Date of Award

Spring 2010

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Micro and Nanoscale Systems

First Advisor

Scott A. Gold

Abstract

Semiconducting and optoelectric conjugated polymers have potential in micro and nano-electronic applications. Their widely tunable physical conformations and orientations make these polymers ideal material for engineering small scale devices. The polymers have been incorporated into several electronic devices including light-emitting diodes, solar cells, and field-effect transistors. Widespread adoption of these materials will not be a reality until the issues of poor device performance, short lifespans, and device degradation are resolved.

Nanostructures have been demonstrated to have improvements in molecular ordering and electronic transport. In the work presented here, tubular nanostructures of conjugated polymers fabricated by the template wetting nanofabrication process are analyzed for their structural and electronic properties. Analysis focused on the optical absorbance of these materials and the effects of confinement, filtration, and solvent choice have on disordered and ordered polymers. Other analyses include the effects of polymer blending, orientation, and solvent evaporation determined from vibrational spectroscopy, thermogravimetric analysis, and mobility characteristics.

Improvements in optical bandgap of MEH-PPV and P3HT ranging from ∼0.12 eV to 0.05 eV, respectively, were found for nanotubes cast chloroform. The improvements in bandgap from all solvents were found to be, on average, 0.08 eV for MEH-PPV and 0.05 eV for P3HT. Enhancement due to nanostructuring of P3HT were found to be independent of pore size but dependent on molecule size as low molecular weight fractions obtained from DC filtration were found to have spectrum similar to drop-cast films. Solvent choice in MEH-PPV was found to be highly important as the non-aromatic solvents THF and chloroform were found to have dichroic ratios greater than 10 indicating a highly aligned structure that increased the optical bandgaps and mobilities of the samples. Order was shown to also be dependent on the gel solvent evaporation time fraction which was 0.17 for chloroform and 0.58 for THF. Solvent choice for P3HT was shown to increase when cast from THF with a dichroic ratio ∼16 and a t gel/ttotal of 0.75. The order in THF samples of P3HT, however, did not noticeably affect the optical properties.

This dissertation shows that the nanostructuring of amorphous and semi-crystalline conjugated polymers leads to increased order, enhanced optical properties, as well as longer effective conjugation lengths that have not been noted in device created from these polymers before.

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