We report the electrical characterization of field effect transistors based on regioregular poly(3-hexylthiophene) (RRP3HT) nanofibers fabricated using nanostencil shadow masks. Mobility values were 0.02 cm2/Vs with on/off current ratios of 106. Current densities of ∼700 A/cm2 were achieved in single nanofibers. A series of Soxhlet extractions was employed to separate RRP3HT into narrow molecular weight fractions. Nanofibers made from the THF fraction exhibited superior electrical properties in terms of increased current levels and decreased activation energy. The lowest activation energies in the nanofibers were achieved by using top contacts (i.e., vapor-deposited metal on top of the nanofibers) and material purified by Soxhlet extraction. Contact effects were eliminated from bottom contact devices (i.e., nanofibers on top of metal electrodes on a dielectric substrate) with a four-probe geometry. Temperature-dependent measurements reveal two distinct regimes of transport. The high-temperature regime (355−245 K) is characterized by activation energies of 62−145 meV depending on contact geometry and RRP3HT purity with a Meyer−Neldel energy of 33 ± 3 meV. The low-temperature regime (235−85 K) has lower activation energies of 31−112 meV. Shifts in the turn-on gate voltage with temperature indicate ∼4.8 × 1012 acceptor-like states/cm2 (or ∼1 per nm of fiber length) and 3.2 × 1012 donor-like states/cm2 exist in the nanofibers. We propose that transport can be explained in terms of the multiple trap and release (MTR) or variable range hopping (VRH) formalisms of transport in a bimodal, exponential distribution of shallow and deep donor-like states.

We report on the electrical conductance of nanofibers of regioregular poly(3‐hexylthiophene) (RRP3HT) as a function of gate‐induced charge. Nanofibers of RRP3HT were deposited onto SiO2/Si substrates by casting from dilute p‐xylene solutions. An analysis of the nanofibers by atomic force microscopy revealed fiber lengths of 0.2–5 μm, heights of 3–7 nm, and widths of approximately 15 nm. A field effect transistor geometry was used to probe the conductance of webs of nanofibers and single nanofibers; in these measurements, gold electrodes served as source and drain contacts, and the doped SiO2/Si substrate served as the gate. Temperature‐dependent transport studies on webs of nanofibers revealed an activation energy of 108 meV at a gate‐induced hole density of 3.8 × 1012 charges/cm2. Pretreating SiO2 with a hydrophobic hexamethyldisilazane (HMDS) layer reduced the activation energy to 65 meV at the same charge density. The turn‐on gate voltage on treated and untreated substrates increased in magnitude with decreasing temperature. Conductance measurements on single nanofibers on HMDS‐treated SiO2 yielded hole mobilities as high as 0.06 cm2/Vs with on/off current ratios greater than 103. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2674–2680, 2003

We report the structural and electrical characterization of two new p-channel organic semiconductors, 5,5'-bis(2-tetracenyl)-2,2'-bithiophene (1) and 5,5'-bis(2-anthracenyl)-2,2'-bithiophene (2). Both compounds exhibited a high degree of thermal stability with decomposition temperatures of 530 degrees C and 425 degrees C for 1 and 2, respectively. The thin-film structures of 1 and 2 were examined using wide-angle X-ray diffraction (XRD), grazing incidence X-ray diffraction (GIXD), and atomic force microscopy (AFM). Films of 1 and 2 pack in similar triclinic unit cells with the long axes of the molecules nearly perpendicular to the substrate. Thin-film transistors (TFTs) based on 1 and 2 exhibit contact-corrected linear regime hole mobility as high as 0.5 cm2/Vs and 0.1 cm2/Vs, respectively. The specific contact resistance at high gate voltages for gold top contacts was 2 x 10(4) Ohms cm and 3 x 10(4) Ohms cm for 35 nm thick films of 1 and 2, respectively. Long-term air stability tests revealed less degradation of the electrical properties of 1 and 2 in comparison to pentacene. Variable temperature measurements revealed activation energies as low as 22 and 27 meV for 1 and 2, respectively. The temperature and gate voltage dependence of the mobility are discussed in terms of a double exponential distribution of trap states and a model accounting for the layered structure of the organic films. The enhanced air and thermal stability over pentacene, combined with good electrical performance characteristics, make 2 a promising candidate for future organic TFT applications.