Date of Award
Doctor of Philosophy (PhD)
Chemistry & Biochemistry
David M. Walba
In a simplified picture of an organic photovoltaic active layer the molecular orbital energies are used to predict photo-induced charge transfer between potential electron donors and acceptors, a crucial step in the generation of electrical power from light. Yet often systems that are projected to work by this metric, in fact, do not work; perhaps there is more to it. The work presented in this thesis aims to understand how solid-state microstructure ultimately affects photoinduced charge transfer. Two distinctive small molecule electron donors are presented and are determined to undergo microstructure-modulated charge transfer for different reasons. Time-resolved microwave conductivity is used to detect photogenerated charges and powder x-ray diffraction, transmission electron microscopy, and solid-state spectroscopy are used to elucidate the film microstructure. In the first system presented, a helical nanofilament heterojunction is shown to yield more charge transfer than a lamellar structuring of the same components; a pathway for charge recombination is proposed to compete with charge generation. In the second system, a series of PBTTT-inspired small molecules with varied alkoxy tail lengths demonstrates systematic differences in the solid-state microstructure, photophysics, and charge transfer driving forces and consequently yield. In both cases it is conclusive that solid-state microstructure has considerable effect on the photophysical properties and charge transfer in organic donor-acceptor blends.
Callahan, Rececca A., "Microstructural Effects on Charge Transfer in Small Molecule Heterojunctions: A Tale of Tails" (2014). Chemistry & Biochemistry Graduate Theses & Dissertations. 115.