Optoelectronic Properties of Microstructures of 3D and 2D Hybrid Perovskites

Solution-processed hybrid organic-inorganic perovskites have attracted great interest from both academia and industry. This class of materials possesses outstanding electrical and optical properties such as high absorption coefficient, large carrier diffusion lengths, and high photoluminescence quantum yield. Changing the chemical compositions or the dimensionality of these materials provides additional knobs for band gap tunability. Many studies have now shown the promise of this material system as viable for applications in photovoltaics, light-emitting diodes, photodetectors, lasers, and transistors.

However, the performance of these devices depends critically on the techniques employed for the synthesis of hybrid organic-inorganic perovskites. Different synthesis techniques result in different thin film morphologies, or different microstructures. So, it is crucial to precisely study and subsequently control the nucleation and crystal growth process, which dictates the overall thin film morphology. Additionally, the nature of charge carriers and the lifetime of the excited states present in these hybrid organic-inorganic materials will dictate the performance of devices. For example, free electrons and holes are ideal charge carriers required for solar cells while Coulomb bonded excitons are ideal for light emitting diodes.

In this dissertation, we synthesize different microstructures of hybrid organic-inorganic lead halide perovskites, namely methyl ammonium lead iodide (MAPbI₃), and 2D layered Ruddlesden-Popper (RP) phase perovskite (BA₂MA₂Pb₃I₁₀). Free electrons and holes are the charge carriers in the case of MAPbI₃, while excitons are dominant in the case of 2D-layered BA₂MA₂Pb₃I₁₀. We developed a novel fabrication strategy to control and grow various phases of MAPbI₃ on the same substrate, allowing us to systematically study the carrier dynamics over a broad spectral and temporal range using ultrafast spectroscopy. All these phases possess similar dynamics and spectral features highlighting the rapid development of the electronic and optical properties during the nucleation and crystal growth stages, occurring often within less than 5 seconds. We explored the emission properties of these phases using several characterization techniques. Angle-resolved cathodoluminescence polarimetry was employed to study the impact of grain boundaries on the polarization state and angular profile of the emission process. We believe that this study serves as blueprint for hybrid perovskite-based flat optics.

Manipulating the orientation of perovskites layer in 2D BA₂MA₂Pb₃I₁₀ perovskite with the goal to control carrier interaction, we study the dynamics of the various excited states — including edge states that have recently shown to dissociate excitons rapidly. With the layers perpendicularly oriented thin film, we clearly show the presence of edge states whereas for the parallel oriented case, we demonstrate that the edge state is suppressed. We used pulse and continuous-wave laser excitation to study the edge states signatures and the properties of the excitons. In addition, the ultrafast dynamics of charge carriers in these films were studied, which further supports the existence of edge states. This study will pave the way for designing light emitting diodes with not only the appropriate hybrid perovskite structures, but with the correct orientation as well.