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

Abstract

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/C60 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 C60 molecules, which by virtue of their symmetry possess no quadrupole. Consequently, at a pentacene/C60 interface, both the vacuum‐level shift and geminate pair dissociation critically depend on the orientation of the pentacene π‐system relative to the adjacent C60.

Field-dependent exciton dissociation in organic heterojunction solar cells

“…In most cases, the weak temperature dependence of the CT-state dissociation process was attributed to additional driving forces for the charge separation such as dipoles 54 and multipoles 55 at the donor acceptor interface, entropy effects, 3 relaxation processes in energetically disordered densities of states, 4,48,56 charge delocalization on polymer chains, 19,44 nonthermalized (hot) CT-states, [57][58][59][60][61][62] and high local values of the mobility. 17 Such additional driving forces increase the CT-state dissociation probability p CT , and if a cell is not limited by the CT-state dissociation, the temperature induced changes of the dissociation efficiency hardly influence the current-voltage (J-V) characteristics.…”

Understanding the Reduced Efficiencies of Organic Solar Cells Employing Fullerene Multiadducts as Acceptors

“…Δ E LUMO represents the potential energy, but according to Marcus theory, the rate in electron transfer that drives charge generation is strongly affected by the difference in Gibbs free energy Δ G 0 between bound and separated state. [ 13 , 44 , 45 ] Factors other than Δ E LUMO , such as nanomorphology, [ 46 ] energetic disorder, [ 38 ] aggregation or crystallinity [ 21 ] and differences in donor -acceptor distances [ 47 ] can contribute to Δ G 0 . These results are in agreement with recent spectroscopic studies of polymer:fullerene systems where we fi nd that materials crystallinity does have a strong impact upon the recombination dynamics of the initially generated polarons.…”

Modeling charge transfer at organic donor-acceptor semiconductor interfaces

“…Vacuum level line-up has indeed been observed at a number of organic-organic semiconductor heterojunctions. [1][2][3] The small potential step of $0.1 eV found at some organic-organic interfaces is explained by small dipoles resulting from polarization of the molecules at the interface 4 and by the weak intermolecular interaction at the interface. 5 A large potential step ( & 0:5 eV) is however observed at a number of organic-organic interfaces.…”