Coherence in Polycrystalline Thin Films of Twisted Molecular Crystals

Helicoidal crystallites
in rhythmically banded spherulites manifest
spectacular optical patterns in small molecules and polymers. It
is shown that concentric optical bands indicating crystallographic
orientations typically lose coherence (in-phase twisting) with growth
from the center of nucleation. Here, coherence is shown to increase
as the twist period decreases for seven molecular crystals grown from
the melt. This dependence was correlated to crystallite fiber thickness
and length, as well as crystallite branching frequency, a parameter
that was extracted from scanning electron micrographs, and supported
by numerical simulations. Hole mobilities for 2,5-didodecyl-3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione
(DPP-C12) measured by using organic field-effect transistors demonstrated
that more incoherent boundaries between optical bands in spherulites
lead to higher charge transport for films with the same twist period.
This was rationalized by combining our growth model with electrodynamic
simulations. This work illustrates the emergence of complexity in
crystallization processes (spherulite formation) that arises in the
extra variable of helicoidal radial twisting. The details of the patterns
analyzed here link the added complexity in crystal growth to the electronic
and optical properties of the thin films.