Electrons and Phonons in Pentacene: Coupling Patterns Reveal the Microscopic Origin of the Phonon Limited Mobility

We present a comprehensive computational study of the vibrational properties and electron-phonon couplings in the three known polymorphs of pentacene. Vibrational patterns and electron-phonon interactions were evaluated at several <i><b>q</b></i> -points of the Brillouin zone, allowing for the detailed mapping of the phonon landscape and the coupling mechanisms relevant to charge transport. Using a pool of postprocessing tools, we analyze the different phonon dispersions and show how low-frequency phonons modulate transport differently in each polymorph through distinct electron-phonon coupling (EPC) signatures in the reciprocal space. In particular, we demonstrate that different phonons dominate in the high-temperature and thin-film polymorphs compared with the high-mobility low-temperature polymorph, leading to different decoherence and localization trends. We describe the microscopic origin of mobility in the bulk phase, showing that polymorphism affects not only the transfer integrals but also the phonon spectrum. Importantly, we find that mobility is not limited by a single "killer" mode but rather by multiple phonons with diverse wave-vectors. We further explain how phonon confinement accounts for the enhanced mobility of the 2D phase. Finally, we address the frequent coexistence of multiple polymorphs in organic crystals, considering the implications of intergrowth, structural defects, and disorder.