Cyano-Substituted Head-to-Head Polythiophenes: Enabling High-Performance n-Type Organic Thin-Film Transistors

Hang Wang, Jun Huang, Mohammad Afsar Uddin, Bin Liu, Peng Chen, Shengbin Shi, Yumin Tang, Guichuan Xing, Shiming Zhang, Han Young Woo, Han Guo, Xugang Guo

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Abstract

Polythiophenes, built on the electron-rich thiophene unit, typically possess high-lying energy levels of the lowest unoccupied molecular orbitals (LUMOs) and show hole-transporting properties. In this study, we develop a series of n-type polythiophenes, P1-P3, based on head-to-head-linked 3,3′-dialkoxy-4,4′-dicyano-2,2′-bithiophene (BTCNOR) with distinct side chains. The BTCNOR unit shows not only highly planar backbone conformation enabled by the intramolecular noncovalent sulfur-oxygen interaction but also significantly suppressed LUMO level attributed to the cyano-substitution. Hence, all BTCNOR-based polymer semiconductors exhibit low-lying LUMO levels, which are -1.0 eV lower than that of regioregular poly(3-hexylthiophene) (rr-P3HT), a benchmark p-type polymer semiconductor. Consequently, all of the three polymers can enable unipolar n-type transport characteristics in organic thin-film transistors (OTFTs) with low off-currents (I off s) of 10 -10 -10 -11 A and large current on/off ratios (I on /I off s) at the level of 10 6 . Among them, polymer P2 with a 2-ethylhexyl side chain offers the highest film ordering, leading to the best device performance with an excellent electron mobility (μ e ) of 0.31 cm 2 V -1 s -1 in off-center spin-cast OTFTs. To the best of our knowledge, this is the first report of n-type polythiophenes with electron mobility comparable to the hole mobility of the benchmark p-type rr-P3HT and approaching the electron mobility of the most-studied n-type polymer, poly(naphthalene diimide-alt-bithiophene) (i.e., N2200). The change of charge carrier polarity from p-type (rr-P3HT) to n-type (P2) with comparable mobility demonstrates the obvious effectiveness of our structural modification. Adoption of n-hexadecyl (P1) and 2-butyloctyl (P3) side chains leads to reduced film ordering and results in 1-2 orders of magnitude lower μ e s, showing the critical role of side chains in optimizing device performance. This study demonstrates the unique structural features of head-to-head linkage containing BTCNOR for constructing high-performance n-type polymers, i.e., the alkoxy chain for backbone conformation locking and providing polymer solubility as well as the strong electron-withdrawing cyano group for lowering LUMO levels and enabling n-type performance. The design strategy of BTCNOR-based polymers provides useful guidelines for developing n-type polythiophenes.

Original languageEnglish
Pages (from-to)10089-10098
Number of pages10
JournalACS Applied Materials and Interfaces
Volume11
Issue number10
DOIs
Publication statusPublished - 2019 Mar 13

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Keywords

  • conformation lock
  • cyano-functionalization
  • head-to-head linkage
  • n-type organic thin-film transistors
  • polythiophene

ASJC Scopus subject areas

  • Materials Science(all)

Cite this

Wang, H., Huang, J., Uddin, M. A., Liu, B., Chen, P., Shi, S., Tang, Y., Xing, G., Zhang, S., Woo, H. Y., Guo, H., & Guo, X. (2019). Cyano-Substituted Head-to-Head Polythiophenes: Enabling High-Performance n-Type Organic Thin-Film Transistors. ACS Applied Materials and Interfaces, 11(10), 10089-10098. https://doi.org/10.1021/acsami.8b22457