Electrons and phonons in pentacene, insights from comparison between experiment and simulations
We have performed 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 calculated at several q-points of the Brillouin zone, allowing for a detailed mapping of the phonon landscape and the associated coupling mechanisms relevant to charge transport. Using a pool of post-processing tools, we analyze the different phonon dispersions. Thus, we shed light on how low-frequency phonons modulate the transport differently in the polymorphs via their distinct electron-phonon coupling (EPC) signatures in reciprocal space. In fact, we show that distinct phonons dominate in high-temperature and thin-film polymorphs with respect to the high mobility low-temperature polymorph, and that these lead to different decoherence/localization trends. We describe the microscopic origin of the mobility in bulk polymorph, demonstrating that polymorphism not only modulates the transfer integral but also the phonon pattern. For the first time we show that different EPC patterns lead to very different mobility values even in similar structures. Also, we explain how phonon confinement is responsible for the increased mobility observed in 2D phase. Finally, we address the problem of possible coexistence of multiple polymorphs within a single specimen, frequently encountered in organic crystals due to their subtle energy landscape and processing conditions. In this context, we consider the implications of polymorph intergrowth, structural defects and disorder.