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 p-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.
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