  Improving the thermoelectric efficiency is one of the greatest challenges in
materials science. The recent discovery of excellent thermoelectric performance
in simple orthorhombic SnSe crystal offers new promise in this prospect [Zhao
et al. Nature 508, 373 (2014)]. By calculating the thermoelectric properties of
orthorhombic IV-VI compounds GeS,GeSe,SnS,and SnSe based on the
first-principles combined with the Boltzmann transport theory, we show that the
Seebeck coefficient, electrical conductivity, and thermal conductivity of
orthorhombic SnSe are in agreement with the recent experiment. Importantly,
GeS,GeSe,and SnS exhibit comparative thermoelectric performance compared to
SnSe. Especially, the Seebeck coefficients of GeS,GeSe,and SnS are even larger
than that of SnSe under the studied carrier concentration and temperature
region. We also use the Cahill's model to estimate the lattice thermal
conductivities at the room temperature. The large Seebeck coefficients, high
power factors, and low thermal conductivities make these four orthorhombic
IV-VI compounds promising candidates for high-efficient thermoelectric
materials.
