Hierarchical Assembly of 2D Nanostructures of Relevance for Organic Solar Cell Design
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Hierarchical Assembly of 2D Nanostructures of Relevance for Organic Solar Cell Design Sarah O'Donnell1, Michael Buettner2, and Petra Reinke3 1 The Mitre Corporation, McLean, VA, 22102 2 University of Virginia, Charlottesville, VA, 22904 3 Department of Materials Science and Engineering, University of Virginia, 385 Mc Cormick Road, Charlottesville, VA, 22904 ABSTRACT The first step in synthesizing a model film morphology via a surface-driven hierarchical assembly process is presented. The goal of the hierarchical assembly is the control of the morphology of complex molecular layers for the investigation of fundamental processes in organic solar cells. Using a focused ion beam (FIB) with Ga+ ions at 30 keV, the surface of highly oriented pyrolitic graphite (HOPG) is patterned with an array of local amorphous carbon ellipsoid spots (ACES), which provide preferential nucleation lines at their perimeter, and thus are instrumental in the control of fullerene island growth. On the undamaged surface regions outside the ACES pattern the fullerene island growth is unperturbed, and presents the wellknown combination of round and fractal island shapes. The fullerene deposition at the periphery of the ACES pattern, which is characterized by single ion impact defects, results in stunted, smaller and irregular islands. Inside the ACES array, the C60 island growth is controlled by the shape of the ACES and is constrained to lobes which form around each ACES spot. The array and C60 lobe morphology and geometry are characterized and a subsequent understanding of the C60 diffusion fields and nucleation lines within the array is discussed. INTRODUCTION Considerable advances in organic solar cells come from improvements in solar cell architecture and materials development1-3. Film morphology of the organic layers across length scales, spanning the range from a hundred nanometers down to the local molecule arrangement, has been recognized as one of the most influential material properties with respect to solar cell performance3, 4,5,6-9. The goal of this work is the two-dimensional morphology control of material phases in a solar cell, allowing the simultaneous investigation of fundamental aspects of exciton transport and charge diffusion which are critical components in the progression of organic solar cell construction. Here, we use hierarchical assembly of material phases, which utilizes surface-driven routes for the synthesis of a large range of film morphologies. In this work, the first step in the hierarchical assembly for one of two material phases in an organic 2-D film is demonstrated. Using a focused ion beam (FIB), a rectangular pattern of amorphous carbon regions is created on highly-oriented pyrolitic graphite10 (HOPG) for the self assembly of C60 regions to achieve control of C60 island morphology. This case serves as a model for testing the principles of hierarchical assembly of organic materials with control over film morphology, and several approaches to add the second molecular phase are currently being inv
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