  The thermoelectric properties of halide perovskites $\mathrm{CsMI_3}$ (M=Sn
and Pb) are investigated from a combination of first-principles calculations
and semiclassical Boltzmann transport theory by considering both the electron
and phonon transport. The electronic part is performed using a modified Becke
and Johnson (mBJ) exchange potential, including spin-orbit coupling (SOC),
while the phonon part is computed using generalized gradient approximation
(GGA). It is found that SOC has remarkable detrimental effect on n-type power
factor, while has a negligible influence in p-type doping, which can be
explained by considering SOC effect on conduction and valence bands. Calculated
results show exceptionally low lattice thermal conductivities in
$\mathrm{CsSnI_3}$ and $\mathrm{CsPbI_3}$, and the corresponding
room-temperature lattice thermal conductivity is 0.54 $\mathrm{W m^{-1}
K^{-1}}$ and 0.25 $\mathrm{W m^{-1} K^{-1}}$. At 1000 K, the maximal figure of
merit $ZT$ is up to 0.63 and 0.64 for $\mathrm{CsSnI_3}$ and $\mathrm{CsPbI_3}$
with scattering time $\tau$=$10^{-14}$ s, and the peak $ZT$ is 0.49 and 0.41
with $\tau$=$10^{-15}$ s. These results make us believe that $\mathrm{CsMI_3}$
(M=Sn and Pb) in perovskite structures may be potential thermoelectric
materials.
