Fluorescence microscopy of materials with energy applications




Geberth, Geoffrey Thomas

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One of the largest drivers of modern materials research is the advancement of renewable energy, particularly solar power. This objective has seen the rise of several different materials systems, each with their own advantages and disadvantages. Before any of these alternatives to traditional inorganic semiconductors can be effectively utilized at a commercial scale, they must first be understood at the fundamental level such that they can be tuned through utilization of the structure-property relationship. This dissertation describes the use of fluorescence-based microscopy techniques to explore material systems relevant to energy production at the smallest possible levels, ranging from single molecules and aggregates to small scale surface structures in order to unravel the microscopic heterogeneities that influence photophysical performance. First, two different conjugated polymers were studied. Poly(3-(2'-methoxy-5'-octylphenyl)thiophene) (POMeOPT) aggregates were studied in bulk solution in order to probe charge transfer character in the excited state. Nonpolar solution environments led to more than hundredfold increases in the fluorescence intensity of this material, demonstrating the importance of the environment in manipulating the photophysics of conjugated polymers and illustrating the role the charge transfer state plays in the excited state. Next, a polyphenylenevinylene (PPV) based block copolymer designed for controlled folding was examined at the single molecule level with excitation polarization spectroscopy, revealing not only the robustness of the folding functionality, but also that the resulting folds spaced the chromophores far enough apart to severely limit interactions between them. Shifting to a different class of energy materials, single perylene diimide (PDI) aggregates were then formed with solvent vapor annealing (SVA) and studied with fluorescence microscopy. These experiments revealed a vast heterogeneity amongst small aggregates as well as provided strong evidence for emissive excited states with triplet character occurring even in small aggregates. The final class of material studied was a Ruddlesden-Popper phase quasi-2D organolead halide perovskite. Confocal fluorescence microscopy was utilized to image film degradation in the presence of moisture, and provided insights into the mechanism behind moisture-driven surface crystallite growth. Taken together these experiments demonstrate the power of fluorescence microscopy to advance the understanding of energy materials systems by examining small scale heterogeneity.



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