Interplay of band occupation, localization, and polaron renormalization for electron transport in molecular crystals: Naphthalene as a case study

Understanding electronic properties and charge transport in organic semiconductors is important for improving organic electronic materials and devices. Here we investigate the impact of electronic band occupation, charge-carrier concentration, and symmetry of phonon modes on the electron mobility in naphthalene crystals for various temperatures. Our theoretical approach is based on the description of the electron-phonon coupling (EPC), where the coupling to low-frequency modes is treated by an effective vibrational disorder potential with local and nonlocal contributions and the coupling to high-frequency modes is included by a polaron treatment. Surprisingly, the coupling to high-frequency modes leads to an increase in the mobility in presence of the low-frequency modes, which is explained by localization and band occupation effects that further depend on the carrier density. A symmetry analysis sheds additional light on the energy dependence of the EPC, which is important to describe transport properties as a function of charge density and temperature. We also find that coupling to low-frequency phonons together with band occupation effects can lead to a vanishing slope of the mobility versus temperature that is known from experiments.