UC Berkeley

Emerging semiconductors have begun to rival conventional silicon with breakthroughs in improved performance and novel and affordable processing techniques. As the applications for emerging semiconductors are explored, there is a clear need to understand the hierarchical nature of these materials to fully optimize their functionality. Consistently reproducingthe assembly and electronic functionality of organic semiconductors, for example, remains difficult given their propensity for variable molecular packing and morphology due to weak bonding interactions. As another case, metal halide perovskite semiconductors demonstrate intriguing photophysics that may change when confined in nanostructure forms. One technique alone cannot comprehensively interrogate the repercussions of changes in the molecular structure to the microstructure, morphology, and ensuing electronic functionality of a semiconductor. Only a multimodal characterization approach enables an extensive multiscale insight into the structure-function dynamics of semiconducting materials and devices.

Chapter 1 first introduces organic semiconductor properties and device applications. Common thin film processing methods and modification strategies are discussed along with the formation of crystalline microstructures in thin films. Next, a suite of characterization techniques that span multiple length and time scales are shared to inform on how one maystrategically combine individual techniques to characterize structure, morphology, and dynamics.

Chapter 2 describes a multimodal imaging study of organic semiconductor, rubrene, thin films. Scanning transmission X-ray microscopy reveals and quantifies the disappearance of orientational discontinuities in a hybrid crystalline microstructure, which is corroborated at higher spatial resolution with 4D-scanning transmission electron microscopy. In situ polarized optical microscopy during thermal annealing uncovers Arrhenius behavior for rubrene crystallization and that the finite substrate thermalization rate leads to the formation of the observed hybrid crystalline microstructure. This study emphasizes the role of temperature in organic semiconductor crystallization, and highlights the challenges and opportunities for thin film design protocols.

Chapter 3 focuses on characterizing the anisotropic behavior of inorganic lead halide perovskite (CsPbBr3) nanowires and nanowire bundles. Stroboscopic scattering interferometric microscopy shows exciton diffusion and trapping in the nanowire bundles. We use transient absorption microscopy to probe the polarization-resolved excited-state dynamics of thenanowire bundles. Coupled with polarization-resolved fluorescence, we determine that the splitting of the band-edge exciton states is driven by the shape anisotropy and a long-range exchange interaction. This study demonstrates the power of optical pump-probe microscopies to resolve charge transport and excited-state dynamics in individual structures.

This dissertation highlights the complexities of characterizing soft semiconductor thin films and nanostructures across multiple length and time scales. Despite the challenges in implementing a multimodal approach, the knowledge acquired about the semiconductors, such as defects in packing, grain boundaries, and charge carrier dynamics, is invaluable for guiding future design principles that incorporate semiconducting materials into novel technology.