  Two-dimensional (2D) metal halides have received more attention because of
their electronic and optoelectronic properties. Recently, researchers are
interested to investigate the thermoelectric properties of metal halide
monolayers because of their ultralow lattice conductivity, high Seebeck
coefficient and figure of merit. Here, we have investigated thermoelectric and
optoelectronic properties of XI$_2$ (X=Sn and Si) monolayers with the help of
density functional theory and Boltzmann transport equation. The structural
parameters have been optimized with relaxation of atomic positions. Excellent
thermoelectric and optical properties have been obtained for both SnI$_2$ and
SiI$_2$ monolayers. For SnI$_2$ an indirect bandgap of 2.06 eV was observed and
the absorption peak was found at 4.68 eV. For this the highest ZT value of 0.84
for p-type doping at 600K has been calculated. Similarly, for SiI$_2$ a
comparatively low indirect bandgap of 1.63 eV was observed, and the absorption
peak was obtained at 4.86 eV. The calculated ZT product for SiI$_2$ was 0.87 at
600K. Both the crystals having high absorbance and ZT value suggest that they
can be promising candidates for optoelectronic and thermoelectric devices.
