Cation replacement effect on the optical and electronic properties of Cs2Ag1-xNaxYCl6 (Y= In, Bi)

Abstract

Lead-free double perovskites are environmentally friendly materials that have attracted enormous attention for their structural stability, long charge carrier lifetime, and good absorption. This work includes two double perovskites Cs2AgBiCl6, Cs2NaBiCl6 with an indirect band gap and two double perovskites Cs2AgInCl6, Cs2NaInCl6 with a direct band gap, considered to make two groups of mixed double perovskites as Cs2Ag1-xNaxBiCl6 and Cs2Ag1-xNaxInCl6 to study the effect of cation replacement on the optical and electronic properties by employing ab-initio approach. The electronic and optical properties of these materials with x = 0, 0.25, 0.50, 0.75, 1 have been studied, and a tunable bandgap from the smallest one to the widest one was found in both groups (1.46–2.85, 0.26–2.87) as they change with different x. The general trend of the band gap in each group shows that by reducing Ag contribution an increase in the band gap happens. But a minimum value at x = 0.25 in the first group, which is absent in the second group comprising In, is due to the different behavior of the half-full Bi-p orbital and full In-s orbital in the edge of the conduction band when the Na-s orbital introduced to the structure. In addition, the study of the absorption coefficient of each group shows that there is a possibility to find a better absorbent in the visible energy range in mixed alloys than the pristine ones.