The Effect of Illumination on the Conformation and Thermodynamics of Conjugated Polymers

This dissertation explores the effects which illumination has on the structure and conformation of conjugated polymers, both in solution and the bulk. Conjugated polymers are an important class of optoelectronic polymeric materials which make up the active layer of key organic electronic devices such as organic photovoltaics, transistors, and light emitting diodes. Considering the strong link between polymer chain structure and device efficiency in these materials, a comprehensive understanding of certain experimental conditions which may influence this conformation and thusly alter the functionality of the devices predicated upon them is of vital importance.

The first part of this dissertation provides a background to conjugated polymers, their role in common electronic applications, and the known effects of illumination on polymeric materials. The next section outlines an initial exploration of light induced effects on benchmark conjugated polymers in solution elucidated via the use of small angle neutron scattering. The third chapter explores the structural evolution of solution gels formed from conjugated polymers at multiple length scales using small and ultra small angle scattering, as well as quantifying modifications to this structural progression brought on by white light exposure. The fourth and fifth chapters further expand our understanding of the proposed thermodynamic driving forces behind the light induced structural changes in conjugated polymer solutions first through alterations in solvent quality, and then through fine-tuning of incident light wavelength, exposure time, and intensity. Finally, the sixth section studies the self-assembly of conjugated polymer composite thin films cast from a single solution through neutron reflectivity, while additionally monitoring the effects of light exposure during the thermal annealing process upon the final film depth profile.

Ultimately, these works cumulatively provide strong evidence indicating that light exposure is in fact an important parameter with far-reaching implications upon the final structure of organic electronic materials. Overlooking illumination conditions and failing to provide a consistent ambient light environment throughout device fabrication will result in non-uniform chain conformations and layering architectures, inevitably impacting device performance. However, if properly understood and harnessed, these light-induced effects could make possible an entirely novel methodology for in-situ tuning of organic electronic device physical parameters.