Organic conjugated compounds are envisaged as functional materials for fabricating devices able to drive low-cost, lowperformance consumer electronics. [1][2][3][4] To reach this goal, however, a better understanding of their electrical behavior is needed. In this view, organic single crystals offer the interesting and unique opportunity to investigate the intrinsic electrical behavior of organic materials, excluding hopping phenomena due to grain boundaries and structural imperfections. Their structural asymmetry also allows the investigation of the correlation between their 3D order and their charge-transport characteristics. [5,6] One useful investigation tool in this sense may be found in organic field-effect transistors (OFETs), which can provide precious information on the nature of the chargetransport phenomena in organic materials. Indeed, single-crystal organic transistors, where the active channel is a single crystal, exhibited up to now the best performances in terms of charge-carriers mobility, reaching time-of-flight (TOF)-measured values as high as 400 cm 2 V À1 s À1 , [7] and FET-measured mobilities of several units, up to tens of cm 2 V À1 s À1. [5,8,9] In this light, macroscopic (millimeter-sized) self-standing crystals suitable for being manipulated and selectively deposited on any surface and in any position with respect to existing electrodes are very useful, and recent studies showed that macroscopic crystals of rubrene present 2D electrical anisotropy. [8,10,11] For obtaining crystals suited for these investigations, vacuumbased methods are up to now the most exploited strategy. [5] However, macroscopic organic crystals may be easily grown also from solution, permitting a considerable degree of control over the final crystal characteristics in terms of dimensions [12] and of the developed crystallographic phase. [13] The suitability of solution grown (SG) organic crystals for electronic studies has been recently confirmed by a report on dicyclohexyl-a-quaterthiophene single crystals. The crystals were grown from solution and used as active materials in FETs, demonstrating 2D electrical anisotropy [14] even though in this case the studied crystals had dimensions in the micrometers domain, and an investigation of their electrical behavior in the third dimension was not presented.Here, we report on millimeter-sized SG organic single crystals based on 4-hydroxycyanobenzene (4HCB, Fig. 1), which exhibited 3D anisotropic electrical properties along the three crystallographic axes a, b (constituting the main crystal flat face), and c (the crystal thickness), measured over several different samples. FET devices were used to estimate the directional carrier mobilities in the dark at room temperature and atmosphere along the two main axes a and b, reaching top values up to 8 Â 10 À2 and 9 Â 10 À3 cm 2 V À1 s À1 for m a and m b , respectively.[15]Along the crystal thickness, axis c, the mobility was determined by means of space-charge-limited current (SCLC) measurements, which delivered a maximum value...

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Steady state photoconductivity and x-ray diffraction combined with optical and electronic spectroscopies were applied to study spin coated poly(3-hexylthiophene) (P3HT) films mechanically rubbed using a teflon bar. As expected, photoconductivity evidenced a strong increase in the photo-response along the rubbing direction. But, while rubbing during annealing at a temperature of 70 • C resulted in a sizeable photocurrent anisotropy along the direction parallel or perpendicular to the rubbing, the samples rubbed at room temperature (RT) showed an isotropic and comparatively stronger enhancement of absorption and photoconductivity spectral features. According to these observations, rubbing at RT resulted in a significant increase in local order and macroscopic alignment of the film, while a higher degree of global order but with a relaxed local structure was obtained after rubbing at 70 • C.

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.