Benefits of Carrier Pocket Anisotropy to Thermoelectric Performance: The case of p-type AgBiSe₂

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We study theoretically the effects of anisotropy on the thermoelectric performance of p-type AgBiSe₂. We present an apparent realization of the thermoelectric benefits of one-dimensional "plate-like" carrier pocket anisotropy in the valence band of this material. Based on first principles calculations we find a substantial anisotropy in the electronic structure, likely favorable for thermoelectric performance, in the valence bands of the hexagonal phase of the silver chalcogenide thermoelectric AgBiSe₂, while the conduction bands are more isotropic, and in our experiments do not attain high performance. AgBiSe₂ has already exhibited a ZT value of 1.5 in a high-temperature disordered fcc phase, but room-temperature performance has not been demonstrated. We develop a theory for the ability of anisotropy to decouple the density-of-states and conductivity effective masses, pointing out the influence of this effect in the high performance thermoelectrics Bi₂Te₃ and PbTe. From our first principles and Boltzmann transport calculations we estimate the performance of p-type AgBiSe₂.