Intrinsic thermoelectric figure of merit enhancement in semiconductors

The recent progress in thermoelectric semiconductor research has resulted in the design of materials with a figure of merit, zT, double that of what existed a decade ago. This progress was based mostly on reductions in the thermal conductivity through nano-patterning. We review here two new techniques that are inspired by nanotechnologies, but apply to bulk materials and do not rely on them to achieve the same results. Bulk semiconductors, such as AgSbTe2, can be selected to have highly anharmonic bonds, resulting in strong phonon-phonon interactions that reduce the lattice thermal conductivity to the amorphous limit. zT values 20% higher than conventional materials have been reached in this class of semiconductors. Another technique is based on creating distortions of the density-of-states by doping with resonant impurities, resulting in a doubling of the zT of PbTe, a semiconductor used for power generation applications near 500degC. Most recently, we found a similar effect in Bi2Te3. The theory behind this approach will be outlined, and its applicability to a wide variety of thermoelectric semiconductors discussed.