Anharmonic Semiconductors

ended

Recent studies of halide perovskite semiconductors (SCs) showed that they exhibit a unique combination of very-low defect density, self-healing properties and low exciton binding energies that result in excellent photovoltaic activity. I hypothezise that the fundamental property that sets the halide perovskites apart from conventional SCs and gives rise to their beneficial properties is strongly anharmonic lattice dynamics. Large amplitude, local polar fluctuations that result from lattice anharmonicity localize the electronic states and enhance the screening of electric charges within the material. In other words, in some aspects, halide perovskites behave more like a liquid than a crystalline solid. Stimulated by the recent discoveries on halide perovskites, I aim to generalize our understanding of the relationship between lattice anharmonicity and the electronic properties of SCs. The potential outcome of this investigation will be a novel scheme to design SCs with desirable properties where lattice anharmonicity is used as a new material-engineering tool. My strategy is to perform comparative studies in both inorganic ionic crystals and small-molecule organic crystals. We will use low-frequency Raman spectroscopy to quantify anharmonic lattice dynamics and compare between different crystals to identify the factors that induce anharmonicity in solids. Photoluminescence, reflectance, time-resolved terahertz and impedance spectroscopies will be used to probe the SCs optical properties, carrier mobilities and lifetimes, and their dielectric response. I expect to find that as anharmonicity increases, the dielectric response and carrier lifetimes increase while carrier mobility decreases. Finally, we will develop a modulated Raman spectroscopic methodology that will identify specific lattice motions that are coupled to band-edge carriers, thus elucidating the microscopic mechanism of carrier-lattice interactions.