  Charge transport in organic semiconductors is notoriously extremely sensitive
to the presence of disorder, both internal and external (i.e. related to the
interactions with the dielectric layer), especially for n-type materials.
Internal dynamic disorder stems from large thermal fluctuations both in
intermolecular transfer integrals and (molecular) site energies in weakly
interacting van der Waals solids and sources transient localization of the
charge carriers. The molecular vibrations that drive transient localization
typically operate at low-frequency (< a-few-hundred cm-1), which renders it
difficult to assess them experimentally. Hitherto, this has prevented the
identification of clear molecular design rules to control and reduce dynamic
disorder. In addition, the disorder can also be external, being controlled by
the gate insulator dielectric properties. Here we report on a comprehensive
study of charge transport in two closely related n-type molecular organic
semiconductors using a combination of temperature-dependent inelastic neutron
scattering and photoelectron spectroscopy corroborated by electrical
measurements, theory and simulations. We provide unambiguous evidence that ad
hoc molecular design enables to free the electron charge carriers from both
internal and external disorder to ultimately reach band-like electron
transport.
