Browsing by Author "Hill, Andrew Hinson"

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Characterizing excited state dynamics and carrier transport in hybrid organic-inorganic lead halide perovskites via ultrafast microscopy
(Montana State University - Bozeman, The Graduate School, 2018) Hill, Andrew Hinson; Chairperson, Graduate Committee: Erik Grumstrup; Kori E. Smyser, Casey L. Kennedy, Eric S. Massaro and Erik M. Grumstrup were co-authors of the article, 'Ultrafast microscopy of methylammonium lead iodide perovskite thin films: heterogeneity of excited state spatial and temporal evolution' which is contained within this thesis.; Kori E. Smyser, Casey L. Kennedy, Eric S. Massaro and Erik M. Grumstrup were co-authors of the article, 'Screened charge carrier transport in methylammonium lead iodide perovskite thin films' in the journal 'Journal of physical chemistry letters' which is contained within this thesis.; Casey L. Kennedy, Eric S. Massaro and Erik M. Grumstrup were co-authors of the article, 'Perovskite carrier transport: disentangling the impacts of effective mass and scattering time through microscopic optical detection' in the journal 'Journal of physical chemistry letters' which is contained within this thesis.; Casey L. Kennedy and Erik M. Grumstrup were co-authors of the article, 'Determining the effects of A-site cation substitution on the optical response and transport properties of lead tri-bromide perovskites' submitted to the journal 'Journal of physical chemistry letters' which is contained within this thesis.
Lead tri-halide perovskites have recently emerged as cost-effective alternatives to silicon for use in photovoltaic devices. A large contributor to their reduced cost compared to silicon is the simple solution processed techniques employed in their fabrication. While these methods can produce effective photovoltaic devices, heterogeneity endemic to solution processing makes characterization of tri-halide perovskites a challenging task. Most spectroscopic techniques use large sample interrogation volumes which often results in the indiscriminate sampling of grain boundaries and other heterogeneities which impact the spectroscopic observable. To circumvent this issue, pump-probe microscopy is used to dramatically shrink the sample volume, reducing the contributions from chemical and morphological heterogeneities and providing a more accurate measure of the sample's inherent properties. This work begins with a study of the recombination and transport dynamics methylammonium lead tri-iodide (MAPbI 3) perovskite. After identifying the main recombination pathways and contributions to the transient signal, experimental focus is shifted to the transport properties of MAPbI 3. The key contributing factors to the high diffusivities reported in MAPbI 3 are found to be strong electron-phonon coupling and a high static dielectric constant which serves to screen carriers from interactions with charged defects and other carriers. Then the development a new all-optical method capable of uniquely determining the two fundamental parameters that govern carrier transport (the mean scattering time and optical mass of photogenerated carriers) is reported. This method was applied to a series of different perovskite materials including MAPbI 3, cesium lead bromide di-iodide (CsPbBrI 2), methylammonium lead tri-bromide (MAPbBr 3), formamidinium lead tri-bromide (FAPbBr 3), and cesium lead tri-bromide (CsPbBr 3). The results of these experiments have led to the characterization of the role each perovskite constituent (namely, the identity of the organic cation and the halide stoichiometry) plays in determining the transport properties of the resulting material. The work presented in this dissertation characterizes the transport properties of lead halide perovskites. Measurements collected across multiple discrete and highly crystalline domains of multiple perovskite species have helped establish a relationship between the functionality and the local structure of these materials. Additionally, the design and first application of a new methodology to disentangle the effects of mean scattering time and the photogenerated carrier mass on carrier transport is reported. This technique will not only continue to aid in the characterization of lead-halide perovskites but will likely also see use on a host of other material systems to advance understanding of carrier transport in a variety of materials.