Electronic and optical properties of inorganic lead-free perovskite Cs3Bi2I9

Organic-inorganic halide perovskite materials are widely used in solar cells because of their excellent photoelectric properties. However, the stability and lead toxicity problems associated with materials and devices have restrict their production and development. Compared with the hybrid perovskite, the inorganic lead-free perovskite Cs₃Bi₂I₉ has attracted wide attention because of its stronger stability and environmental friendliness. The Cs₃Bi₂I₉ has three crystal types: monoclinic type, trigonal type, and hexagonal type. At present, the researches of Cs₃Bi₂I₉ focus mainly on the hexagonal phase. In this paper, based on the first principles of density functional theory, the electronic properties, carrier effective mass values, stabilities, and optical properties of Cs₃Bi₂I₉ monoclinic, trigonal, and hexagonal phases are studied theoretically. It is suggested that the stabilities of the three crystal phases are similar, and the direct band gap (1.21 eV) of the trigonal phase would be noticeable. For the three phases, their effective mass values show that their properties are the same along both the <i>a</i> direction and the <i>b</i> direction, but different along the <i>c</i> direction. The effective mass of electron of the trigonal phase is significantly smaller along the <i>a</i>-direction than along the <i>c</i>-direction. Corresponding to the red shift phenomenon of optical properties, the trigonal phase shows the better optical absorption performance than other phases. In addition, the optical properties also show that the properties are the same along the <i>a</i> direction and the <i>b</i> direction, but different along the <i>c</i> direction, and the optical absorption performance is better along the <i>a</i>-direction than along the <i>c</i>-direction.