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 [ 1 1 ¯ 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.