Date of Award
Doctor of Philosophy (PhD)
Chemistry & Biochemistry
David M. Walba
Organic electronic materials offer several advantages when compared to inorganic materials, but they suffer from low charge carrier mobility. Two major factors hindering effective charge transport in organic materials are: 1) effective wavefunction overlap in organic crystals and 2) the domain morphology of thin films. Charge transport in organic materials occurs via a hopping mechanism along the conjugated π system. Often, rigid, aromatic organic materials crystallize in a herringbone, edge-to-face orientation, limiting π-π stacking and decreasing charge carrier mobility. Face-to-face orientation of aromatic rings decreases intermolecular π-π distances and increases wavefunction overlap. Control of the crystal structure can be achieved to some extent by tuning structural features of the molecule, like increasing the ratio of carbon atoms to hydrogen atoms in the aromatic rings; this is often achieved by introducing heteroatoms like sulfur and oxygen into the aromatic ring structure.
Thin films of organic materials often contain many unaligned domains; this is caused by rapid crystallization. Control of the domain morphology of thin films has been shown to increase charge carrier mobility by 6 orders of magnitude for thin films of the same material. Liquid crystal phases allow a slow process of crystallization, whereby the molecules in a thin film can be slowly aligned into a monodomain before crystallization. The crystal-smectic phases, like smectic E, are particularly attractive for this strategy due to their high degree of intermolecular order.
This project describes the synthesis and characterization of organic semiconductors designed to exhibit short π-π distances and highly ordered crystalsmectic phases to obtain thin films with high charge carrier mobility. The n,2-OBTTT series contains 15 newly designed and synthesized mesogens. The liquid crystal and solid crystal structures of these mesogens are examined and deposition conditions are optimized for the production of highly ordered, monodomain thin films.
Springer, Mike T., "Utilizing Liquid Crystal Phases to Obtain Highly Ordered Thin Films for Organic Electronics" (2014). Chemistry & Biochemistry Graduate Theses & Dissertations. 4.