Computational Study of Structural and Electronic Properties of Lead-Free CsMI₃ Perovskites (M = Ge, Sn, Pb, Mg, Ca, Sr, and Ba)

Electronic structure calculations of five crystallography-imitated structures of CsMI₃ perovskites with M = Ge, Sn, Pb, Mg, Ca, Sr, and Ba were performed. The formation energy of different perovskite phases, their relative stability, and structural and electronic properties were explored. The sensitivity of the calculations to the choice of the density functional was investigated, and the predictions were compared with experimental results. The outcome of this study is that Mg and Ba perovskites are unlikely to form in the cubic, tetragonal, or orthorhombic phases because they have positive formation energies. Although Ca and Sr perovskites have negative formation energies with respect to the metal-iodide precursors, they exhibit wide band gaps and high hygroscopicity, making these unlikely candidates for applications in photovoltaic devices. Our results suggest that the performance of a local density functional with a nonseparable gradient approximation (NGA) is similar to that of hybrid functionals in terms of band gap predictions, when M in CsMI₃ is a p-block element (Pb, Sn, and Ge). However, local density functionals with NGA predictions for the band gap are similar to other local functionals with a generalized gradient approximation (PBE, PBEsol, and PBE-D3) and are worse than those of HSE06, when M is an s-block element (Mg, Ca, Sr, and Ba).