Abstract

Despite the poor screening of the Coulomb potential in organic semiconductors, excitons can dissociate efficiently into free charges at a donor-acceptor heterojunction. In the first part of this colloquium I will present a kinetic two-particle Monte Carlo model that quantitatively explains this high efficiency as a two-step hopping process. Driven by the band offset between donor and acceptor, one of the charge carriers first hops across the interface, forming a charge transfer (CT) complex. Since the electron and hole forming the CT complex have typically not relaxed within the disorder-broadened density of states (DOS), the remaining Coulomb binding energy can be overcome by further relaxation in the DOS . As this process is driven by the internal energy of the non-relaxed system the dissociation yield therefore only shows a moderate thermal activation and a weak dependence on electric field. The model predicts dissociation yields in excess of 90% for a prototypical heterojunction. In the second part of the colloquium I will address charge sepa-ration in the more realistic situation where free charges and excitons are simultaneously present.