A Comparative Study of the Structural and Electronic Properties of Orthorhombic and Cubic CsPbI3 and Trigonal CsGeI3 using First-Principles Calculations

Lead-free halide perovskites are of great importance as prospective materials for efficient solar cells. Germanium is a very promising non-toxic alternative to lead. In this study, the crystal configuration, projected density of states and band structure of the trigonal CsGeI3, the yellow and the quenched black orthorhombic CsPbI3, and the cubic CsPbI3 were investigated using the density functional theory with Perdew-Burke-Ernzerhof functional. Our calculations showed that for the CsGeI3, the valence band maximum is mainly contributed by the I 5p and Ge 4s orbitals whereas the conduction band minimum is mainly contributed by the Ge 4p orbitals. The replacement of Pb with Ge results in a narrower bandgap.