Electronic Structure and Geminate Pair Energetics at Organic-Organic Interfaces: The Case of Pentacene/C60 Heterojunctions

Organic semiconductors are characterized by localized states whose energies are predominantly determined by electrostatic interactions with their immediate molecular environment. As a result, the details of the energy landscape at heterojunctions between different organic semiconductors cannot simply be deduced from those of the individual semiconductors, and they have so far remained largely unexplored. Here, microelectrostatic computations are performed to clarify the nature of the electronic structure and geminate pair energetics at the pentacene/C-60 interface, as archetype for an interface between a donor molecule and a fullerene electron acceptor. The size and orientation of the molecular quadrupole moments, determined by material choice, crystal orientation, and thermodynamic growth parameters of the semiconductors, dominate the interface energetics. Not only do quadrupoles produce direct electrostatic interactions with charge carriers, but, in addition, the discontinuity of the quadrupole field at the interface induces permanent interface dipoles. That discontinuity is particularly striking for an interface with C-60 molecules, which by virtue of their symmetry possess no quadrupole. Consequently, at a pentacene/C-60 interface, both the vacuum-level shift and geminate pair dissociation critically depend on the orientation of the pentacene pi-system relative to the adjacent C-60.