Charge Transport in Organic Crystals: Role of Disorder and Topological Connectivity Thorsten Vehoff, † Bjo ¨ rn Baumeier,* ,† Alessandro Troisi, ‡ and Denis Andrienko* ,† Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany, and Department of Chemistry and Centre of Scientific Computing, UniVersity of Warwick, CoVentry CV4 7AL, United Kingdom Received May 20, 2010; E-mail: baumeier@mpip-mainz.mpg.de; denis.andrienko@mpip-mainz.mpg.de Abstract: We analyze the relationship among the molecular structure, morphology, percolation network, and charge carrier mobility in four organic crystals: rubrene, indolo[2,3-b]carbazole with CH 3 side chains, and benzo[1,2-b:4,5-b′]bis[b]benzothiophene derivatives with and without C 4 H 9 side chains. Morphologies are generated using an all-atom force field, while charge dynamics is simulated within the framework of high-temperature nonadiabatic Marcus theory or using semiclassical dynamics. We conclude that, on the length scales reachable by molecular dynamics simulations, the charge transport in bulk molecular crystals is mostly limited by the dynamic disorder, while in self-assembled monolayers the static disorder, which is due to the slow motion of the side chains, enhances charge localization and influences the transport dynamics. We find that the presence of disorder can either reduce or increase charge carrier mobility, depending on the dimensionality of the charge percolation network. The advantages of charge transporting materials with two- or three-dimensional networks are clearly shown. 1. Introduction Among organic semiconducting materials, single crystals created by vapor deposition have record charge carrier mobilities. 1-4 A representative example is rubrene for which mobilities up to 15 cm 2 /(V s) have been reported. 5-7 As a result, performance of OFETs based on single crystals is comparable to that of amorphous silicon-based TFTs. Such devices, however, are of almost no use in practical applications. In contrast, thin-film-based OFETs, 8-11 while having many po- tential applications, have mobilities of active layers on the order of 1 cm 2 /(V s) only. To assist the design of compounds suitable for thin organic layers, it would be helpful to understand what limits charge transport in self-assembled monolayers and, eventually, formu- late design rules for organic semiconductors of this kind. This is a nontrivial task, since several factors can influence charge carrier mobility: (i) the molecular electronic structure, (ii) the relative positions of molecules in the crystal structure, and (iii) the disorder in the morphology arising from static or dynamic deviations from optimal single-crystal structures. In this situa- tion, computer simulations can assist with the morphology characterization and can help to link electronic structure and morphology to charge mobility. 12 This is particularly challenging in the case of charge transport in organic materials, since even the type of transport can change depending on the degree of molecular ordering and temperature. For perfectly ordered defect-free crystals at low temperatures the Drude model based on band theory 5-7,13-21 or its extensions which account for local † Max Planck Institute for Polymer Research. ‡ University of Warwick. (1) Haemori, M.; Yamaguchi, J.; Yaginuma, S.; Itaka, K.; Koinuma, H. Jpn. J. Appl. Phys. 2005, 44, 3740–3742. (2) Herwig, P. T.; Mu ¨llen, K. AdV. Mater. 1999, 11 (6), 480–483. (3) Murphy, A. R.; Fre ´chet, J. M. J.; Chang, P.; Lee, J.; Subramanian, V. J. Am. Chem. Soc. 2004, 126 (6), 1596–1597. (4) Kloc, C.; Tan, K.; Toh, M.; Zhang, K.; Xu, Y. Appl. Phys. A: Mater. Sci. Process. 2008, 95 (1), 219–224. 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