  We theoretically investigate how each orbital and valley play a role for high
thermoelectric performance of SnSe. In the hole-doped regime, two kinds of
valence band valleys contribute to its transport properties: one is the valley
near the U-Z line, mainly consisting of the Se-$p_z$ orbitals, and the other is
the one along the $\Gamma$-Y line, mainly consisting of the Se-$p_y$ orbitals.
Whereas the former valley plays a major role in determining the transport
properties at room temperature, the latter one also offers comparable
contribution and so the band structure exhibits multi-valley character by
increasing the temperature. In the electron-doped regime, the conduction band
valley around the $\Gamma$ point solely contributes to the thermoelectric
performance, where the quasi-one-dimensional electronic structure along the
$a$-axis is crucial. This study provides an important knowledge for the
thermoelectric properties of SnSe, and will be useful for future search of
high-performance thermoelectric materials.
