Radically Tunable n‑Type Organic Semiconductor via Polymorph Control

Polymorphism has emerged as an important
design consideration in
organic semiconductors (OSCs). Previously, in many OSCs, even small
changes in molecular stacking can cause drastic changes to the optical
and electronic properties. However, investigation into n-type semiconductors
has significantly lagged behind their p-type counterparts. In this
work, we present the prolific polymorphism of 2-dimensional quinoidal
terthiophene (2DQTT-o-B) and systematically investigate each of 5
polymorphs, 3 of which have been previously unreported. Grazing incidence
X-ray diffraction provided a key method to understanding the structure
of each polymorph. Via the polymorphic transitions mapped, we tuned
the electron mobility by 5 orders of magnitude, from 5.63 × 10–5 to 0.22 cm2 V−1 s–1. These were accompanied by modifications to the optical
properties, namely we observed substantial differences in the refractive
index noted by intensity differences under polarized optical microscopy
and a large shift in optical band gap from 1.18 eV up to 1.40 eV.
Finally, we suggest that changes to these properties may be related
to the unique quinoidal to aromatic transition observed in quinoidal
molecules.