Three-dimensional anisotropic density of states distribution and intrinsic-like mobility in organic single crystals B. Fraboni a, * , A. Fraleoni-Morgera b , A. Cavallini a a Dipartimento di Fisica, Università di Bologna, viale Berti Pichat 6/2, 40127 Bologna, Italy b Sincrotrone Trieste, Strada Statale Km 163.5, 34102 Basovizza (Trieste), Italy article info Article history: Received 27 May 2009 Received in revised form 10 September 2009 Accepted 15 September 2009 Available online 19 September 2009 Keywords: Molecular electronics Anisotropic mobility Density of states distribution Organic single crystal Solution-grown crystals abstract Organic semiconducting molecules are receiving a large attention because of their poten- tial applications, spanning from OLEDs to plastic photovoltaics to bio-chemical sensors. However, the electronic transport properties of these materials are still not fully under- stood, and organic single crystals (OSCs) may represent model materials for assessing the charge transport mechanisms, thanks to their high purity and molecular order. Here we show for the first time that solution-grown, millimiter-sized organic single crystals of 4-hydroxycyanobenzene (4HCB) possess a clear and reproducible three-dimensional anisotropy in their main transport parameters: (i) charge carrier mobility, (ii) distribution of the electronic density of states and (iii) deep traps energy and concentration, and we report intrinsic-like three-dimensional mobility values for these crystals. These findings demonstrate that the electronic spatial anisotropy of OSCs extends well beyond the carrier mobility, and open the way to the development of novel electronic device architectures based on the simultaneous exploitation of different electronic responses along the three spatial directions of the crystal. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction Organic semiconductors are considered promising mate- rials for implementing low-cost and large-scale produced electronics [1–8]. Among the most attractive prospects in this field are the possibility of tailoring functionalities by molecular design [9] and the realization of flexible devices [10–12], distinctive and unique features that open the way to the attainment of versatile and multifunctional electronic systems. However, in order to achieve this goal, a number of issues relative to the charge transport in organic materials are yet to be clarified. In particular, a clear and well defined comprehension of the electronic behaviour of these materi- als (including a complete description of the fundamental electronic states of the charge carriers) has still to be gained and related to their structural properties. In this view, organic single crystals (OSCs) offer the possibility to effectively probe the electronic behaviour of organic materials. In fact, thanks to their long-range molec- ular order, they limit charge carrier trapping and hopping phenomena due to grain boundaries, interfaces and struc- tural imperfections, and provide an almost ideal work bench for the study of intrinsic electronic transport. None- theless, the low symmetry of organic molecules leads to anisotropically packed crystal structures, that affect their transport properties (for example, the direction of the strongest p-orbitals overlap usually coincides with the direction of the highest carrier mobility [13–19]). This asymmetry, on one hand introduces difficulties in a clear understanding of the transport behaviour of organic semi- conductors, but on the other hand it offers the tool to inves- tigate the correlation between the three-dimensional molecular stacking order of OSCs and their recently found 1566-1199/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2009.09.014 * Corresponding author. Address: Department of Physics, University of Bologna, viale Berti Pichat 6/2, Bologna 40127 Italy. Tel.: +39 0512095806; fax: +39 0512095113. E-mail address: beatrice.fraboni@unibo.it (B. Fraboni). Organic Electronics 11 (2010) 10–15 Contents lists available at ScienceDirect Organic Electronics journal homepage: www.elsevier.com/locate/orgel