Structure-Function Relationships in Semiconducting Polymers for Organic Photovoltaics
The major body of this work investigates how the chemical structure of conjugated polymers relates to the fundamental operating mechanism of organic photovoltaic devices. New conjugated polymers were characterized and their optical and electronic properties tested and correlated with their power conversion efficiencies as the active layer in polymer solar cells. From these experiments general structure/function relationships are drawn with an eye toward developing universal guidelines for conjugated polymer design and synthesis.
Starting with light absorption, three major steps in the photovoltaic mechanism are examined. First, photogeneration of excited states and the migration of these states through the active layer are correlated to the polymeric backbone chemistry and the resulting device performance. Next, separation of these excited states at an interface between electron donors and electron acceptors is examined as a function of donor-acceptor distance and active layer dielectric constant. These two variables were tuned by chemical modification of polythiophene side groups. Third, charge carrier conduction is related to both polymer electronic states and to solid-state packing morphology. Design principles for effective conduction of both holes and electrons are outlined and the ambipolar nature of conjugated organic materials is discussed.
In the final chapters, the solid-state polymer morphology in a solution processed thin film is examined. The impact that this morphology has on all steps in the photovoltaic mechanism is highlighted. How chemical modification of the polymer can influence this packing structure is examined as well as how new fabrication procedures can be used to pre-form nanostructured materials in solution before thin film deposition.