A first-principles study of the Mechanical Stability and Electronic Properties of Lead-free Halide Inorganic Double Perovskites Cs2InAgX6 (X =F, Br, Cl, I)

Abstract

Lead-free double perovskites (DPs), Cs₂InAgX₆, specifically, were studied as candidate materials for photo voltaic cells (PVC). The density functional theory (DFT) methods as implemented in the Quantum Espresso suite was applied for the study. Equilibrium lattice parameters for the DP which are all face-centered cubic crystals, were determined as follows: a = 9.13 Å for X=F, a = 10.38 Å for X= Cl, a = 11.07 Å for X = Br and a = 11.39 Å for X = I. These findings align well with reported values. Additionally, elastic constants were calculated and found to be in fair agreement with reported values as well. Using the calculated elastic constants, all DPs were found to be ductile. The values of energy band gaps, Eg, calculated were Eg = 1.7 eV for X = F, Eg = 1.5 eV for X = Cl, Eg =0.9 for X = Br and Eg = 0.7 eV. It is noteworthy that, various studies, the experimentally reported Eg value for X = Cl, (in the range of 3.23-3.3 eV) significantly exceed the calculated Eg values in this work. The experimental Eg value for X = Cl as reported by several works is 3.23 - 3.3 eV, much higher than this and all calculated Eg values reported in this work. In summary, Cs₂InAgCl₆ emerges as a robust candidate material for PVCs, showcasing favorable mechanical stability and band gap characteristics. On the other hand, Cs₂InAgBr₆ would require band gap engineering to be a suitable material.