  In the search for stable lead (Pb) free perovskites, Vacancy ordered double
perovskite (VODP), A$_2$BX$_6$ has emerged as a promising class of materials
for solar harvesting owing to their nontoxicity, better stability, and unique
optoelectronic properties. Here, we present the stability and the key physical
attributes of few selected compounds in a systematic manner using
state-of-the-art first-principle calculations. A careful structural and
stability analysis via simulating convex hull and compositional phase diagrams
for different structural prototypes discloses 14 stable and 1 metastable
compounds in this class. The electronic structure calculations using hybrid
functional reveals six compounds to acquire band gap in the ideal visible
region. These six compounds, namely Cs$_2$SnI$_6$, Cs$_2$PdI$_6$,
Cs$_2$TeI$_6$, Cs$_2$TiI$_6$, Cs$_2$PtI$_6$, and Cs$_2$PdBr$_6$, show high
optical absorption ($\approx$ 10$^{5}$ cm $^{-1}$) giving rise to high
spectroscopic limited maximum efficiency, SLME (15-23\%) in the thin-film
thickness range. Close inspection of transport properties reveals polar optical
phonon scattering to be the dominant mechanism limiting the overall mobility.
Further analysis of the polaron excitations discloses the possibility of large
polaron formation at low to moderate defect concentrations. At high defect
concentrations, ionized impurity scattering takes over. This suggests that, a
simulation based guided control of defect concentrations during synthesis can
yield a desired candidate for promissing device application. Additionally, few
selected compounds show moderate to high electron mobility values ($\sim$13-63
cm$^2$V$^{-1}$ s$^{-1}$) at room temperature. Overall, the present study paves
an important path to help design VODP as Pb-free potential candidates for
future optoelectronic applications.
