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Open Access Publications from the University of California

Synthetic Control of Organic Semiconductor Excited States

  • Author(s): Clem, Tabitha Ann
  • Advisor(s): Frechet, Jean M.J.
  • et al.

This work describes the synthesis, and study of new organic and organometallic compounds designed to possess modified excited states for applications in electronic devices, with a particular focus on photovoltaics. The first chapter introduces the origin of semiconducting behavior in conjugated polymers, and how the properties of these semiconductors ultimately impact the fundamental physical processes occurring in organic electronic devices. The second chapter describes the synthesis, optoelectronic properties, and photovoltaic performance of a small molecule oligothiophene analog that possesses a considerable redshift in absorption relative to an unmodified oligothiophene. This redshift in absorption is the result of using a fused thiophene unit that acts as a driving force to enable dearomatization of the oligothiophene unit. The third chapter describes the preparation of wide band gap, phosphorescent cyclometalated platinum polymers and their application to bilayer photovoltaic devices where an increase in photocurrent is observed, and is attributed to an increase in the exciton diffusion length arising from formation of triplet excitons. The fourth chapter presents further development of cyclometalated platinum polymers, with a focus on tuning the photophysical properties and studying these materials in bulk heterojunction photovoltaic devices. The fifth chapter presents a study of cyclometalated platinum and iridium small molecules, describing their photophysical properties and solid-state structure, and exploring the application of these complexes to light emitting diodes. The final chapter describes the development of polythiophenes containing highly polarizable functional groups, with particular attention to the effect of these modifications on the optoelectronic properties and the dielectric constants of the resulting polymers. Together, these chapters present a study encompassing the modification of excited states in organic semiconductors, beginning with light absorption, continuing to modification of the spin state of the exciton, and ultimately modifying the dielectric constant of the active layer materials to affect charge separation and transport. Utilizing materials developed based on these principles provides an attractive new route to altering and enhancing the basic steps involved in the operation of organic electronic devices, ultimately producing both an improved understanding of the mechanism of device operation, and improved device performance.

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