UC Berkeley

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 splits into two triplet excitons, initially forming a short-lived and poorly understood correlated triplet pair. By exploiting the inherent anisotropy within an individual crystalline domain we obtain unprecedented insight into nature of the correlated triplet pair. We resolve a disagreement related to its formation timescale, quantify the triplet-triplet binding energy, and measure how the charge transfer character of the correlated triplet pair perturbs its electronic structure and hence absorption spectrum.

Chapter 5 takes a step back to study the self-assembly process in the organic semiconductor rubrene. We use a variety of spatially- and temporally-resolved techniques to study the growth and morphology of spherulites, a kinetically trapped polycrystalline structure that could allow for both rapid and uniform charge transport. We use time-resolved wide-angle in-situ X-ray scattering, wide-angle X-ray microdiffraction, and atomic force microscopy to identify shear strain localized along lines of crystalline misorientation. This strain templates upon nucleation and is kinetically trapped only at higher annealing temperatures, possibly promoting the spherulite structure. Steps to extend this project are also described, using scanning transmission electron microscopy to measure the crystal structure with nanoscale resolution and TA microscopy to connect this local structure with electronic dynamics.

Chapter 6 looks at nanowires of the inorganic perovskite CsPbBr3, in which quantum confinement along two dimensions changes the excited-state dynamics. We align many nanowires into micron-scale bundles, which allows them to be studied with optical techniques while still retaining anisotropic behavior. We use TA microscopy to study the polarization-resolved ultrafast electronic dynamics of the nanowires, and find among other things that the nanowire geometry splits the degeneracy of the lowest excited states. We also use stroboscopic interferometic scattering to observed exciton diffusion within nanowire bundles, and measure the diffusivity both along and between the nanowires.

Taken together, this dissertation illustrates specific examples of the relationship between semiconductor structure and function. Small changes in processing conditions can drastically affect morphology, and small changes in morphology, whether they be defects, interfaces, or tweaks to the unit cell, can drastically affect electronic properties. Understanding these relationships is vital to incorporating novel semiconducting materials into next-generation optoelectronic devices.