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

Research output: Contribution to journalArticlepeer-review

29 Citations (Scopus)

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

Bibliographical note

Funding Information:
S.Z. acknowledges the financial support from the National Key R&D Program of “Strategic Advanced Electronic Materials (No. 2016YFB0401100) and the National Natural Science Foundation of China (Grant No. 61574077). X.G. is grateful to the National Science Foundation of China (NSFC, 21774055), Shenzhen Basic Research Fund (JCYJ20170817105905899), and Shenzhen Peacock Plan Project (KQTD20140630110339343). H.Y.W. is grateful to the financial support from the NRF of Korea (2016M1A2A2940911 and 2015M1A2A2057506).

Funding Information:
S.Z. acknowledges the financial support from the National Key R&D Program of “Strategic Advanced Electronic Materials” (No. 2016YFB0401100) and the National Natural Science Foundation of China (Grant No. 61574077). X.G. is grateful to the National Science Foundation of China (NSFC, 21774055), Shenzhen Basic Research Fund (JCYJ20170817105905899), and Shenzhen Peacock Plan Project (KQTD20140630110339343). H.Y.W. is grateful to the financial support from the NRF of Korea (2016M1A2A2940911 and 2015M1A2A2057506).

Publisher Copyright:
© 2019 American Chemical Society.

Keywords

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

ASJC Scopus subject areas

  • General Materials Science

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