UNC-CH Physics and Astronomy PhD Defense

Mark Moog

“Poly(3-methylthiophene) Brushes: Structure, Morphology, and Electronic Transport”

Low charge carrier mobility is a major obstacle that limits the performance of organic electronic devices. For conjugated polymers, increasing the contribution from intramolecular charge transport channels to overall charge transport is expected to increase the mobility of the overall device. This is because the transport of charge carriers along conjugated polymer backbones is expected to be much faster than transport between the polymers. However, designing devices that take advantage of intramolecular charge transport has proven difficult, and in virtually all organic electronic devices charge must travel not only along individual polymer backbones, but must hop inefficiently between polymers as well due to film morphology.

Conjugated polymer brushes are films comprising densely packed conjugated polymers that are tethered to a substrate, causing the polymers to elongate creating a brush-like morphology. Due to their morphology, polymer brush devices are expected to exhibit enhanced intramolecular charge transport. Electronic devices can be made from polymer brush films by growing the polymer from a conducting substrate and attaching a metal electrode to the top of the brush film, forming the vertical electrode-polymer-electrode structure. These devices make it possible to investigate intramolecular charge transport processes, because each polymer which contacts the top electrode is also contacting the bottom electrode (i.e. the substrate).

Polythiophene is a model conjugated polymer that has received extensive research interest as an active layer in organic electronic devices due to their electronic properties and environmental and thermal stability. In this work the morphology and electronic properties of poly(3-methylthiophene) (P3MT) brushes are studied using the vertical electrode-polymer-electrode devices. The bulk resistivity of P3MT brush devices was found to be 1.4*10^5 Ω-cm, two orders of magnitude lower than that of spun cast films of comparable polythiophene films. The resistivity of the brush films has been analyzed on a per-molecule basis, and the resulting ‘molecular resistivity’ value of 180 GΩ/nm is comparable to those obtained from transport studies of molecular wires. The significantly reduced bulk resistivity along with a molecular resistivity on par with molecular wires indicate an enhanced contribution from intramolecular charge transport in conjugated polymer brush devices.