  Using first-principles pseudopotential method and Boltzmann transport theory,
we give a comprehensive understanding of the electronic and phonon transport
properties of thermoelectric material BiCuSeO. By choosing proper hybrid
functional for the exchange-correlation energy, we find that the system is
semiconducting with a direct band gap of ~0.8 eV, which is quite different from
those obtained previously using standard functionals. Detailed analysis of a
three-dimensional energy band structure indicates that there is a valley
degeneracy of eight around the valence band maximum, which leads to a sharp
density of states and is responsible for a large p-type Seebeck coefficient.
Moreover, we find that the density of states effective masses are much larger
and results in very low hole mobility of BiCuSeO. On the other hand, we find
larger atomic displacement parameters for the Cu atoms, which indicates that
the stronger anharmonicity of BiCuSeO may originate from the rattling behavior
of Cu instead of previously believed Bi atoms.
