(185m) A Computational Study of C60-Pentacene Heterojunction Interfaces

The morphology of the C₆₀/pentacene heterojunction, of interest for organic photovoltaics applications, is not well understood. On an atomic scale, the supplement of molecular simulation to experimental evidence provides insight into an otherwise enigmatic system. All-atom Molecular Dynamics simulation techniques were used to elucidate the dynamic 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 single molecule 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 that intercepts the 0, ½, 0 point in the unit cell. An Arrhenian analysis of this diffusion data gave estimates for the prefactor, D₀, and energy barrier, E_a , 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, a practical limit of an all-atom approach, reinforced this suggestion that C₆₀ likes to dewet a pentacene surface. Collaborators Dan Dougherty and Steve Robey have deposited C₆₀ on two layers of flat pentacene on Ag(111), and have observed (via low temperature STM images) nanowires of C₆₀ in between pentacene rows at relatively low C₆₀ coverages; at higher C₆₀ coverages, they observe a combination of nanowires in between pentacene rows and disordered C₆₀ thin films. While the systems considered in experiment are not exactly the same as those in simulation, both suggest that both the C₆₀-pentacene adhesion and the C₆₀-C₆₀ cohesion influence the C₆₀ thin film structure. The question of what exactly determines the relative order of the C₆₀ thin films on pentacene is yet to be answered.