Semiconductors for thermoelectric and spin-thermal solid-state energy conversion

The recent decade has seen a doubling of the efficiency of thermoelectric converters through the use of nanotechnology. Here we first outline how specific doping impurities in semiconductors, called resonant levels, mimic the effects of nanostructures and give the same benefits in bulk semiconductors. Resonant impurities catalyse the formation of new bands in semiconductors, which can be engineered to tailor the thermopower. The result is an efficiency increase of 50% in BiSb and more in PbTe. Then, we describe a new solid-state heat-to-electricity conversion mechanism, the spin-Seebeck effect, which arises from driving spinpolarized particles out of thermal equilibrium. This generates a spin flux that is converted into a voltage in a non-polarized adjacent material; the effect can be as large as the highest thermoelectric voltages in semiconductors. Nanotechnologies and these new effects add new design variables to the old problem of optimizing the thermoelectric figure of merit, resulting in much improved thermal efficiencies.