Extremely Small and Incredibly Fast: Combining Spectroscopy and Microscopy to Reveal Local Excited State Dynamics in Disordered Semiconductors

Despite years of intense study, even today there are many novel semiconductor materials whose unique properties raise fundamental scientific questions about the relationship between structure and function. Two examples are organic semiconductors and semiconducting nanostructures. Organic semiconductors can be solution processed at room temperature, leading to thin films that are flexible, inexpensive, and more sustainable when compared to inorganic alternatives. Yet solution processing leads to complex microstructures, and the resulting polymorphism, interfaces, and defects all affect the material's behavior in complex ways. In semiconducting nanostructures, the novel behavior arises due to their nanometer-scale dimensions. Confinement and enhanced surface interactions affect the electronic structure and dynamics in sometimes unexpected ways and can produce new physics.Chapters 1 and 2 provide the necessary background for this dissertation. Chapter 1 provides an overview of semiconductors and what makes organic semiconductors distinct. The dynamic processes that are relevant for subsequent chapters are introduced. Chapter 2 describes a home-built transient absorption (TA) microscope that was used in each project discussed here. TA is an important tool for studying a material's ultrafast excited state dynamics, and by combining TA with microscopy we have been able to investigate how those dynamics depend on the local morphology.Chapters 3 and 4 discuss the use of TA microscopy to probe the ultrafast electronic dynamics of individual crystalline domains of organic semiconductors. In Chapter 3 we find that different domains of the material diF-TES-ADT can display different dynamics. We fit a kinetic model to the observed behaviors, quantify the amount of heterogeneity, and propose that the observations are due to polymorphism. In Chapter 4 we study singlet fission in single crystalline domains of the material TIPS-Pentacene. During singlet fission, a singlet exciton sp