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

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 (Δ E) of PbSe around room temperature; however, with rising temperature it makes Δ E 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 PbSeTe decreases Δ E and enhances the Seebeck coefficient at all temperatures. Excess Cd precipitates out as CdSeTe , whose valence band aligns with that of the p-type Na-doped PbSeTe 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 PbSeTe is significantly decreased to its amorphous limit of 0.5 W m K. 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 PbNaCdSeTe, which are ∼70% and ∼50% higher than those of PbNaSe control sample, respectively.