Organic Heterojunctions for Photovoltaic Applications: C₆₀ Growth on Pentacene

Abstract

Using atomic-scale Molecular Dynamics (MD) and energy minimization techniques in conjunction with semi-empirical MM3 potential energy functions, we consider the adsorption of a C60 molecule on a series of hypothetical pentacene structures that vary only in the tilt of the angle that the short axis of the pentacene molecules makes with the underlying surface (the long axis lying essentially flat, as on a metal substrate). Important relationships were discovered between the angle adopted by the short axis of pentacene on the surface, φ1, and the adsorption and diffusion characteristics of C60. Static energy calculations show that there is a transition of the deepest energy minima from between the pentacene rows at low values of φ1 to within the rows at high values of φ1, where φ1 is the angle the pentacene short axis makes with the surface. MD confirms this trend by the predominant residence locations at the extreme φ1 values. Furthermore, MD results suggest that the C60 traverses the pentacene surface in the east-west direction for lower φ1 values (φ1 ≤ 40°) and in the north-south direction for higher φ1 values (φ1 ≥ 70°). Taking both static and dynamic results together, the most favorable tilt angles for mono-directional nanowire growth should occur between 70° and 80° off-normal.

Computationally Derived Rules for Persistence of C60 Nanowires on Recumbent Pentacene Bilayers

“…A key component of MD simulations is the choice of intermolecular potential model. The molecular mechanics 3 (MM3) potential model used in this paper is the same as that reported in our previous papers. − We chose this semiempirical MM3 force field since it has been shown by its developers (and by us) to accurately describe a variety of thermodynamic properties of hydrocarbons, namely, three-, four-, five-, and six-ringed structures. − MM3 incorporates stretching, bending, and torsional energies as well as the van der Waals interaction energies based on atom–atom parameters related to their chemical environment …”

“…We have been studying the structural properties of all-organic heterojunctions, in particular, C 60 /pentacene, − motivated by their potential as prototypical planar p–n junctions. − However, this system, like many small-molecule organic semiconductor systems, shows a tendency for C 60 to dewet pentacene surfaces in upright-standing polymorphs (namely, the thin film and bulk phases), leading to undesirable three-dimensional growth rather than a more ordered, and hence higher mobility, layer-by-layer growth. ,, Two-dimensional growth is preferred based on a rubric that links increased structural order to higher electron and hole mobilities. Dewetting occurs due to the consequences of the stronger C 60 –C 60 interactions in comparison to those for C 60 /pentacene, exemplifying the balance of forces that drives the preferred morphology of thin film growth in many such small-molecule organic semiconductor systems.…”

“…Our initial prior study of the tendency of C 60 molecules to form nanowires involved using computationally efficient static energy calculations to produce the potential energy surface between a single C 60 molecule and a set of variably angled recumbent pentacene structures . Subsequent molecular dynamics (MD) simulations also involved a single C 60 molecule moving over a recumbent pentacene substrate (free to move in response to the intermolecular forces) in which the tilt angle was controlled . A fixed pentacene layer beneath these dynamic layers offered an underlying template substrate.…”

“…In ref , we introduced the possibility of forming nanowires oriented in a N-S direction, in addition to E-W ones. At higher ϕ 1 values (ϕ 1 ≥ 70°), we predicted that N-S nanowires would form preferably compared to E-W configurations because the C 60 molecules are more energetically confined in a N-S orientation at higher ϕ 1 values.…”

Striped domains at the pentacene:C60 interface

“…This is consistent with angular changes observed in semiempirical energy minimization calculations of the restructuring of C 60 -pentacene interfaces. 20 Figure 3͑b͒ shows a schematic of the long range modulation in the pentacene layer on C 60 .…”