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 (ΔE_V) of PbSe around room temperature; however, with rising temperature it makes ΔE_V 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 PbSe_0.85Te_0.15 decreases ΔE_V and enhances the Seebeck coefficient at all temperatures. Excess Cd precipitates out as CdSe_1-yTe_y, whose valence band aligns with that of the p-type Na-doped PbSe_0.85Te_0.15 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 PbSe_0.85Te_0.15 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 Pb_0.95Na_0.02Cd_0.03Se_0.85Te_0.15, which are ∼70% and ∼50% higher than those of Pb_0.98Na_0.02Se control sample, respectively.