Enhanced Thermoelectric ZT in the Tails of the Fermi Distribution via Electron Filtering by Nanoscale Defects — Model Electron Transport in Nanostructured Materials

The addition of nanoscale defects to thermoelectric materials can significantly increase the figure of merit, ZT, by reducing the thermal conductivity. Unfortunately, these defects are also detrimental to the thermoelectric power factor in the numerator of the figure of merit ZT. We have derived strategies to recoup and even enhance the electrical performance of nanostructures Si containing additive SiC nanoparticles/ porosity by fine tuning the carrier concentration and through judicious design of the defects size and shape so as to provide energy selective electron filtering. A semiclassical Boltzmann transport equation is used to model the electrical transport properties of the Seebeck coefficient and electrical conductivity. The model is validated against a set of experiments on Si/SiC nanocomposites. The nanoinclusions lead to a significant improvement of the Seebeck coefficient. The theoretical analysis confirms that the enhancements in the thermoelectric properties are consistent with the energy selective scattering of electrons induced by the offset between the silicon Fermi level and the carbide conduction band edge. We also considered Si membranes containing discrete pores that are either spheres, cylinders, cubes, or triangular prisms. This model reveals three key results: The largest enhancement in the Seebeck coefficient occurs with cubic pores. The fractional improvement is about 15% at low carrier concentration up to 60% at high carrier population with characteristic length around 1 nm. To obtain the best energy filtering effect at room temperature, nanoporous Si needs to be doped to a higher carrier concentration than is optimal for bulk Si. Finally, in n-type Si thermoelectrics the electron filtering effect that can be generated with nanoscale porosity is significantly lower than the ideal filtering effect; nevertheless, the enhancement in the Seebeck coefficient that can be obtained is large enough to offset the reduction in electrical conductivity caused by porosity. This study proves that careful engineering of the energy-dependent electron scattering rate can provide a route towards relaxing long-standing constraints in the design of thermoelectric materials.