  There is compelling evidence that charge carriers in organic semiconductors
(OSs) self-localize in nano-scale space because of dynamic disorder. Yet, some
OSs, in particular recently emerged high-mobility organic molecular crystals,
feature reduced mobility at increasing temperature, a hallmark for delocalized
band transport. Here we present the temperature-dependent mobility in two
record-mobility OSs: DNTT (dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]-thiophene),
and its alkylated derivative, C8-DNTT-C8. By combining terahertz
photoconductivity measurements with fully atomistic non-adiabatic molecular
dynamics simulations, we show that while both crystals display a power-law
decrease of the mobility (\mu) with temperature (T, following: \mu \propto
T^(-n)), the exponent n differs substantially. Modelling provides n values in
good agreement with experiments and reveals that the differences in the falloff
parameter between the two chemically closely related semiconductors can be
traced to the delocalization of the different states thermally accessible by
charge carriers, which in turn depends on the specific electronic band
structure of the two systems. The emerging picture is that of holes surfing on
a dynamic manifold of vibrationally-dressed extended states with a
temperature-dependent mobility that provides a sensitive fingerprint for the
underlying density of states.
