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.
Bibliographical noteFunding Information:
G.T. acknowledges the financial support from the Natural Science Foundation of China (Grant no. 11804261). At Northwestern University the work was supported primarily by the Department of Energy, Office of Science, Basic Energy Sciences under Grant DE-SC0014520 (synthesis, sample processing, transport, and TEM and DFT calculations). Photoemission yield spectroscopy measurements were carried out with equipment acquired by ONR Grant N00014-18-1-2102. Transmission electron microscopy work was partially performed in the EPIC facility of the NUANCE Center at Northwestern University. Access to facilities of high-performance computational resources at Northwestern University is acknowledged.
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