Bandlike charge transport and electron-phonon coupling in organic molecular crystals.

Abstract

Charge transport is important in organic molecular crystals (OMCs), where high carrier mobilities are desirable for a range of applications. However, modeling and predicting the mobility is chal- lenging in OMCs due to their complex crystal and electronic structures and electron-phonon (e-ph) interactions. Here we show accurate first-principles calculations of electron and hole carrier mobility in several OMCs: benzene, anthracene, tetracene, pentacene, and biphenyl. Our calculations use the Boltzmann transport equation (BTE) formalism with e-ph interactions computed from first principles. These calculations describe transport in the bandlike, weak e-ph coupling regime, and include all phonon modes and electronic bands on equal footing. In all systems studied, we predict the mobility and its temperature dependence in very good agreement with experiments between 100-400 K, where transport is phonon-limited. We show that e-ph scattering from low-frequency (LF) phonons with energy below 150 cm-1 primarily limits the mobility, even though these modes are not the ones with the strongest e-ph coupling. These LF modes are shown to consist mainly of intermolecular vibrations, with admixed long-range intramolecular character in OMCs with larger molecules. Furthermore, we find that the LF-mode scattering rates vary significantly with strain, suggesting that strain engineering can effectively modulate e-ph coupling and enhance the mobility. This work sheds light on bandlike transport mechanisms in OMCs and advances the rational design of high-mobility organic semiconductors.