Structure-property relationships for organic semiconductor materials: From holes to electrons to ambipolar behavior in high performance acenes
Organic transistors offer the potential for low-cost, large area, flexible electronic applications. However the structure-property relationships that govern thin film packing and hence mobility are not clear, even for small molecules. In this dissertation, I investigate these relationships by designing, synthesizing and characterizing a family of acenes. I have designed a new conjugated core, tetraceno[2,3-b]thiphene that is more stable and easily functionalized than pentacene, the workhorse of organic transistors, but retains its high mobility. By incorporating various functional groups on this core, I am able to show that by increasing the van der Waals forces between molecules, intermolecular interaction is increased, and hence thin film mobility. By adding other functional groups like halogen atoms, I am able to induce electron transport by lowering molecular orbital energy levels. Hole and electron mobilities from 0.l-0.6cm2/Vs are measured for a series of acenes. This is interesting, because ambipolar organic thin film transistors consisting of a single material having high and balanced hole and electron mobilities are rare, with pentacene being the sole exception. Lastly, a series of 20 acenes are examined with respect to their frontier orbital energy levels and carrier type. In accordance with theory, molecules with higher energies only transport holes, while molecules relatively low in energy (like perfluoropentacene) are solely electron transporting. Molecules with energy levels in between are ambipolar. This is the first time the transition between charge carrier type is directly correlated with the measured HOMO/LUMO levels in the field of organic transistors.
0542: Chemical engineering