  The electronic, phonon and thermoelectric transport properties of (PbTe)2
layer are systematically investigated by using first-principles pseudopotential
method and Boltzmann transport equation. Our calculations demonstrate that
there is a valley degeneracy of six for the top valence band, which leads to
larger carrier concentration and thus higher electrical conductivity without
obvious reduction in the Seebeck coefficient. Moreover, the intrinsic van der
Waals interactions between neighboring Pb layers induce additional phonon
scattering and thus ultrasmall lattice thermal conductivity. As a consequence,
a maximum p-type ZT value of 2.9 can be achieved at 1000 K. Moreover, we find
almost identical n- and p-type ZT in the temperature range from 300 K to 800 K.
