Temperature-Dependent Behavior of Organic Semiconductors

Many properties of organic semiconductors, such as charge transport or optical absorbance, are governed by the solid-state organization of the material, which is in turn determined by its chemical structure. Developing a fundamental understanding of molecular features that can be used to control the solid-state properties of organic semiconductors is key to designing materials with improved performance or unique new properties. In my research, I have employed a diverse set of techniques, such as theory, experiment and device characterization, to better understand the relationship between molecular structure and solid-state properties of molecular organic semiconductors. Herein, the relationship between the chemical structure of molecular and oligomeric organic semiconductors and their response to thermal stresses and changes in temperature are described. First, I will describe the tolerance of an organic field-effect transistor to operation at temperatures up to 200 ºC and over multiple high-temperature cycles, with only slight changes in charge carrier mobility and properties. The active layer of this device is composed of an oligomeric organic semiconductor whose structure is very similar to that of two well-known ambipolar conjugated polymers. Heating to 200 ºC or above results in appearance ambipolar charge transport in the oligomeric active layer as well. Then, the solid-state organization of a nearly isostructural series of oligomeric organic semiconductors–including the material discussed above–is characterized. As-cast, the first half of the series orients edge-on relative to the substrate while the second half is oriented face-on. This difference is likely due to kinetic trapping during the spin-coating process because melt-annealing of thin films of the four different materials results in edge-on orientation of crystallites for all four molecules. Finally, I describe a combined theoretical and experimental approach to probing aspects of molecular organization and topology based on solid-state NMR. Using this approach, a previously hypothesized change in molecular shape upon crystallization of a molecular organic semiconductor is confirmed. The novel molecular design element of a tail-to-tail couple hexyl-bithiophene core removes the trade-off between solubility and solid-state order in molecular semiconductors