. Furthermore, the NDTI-based polymer also demonstrates near-perfect shelf-life stability over a month-long test, exhibiting a negligible decrease in both the maximum on-current and transconductance. Our results highlight the importance of polymer backbone design for developing stable, high-performing n-type materials with mixed ionic-electronic conduction in aqueous media.
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