Organic Heterojunctions for Photovoltaic Applications: C 60 Growth on Pentacene
- PDF / 332,914 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 2 Downloads / 240 Views
1263-Y05-06
Organic Heterojunctions for Photovoltaic Applications: C60 Growth on Pentacene Rebecca A. Cantrell and Paulette Clancy Cornell University, Department of Chemical and Biomolecular Engineering, Ithaca, NY 14853. ABSTRACT Using atomic-scale Molecular Dynamics (MD) and energy minimization techniques in conjunction with semi-empirical MM3 potential energy functions, we consider the adsorption of a C60 molecule on a series of hypothetical pentacene structures that vary only in the tilt of the angle that the short axis of the pentacene molecules makes with the underlying surface (the long axis lying essentially flat, as on a metal substrate). Important relationships were discovered between the angle adopted by the short axis of pentacene on the surface, φ1, and the adsorption and diffusion characteristics of C60. Static energy calculations show that there is a transition of the deepest energy minima from between the pentacene rows at low values of φ1 to within the rows at high values of φ1, where φ1 is the angle the pentacene short axis makes with the surface. MD confirms this trend by the predominant residence locations at the extreme φ1 values. Furthermore, MD results suggest that the C60 traverses the pentacene surface in the east-west direction for lower φ1 values (φ1 ≤ 40°) and in the north-south direction for higher φ1 values (φ1 ≥ 70°). Taking both static and dynamic results together, the most favorable tilt angles for monodirectional nanowire growth should occur between 70º and 80º off-normal. INTRODUCTION In this paper, we focus on the interactions between p-type pentacene (abbreviated hereafter to “Pn”) and n-type C60 as an exemplar for a p-n heterojunction in an all-organic semiconductor system. In the literature, C60 growth is almost always reported on thin film or bulk phases of pentacene where the long axes of the pentacene molecules is nearly vertical, such that C60 interacts only with the hydrogen atoms at the tips of the pentacene molecules which constitutes an extremely weak van der Waals interaction. This weak interaction, in contrast to the relatively strong C60-C60 interactions, is largely responsible for experimental difficulties in growing ordered layers of C60 on top of thin film and bulk phases of pentacene films in an ideal layer-by-layer manner. C60 shows a tendency to cluster and dewet the pentacene surface, invariably growing in an undesirable three-dimensional fashion [1]. To improve the growth behavior of C60 on pentacene, one strategy is to increase the C60-Pn interaction sufficiently to compete with the very strong C60-C60 interaction [2,3], while maintaining enough mobility of the C60 molecules on the pentacene surface so that the growth mode is not “hit and stick” (ballistic deposition), which leads to undesirable fractal growth patterns. One way to accomplish this is to orient the pentacene molecules such that their long axes are parallel to the substrate below (as occurs when Pn is adsorbed on metals or Si, for instance) so that C60 molecules are able to access, and hence b
Data Loading...
