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

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.

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