Investigation of material properties of halide mixed lead-Free double perovskite for optoelectronic applications using first-principles study

All-inorganic halide double perovskite has recently emerged as a potential substitute for lead halide perovskite capable of solving the problem of toxicity associated with lead content and its inherent instability issue. However, most double perovskites possess wide bandgaps having drastic consequences on their efficiency, hindering their usage as solar absorbers in the photovoltaic industry. Halide mixing has been identified as a viable means of engineering bandgaps of all-inorganic halide double perovskite. This study investigates material properties of allinorganic double perovskite Cs2NaM(IxBr1-x)6 [M = Bi, In] for optoelectronic applications. Structural, electronic, and optical properties of the materials are investigated using first-principles density functional theory with the help of virtual crystal approximation for modeling the virtual atoms IxBr1_x. The results show a linear increase in lattice parameters of the compounds as iodine content is increased while the bandgaps decrease following a quadratic function. For M = Bi, indirect band nature is observed while for M = In, direct band nature is observed all through the range. With increased iodine content, the optical absorption of the perovskites is observed to enhance weakly. The static dielectric constants increase quadratically with increased iodine content. Phase decomposition analysis of the materials shows that CNBX materials remain stable up to x = 0.2 while CNIX remains stable up to x = 0.6 against phase decomposition. Thus, halide mixing of iodine with bromine may be a viable means of bandgap reduction and enhancing optical properties of double perovskites for optoelectronic applications.