  The lead-free double perovskite material (viz. Cs$_2$AgBiCl$_6$) has emerged
as an efficient and environmentally friendly alternative to lead halide
perovskites. To make Cs$_2$AgBiCl$_6$ optically active in the visible region of
solar spectrum, band gap engineering approach has been undertaken. Using
Cs$_2$AgBiCl$_6$ as a host, band gap and optical properties of Cs$_2$AgBiCl$_6$
have been modulated by alloying with M(I), M(II), and M(III) cations at
Ag-/Bi-sites. Here, we have employed density functional theory (DFT) with
suitable exchange-correlation functionals in light of spin-orbit coupling (SOC)
to determine the stability, band gap and optical properties of different
compositions, that are obtained on Ag-Cl and Bi-Cl sublattices mixing. On
analyzing the 64 combinations within Cs$_2$AgBiCl$_6$, we have identified 19
promising configurations having band gap sensitive to solar cell applications.
The most suitable configurations with Ge(II) and Sn(II) substitutions have
spectroscopic limited maximum efficiency (SLME) of 32.08% and 30.91%,
respectively, which are apt for solar cell absorber.
