Three-dimensional anisotropic electronic properties of solution grown organic single crystals measured by Space-Charge L
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1154-B05-118
Three-dimensional anisotropic electronic properties of solution grown organic single crystals measured by Space-Charge Limited Current (SCLC) Beatrice Fraboni1 , Alessandro Fraleoni-Morgera2 and Anna Cavallini1 1 Dipartimento di Fisica, Università di Bologna, viale Berti Pichat 6/2, 40127 Bologna, Italy 2 Sincrotrone Trieste, Strada Statale Km 163.5, 34102 Basovizza (Trieste), Italy
ABSTRACT Organic single crystals offer the interesting and unique opportunity to investigate the intrinsic electrical behaviour of organic materials, excluding hopping phenomena due to grain boundaries and structural imperfections. Their structural asymmetry permits also to investigate the correlation between their three-dimensional order and their charge transport characteristics. Here we report on millimeter-sized, solution-grown organic single crystals, based on 4hydroxycyanobenzene (4HCB), which exhibit three-dimensional 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. The carrier mobility was determined by means of space charge limited current (SCLC) and air-gap field effect transistors fabricated with 4HCB single crystals and the measured values well correlate with the structural packing anisotropy of the molecular crystal. A differential analysis of SCLC curves allowed to determine the distribution and the concentration of the dominant electrically active density of states within the gap. INTRODUCTION Organic semiconductors are considered promising materials for implementing low-cost and large–scale produced electronics [1,2] and are receiving a large attention because of their potential applications, spanning from OLEDs to plastic photovoltaics and to bio-chemical sensors. However, the electronic transport properties of these materials are still not fully understood, and organic single crystals (OSCs) may represent model materials for assessing the charge transport mechanisms, thanks to their high purity and molecular order. Moreover, their structural asymmetry permits to investigate the correlation between their three-dimensional molecular stacking order and their charge transport characteristics [3,4].Recent studies showed that macroscopic crystals of rubrene present a two-dimensional electrical anisotropy [5,6] and single crystal organic transistors exhibited up to now the best performances in terms of charge carriers mobility, reaching time-of-flight-measured values as high as 400 cm2V-1s-1, and FETmeasured mobilities up to several cm2V-1s-1.[3,5,7] In this view, the prospect of developing 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, is very attractive. The more widely diffused method for obtaining crystals suited for these investigations is vacuum-based deposition, even if macroscopic organic crystals may be easily grown also from solu
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