Regioregular poly(3-alkylthiophene)s are promising candidates for plastic electronics. In particular, poly(3-
hexylthiophene) (P3HT) has attracted considerable interest due to its excellent field effect mobility. However, not much
attention has been given to poly(3-alkylthiophene)s with longer alkyl side chains, mainly because a few studies reported
that longer side chain length was detrimental to field effect mobility. However, these past studies used untreated SiO2 as
the gate dielectric and commercially available polymers synthesized by the Rieke method. Here, all polymers tested
were synthesized in house using our quasi-living GRIM method to yield well-defined clean polymers. For our first study,
we synthesized P3HT and regioregular poly(3-octylthiohene) (P3OT), both with a high molecular weight (Mn of 40kDa
and 35kDa, respectively). Regioregular P3HT required some sonication to dissolve in chloroform, whereas P3OT
dissolved readily in chloroform at room temperature. Hole mobility was measured using bottom-contact geometry, with
SiO2 as gate dielectric. The SiO2 surface was either untreated, or chemically treated with octyltrichlorosilane. Average
mobility on untreated SiO2 was 0.09 cm2/Vs for P3HT and 0.03 cm2/Vs for P3OT. Average mobility on OTS-8 treated
SiO2, on the other hand, was 0.13 cm2/Vs for P3HT and 0.19 cm2/Vs for P3OT. The OTS-8 treatment therefore
dramatically improved the mobility of P3OT. Both polymers had a maximum mobility of about 0.2 cm2/Vs for channel
lengths ≥10 μm, and a mobility of 0.22 cm2/Vs was obtained several times for P3OT. To our knowledge, this is by far
the highest mobility reported for P3OT.
Regioregular poly(3-hexylthiophene) (RR-P3HT) has been a benchmark for organic field effect transistors containing a
conjugated polymer. Here, we focus on a new class of materials that we have developed: block copolymers of RRP3HT
and of a non-conjugated polymer. By varying the physical properties of the non-conjugated segment, it is
possible to fine-tune the physical and electrical properties of the copolymer for various device applications. Here, we
report the synthesis, UV-Vis, and conductivity of a series of block copolymers of RR-P3HT containing poly(methyl
acrylate) (PMA) as the second segment. We also present preliminary results of these polymers in FET devices. We find
that despite the presence of the insulating segment, these polymers have relatively high mobilities, especially when the
substrate is treated with octyltrichlorosilane. For example, a mobility of 0.03 cm2V-1s-1 is reported for the block
copolymer P3HT-b-PMA containing 41 mole % PMA, for a film drop-cast from a chloroform solution under ambient
conditions. This work demonstrates that using block copolymers is a good approach for improving the material's
properties while maintaining a high solid-state order that is critical for good OFET performance.
Richard McCullough, Genevieve Sauve, Bo Li, Malika Jeffries-El, Suresh Santhanam, Lawrence Schultz, Rui Zhang, Mihaela Iovu, Jessica Cooper, Prathapan Sreedharan, Joseph Revelli, Aaron Kusner, Tomasz Kowalewski, Jay Snyder, Lee Weiss, David Lambeth, Gary Fedder
We have developed and synthesized highly conductive regioregular poly(3-alkylthiophene) (rr-PAT) derivatives for use in sensor arrays on a chip. Poly(3-alkylthiophene)s are ideally suited for this application because of their excellent electrical properties, solution processability and our ability to modify their chemical structure. Here, we synthesized rr-PATs that have different side chains and different end groups. The polymers were ink-jetted onto ChemFET devices on a chip and their chemical sensing properties were tested to a variety of volatile organic compounds (VOCs). The sensors demonstrated ppm level sensitivity and selectivity to all VOCs tested, including both polar and non-polar compounds.
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