  Two-dimensional (2D) materials may have potential applications in
thermoelectric devices. In this work, we systematically investigate the
thermoelectric properties of orthorhombic group IV-VI monolayers $\mathrm{AB}$
(A=Ge and Sn; B=S and Se) by the first-principles calculations and
semiclassical Boltzmann transport theory. The spin-orbit coupling (SOC) is
included to investigate their electronic transport, which produces observable
effects on power factor, especially for n-type doping. According to calculated
$ZT$, the four monolayers exhibit diverse anisotropic thermoelectric
properties, although they have similar hinge-like crystal structure. The GeS
along zigzag and armchair directions shows the strongest anisotropy, while SnS
and SnSe show mostly isotropic efficiency of thermoelectric conversion, which
can be understood by the strength of anisotropy of their respective power
factor, electronic and lattice thermal conductivities. Calculated results show
that $ZT$ for different carriers of n- and p-type has little difference for
GeS, SnS and SnSe. It is found that GeSe, SnS and SnSe show better
thermoelectric performance compared to GeS in n-type doping, and SnS and SnSe
exhibit higher efficiency of thermoelectric conversion in p-type doping.
Compared to a lot of 2D materials, orthorhombic group IV-VI monolayers
$\mathrm{AB}$ (A=Ge and Sn; B=S and Se) may possess better thermoelectric
performance due to higher power factor and lower thermal conductivity. Our work
would be beneficial to further experimental study.
