Singlet Exciton Diffusion in Organic Crystals Based on Marcus Transfer Rates.

Exciton diffusion is a critical step for energy conversion in optoelectronic devices. This spawns the desire for theoretical approaches that allow for fast but reliable determinations of the material-dependent exciton transport parameters. For this purpose, the Marcus theory, which is widely used in the context of charge transport, is adapted to exciton diffusion. In contrast to the common approach of calculating the exciton hopping rate via the coupling and the spectral overlap, this alternative approach is less costly, because, instead of the spectral overlap, only the reorganization energy is needed. To demonstrate the capability of the approach, the diffusion constants for naphthalene, anthracene, and diindenoperylene crystals are calculated and compared with both calculations conducted with the well-established exciton hopping rate, including coupling and spectral overlap, and with experimental data. These test calculations show that Marcus-based exciton diffusion properties tend to be too small but are qualitatively correct (i.e., they seem to be useful to predict trends). Nevertheless, for reliable results, high-level quantum chemical approaches are necessary for the computation of the reorganization energies. However, they have to be calculated only once. Coupling constants, which are needed for all pairs of monomers, have a considerably smaller influence, i.e., they can be computed by a lower level approach, which makes the method even less costly.