A computational study of surface diffusion of C60 on pentacene

The morphology of the C-60/pentacene heterojunction is of interest for organic photovoltaic applications, yet is not well characterized. With that in mind, all-atom molecular dynamics simulation techniques were used to elucidate the diffusional behavior of small numbers of C-60 molecules on the surface of crystalline pentacene as a probe of the molecular-level interactions between C-60 and pentacene. The ultimate molecular probe of the pentacene surface, a single C-60 admolecule, exhibited an anisotropic diffusion pattern that lingered in energetically preferred sites in the (110) direction, intercepting the (0. 1/2, 0) point in the unit cell. An Arrhenian analysis of this diffusion data gave estimates for the prefactor, D-0, and energy barrier, E-a, of 2 x 10(-3) cm(2)/s and 0.1 eV, respectively. Surface diffusion of one C-60 molecule on pentacene is significantly more rapid (by about 1-2 orders of magnitude) than if even one additional C-60 admolecule is present, implying that the C-60-C-60 cohesion interaction is stronger than the C-60-pentacene adhesion interaction. Simulations with up to four C-60 molecules, the practical limit of an all-atom approach, reinforced the suggestion that C-60 likes to dewet a pentacene surface and will show a preference for forming small 3D nuclei. (C) 2008 Elsevier B.V. All rights reserved