Influence of Sn/Ge Cation Exchange on Vacancy-Ordered Double Perovskite Cs₂Sn_(1−x)Ge_xI₆: A First-Principles Theoretical Study

The toxicity of lead (Pb) and the volatility of organic cations in the typical Pb-based organic–inorganic hybrid perovskite materials are the two key challenges in the emerging perovskite solar cells (PSCs). Thus, the development of lead-free and inorganic perovskite materials for solar cells is very important. Here, the mixed vacancy-ordered inorganic double perovskite Cs₂Sn_(1−x)Ge_xI₆ is studied to explore how gradual substitution of tetravalent gemanium (Ge⁴⁺) for tetravalent tin (Sn⁴⁺) influences the structure, as well as thermodynamic, electronic and mechanical properties of Cs₂Sn_(1−x)Ge_xI₆ on the basis of first-principles theory calculation. By changing the concentration of doping Ge⁴⁺, it is shown that the values of band gap and concentration of Ge⁴⁺ are in a linear relationship, satisfying the fitting equation: y = 1.04 − 0.704x, where y is the value of band gap, and x is the concentration of Ge⁴⁺. Moreover, it is found that the volume of crystal cell generally decreases with the concentration of doping Ge⁴⁺ increasing. This indicates that the reduction of band gap relates to contraction of crystal cell. In addition, in this study, it is shown that Cs₂Sn_0.75Ge_0.25I₆ exhibits the best thermodynamics stability and the best mechanical ductility and flexibility.