Designing Organic A-site Cations for Emerging Optoelectronic Properties of Hybrid Halide Perovskites: Reversible Melting, Non-Centrosymmetry, and Chirality

Abstract

In typical hybrid metal halide perovskites, including layered 2D A2PbX4 (A: organic cation, X: I, Br), the valance band maximum and conduction band minimum are composed of orbitals of the inorganic sublattice {PbX4}2-. Therefore, the optoelectronic properties are predominantly determined by the inorganic sublattice, and the main role of organic A-site cation is to stabilize the structure. In this thesis work, we designed innovative organic A-site cations that leads to emerging optoelectronic properties, expanding their role beyond mere structural support. The A-site cations are rationally designed to tailor the hydrogen- and halogen-bonding non-covalent interactions at the organic-inorganic interface. Consequently, physical and chemical properties of hybrid perovskites are rationally controlled by the choice of the A-site cation. Low-dimensional perovskites (2D, 1D and 0D) do not need to follow the stringent structural requirements of Goldschmidt's tolerance factor, and therefore, the scope for designing such functional A-site cations is enormous for such low-dimensional perovskites. In chapter 2, we designed a series of 2D A2PbX4 perovskites, where entropy of fusion drives reversible melting and recrystallization at a moderate temperature ~ 110 oC. Subsequently, melt-pressed (solvent- and vacuum-free fabrication) perovskite films were fabricated for photodetector applications. Chapter 3 demonstrates new synthesis methodologies of hybrid Pb-halide perovskites in the molten state. Then the specially designed A-site cations were used to introduce non-centrosymmetry in crystal structure of 1D Bi-halide hybrid perovskite (chapter 4), and chirality in 0D Bi-halide hybrid perovskites (chapter 5). Subsequently, nonlinear optical properties and chiroptic properties were observed in these hybrid perovskites.