First principle insight on the structural, mechanical, electronic and optical properties of indirect band gap photovoltaic material Cs2NaBiX6 (X= Cl, Br, I)

In recent years, the inorganic perovskite materials have gained a lot of attention in scientific community due to their structural stability in photovoltaic applications. In this work, we explore the structural, mechanical, electronic and optical properties of Cs2NaBiCl6, Cs2NaBiBr6 and Cs2NaBiI6 through the density functional theory calculations. The effect of anion exchange and the impact on the band gaps are studied from band structure calculations. Mechanically, all these materials are classified as ductile in nature. The band gap of these materials are linearly decreased from 2.91 eV to 1.56 eV, when we exchange the halide atom from Cl to I. All the materials have desirable optical absorption in the ultraviolet regions, and Cs2NaBiI6 absorption was red-shifted towards the visible regions in the range of 220-520 nm. All these studies and comparison results confirm that Cs2NaBiI6 is suitable material for single junction solar cell applications.