  A comprehensive study of the structural, electronic, and optical properties
of lead-free perovskites has been carried out by means of first principles
method based on DFT. The calculations are performed for the compound of the
type A2BX6 with A=Rb, and Cs; B=Sn, Pd, and Pt; and X=Cl, Br, and I. The
calculated structural parameters (lattice constants and bond lengths) agree
well with the experiments. The computed band gap reveals a semiconducting
profile for all these compounds showing a decreasing trend of the band gap
energy by changing the halide ions consecutively from Cl to Br and Br to I.
However, for variation in the B-site cation, the band gap increases by changing
the cation from Pd to Pt via Sn. The most likely compounds, Rb2PdBr6 and
Cs2PtI6, exhibit a band gap within the optimal range of 0.9-1.6 eV for
single-junction photovoltaic applications. The optical properties in terms of
the optimal value of the dielectric constant, optical conductivity, and
absorption coefficient are also investigated upto the photon energy of 10 eV.
Our results indicate that upon changing the halogen ions (Cl by Br and Br by I)
the optical properties altered significantly. Maximum dielectric constants and
high optical absorption are found for Rb2PdI6 and Cs2PtI6. The unique
optoelectronic properties such as ideal band gap, high dielectric constants,
and optimum absorption of A2BX6 perovskites could be efficiently utilized in
designing high performance single and multi-junction perovskite solar cells.
