We report on the preparation of high performance field-effect transistors (FETs) based on large areas
of aligned films of a TTF derivative, namely, tetrakis-(octadecylthio)-tetrathiafulvalene (TTF-4SC18).
TTF-4SC18 assembles into one-dimensional stacks in which the long alkyl chains promote intermolecular
π−π overlapping due to their extremely closely packed nature. The films were prepared from solution
by zone-casting, a simple technique that does not require the use of preoriented substrates. The films
were characterized by AFM and X-ray, indicating an extremely high crystalline quality. The TTF molecules
are tilted with respect to the substrate surface and are well-aligned in the casting direction. More than 40
FETs were measured, showing a remarkable reproducibility of their performance. The average charge
carrier mobility value measured along the casting direction was about 0.006 cm2/V s for a channel length
L = 100 μm and about 0.01 cm2/V s for L = 80 μm and L = 50 μm. The FET mobilities determined in
the direction perpendicular to the orientation were ca. 1 order of magnitude lower. We found that all the
devices after annealing exhibited an enhanced performance with FETs mobilities about 1 order of
magnitude higher. The best devices revealed a charge carrier mobility close to 0.1 cm2/V s with an
on/off ratio of the order of 104.
In this paper we present that the surface energy of silicon dioxide employed as the dielectric in bottom gate organic field effect transistors has large impact on the device performance. By the use of the zone-casting simple solution processing technique, we ensured reproducibility of active layer preparation confirmed by the atomic force microscopy and x-ray diffraction that showed high crystalline quality. Electrical measurements revealed that charge carrier mobility based on highly ordered zone-cast tetrakis-(octadecylthio)-tetrathiafulvalene layer was increased 30 times to 0.2 cm2/V s, when dielectric surface energy decreased from 51.8 to 40.1 mN/m.
We present an approach to improving the performance of solution processed organic semiconductor transistors based on a dual solvent system. We here apply this to a blend containing the p-conjugated small molecule 6,13 bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) and polystyrene, which acts as an inert binder. Using a semiconductor-binder solution of two solvents, where the main solvent is a better solvent of the small molecule and second solvent is a better solvent of the polymer, crystal morphologies can be altered and transistor mobilities increased by almost an order of magnitude. In this way, air-ambient and solution-processed transistors with linear and saturation mobilities higher than 1 cm 2 V À1 s À1 have been fabricated. We discuss how the solubility properties of the formulation components can be used to identify solvent candidates that promote an efficient self-assembly of the small molecule.
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