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

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.