Influence of Sn/Ge Cation Exchange on Vacancy-Ordered Double Perovskite Cs2Sn(1-x)GexI6: 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)GexI6 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)GexI6 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 Cs2Sn0.75Ge0.25I6 exhibits the best thermodynamics stability and the best mechanical ductility and flexibility.