Structures and Defects at Interfaces in Organic Molecular Heterostructures Paul G. Evans, Department of Materials Science and Engineering, University of Wisconsin, Madison, WI 53706 The mobilities of charge carriers in organic semiconductors is dramatically higher in single crystals than in thin films. In large part, this difference is thought to arise from the formation of structural defects during the growth of the thin film form. The image at right (ref. 1) shows the nanometer-scale structure of vacancies formed in a thin film crystal of the organic semiconductor pentacene. A variety of larger 2D defects, including stacking faults and grain boundaries also occurs in pentacene layers (ref. 2). Understanding and potentially either eliminating or passivating these defects could have an important role in improving the performance of organic electronic and optoelectronic devices, including photovoltaics. 1. S. Seo and P. G. Evans, “Molecular structure of extended defects in monolayer-scale pentacene thin films,” J. Appl. Phys. 106, 103521 (2009). 2. S. Seo, L. C. Grabow, M. Mavrikakis, R. J. Hamers, N. J. Thompson, and P. G. Evans, “Molecular-scale structural distortion near vacancies in pentacene,” Appl. Phys. Lett. 92, 153313 (2008).