Magneto-Optical Investigations of Lead-Free Metal Halide Perovskite Nanocrystals

Abstract

Inorganic lead-free metal halide perovskites have garnered much attention as low-toxicity alternatives to lead halide perovskite for luminescence and photovoltaic applications. However, the electronic structure and properties of these materials, including the composition dependence of the band structure, spin-orbit coupling, and Zeeman effects remain poorly understood. In this thesis, we focus on two specific systems: Cs3Bi2X9 (X = Cl, Br) and double perovskite, including Cs2AgBiX6 (X = Cl, Br), Cs2AgInCl6 and its Bi-alloyed analogue (Cs2AgIn0.5Bi0.5Cl6). Using magnetic circular dichroism (MCD) spectroscopy, we investigate the electronic structure, magneto-optical properties, and excitonic transitions in these lead-free perovskite NCs. Our results reveal that the excitonic spectra of Cs3Bi2X9 are predominantly characterized by both direct and indirect band-gap transitions, with only a minor contribution from excitons localized on Bi3+ sites. In contrast, the excitonic transitions in Cs2AgBiX6 are primarily derived from direct free- and bound- exciton transition. Additionally, our results demonstrate that halide composition significantly influences the Zeeman splitting energy and g-factors, with Cs3Bi2Br9 and Cs2AgBiBr6 exhibiting stronger spin-orbit coupling compared to their chloride counterparts. Moreover, introducing bismuth ion (Bi3+) into Cs2AgInCl6 NCs can enhance the spin-orbit coupling and modify the electronic structure, demonstrating the potential for compositional tuning to optimize these materials for specific applications. Furthermore, temperature-dependent MCD measurements were conducted to further explore the excitonic behavior of these materials, providing insights into their suitability for further applications. In conclusion, this thesis provides detailed insights into lead-free halide perovskite NCs, emphasizing their potential as environmentally friendly alternatives to lead-based perovskite. These findings offer valuable guidance for the design of low-toxicity, high-performance materials for applications in spintronics, photovoltaics, and optoelectronics.