The Exchange Mechanism and Exciton Migration in Organic Semiconductors

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Nontechnical Abstract:Organic semiconductors remain the focus of a broad range of next generation optical and electronic materials applications. Compared with inorganic semiconductors such as silicon, the molecular variety available for organic semiconductors, thanks to synthetic organic chemistry, offers near limitless potential. When combined with materials science, this rich molecular tapestry suggests availability of extraordinary control over optical and mechanical properties, materials cost, environmental impact, and simplicity of device construction. Efficient transfer of energy in the solid state is one of the fundamental aspects of both light-emitting and light harvesting devices. A better understanding of the relationship between molecular design and the transfer of energy in organic materials enables design of a broad range of more efficient next-generation optical and electronic devices. This research targets one of the energy transfer mechanisms that historically received less attention, a mechanism known as exchange. This mechanism is operative in all molecular systems, and a better understanding of how it contributes to energy transfer in the solid state will open the door to a broad range of new, and more efficient organic electronics. This project includes introducing thousands of third through sixth grade students from disadvantaged socioeconomic backgrounds to the fundamental concept of energy and the basic idea of pursuing a college education in a STEM field.Technical Abstract:The first step in exciton transport is inter-chromophore energy transfer. Historically, simplicity has promoted binary association of emissive materials with an inductive transfer mechanism and non-emissive materials with an exchange mechanism. This distinction has not always been consistent with observation. This research is developing a better understanding of the role of each mechanism. Experimental accessibility has focused previous attention on luminescent systems. Realizing that the high molecular density inherent to devices facilitates exchange, and acknowledging that exchange does not have the same limitations as induction, this work is filling gaps in the understanding of transport in non-luminescent materials. At the same time, this work is elucidating the combined roles of both mechanisms. Employing methodologies to probe exciton diffusion in both luminescent and non-luminescent materials, this work is elucidating ways to exploit combined approaches to transport, quantifying parasitic exciton quenching via annihilation and polaron trapping, and measuring the effects of exciton fission on energy transport. Taken in aggregate, this research is providing a significant contribution to the understanding of exciton transport in organic semiconductors and developing design principles to enhance the next generation of light harvesting and light-emitting devices. The project is simultaneously training multiple graduate students in a wide range of state-of-the-art physical and materials science theory and experimental techniques.