Thermoelectric optimization of AgBiSe2 by defect engineering for room-temperature applications

The hexagonal phase of $\mathrm{AgBiS}{\mathrm{e}}_{2}$ has been discovered as a promising thermoelectric material for room-temperature applications. However, its basic conduction type is still ambiguous, and its current ZT value is pretty low. To improve the thermoelectric performance of $\mathrm{AgBiS}{\mathrm{e}}_{2}$, we apply band engineering to modify its band structure by introducing defects to increase the band degeneracy. From the calculated intrinsic point defect formation energies of $\mathrm{AgBiS}{\mathrm{e}}_{2}$ at different growth conditions, we clarify that the conducting behavior of $\mathrm{AgBiS}{\mathrm{e}}_{2}$ is a $p$-type semiconductor, and the Ag vacancy is the dominated acceptor. Based on scrutinizing the band structure of $\mathrm{AgBiS}{\mathrm{e}}_{2}$, two kinds of methodologies can be used to modify its band structure to achieve high band degeneracy: (i) shifting the Fermi level into the valence band using intrinsic defects, and (ii) converging several valence-band maxima by introducing extrinsic defects. We find that the intrinsic Ag vacancy is helpful to significantly increase the power factor, leading to a large ZT for Ag vacancy-doped $\mathrm{AgBiS}{\mathrm{e}}_{2}$: the maximum ZT value is increased to 0.3--0.5 at near room temperature. Based on analyzing the bonding characters and atomic energy levels in the compound, we predict several extrinsic dopants (Cu, Rh, and Pd) that can be used to converge three valence-band maxima. Our work provides methodologies to improve the room-temperature thermoelectric applications of $\mathrm{AgBiS}{\mathrm{e}}_{2}$ by tuning its band structures using intrinsic or extrinsic defects.