Calculating the Efficiency of Exciton Dissociation at the Interface between a Conjugated Polymer and an Electron Acceptor

The decisive, feature of any material designed for photovoltaic applications is the dissociation efficiency of photogenerated excitons. This efficiency is essentially governed by the Coulomb attraction between electrons and holes. Because the dielectric constant in organic materials is rather low (epsilon(r) approximate to 3), the exciton binding energy is much larger than the thermal energy at room temperature, so that thermally governed dissociation seems improbable. Experiments show that while the dissociation probability for electron-hole pairs is indeed very low in the bulk samples, it becomes close to unity at intrinsic interfaces between two organic materials, an electron donor (usually a conjugated polymer) and an electron acceptor (usually a fullerene derivative). The driving force for this dissociation is still a matter of controversy. This Perspective provides a theoretical analysis of possible mechanisms for the efficient dissociation of electron-hole pairs at internal organic interfaces despite the strong Coulomb attraction between the charges.