Anisotropic Charge Transport in Organic Single Crystals Based on Dipolar Molecules

Abstract

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.

Solution growth of single crystals of 4-hydroxycyanobenzene (4HCB) suitable for electronic applications

“…This finding confirms that the results previously obtained for silver-paste-contacted, 4HCB solution-grown crystals [13][14][15] may be attributed to the actual bulk properties of the crystals and not to any surface contact effect. Moreover, the fact that the found linearity was obtained working on samples grown on different batches, in terms of both growth runs and recipes (i.e.…”

“…Therefore, charge transport studies along all their three dimensions were previously conducted, revealing interesting aspects of their electronic behaviour. In particular, the measurements showed clear three-dimensional anisotropy in electronic transport parameters such as charge carrier mobility, density of states (DOS) and carrier trap density and energy [13][14][15]. Correlations between the crystal structure (already known from the 1970s [16], and confirmed by XRD measurements carried out on solution-grown 4HCB crystals discussed here) and the electronic behaviour were also brought to evidence [13,14].…”

“…In addition to electrical transport along the thickness, SCLC analysis allowed one to verify the already reported [12][13][14] anisotropic carrier mobilities along the three dimensions for the crystals grown with the described recipes. The results of these measurements are shown in Fig.…”

“…Finally, since along the crystal thickness there is no appreciable overlap of the 4HCB benzenic p-electrons (see Refs. [13,14] for a detailed explanation of the crystal structure), these results may be attributable to transport occurring without any band-like mechanism, hence in a likely pure hopping mode.…”

“…The structure of 4HCB makes it attractive for exploring the effect of an intrinsic, static dipole on the electronic properties of organic single crystals [13]. As was already shown in previous works, [13,14], the crystal shape (squared platelets, with side lengths of 1-3 mm and thicknesses of 0.3-1 mm) reflects the molecular packing, with the largest dimensions of the crystals (the ones defining the main squared face of the platelet, axes a and b) corresponding to the directions where p-electron stacking of the 4HCB benzenic rings is appreciable, and the crystal thickness (axis c), which is much shorter with respect to the side length, coincident with negligible p-stacking interactions between the molecules.…”