Anisotropic charge transport in organic single crystals based on dipolar molecules

We studied the anisotropic charge transport properties of solution-grown organic single crystals based on a dipolar molecule 4HCB (4-hydroxy- cyanobenzene) by electrical transport measurements, current-voltage and space charge limited current (SCLC), and by X-ray diffraction analyses. Optical excitation differently affects the flow of charge carriers along the two main planar crystal axis, 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. The anisotropy of transport along the three crystallographic directions has been assessed by electrical characterization and correlated to the crystal molecular packing as determined by X-ray analyses.