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

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

Authors 

Cantrell, R. A. - Presenter, Cornell University
Dougherty, D. - Presenter, National Institute of Standards and Technology
Robey, S. W. - Presenter, National Institute of Standards and Technology


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, 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, 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.