Hyperspectral Imaging at High Spatial and Temporal Resolution: A Case for Hybrid Perovskite

Hybrid organic-inorganic perovskites have gained significant interest because of their outstanding optoelectronic properties, which have led to enhanced performance in applications such as photovoltaics and light-emitting diodes. However, their properties are highly sensitive to chemical composition and crystal structure, affecting exciton binding energy, charge carrier generation and recombination at the micron scale. Most investigations on the microstructure of hybrid perovskites have centered on thin films having small grains (typically, < 1 µm²), where spatially averaged measurements are linked indirectly to grain boundary density and its impact on degradation, trap formation, and charge transport. With novel synthetic strategies that yield grains that are > 100 µm², there is a unique opportunity to understand grain boundaries modification of optoelectronic properties at the single boundary level.

This dissertation focuses on understanding grain boundaries of methylammonium lead iodide at various spatial and temporal resolutions to reveal the impact of single grain boundaries on charge-carrier recombination, long-term film stability, and nonlinear optical responses. In Chapter 2, we use hyperspectral electron cathodoluminescence microscopy, coupled with matrix decomposition techniques to map the degradation process as a function of grain boundary and intra-grain structures, highlighting the spatial distribution of intermediate degradation phases. In Chapter 3, a novel optical photon-correlation microscope is developed to investigate, at the single grain level, the effect of interfacial boundaries on charge carrier dynamics and transport behavior with high temporal resolution. In Chapter 4, we develop an interferometric femtosecond frequency-resolved autocorrelation microscope to investigate how grain boundaries modify the nonlinear optical coupling of excited states, revealing that hybrid perovskites nonlinear response is robust to boundaries, as also supported via density matrix simulations.

Overall, this dissertation reveals that grain boundaries in large grains act as focal points for degradation, function as defect-related recombination centers, and hinder charge carrier diffusion; however, their minimal impact on nonlinear optical coupling highlights promising opportunities for multiphoton absorption applications.