Dimension-Dependent Bandgap Narrowing and Metallization in Lead-Free Halide Perovskite Cs(3)Bi(2)X(9) (X = I, Br, and Cl) under High Pressure

Low-toxicity, air-stable cesium bismuth iodide Cs(3)Bi(2)X(9) (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 Cs(3)Bi(2)X(9) 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 Cs(3)Bi(2)I(9), one-dimensional Cs(3)Bi(2)Cl(9), and two-dimensional Cs(3)Bi(2)Br(9) 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 Cs(3)Bi(2)I(9), Cs(3)Bi(2)Br(9), and Cs(3)Bi(2)Cl(9) perovskites can be reached at 2–3, 21–26, and 25–29 GPa, respectively. The Cs(3)Bi(2)I(9) 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 Cs(3)Bi(2)X(9) perovskites exhibit a strong dependence on dimension. Additionally, the Cs(3)Bi(2)X(9) 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 Cs(3)Bi(2)X(9) perovskites at high pressures, providing a theoretical support for the design of materials under high pressure.