  We employed first-principles calculations to investigate the thermoelectric
transport properties of the compound As$_2$Se$_3$. Early experiments and
calculations have indicated that these properties are controlled by a kind of
native defect called antisites. Our calculations using the linearized Boltzmann
transport equation within the relaxation time approximation show good agreement
with the experiments for defect concentrations of the order of 10$^{19}$
cm$^{-3}$. Based on our total energy calculations, we estimated the equilibrium
concentration of antisite defects to be about 10$^{14}$ cm$^{-3}$. These
results suggest that the large concentration of defects in the experiments is
due to kinetic and/or off-stoichiometry effects and in principle it could be
lowered, yielding relaxation times similar to those found in other chalcogenide
compounds. In this case, for relaxation time higher than 10 fs, we obtained
high thermoelectric figures of merit of 3 for the p-type material and 2 for the
n-type one.
