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All-Scale Hierarchically Structured p-Type PbSe Alloys with High Thermoelectric Performance Enabled by Improved Band Degeneracy

Tan, Gangjian, Hao, Shiqiang, Cai, Songting, Bailey, Trevor P., Luo, Zhongzhen, Hadar, Ido, Uher, Ctirad, Dravid, Vinayak P., Wolverton, Christopher, Kanatzidis, Mercouri G.
Journal of the American Chemical Society 2019 v.141 no.10 pp. 4480-4486
alloys, ambient temperature, cadmium, energy, lead, thermal conductivity
We show an example of hierarchically designing electronic bands of PbSe toward excellent thermoelectric performance. We find that alloying 15 mol % PbTe into PbSe causes a negligible change in the light and heavy valence band energy offsets (ΔEV) of PbSe around room temperature; however, with rising temperature it makes ΔEV decrease at a significantly higher rate than in PbSe. In other words, the temperature-induced valence band convergence of PbSe is accelerated by alloying with PbTe. On this basis, applying 3 mol % Cd substitution on the Pb sites of PbSe₀.₈₅Te₀.₁₅ decreases ΔEV and enhances the Seebeck coefficient at all temperatures. Excess Cd precipitates out as CdSe₁–yTey, whose valence band aligns with that of the p-type Na-doped PbSe₀.₈₅Te₀.₁₅ matrix. This enables facile charge transport across the matrix/precipitate interfaces and retains the high carrier mobilities. Meanwhile, compared to PbSe the lattice thermal conductivity of PbSe₀.₈₅Te₀.₁₅ is significantly decreased to its amorphous limit of 0.5 W m–¹ K–¹. Consequently, a highest peak ZT of 1.7 at 900 K and a record high average ZT of ∼1 (400–900 K) for a PbSe-based system are achieved in the composition Pb₀.₉₅Na₀.₀₂Cd₀.₀₃Se₀.₈₅Te₀.₁₅, which are ∼70% and ∼50% higher than those of Pb₀.₉₈Na₀.₀₂Se control sample, respectively.