High Pressure and Low Temperature Studies on Lead Free Halide Perovskites

Samanta, Debabrata (2024) High Pressure and Low Temperature Studies on Lead Free Halide Perovskites. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Debabrata Samanta (18RS023)) 18RS023.pdf - Submitted Version Restricted to Repository staff only Download (13MB)

Abstract

The exploration of energy-harvesting materials for photovoltaic and optoelectronic applications has inspired intense research interest in the scientific community. In this search, lead-halide perovskites have shown excellent photovoltaic properties including direct bandgap, high absorption coefficient, high photoluminescence (PL), long charge diffusion lengths, and high charge carrier mobility. However, the toxicity and stability issues prevented them in commercial applications. Therefore, in this thesis work, a few lead-free halide perovskites have been synthesized and investigated using Raman scattering, photoluminescence, absorption, and x-ray diffraction (XRD) measurements at high-pressures and low-temperatures. To explain experimental results, electronic band structure, the density of states, and phonon calculations at different pressures have also been performed using density functional theory (DFT). These computations are carried out using the QUANTUM ESPRESSO software. The structural, electronic, and optical properties of Cs₃Bi₂Br₉ and Cs₃Sb₂Br₉ are investigated in detail under external stimuli such as pressure and temperature. Cs₃Bi₂Br₉ undergoes two isostructural transitions at about 0.9 and 2.4 GPa, respectively. The sample shows the appearance of PL at about 1.4 GPa driven by the isostructural transition. The enhancement of PL intensity and blue shift in PL peaks are related to the distortion and quadratic elongation of BiBr6 octahedra. In the case of Cs3Sb2Br9, at about 3 GPa, an electronic transition manifests itself through a broad minimum in linewidth, a maximum in the intensity of Eg, A1g Raman modes, and the unusual change in the c/a ratio of the trigonal structure. The observed below bandgap broadband emission in the PL measurement indicates the recombination of self-trapped excitons. The first-principles DFT calculations reveal that the electronic transition is associated with direct-toindirect bandgap transition due to changes in the hybridization of Sb−5s and Br − 4p orbitals near the Fermi level in the valence band. In addition, the DFT calculations predict a 27.5% reduction of the bandgap in the pressure range 0-8 GPa. Trigonal to monoclinic structural transition in Cs₃Bi₂Br₉ at around 95 K is discussed and shown to be driven by the incomplete soft mode. The anharmonic scattering of phonons due to the nucleation of the new phase leads to the incomplete soft mode. Some of the Raman mode frequencies exhibit anomalous temperature dependence due to strong anharmonic phonon-phonon coupling. In the trigonal phase, the broadening of the full width at half maximum with an increase in temperature for Eg and A1g modes are mostly accompanied by the decaying of one optical phonon into two acoustic phonons. In the monoclinic phase, the strength of four phonon processes to the frequency shift and linewidth broadening is much smaller than that for three phonon processes. We have also investigated the emission characteristics and phonon anharmonicity in Cs₃Sb₂Br₉. The exciton-optical phonon coupling leads to below bandgap broad emissions, arising from selftrapped excitons recombination. The anomalous temperature dependence of the lowest frequency Raman mode is attributed to the phonon-phonon and electron-phonon interactions. The temperature-dependent XRD measurement reveals a minimum in the volume thermal expansion coefficient at around 120 K. We also quantify the quasiharmonic contributions to the phonon frequency shift for all Raman modes.

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