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Photocurrent (PC) spectroscopy is proposed as a reliable tool in the investigation of the transport properties of organic thin-film transistors (OFETs). We have applied PC analyses to the study of the electronic density of states (DOS) distribution of the OFETs with crystalline-mixed films of two derivatives of a conjugated molecule [ α-sexithiophene (6T), and its alkylated analogue α, ω-dihexylsexithiophene (DH6T)] as the active semiconductor. We have investigated the modifications in the DOS distribution associated to variations in the carrier density in the OFET channel and we have detected the formation of deep electrically active states in the below-band-gap region, associated to polaron states induced by a prolonged exposure of the device to atmosphere. A clear correlation between the PC results and the electrical characteristics of the corresponding FET devices has been observed. [ABSTRACT FROM AUTHOR]

Abstract: We studied the anisotropic charge transport properties of solution-grown organic single crystals based on a dipolar molecule (4-hydroxy-cyanobenzene) by field effect devices and by spectral photocurrent analyses. Optical excitation differently affects the flow of charge carriers along the two main planar crystal axes, altering the charge transport anisotropy induced by the molecular π-orbitals stacking. We attribute this behaviour to the presence of an intrinsic molecular dipole and to its different orientation within the crystal lattice and we advance a hypothesis on the nature of the observed band of deep trapping states. [Copyright &y& Elsevier]