Do charges delocalize over multiple molecules in fullerene derivatives?

Abstract

We address the question of charge delocalization in amorphous and crystalline fullerene solids by performing state of the art calculations encompassing force-field molecular dynamics, microelectrostatic and quantum-chemical methods. The solution of a tight-binding model built from spatially (down to atomistic scale) and time (down to fs) resolved calculations yields the density of electronic states for the charge carriers and their energy-dependent intermolecular delocalization. Both pristine C₆₀ and the soluble PC₆₁BM/PC₇₁BM acceptors may sustain high-energy states that spread over a few tens of molecules irrespective of morphology, yet electrostatic disorder (mostly dipolar and static in nature) makes the thermally available electron states collapse to hardly more than one molecule in PC₆₁BM/PC₇₁BM, while it has a much more limited impact in the case of the bare C₆₀. Implications of these results for charge transport and exciton dissociation at donor–fullerene interfaces are discussed.