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405d

Donor-Acceptor Conjugated Copolymers for Ambipolar Field-Effect Transistors

Richard D. Champion1, Pei-Tzu Wu1, Hung-Ren Wang1, and Samson A. Jenekhe2. (1) Department of Chemical Engineering, University of Washington, Seattle, WA 98195, (2) Department of Chemical Engineering and Department of Chemistry, University of Washington, Seattle, 98195-1750

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 1.410-3 to 4.410-3 cm2˙V-1s-1 with on/off current ratios of 105 5106 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 cm2V-1s-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. References

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  2. Champion, R.D.; Cheng, K.-F.; Pai, C.-L.; Chen, W.-C.; Jenekhe, S.A. Macromol. Rapid Commun. 2005, 26, 1835.