Positional disorder enhancement of exciton dissociation at donor/acceptor interface

We investigate the dissociation of a Coulomb bounded electron-hole geminate pair at a disordered donor/acceptor (D-A) heterojunction by extending a previous proposal in the literature [Arkhipov , Appl. Phys. Lett. 82, 4605 (2003)] and using Monte Carlo simulations. We show that the presence of a layer of randomly distributed dipoles at the D-A interface creates both a potential well that confines the hole and a repulsive barrier that prevents the geminate pair recombination, even when the effective mass of the hole is around the electron rest mass. Our calculations depend strongly on the heterojunction morphology. However, contrary to what is generally believed, we find that the disorder in the position of the dipoles along the D-A interface axis enhances the pair dissociation. Inhomogeneities in the acceptor concentration at the heterojunction can then create highly efficient centers for exciton dissociation. The model explains recent experimental results for organic D-A heterojunctions and has important consequences on the design of organic photovoltaic devices.