Phase Transition and Physical Properties of Mixed- Halide Hybrid Perovskites

Hybrid halide perovskites have emerged as highly promising photovoltaic materials due to their exceptional optoelectronic properties, including low defect density, tunable bandgap, large absorption coefficient, high carrier mobility, and long charge carrier lifetime, which are often optimized via compositional engineering such as mixing halides. It is well established that hybrid halide perovskites undergo a series of structural phase transitions from cubic to tetragonal and orthorhombic as temperature varies. However, the phase transition and molecular dynamics of mixed-halide hybrid perovskites are still unknown, and a comprehensive study on this category of materials is urgently needed as they play an important role in optoelectronic and photovoltaic applications. In this thesis, we find that phase transitions are substantially suppressed in mixed- halide hybrid perovskite single crystals of MAPbI3-xBrx (MA = CH3NH3+ and x = 1 or 2). The temperature-dependent Powder X-ray Diffraction (PXRD) refinement and simulations of single-crystals, grown by Inverse Temperature Crystallization (ITC) method, showed that multiple crystallographic phases coexist in mixed-halide perovskites over a wide temperature range, and a slightly distorted monoclinic phase, hitherto unreported for hybrid perovskites, is dominant at application-relevant temperatures. These were further confirmed by Photoluminescence (PL) and dielectric permittivity measurements at various temperatures. The slower MA reorientational motions are demonstrated in mixed-halide perovskites compared to pure-halide counterparts. Moreover, the thoretical simulations, Brillouin microspectroscopy, and inelastic neutron scattering, revealed that the anomalous structural evolution is correlated with the glassy behavior of organic cations and optical phonons in mixed-halide perovskites. This work demonstrates the complex interplay between composition engineering and lattice dynamics in hybrid perovskites, shedding new light on their unique properties. In fact, molecular dynamics in a crystal lattice could impact optoelectronic properties. Therefore, the photovoltaic and optoelectronic performances are affected by any alterations in chemical composition and temperature-dependent changes in crystal symmetry. These phase transition behaviours should be considered to optimize the operation of photovoltaic devices, especially around room temperature