Recent Developments in Semiconductor Thermoelectric Physics and Materials

Recent advances in semiconductor thermoelectric physics and materials are reviewed. A key requirement to improve the energy conversion efficiency is to increase the Seebeck coefficient (5) and the electrical conductivity (sigma) while reducing the electronic and lattice contributions to thermal conductivity (kappa(e) + kappa(L)). Some new physical concepts and nanostructures make it possible to modify the trade-offs between the bulk material properties through changes in the density of states, scattering rates, and interface effects on electron and phonon transport. We review recent experimental and theoretical results on nanostructured materials of various dimensions: superlattices, nanowires, nanodots, and solid-state thermionic power generation devices. Most of the recent success has been in the reduction of lattice thermal conductivity with the concurrent maintenance of good electrical conductivity. Several theoretical and experimental results to improve the thermoelectric power factor (S-2 sigma) and to reduce the Lorenz number (sigma/kappa(e)) are presented. We briefly describe recent developments in nonlinear thermoelectrics, as well as the generalization of the Bergman theorem for composite materials. Although the material thermoelectric figure of merit Z [= S-2 sigma/(kappa(e) + kappa(L))] is a key parameter to optimize, one has to consider the whole system in an energy conversion application. A rarely discussed but important efficiency/cost trade-off for thermoelectric power generation is briefly reviewed, and research directions for the development of low-cost thermoelectric materials are identified. Finally, we highlight the importance of the figure of merit, Z, beyond macroscale energy conversion applications in describing the microscopic coupling between charge and energy transport in materials.