Intermolecular Charge Transfer Parameters, Electron-Phonon Couplings, and the Validity of Polaron Hopping Models in Organic Semiconducting Crystals: Rubrene, Pentacene, and C60

We evaluate the validity of the commonly assumed polaron hopping model for some of the most popular organic semiconductors, rubrene, pentacene, and C-60. This model is based on the assumption that the charge carrier is localized, i.e., forms a polaron that hops from one molecule to the next. We have calculated the relevant intermolecular charge transfer parameters that determine whether a polaron forms or not: electronic coupling matrix element and reorganization energy for the above materials using quantum chemical calculations and molecular dynamics simulations. We find that neither for rubrene nor pentancene the hopping model is justified due to the relatively large electronic couplings between molecules in the respective herringbone layers. For C-60 the coupling matrix elements are smaller, and a small but finite barrier for charge transport exists in any transport direction. Despite the theoretical problems surrounding the polaron transport model, we find that mobilities based on this model (as obtained from Kinetic Monte Carlo simulation) reproduce very well the room temperature experimental mobility and anisotropy in pentacene and rubrene. However, it fails to reproduce the correct temperature dependence of mobility, predicting a too shallow decay with temperature compared to experiment. Our results call for further development of more advanced simulation approaches, such as nonadiabatic molecular dynamics simulation and their scale-up to large, application-relevant systems.