  Charge transport in organic semiconductors is notoriously extremely sensitive
to the presence of disorder, both intrinsic and extrinsic, especially for
n-type materials. Intrinsic 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 extrinsic, 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
intrinsic and extrinsic disorder to ultimately reach band-like electron
transport.
