Optimization of the thermoelectric properties of nanostructured silicon

We analyse the possibilities opened by nanostructuring for the efficient use of silicon as a thermoelectric material. Nanostructured silicon does not offer significant opportunities from the point of view of an increased Seebeck coefficient; however, nanostructuring allows an important advantage in terms of the reduction of thermal conductivity, which is a key factor for increasing the thermoelectric figure of merit. We will show that when the phonon contribution to the thermal conductivity is reduced down to the order of (or below) 1 W/(mK), doping of silicon can be tailored to optimize the figure of merit. In particular, the figure of merit can increase by more than a factor of two if the doping concentration varies by an order of magnitude. We report the numerical calculation of the efficiency for a thermoelectric generator based on silicon nanowires, taking into account the dependence of thermoelectric parameters on temperature. Finally, we show that, for a given thermal conductivity, the optimal doping concentration depends on the nanowire width and on the temperature difference between the hot and cold sources.