  Hybrid Lead halide perovskites, despite having unique intrinsic properties
with the possibility of flexible synthesis and device fabrication, still suffer
from two fundamental issues, i.e. stability in external environment and
toxicity due to lead. More recently, double perovskite materials have emerged
as a promising choice. The main outcome from various studies on this class can
essentially be summarized into two categories, (i) either they have indirect
band gap or (ii) direct but large optical band gap, which are not suitable for
solar devices. Here we propose a combinatorial set of stable double perovskite
materials, Cs$_2$BB$^{'}$X$_6$ (for various B, B$^{'}$) (X=Cl,Br,I), which show
indirect to direct band gap transition via small Pb$^{+2}$ doping. This kind of
doping has helped to change the topology of band structure triggering an
optically allowed transition from valence band maxima to conduction band
minima. It also reduces the band gap significantly, bringing it well in the
visible region. Simulation reveals comparable/higher absorption coefficient and
solar efficiency with respect to the state of the art photovoltaic absorber
material CH$_3$NH$_3$PbI$_3$. Our experimentally measured properties on
Cs$_2$(Ag$_{0.75}$Pb$_{0.25}$)(Bi$_{0.75}$Pb$_{0.25}$)Br$_6$ agrees fairly well
with the theoretical predictions. With higher stability than
CH$_3$NH$_3$PbI$_3$, this material shows the potential to be a better
candidate.
