Mechanistic Insights into Lead Halide Perovskite Transformations

Lead halide perovskites have been shown to be a promising class of semiconductors for use in light-harvesting and light-emitting applications. While these materials have been employed in high efficiency devices in laboratory settings, instability issues inhibit the adoption of perovskites into market applications. This thesis investigates the transformations perovskites undergo that lead to operational instability. The research presented in this thesis approaches the study of perovskites through optical measurements and materials characterization methods. Using these techniques, both cesium and methylammonium lead halide perovskites are studied.

Physical changes that occur in lead halide perovskites are investigated through three different studies.Cesium lead bromide nanocrystals are studied under elevated temperatures and the transformation process they undergo to change into bulk perovskites films is monitored. Physical changes in perovskites are further studied by investigating the movement of halide ions between perovskite heterostructures. This thrust of research focuses on both cesium and methylammonium based perovskites. The movement of halide ions in these two species are compared.

The excited state properties are perovskites are also elucidated using transient absorption spectroscopy. The underlying theory of transient absorption spectroscopy is discussed as well as an explanation of interpretation of transient absorption data. Transient absorption spectroscopy is used to find optical properties of perovskite nanostructures and a band filling model is proposed.

Inhibiting anion migration between perovskite nanocrystals through PbSO₄-oleate capping is studied. The optical properties of the novel nanostructure made with PbSO₄-oleate capping are explored. The ability of these nanostructures to inhibit anion movement in thin films is also discussed. The potential for tandem photovoltaic devices of this material is also elucidated using transient absorption spectroscopy to study the excited state.