Electronic structures and properties of copper, germanium, or tin-based MA/Pb-free perovskite halides

Perovskite halides with a typical composition of CH₃NH₃PbI₃ provide high photoconversion efficiencies and lightweight/flexible solar cells. Since the main element lead (Pb) is toxic, perovskites without Pb (Pb-free) should be developed from the viewpoint of natural environments and human health. In addition, methyl ammonium (CH₃NH₃, MA) is an unstable molecule in the crystal, and MA-free perovskites are also mandatory to improve their structural stabilities. The aim of the present study is to clarify the electronic structures and properties of copper (Cu), germanium (Ge), or tin (Sn)-based MA/Pb-free perovskite halides using first principles calculations. Monovalent copper (Cu⁺), rubidium (Rb), and cesium (Cs) were introduced at the MA sites, and various transition elements and typical elements such as Ni, Cr, Fe, Zn, and others were also induced at the Pb site. Pb-free double perovskite bromides were also found to be the suitable photovoltaic materials, which would be due to high electron density of Ge compared with Sn. The double perovskites have wide energy gaps and stabilities compared with the ordinary perovskites, and the hybridization of Ge/Sn would influence the electronic structures. Total energies of Cs-based perovskites were reduced by the Cu⁺ addition. The band gap energies of Cu-based Pb-free chlorides with transition metals provided suitable values for solar cells. Carrier mobilities and crystal structures of the perovskites could be stabilized by the overlapping of electron orbitals between the chloride octahedron and Cu⁺. The Cu⁺ at the MA-site would be effective to control the structures and stabilities of the all-inorganic perovskites, which would expand the multiplicity of the perovskites; as a result, the α-formamidinium cesium lead triiodide was stably formed by the addition of Cu⁺.