  Researchers have addressed the non-traditional power generation schemes as
alternatives to the traditional fossil-fuel methods enormously since the
scientific community has serious concerns about shortages of energy on our
planet for future generations. In this scenario, the innovative materials for
photovoltaic and thermoelectric device applications are required by addressing
current issues of instability and efficiency. Perovskites are very popular in
this regard particularly having higher power conversion efficiency of 25.2% in
the case of solar cells. In the current article, we investigated innovative
small direct band gap double perovskites (elapsolite) Cs$_2$AgTlX$_6$ (X= Cl,
Br) with a comprehensive discussion on structural, electronic, optical, and
thermoelectric properties using a first-principles approach. The compounds
under investigation are found stable, efficient, and economical with alluring
optical and thermoelectric properties. The higher absorption peaks in the
visible range, substantial optical conductivities (~10$^{16}$ sec$^{-1}$), and
a lower percentage of reflection in the visible range make these compounds
fascinating for solar cell applications. Whereas large values of Seebeck
coefficients, electrical conductivities, the figure of merits (greater than
unity), and small values of thermal conductivities suggest the applications of
these compounds in thermoelectric generators.
