A computational study of surface diffusion of C60 on pentacene

The morphology of the C60/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 C60 molecules on the surface of crystalline pentacene as a probe of the molecular-level interactions between C60 and pentacene. The ultimate molecular probe of the pentacene surface, a single C60 admolecule, exhibited an anisotropic diffusion pattern that lingered in energetically preferred sites in the [11¯0] direction, intercepting the (0, 1/2, 0) point in the unit cell. An Arrhenian analysis of this diffusion data gave estimates for the prefactor, D0, and energy barrier, Ea, of 2 × 10−3 cm2/s and 0.1 eV, respectively. Surface diffusion of one C60 molecule on pentacene is significantly more rapid (by about 1-2 orders of magnitude) than if even one additional C60 admolecule is present, implying that the C60-C60 cohesion interaction is stronger than the C60-pentacene adhesion interaction. Simulations with up to four C60 molecules, the practical limit of an all-atom approach, reinforced the suggestion that C60 likes to dewet a pentacene surface and will show a preference for forming small 3D nuclei.