We report structural and electrical transport properties of a family of π-stacking soluble organic semiconductors, N,N‘-dialkyl-3,4,9,10-perylene tetracarboxylic diimides (alkyl − pentyl [1], octyl [2], and dodecyl [3]). The structures of evaporated polycrystalline films of 1−3 were studied using X-ray diffraction and atomic force microscopy. Films of 1−3 pack similarly with the direction of π−π overlap in the substrate plane. Organic thin film transistors (OTFTs) based on 1−3 deposited on SiO2 gate dielectric showed linear regime electron mobilities of 0.1, 0.6, and 0.2 cm2/(V s), respectively, corrected for contact resistance. OTFTs of 2 had saturation electron mobilities as high as 1.7 cm2/(V s) with on-to-off current ratios of 107. Variable temperature measurements were used to examine the charge transport kinetics in the range 80−300 K and revealed (1) thermally activated electron mobilities with activation energies dependent on gate voltage and (2) the presence of well-defined isokinetic points, i.e., temperatures at which Arrhenius plots at different gate voltages intersect for a given film. Isokinetic points indicate a common charge transport mechanism and can be explained in terms of the multiple trapping and release (MTR) transport model. MTR assumes trap-limited band transport, and quantum chemical calculations were used to verify that delocalized transport is likely in 1−3; a conduction bandwidth of 0.58 eV was calculated for 1. Using MTR, the trap concentrations were estimated to be ∼1012 cm-2 for deep traps, and ∼6 × 1013 cm-2 for shallow traps. However, a nonmonotonic dependence of the electron mobility on gate voltage was also observed, which is not predicted by MTR and suggests that the transport mechanism is more complicated, perhaps due to the discrete layered structure of these materials. The high values for the electron mobility and on-to-off current ratio suggest that substituted perylene diimides represent a promising class of n-channel conductors for OTFTs.

We describe gated four-probe measurements designed to measure contact resistance in pentacene-based organic thin-film transistors (OTFTs). The devices consisted of metal source and drain electrodes contacting a 300-Å-thick pentacene film thermally deposited on Al2O3 or SiO2 dielectrics with a p-doped Si substrate serving as the gate electrode. Voltage-sensing leads extending into the source-drain channel were used to monitor potentials in the pentacene film while passing current during drain voltage (VD) or gate voltage (VG) sweeps. We investigated the potential profiles as a function of contact metallurgy (Pt, Au, Ag, and Ca), substrate chemistry, VG, and temperature. The contact-corrected linear hole mobilities were as high as 1.75cm2∕Vs and the film sheet resistance and specific contact resistance were as low as 600kΩ∕◻ and 1.3kΩ-cm, respectively, at high gate voltages. In the temperature range of 50–200K, the pentacene OTFTs displayed an activated behavior with activation energies of 15–30meV. Importantly, the activation energy associated with the contact resistance showed no dependence on contact metal type at high gate voltage. Also, the activation energies of the contact resistance and film resistance were approximately the same. Above approximately 200K and below 50K, the mobility was essentially temperature independent.

Thin‐film transistors (TFTs) based on a new n‐channel organic semiconductor (DCMT; see Figure) are reported. An electron mobility as high as 0.2 cm2/V s was observed, as well as ambipolar TFT behavior. Variable temperature measurements reveal that electron conduction is activated, with a small activation energy of 35 ± 10 meV. These results demonstrate that quinoidal oligothiophenes are a promising new class of organic semiconductors for TFTs.

A terthiophene-based quinodimethane, 3',4'-dibutyl-5,5' '-bis(dicyanomethylene)-5,5' '-dihydro-2,2':5',2' '-terthiophene (1) was synthesized and crystallized. Compound 1 has a planar quinoid geometry that is stabilized by dicyanomethylene groups at each end of the molecule. In the crystal each molecule is part of a dimerized face-to-face pi-stack, with intermolecular spacings of 3.47 and 3.63 A, respectively. Cyclic voltammetry showed that 1 could be reversibly reduced and oxidized in methylene chloride solution. Thin film transistors (TFTs) were prepared by vacuum evaporation of 1 onto SiO2(300 nm)/Si substrates, followed by evaporation of Ag source and drain contacts. The doped Si substrate served as the gate electrode. X-ray diffraction and atomic force microscopy indicate the films are polycrystalline, with the long axes of the molecules approximately perpendicular to the substrate. The TFT measurements revealed n-channel conduction in films of 1, with room-temperature electron field effect mobilities as high as 0.005 cm2/Vs. The butyl side chains give 1 appreciable solubility in a range of common solvents, and preliminary TFT results on films cast from chlorobenzene show electron mobility as high as 0.002 cm2/Vs. These results indicate that pi-stacked quinoidal thiophene oligomers are a promising new class of soluble n-channel organic semiconductors.