(405d) Donor-Acceptor Conjugated Copolymers for Ambipolar Field-Effect Transistors

Champion, R. D. - Presenter, University of Washington
Wu, P. - Presenter, University of Washington
Wang, H. - Presenter, University of Washington
Jenekhe, S. A. - Presenter, University of Washington

Conjugated copolymers that have
donor-acceptor architectures are of growing interest as an approach towards
ambipolar charge transport for device applications such as light-emitting
diodes, solar cells, and thin-film transistors.1 Here we describe
the synthesis and characterization of thiophene-pyridopyrazine and
thiophene-quinoxaline conjugated copolymers, including  poly[(thiophene-2,5-diyl-alt-(2,3-dialkylpyrido[3,4-b]pyrazine-5,8-diyl)]
(PTHPPz and PTDPPz), poly[(thiophene-2,4-diyl-alt-(2,3-alkylquinoxaline-4,8-diyl)]
(PTHQx and PTDdQx), and poly[(2,2')bithiophenyl-5,5'-diyl-alt-(2,3-didodecylquinoxaline-4,8-diyl)] (P2TDdQx and their use as semiconductors in organic field-effect transistors (OFETs). Pyrido[3,4-b]pyrazine and quinoxaline
units function as the acceptor, whereas thiophene or bithiophene units are the
donor units in these donor-acceptor (D-A) copolymers. The copolymers had
thin-film absorption maxima at 630-660 nm and had electron affinity (EA) and
ionization potential (IP) of 2.7-3.2 eV and 4.9-5.4 eV, respectively. The
optical band gaps derived from the absorption edge are 1.7-1.8 eV. The experimental
electronic structure (EA, IP) and optical band gap agree well with the calculated
results based on the density functional theory (DFT).2 The copolymers
were explored as semiconductors in OFETs. Hole mobilities of 1.4×10-3
to 4.4×10-3 cm2˙V-1·s-1 with on/off current ratios of 105 ? 5×106 were observed in
p-channel OFETs made from spin-coated PTDdQx, P2TdQx, and PTDPPz thin-films
prepared from hot 1,2,4-trichlorobenzene (TCB) solutions. PTHPPz was insoluble
in TCB and other higher boiling-point solvents and thus thin-films from
trifluoroacetic acid (TFA) solutions on substrates with an octyltrichlorosilane
(OTS) self-assembled monolayer (SAM) modified substrates had a lower saturation
hole mobility of 3x10-4 cm2·V-1·s-1 with an on/off current ratio of 5x104. The use of a high
boiling-point solvent (TCB) with annealing treatment and the use of OTS SAMs on
the SiO2 dielectric surface were shown to improve the OFET
performance. Atomic force microscopy of these D-A copolymer thin-films showed
aggregates and crystallites whose morphology was dependent upon the solvent and
the dielectric surface treatment. Different from previously studied PTHQx
thin-films,2 no densely-packed polycrystalline grain morphology was
observed for PTDPPz, P2TDdQx, and PTDdQx thin-films.


  1. (a) Yamamoto,
    T.; Yasuda, T.; Sakai, Y.; Aramaki, S.; Macromol. Rapid
    2005, 26, 1214 (b) Lee, B.-L.; Yamamoto, T. Macromolecules
    1999, 32, 1375; (c) Kanbara, T.; Miyazaki, Y.; Yamamoto, T. J.
    Polym. Sci. A: Polym. Chem.
    1995, 33, 999. (d) Jenekhe,
    S. A.; Lu, L.; Alam, M. M. Macromolecules 2001, 34,
    7315; (e) Alam, M.M.; Jenekhe, S.A. Chem. Mater. 2004, 16,
    4647. (f) Thompson, B.C.; Madrigal, L.G.; Pinto, M.R.; Kang, T.-S.;
    Schanze, K. S.; Reynolds, J.R. J. Polym. Sci. A: Polym. Chem. 2005,
    43, 1417; (g) Kulkarni, A.P.; Zhu, Y.; Jenekhe, S.A. Macromolecules 2005, 38, 1553.
  2. Champion, R.D.; Cheng, K.-F.;
    Pai, C.-L.; Chen, W.-C.; Jenekhe, S.A. Macromol. Rapid Commun. 2005,
    26, 1835.