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

SnSe doped a different way: Heat can be converted into electricity by thermoelectric materials. Such materials are promising for use in solid-state cooling devices. A challenge for developing efficient thermoelectric materials is to ensure high electrical but low thermal conductivity. Chang et al. found that bromine doping of tin selenide (SnSe) does just this by maintaining low thermal conductivity in the out-of-plane direction of this layered material. The result is a promising n-type thermoelectric material with electrons as the charge carriers—an important step for developing thermoelectric devices from SnSe. Science , this issue p. 778

Thermoelectric materials have drawn vast attentions for centuries, because thermoelectric effects enable direct conversion between thermal and electrical energy, thus providing an alternative for power generation and refrigeration. This review summaries the thermoelectric phenomena, applications and parameter relationships. The approaches used for thermoelectric performance enhancement are outlined, including: modifications of electronic band structures and band convergence to enhance Seebeck coefficients; nanostructuring and all-scale hierarchical architecturing to reduce the lattice thermal conductivity. Several promising thermoelectric materials with intrinsically low thermal conductivities are introduced. The low thermal conductivities may arise from large molecular weights, complex crystal structures, liquid like transports or high anharmonicity of chemical bonds. At the end, a discussion of future possible strategies is proposed, aiming at further thermoelectric performance enhancements

This review discusses recent developments and current research in high performance bulk thermoelectric materials, comprising nanostructuring, mesostructuring, band alignment, band engineering and synergistically defining key strategies for boosting the thermoelectric performance. To date, the dramatic enhancements in the figure of merit achieved in bulk thermoelectric materials have come either from the reduction in lattice thermal conductivity or improvement in power factors, or both of them. Here, we summarize these relationships between very large reduction of the lattice thermal conductivity with all-scale hierarchical architecturing, large enhanced Seebeck coefficients with intra-matrix electronic structure engineering, and control of the carrier mobility with matrix/inclusion band alignment, which enhance the power factor and reduce the lattice thermal conductivity. The new concept of hierarchical compositionally alloyed nanostructures to achieve these effects is presented. Systems based on PbTe, PbSe and PbS in which spectacular advances have been demonstrated are given particular emphasis. A discussion of future possible strategies is aimed at enhancing the thermoelectric figure of merit of these materials.