Divalent doping-induced thermoelectric power factor increase in p-type Bi2Te3 via electronic structure tuning

Abstract

We use first-principles calculations to reveal the effects of divalent Pb, Ca, and Sn doping of Bi2Te3 on the band structure and transport properties, including the Seebeck coefficient, α, and the reduced power factor, α2σ/τ, where σ is the electrical conductivity and τ is the effective relaxation time. Pb and Ca additions exhibit up to 60%–75% higher peak α2σ/τ than that of intrinsic Bi2Te3 with Bi antisite defects. Pb occupancy and Ca occupancy of Bi sites increase σ/τ by activating high-degeneracy low-effective-mass bands near the valence band edge, unlike Bi antisite occupancy of Te sites that eliminates near-edge valence states in intrinsic Bi2Te3. Neither Pb doping nor subatomic-percent Ca doping increases α significantly, due to band averaging. Higher Ca levels increase α and diminish σ, due to the emergence of a corrugated band structure underpinned by high-effective-mass bands, attributable to Ca–Te bond ionicity. Sn doping results in a distortion of the bands with a higher density of states that may be characterized as a resonant state but decreases α2σ by up to 30% due to increases in the charge carrier effective mass and decreases in both spin–orbit coupling and valence band quasidegeneracy. These results, and thermal conductivity calculations for nanostructured Bi2Te3, suggest that Pb or Ca doping can enhance the thermoelectric figure of merit ZT to values up to ZT ∼ 1.7, based on an experimentally determined τ. Our findings suggest that divalent doping can be attractive for realizing large ZT enhancements in pnictogen chalcogenides.