Dimension-Dependent Bandgap Narrowing and Metallization in Lead-Free Halide Perovskite CsBiX (X = I, Br, and Cl) under High Pressure.

Low-toxicity, air-stable cesium bismuth iodide CsBiX (X = I, Br, and Cl) perovskites are gaining substantial attention owing to their excellent potential in photoelectric and photovoltaic applications. In this work, the lattice constants, band structures, density of states, and optical properties of the CsBiX under high pressure perovskites are theoretically studied using the density functional theory. The calculated results show that the changes in the bandgap of the zero-dimensional CsBiI, one-dimensional CsBiCl, and two-dimensional CsBiBr perovskites are 3.05, 1.95, and 2.39 eV under a pressure change from 0 to 40 GPa, respectively. Furthermore, it was found that the optimal bandgaps of the Shockley-Queisser theory for the CsBiI, CsBiBr, and CsBiCl perovskites can be reached at 2-3, 21-26, and 25-29 GPa, respectively. The CsBiI perovskite was found to transform from a semiconductor into a metal at a pressure of 17.3 GPa. The lattice constants, unit-cell volume, and bandgaps of the CsBiX perovskites exhibit a strong dependence on dimension. Additionally, the CsBiX perovskites have large absorption coefficients in the visible region, and their absorption coefficients undergo a redshift with increasing pressure. The theoretical calculation results obtained in this work strengthen the fundamental understanding of the structures and bandgaps of CsBiX perovskites at high pressures, providing a theoretical support for the design of materials under high pressure.