Heat conversion gets a power boost Thermoelectric materials convert waste heat into electricity, but often achieve high conversion efficiencies only at high temperatures. Zhao et al. tackle this problem by introducing small amounts of sodium to the thermoelectric SnSe (see the Perspective by Behnia). This boosts the power factor, allowing the material to generate more energy while maintaining good conversion efficiency. The effect holds across a wide temperature range, which is attractive for developing new applications. Science , this issue p. 141 ; see also p. 124

Materials and Methods Tetrahedrite synthesis: Cu 12-x (Fe,Zn) 2-x Sb 4 S 13 samples were synthesized by direct reaction of the starting elements-Cu(99.99 %, Alfa-Aesar), Sb(99.9999 %, Alfa-Aesar), and S, Zn, Fe (99.999%, Alfa-Aesar). The elements were weighed out in stoichiometric proportions using a high-precision Mettler balance; typical charges were on the order of 5 grams total, with individual element masses weighed out with an accuracy of 0.0005 g (0.5 mg). The stoichiometric proportions of the elements were placed into quartz ampoules of inside diameter 10 mm and wall thickness 0.5 mm. The ampoules were evacuated of air using a turbo molecular pump; typical final pressures were <10-5 Torr. The ampoules were sealed under dynamic vacuum using an oxygen/methane torch and provided with a small quartz hook on the top. A wire was attached to this hook and the ampoules were suspended in a vertical Thermolyne tube furnace at room temperature. The furnace was heated at 0.3 ºC min-1 to 650 ºC and held at that temperature for 12 hours. Subsequently, the furnace was cooled to room temperature at the rate of 0.4 ºC min-1. Natural tetrahedrite mineral: A natural specimen of tetrahedrite (photograph, Figure S1) was obtained from a mineral dealer [S1]. This specimen, which is identified as tetrahedrite from its geological characteristics, originated from Casapalca region of Peru. It also contained small regions of quartz (small white crystals, Figure S1), and pyrite (not shown in Figure S1).

We report a significant enhancement of the thermoelectric performance of p-type SnTe over a broad temperature plateau with a peak ZT value of ∼1.4 at 923 K through In/Cd codoping and a CdS nanostructuring approach. Indium and cadmium play different but complementary roles in modifying the valence band structure of SnTe. Specifically, In-doping introduces resonant levels inside the valence bands, leading to a considerably improved Seebeck coefficient at low temperature. Cd-doping, however, increases the Seebeck coefficient of SnTe remarkably in the mid- to high-temperature region via a convergence of the light and heavy hole bands and an enlargement of the band gap. Combining the two dopants in SnTe yields enhanced Seebeck coefficient and power factor over a wide temperature range due to the synergy of resonance levels and valence band convergence, as demonstrated by the Pisarenko plot and supported by first-principles band structure calculations. Moreover, these codoped samples can be hierarchically structured on all scales (atomic point defects by doping, nanoscale precipitations by CdS nanostructuring, and mesoscale grains by SPS treatment) to achieve highly effective phonon scattering leading to strongly reduced thermal conductivities. In addition to the high maximum ZT the resultant large average ZT of ∼0.8 between 300 and 923 K makes SnTe an attractive p-type material for high-temperature thermoelectric power generation.

The broad-based implementation of thermoelectric materials in converting heat to electricity hinges on the achievement of high conversion efficiency. Here we demonstrate a thermoelectric figure of merit ZT of 2.5 at 923 K by the cumulative integration of several performance-enhancing concepts in a single material system. Using non-equilibrium processing we show that hole-doped samples of PbTe can be heavily alloyed with SrTe well beyond its thermodynamic solubility limit of <1 mol%. The much higher levels of Sr alloyed into the PbTe matrix widen the bandgap and create convergence of the two valence bands of PbTe, greatly boosting the power factors with maximal values over 30 μW cm−1 K−2. Exceeding the 5 mol% solubility limit leads to endotaxial SrTe nanostructures which produce extremely low lattice thermal conductivity of 0.5 W m−1 K−1 but preserve high hole mobilities because of the matrix/precipitate valence band alignment. The best composition is hole-doped PbTe–8%SrTe.

Iodine-doped Cu2 Se shows a significantly improved thermoelectric performance during phase transitions by electron and phonon critical scattering, leading to a dramatic increase in zT by a factor of 3-7 times culminating in zT values of 2.3 at 400 K.