posted on 2023-11-14, 19:04authored byLaura
E. Dickson, Rosemary R. Cranston, Hao Xu, Sufal Swaraj, Dwight S. Seferos, Benoît H. Lessard
Poly(3-hexylthiophene)
is one of the most prevalent and promising
conjugated polymers for use in organic electronics. However, the deposition
of this material in thin films is highly dependent on the process,
such as blade coating versus spin coating and material properties
such as molecular weight. Typically, large polymer dispersity makes
it difficult to isolate the effect of molecular weight without considering
a distribution. In this study, we characterize oligothiophenes of
exactly 8, 11, and 14 repeat units, which were deposited into thin
films by varying blade coating conditions and postdeposition annealing.
From synchrotron-based grazing incidence wide-angle X-ray scattering
(GIWAXS), scanning transmission X-ray microscopy (STXM) and near-edge
X-ray absorption fine structure spectroscopy (NEXAFS), Raman microscopy,
optical microscopy, and X-ray diffraction (XRD), it was suggested
that higher molecular weight polymers exhibit a fast-forming crystalline
polymorph (form-1) while low molecular weight polymers exhibit a slow
forming polymorph (form-2) with large domain boundaries. As molecular
weight is gradually increased, the polymorph formed transitions from
form-1 and form-2, where 11 repeat unit oligomers display both polymorphs.
We also found that processing conditions can increase the formation
of the form-2 polymorph. We also report improved organic thin film
transistor (OTFT) performance when form-1 is present. Overall, oligothiophene
polymorph formation is highly dependent on the molecular weight and
processing conditions, providing critical insight into the importance
of polymer weight control in the development of thin-film electronics
based on conjugated polymers.