Anisotropic Charge Transport in Organic Single Crystals Based on Dipolar Molecules
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Anisotropic Charge Transport in Organic Single Crystals Based on Dipolar Molecules Beatrice Fraboni1, Riccardo Di Pietro1, Antonio Castaldini1, Anna Cavallini1, Alessandro Fraleoni2, Leonardo Setti2, Ivan Mencarelli2, and Cristina Femoni3 1 Physics, University of Bologna, viale Berti Pichat 6/2, Bologna, 40127, Italy 2 Industrial and Material Chemistry, University of Bologna, Bologna, 40137, Italy 3 Physical and Inorganic Chemistry, University of Bologna, Bologna, 40126, Italy 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. INTRODUCTION Organic conjugated compounds are envisaged as functional materials for fabricating devices able to drive low-cost, low-performance consumer electronics[1,2]. To reach this goal, however, a better understanding of the electrical behaviour of organic materials is needed. It has been shown that organic single crystals offer the possibility of studying the intrinsic properties of organic materials, thanks to their high purity and molecular order [3,4]. The charge transport parameters may be investigated with the aid of a transverse electric field effect, which can be implemented by fabricating field effect transistors (FETs). The fabrication of the first single crystal FET (SCFET) was reported just a few years ago [3,5] and a device geometry that uses a thin air gap as the gate dielectric was shown to strongly reduce the defective states induced by standard lithographic fabrication processes [6], a factor significantly affecting transport processes in organic materials. The low symmetry of molecules, that leads to anisotropically packed crystal structures results in a strong anisotropy of their transport properties [4,7]. Macroscopic (millimeter-sized), self-standing crystals, suitable for being manipulated and selectively deposed on any surface and in any position with respect to existing electrodes, are very useful, and indeed, recent studies showed that macroscopic crystals of rubrene grown by vacuum-based methods present a two-dimensional electrical anisotropy [6,8,9]. Vacuum based methods are up to now the ones of choice[4] for obtaining crystals suited for these investigations. However, macroscopic organic crystals may also be easily grown from solution, permitting a considerable degree of control over
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