Organic field-effect transistors, in which the active semiconductor is made of oligothiophenes of various lengths, have been fabricated and characterized. A method is developed to estimate the field-effect mobility μ corrected for the contact series resistance. The mobility is found to increase by a factor of nearly 100 from quaterthiophene (4T) to octithiophene (8T). More importantly, μ increases quasilinearly with gate voltage. The origin of this gate bias dependence is discussed. One explanation could be the presence of traps that limit charge transport. Alternatively, the gate-voltage dependence is tentatively attributed to a dependence of the mobility with the concentration of carriers in the accumulation layer.

End-substitution of quaterthiophene with hexyl groups leads to a highly soluble conjugated oligomer, α,ω-dihexylquaterthiophene (DH4T), which has been characterized for its thermal, structural, and electrical properties. Differential scanning calorimetry indicates the existence of a 3-dimension (3D)-to-mesophase transition, occurring at 84 °C, below the melting temperature of the material (179 °C). X-ray diffraction patterns performed on crude and thermally treated films of DH4T confirm the existence of a smectic phase with two spacings that increase with temperature. This result is interpreted by a model involving alkyl chain movements, which result in spacing shrinkage, whereas the thienylene sequence remains faced at typical van der Waals distances. DH4T thus forms a 2-dimensional (2D) semiconductor with a liquid crystal-like structural organization. DH4T can be deposited as active semiconducting layer in thin-film transistors, either by vacuum evaporation or by spin coating on an octylsilane-pretreated surface. A high-field-effect carrier mobility has been obtained for both deposition techniques, μ = 3 × 10-2 and 1.2 × 10-2 cm2 V-1 s-1, respectively, together with an interesting I on/I off ratio of 105. These data are discussed together with literature results on unsubstituted quaterthiophene (4T) and the parent sexithiophenes (DH6T and 6T). On one hand, results show that the semiconducting quaterthiophene core in DH4T is large enough to allow high charge transport properties, comparable to those of a sexithiophene core, whereas the core is also short enough to allow its α,ω-dialkylated derivative to be highly soluble and solution processable, contrary to the case of DH6T. Results also suggest that the larger band gap of the shorter conjugated quaterthiophenes is responsible for their lower dopant concentration, and hence of their higher dynamic ratio.

Organic field-effect transistors (OFETs) made of conjugated oligomers (oligothiophenes, pentacene), which were first fabricated in this laboratory eight years ago, ['] have now been reproduced by several group^,[^-^^ and proved to be promising devices with a field-effect mobility pFE well into the cm2V-' s-' range and an on/off current ratio of up to lo5. Enhanced performance of the sexithiophene (6T) OFET has been achieved by improving the ordering of the evaporated film, from a disordered three-dimensional(3D) structure to a well-ordered two-dimensional organization, where all the molecules align along a direction nearly perpendicular to the film and form layers parallel to it, with a preferential charge transport along the direction of the layers.r61 6T belongs to the family of the oligothiophenes (nT, where n stands for the number of a-linked thiophene rings). Early results on shorter (3T to 5T)[2371 and longer (8T)171 oligomers than 6T revealed a very steep increase of pFE-by up to two order of magnitude for each additional ring-from 3T to 6T, followed by a slight decrease from 6T to 8T. The latter feature was attributed to the poor structural organization of evaporated films of longer oligomers, due to the lower purity of the raw material and a partial decomposition during the evaporation. The former behavior, namely the huge increase of pFE with n, is still unexplained. It is at variance with what has been observed for a long time on another intensively studied family of molecular crystals, the polyacenes, where the time-offlight (TOF) mobility at room temperature is found to lie in the range 0.1-1 cm2 V-' s-', whatever the length of the molecule, from naphthacene to tetracene.['] (We note that, to the best of our knowledge, field-effect devices with these well-known molecular crystals have only been realized with polycrystalline penta~ene. [~,~]) Charge injection in organic semiconductors has long been shown to be a crucial problem in organic-based devices, especially for large-bandgap compounds. This could

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