A theoretical study of enhancing thermoelectric efficiency in pnictogen-chalcogen alloys via doping, strain, and nanostructuring

Abstract

Finally, using similar DFT-based transport calculations with supercells, this thesis explores the effects of divalent, p-type dopants Pb, Ca, and Sn, to replicate enhanced band quasi-degeneracy behavior under ambient conditions. When compared to intrinsic Bi2Te3 with p-type Bi anti-site defects, dilute (sub-atomic-percent) Pb doping enhances α2σ up to 75%, in agreement with experiment. This is due to the preservation of highly-degenerate, low-effective-mass valence bands, which enhance σ without degrading α. Thermoelectric transport in Ca-doped Bi2Te3 has yet to be experimentally measured. This thesis therefore predicts that Ca doping provides up to 60% α2σ enhancement, owed to increase in valence band quasi-degeneracy. Increasing Ca concentration leads to the formation of high-effective-mass bands, which enhance α instead of σ. Lastly, dilute Sn doping causes up to 30% degradation in α2σ, due to decreased valence band quasi-degeneracy and increased hole effective mass. Contrary to popular hypotheses, the Sn-induced resonant state does not enhance α2σ. Rather, it produces a lower secondary α2σ peak at higher hole concentration. It is therefore not resonant states but increased band-edge quasi-degeneracy which transcends single crystal α2σ limits in Bi2Te3. When combined with nanostructuring, Pb and Ca doping can achieve ZT ~ 1.7.