Phonon scattering by nanostructures and point defects has become the primary strategy for minimizing the lattice thermal conductivity (κ ) in thermoelectric materials. However, these scatterers are only effective at the extremes of the phonon spectrum. Recently, it has been demonstrated that dislocations are effective at scattering the remaining mid-frequency phonons as well. In this work, by varying the concentration of Na in Pb Eu Te, it has been determined that the dominant microstructural features are point defects, lattice dislocations, and nanostructure interfaces. This study reveals that dense lattice dislocations (≈4 × 10 cm ) are particularly effective at reducing κ . When the dislocation concentration is maximized, one of the lowest κ values reported for PbTe is achieved. Furthermore, due to the band convergence of the alloyed 3% mol. EuTe the electronic performance is enhanced, and a high thermoelectric figure of merit, zT, of ≈2.2 is achieved. This work not only demonstrates the effectiveness of dense lattice dislocations as a means of lowering κ , but also the importance of engineering both thermal and electronic transport simultaneously when designing high-performance thermoelectrics.

To minimize the lattice thermal conductivity in thermoelectrics, strategies typically focus on the scattering of low-frequency phonons by interfaces and high-frequency phonons by point defects. In addition, scattering of mid-frequency phonons by dense dislocations, localized at the grain boundaries, has been shown to reduce the lattice thermal conductivity and improve the thermoelectric performance. Here we propose a vacancy engineering strategy to create dense dislocations in the grains. In Pb1−xSb2x/3Se solid solutions, cation vacancies are intentionally introduced, where after thermal annealing the vacancies can annihilate through a number of mechanisms creating the desired dislocations homogeneously distributed within the grains. This leads to a lattice thermal conductivity as low as 0.4 Wm−1 K−1 and a high thermoelectric figure of merit, which can be explained by a dislocation scattering model. The vacancy engineering strategy used here should be equally applicable for solid solution thermoelectrics and provides a strategy for improving zT.

Diffuson-mediated thermal transport suggests a lower minimum thermal conductivity than phonon models.

By utilizing the interaction between Cu and CNTs, a record-high zT of 2.4 has been achieved in Cu2Se/CNT hybrid materials.

Thermoelectric materials require an optimum carrier concentration to maximize electrical transport and thus thermoelectric performance. Element-doping and composition off-stoichiometry are the two general and effective approaches to optimize carrier concentrations, which have been successfully applied in almost all semiconductors. In this study, we propose a new strategy coined as bonding energy variation to tune the carrier concentrations in Cu2Se-based liquid-like thermoelectric compounds. By utilizing the different bond features in Cu2Se and Cu2S, alloying S at the Se-sites successfully increases the bonding energy to fix Cu atoms in the crystal lattice to suppress the formation of Cu vacancies, leading to much lowered carrier concentrations toward the optimum value. Combing the lowered electrical and lattice thermal conductivities, and the relatively good carrier mobility caused by the weak alloy scattering potential, ultrahigh zTs are achieved in slightly S doped Cu2Se with a maximum value of 2.0 at 1000 K, 30% higher than that in nominallystoichiometric Cu2Se.The table of contents entry: Beyond element-doping and composition off-stoichiometry, we propose a new strategy coined as bonding energy variation to tune the carrier concentrations in Cu2Se-based liquid-like thermoelectric compounds, leading to a maximum zT value of 2.0 at 1000 K, 30% higher than that in nominally-stoichiometric Cu2Se.