Improving the thermoelectric performance of PbSe over its previously reported maximum zT can be achieved by engineering its electronic band structure. We demonstrate here, using optical absorption spectra, first principles calculations, and temperature dependent transport measurements, that alloying PbSe with SrSe leads to a dramatic change of the band structure that increases the thermoelectric figure of merit, zT. The temperature where the two valence bands converge decreases with Sr addition. The zT value, when the carrier density is optimized, increases with Sr addition in Pb 1Àx Sr x Se and when x ¼ 0.08 a maximum zT of 1.5 at 900 K is achieved. The net benefit in zT comes from the band structure tuning even though in other thermoelectric solid solutions it is the thermal conductivity reduction from disorder that leads to net zT improvement. Broader contextBand engineering in semiconductors is important for their application in electronic or optoelectronic devices. For heavily doped thermoelectric semiconductors it is also crucial for the high zT found in PbTe 1Àx Se x , Pb 1Àx Mg x Te, and Mg 2 Si 1Àx Sn x . For high temperature bulk thermoelectrics, most of such engineering is realized by forming solid solutions. In this study we demonstrate successful band tuning of p-type PbSe, the slightly lower zT analog of the well-known thermoelectric compound PbTe, using Pb 1Àx Sr x Se solid solutions. It is well known that formation of solid solutions is desirable for thermoelectrics due to their lower thermal conductivities. We demonstrate here that the ability to change not only the band gap but also the relative positions of different band maxima provides another important benet for solid solutions as thermoelectrics. Actually, we found in these alloys that the reduction of lattice thermal conductivity by alloying has been compensated by the counter effect of the reduced carrier mobility, as also been found in the n-type solid solutions PbTe 1Àx Se x and PbSe 1Àx S x where simply forming solid solutions without the band engineering effect does not improve zT. Therefore we conclude that the change in the band structure with formation of solid solution accounts for the improvement of zT in p-type PbSe from 1.1 to 1.5 at 900 K.

Many monumental breakthroughs in p-type PbTe thermoelectrics are driven by optimizing a Pb 0.98 Na 0.02 Te matrix. However, recent works found that x > 0.02 in Pb 1−x Na x Te further improves the thermoelectric figure of merit, zT, despite being above the expected Na solubility limit. We explain the origins of improved performance from excess Na doping through computation and experiments on Pb 1−x Na x Te with 0.01 ≤ x ≤ 0.04. High temperature X-ray diffraction and Hall carrier concentration measurements show enhanced Na solubility at high temperatures when x > 0.02 but no improvement in carrier concentration, indicating that Na is entering the lattice but is electrically compensated by high intrinsic defect concentrations. The higher Na concentration leads to band convergence between the light L and heavy Σ valence bands in PbTe, suppressing bipolar conduction and increasing the Seebeck coefficient. This results in a high temperature zT nearing 2 for Pb 0.96 Na 0.04 Te, ∼25% higher than traditionally reported values for pristine PbTe-Na. Further, we apply a phase diagram approach to explain the origins of increased solubility from excess Na doping and offer strategies for repeatable synthesis of high zT Na-doped materials. A starting matrix of simple, high performing Pb 0.96 Na 0.04 Te synthesized following our guidelines may be superior to Pb 0.98 Na 0.02 Te for continued zT optimization in p-type PbTe materials.

Chemical composition alteration is a general strategy to optimize the thermoelectric properties of a thermoelectric material to achieve high-efficiency conversion of waste heat into electricity. Recent studies show that the Al 2 Fe 3 Si 3 intermetallic compound with a relatively high power factor of ∼700 μW m −1 K −2 at 400 K is promising for applications in low-cost and nontoxic thermoelectric devices. To accelerate the exploration of the thermoelectric properties of this material in a mid-temperature range and to enhance its power factor, a machine-learning method was employed herein to assist the synthesis of off-stoichiometric samples (namely, Al 23.5+x Fe 36.5 Si 40−x ) of the Al 2 Fe 3 Si 3 compound by tuning the Al/Si ratio. The optimal Al/Si ratio for a high power factor in the mid-temperature range was found rapidly and efficiently, and the optimal ratio of the sample at x = 0.9 was found to increase the power factor at ∼510 K by about 40% with respect to that of the initial sample at x = 0.0. The possible mechanism for the enhanced power factor is discussed in terms of the precipitations of the metallic secondary phases in the Al 23.5+x Fe 36.5 Si 40−x samples. Furthermore, the maximum achievable thermal conductivity of Al 2 Fe 3 Si 3 estimated by the Slack model is ∼10 W m −1 K −1 at the Debye temperature. An avoided-crossing behavior of the acoustic and the low-lying optical modes along several crystallographic directions is found in the phonon dispersion of Al 2 Fe 3 Si 3 calculated by ab initio density functional theory method. These preliminary results suggest that Al 2 Fe 3 Si 3 can have a low thermal conductivity. The calculated formation energies of point defects suggest that the antisite defects between Al and Si are likely to cause the Al and Si off-stoichiometries in Al 2 Fe 3 Si 3 . The theoretically obtained insight provides additional information for the further understanding of Al 2 Fe 3 Si 3 .

a Yb 9 Mn 4.2 Sb 9 has been shown to have extremely low thermal conductivity and a high thermoelectric figure of merit attributed to its complex crystal structure and disordered interstitial sites. Motivated by previous work which shows that isoelectronic substitution of Mn by Zn leads to higher mobility by reducing spin disorder scattering, this study investigates the thermoelectric properties of the solid solution, Yb 9 Mn 4.2Àx Zn x Sb 9 (x ¼ 0, 1, 2, 3 and 4.2). Measurements of the Hall mobility at high temperatures (up to 1000 K) show that the mobility can be increased by more than a factor of 3 by substituting Zn into Mn sites. This increase is explained by the reduction of the valence band effective mass with increasing Zn, leading to a slightly improved thermoelectric quality factor relative to Yb 9 Mn 4.2 Sb 9 . However, increasing the Zn-content also increases the p-type carrier concentration, leading to metallic behavior with low Seebeck coefficients and high electrical conductivity. Varying the filling of the interstitial site in Yb 9 Zn 4+y Sb 9 (y ¼ 0.2, 0.3, 0.4 and 0.5) was attempted, but the carrier concentration ($10 21 cm À3 at 300 K) and Seebeck coefficients remained constant, suggesting that the phase width of Yb 9 Zn 4+y Sb 9 is quite narrow.

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