Title: Transport Phenomena in Lead Halide Perovskites and Layered Materials
Description: This thesis focuses on the electrical and thermal transport properties of two distinct systems: lead halide perovskites and layered materials. While unrelated, each system relies on diffusion phenomena in several ways. The first part of this work therefore explores particulate, charge carrier and thermal diffusion to establish a framework on which the rest of this thesis lays. In this first section, an introduction to the many measurement techniques are also included. The interested reader and future members of the lab will hopefully find this as a useful primer and can also find relevant and practical information involving the manipulation of some of these instruments in the Appendix as well. The second part of this thesis focuses entirely on lead halide perovskites. Despite its complex nature, or perhaps because of it, lead halide perovskites have recently enjoyed increasing attention from the scientific community at large for photovoltaic, thermoelectric and optoelectronic applications.

Although photovoltaic efficiencies over 20 % have been reported and continue to rise, very little is still understood about the mechanism of transport within the system as a whole. Debates on improving performances have focused primarily on the A-type cation in the APbX3 perovskite structure, often pointing to the organic cation as the magical ingredient that lends lead halide perovskites their superpowers. We explore this notion by studying the diffusion lengths of charge carriers and mean free paths of phonons in a series of lead halide perovskites, focusing both on the A-type cation and the halide anion composition. Using several optical and optoelectronic techniques, we show that that the composition of the A-type cation has only a secondary effect on thermal and charge carrier transport, and note that the halide is a stronger influencing factor for both means of transport. We deconstruct the transport distances into individual contributions from speed and lifetime, and note differences not only across the series of perovskites but also between charge carrier types, ultimately allowing us to suggest areas of improvement for future photovoltaic and thermoelectric device designs.

Finally, we begin the exploration of the interplay between structure and transport through a detailed study of the crystal structure via SCXRD as well as the transport phenomena, both as a function of temperature. The third and final part of this thesis shifts gears and looks at the work that we’ve done with layered materials and intercalation. The intercalation of layered materials is a time-honored tradition in chemistry and has proven to be an effective and reversible doping method for many solid-state materials. This sections begins with a discussion of more traditional materials and the development of techniques within our lab that can now be used to intercalate mesoscopic samples electrochemically. We then expand this study to include van der Waals heterostructures, showing for the first time ever, the intercalation of a heterointerface of this nature. We then conclude with preliminary work that has been done to extend the traditional notions of layered materials and their intercalation to superatomic structures. Both of these systems represent a path to new class of exciting and yet-to-be-studied materials.