Electrostatic Effects at Organic Semiconductor Interfaces: A Mechanism for “Cold” Exciton Breakup

Abstract

Exciton dissociation at organic semiconductor interfaces is an important process for the design of future organic photovoltaic (OPV) devices, but at present, it is poorly understood. On the one hand, exciton breakup is very efficient in many OPVs. On the other, electron–hole pairs generated by an exciton should be bound by Coulombic attraction, and therefore difficult to separate in materials of such low dielectric. In this paper, we highlight several electrostatic effects that appear commonly at organic/organic interfaces. Using QM/MM simulations, we demonstrate that the electric fields generated in this fashion are large enough to overcome typical electron–hole binding energies and thus explain the high efficiencies of existing OPV devices without appealing to the existence of nonthermal (“hot”) carrier distributions. Our results suggest that the classical picture of flat bands at organic/organic interfaces is only qualitatively correct. A more accurate picture takes into account the subtle effects of electrostatics on interfacial band alignment.