We have performed current–voltage measurement on polycrystalline sexithiophene (6 T) thin film transistors at temperatures ranging from 10 to 300 K. A method is developed to extract the carrier mobility from an analysis of the transfer characteristics. In particular, data are corrected for contact resistance. The carrier mobility is found to increase quasilinearly with gate voltage at room temperature. The dependence becomes superlinear at low temperatures. The temperature dependence shows three domains. For 100 K<T<300 K, the mobility is thermally activated with an activation energy of around 0.1 eV. The activation energy reduces to 5 meV for 25 K<T<100 K. Finally, the mobility is practically temperature independent for temperatures lower than 25 K. The data are explained by a model where charge transport is limited by a high concentration of traps at grain boundaries. At high temperatures, charge transfer at boundaries occurs via thermionic emission, while tunnel effect takes place at low temperatures. The energy distribution of traps is determined, and various features predicted by the model are outlined.

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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.

The photorefractive (PR) effect involves a photoinduced change of the refractive index in an optically nonlinear and photoconducting material. [1] This effect arises when photocharges, generated by spatially modulated light intensity, migrate through drift and/or diffusion processes and become trapped to produce a non-uniform charge density distribution. This charge separation then creates an internal space-charge field, which modulates the refractive index of the material through the linear electro-optic (EO) effect, also known as Pockels effect. The refractive index grating

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