posted on 2020-12-29, 12:35authored byKaichen Gu, Yucheng Wang, Ruipeng Li, Esther Tsai, Jonathan W. Onorato, Christine K. Luscombe, Rodney D. Priestley, Yueh-Lin Loo
The
performance of electronic devices comprising conjugated polymers
as the active layer depends not only on the intrinsic characteristics
of the materials but also on the details of the extrinsic processing
conditions. In this study, we examine the effect of postdeposition
thermal treatments on the microstructure of poly(3-hexylthiophene)
(P3HT) thin films and its impact on their electrical properties. Unsurprisingly,
we find thermal annealing of P3HT thin films to generally increase
their crystallinity and crystallite coherence length while retaining
the same crystal structure. Despite such favorable structural improvements
of the polymer active layers, however, thermal annealing at high temperatures
can lead to a net reduction in the mobility of transistors, implicating
structural changes in the intercrystallite amorphous regions of these
semicrystalline active layers take place on annealing, and the simplistic
picture that crystallinity governs charge transport is not always
valid. Our results instead suggest tie-chain pullout, which occurs
during crystal growth and perfection upon thermal annealing to govern
charge transport, particularly in low-molecular-weight systems in
which the tie-chain fraction is low. By demonstrating the interplay
between intracrystallite and intercrystallite structuring in determining
the macroscopic charge transport, we shed light on how structural
evolution and charge-transport properties of nominally the same polymer
can vary depending on the details of processing.