Linear and Nonlinear Optical Properties and Ultrafast Dynamics of Hybrid Lead Halide Perovskites

Abstract

Hybrid lead halide perovskites with the chemical formula ABX3 have now been established as a class of materials with astonishing optoelectronic and photovoltaic properties ideal for applications ranging from X-ray scintillation detectors to thermoelectric materials. Using halide variation (I, Br, Cl) and dimensionality reduction (2D, 1D, and 0D), easy tunability in the bandgap can be incorporated over a significantly wide wavelength range. Compositional engineering brings about changes not only in the bandgap but in the linear and nonlinear optical optical properties. In linear regime, unique emissive properties are observed arising from the process of exciton self-trapping. Additionally, in nonlinear regime, potential applications in energy up-conversion and low-threshold lasing have been observed. This thesis focuses on understanding the underlying fundamental mechanisms of various exciting linear and nonlinear optical properties in lower dimensional hybrid lead halide perovskites and ultrafast carrier and phonon dynamics of 3D perovskite nanocrystals. We utilized a variety of ultrafast spectroscopic techniques to approach these processes, such as time-resolved terahertz spectroscopy, optical Kerr effect spectroscopy, and temperature-dependent photoluminescence spectroscopy. Understanding the photo-physics of semiconductors is crucial to their better utilization in photovoltaic and photocatalytic devices. Formamidinium-based lead bromide nanocrystal films were studied to explore the carrier and phonon dynamics, recombination mechanisms, and carrier phonon coupling interactions using optical pump terahertz probe spectroscopy. A comparison with all-inorganic counterpart indicates that organic cation affects the above-mentioned processes. Upon incorporating aromatic pyridinium cation, 1D lead halide perovskites were synthesized, showing exciton self-trapping (STE). Ultrafast optical Kerr effect spectroscopy and temperature-dependent PL measurements were used to decipher the mechanism of STE formation in pyridinium lead bromide and iodide single crystals, respectively. At room temperature, the significant role of phonon modes due to cation motions in addition to lead halide phonon modes in STE formation was observed. Additionally, we observed that an interplay between anharmonicity and dynamic disorder dictates the emission properties, which gets further complicated by an isostructural phase transition at ~170 K. With reduced dimensionality, intriguing light-matter interactions evolve in lead halide perovskites. In another project, we employed dimensionality control and cation engineering to explore the nonlinear optical (NLO) behaviour in a wide class of 1D lead halide perovskites. By employing highly polarizable cations, we were able to enhance the NLO up-conversion efficiency and optical stability.