A large thermoelectric power factor in heavily boron-doped p-type nanograined Si with grain sizes ∼30 nm and grain boundary regions of ∼2 nm is reported. The reported power factor is ∼5 times higher than in bulk Si. It originates from the surprising observation that for a specific range of carrier concentrations, the electrical conductivity and Seebeck coefficient increase simultaneously. The two essential ingredients for this observation are nanocrystallinity and extremely high boron doping levels. This experimental finding is interpreted within a theoretical model that considers both electron and phonon transport within the semiclassical Boltzmann approach. It is shown that transport takes place through two phases so that high conductivity is achieved in the grains, and high Seebeck coefficient by the grain boundaries. This together with the drastic reduction in the thermal conductivity due to boundary scattering could lead to a significant increase of the figure of merit ZT. This is one of the rare observations of a simultaneous increase in the electrical conductivity and Seebeck coefficient, resulting in enhanced thermoelectric power factor.

This paper will show that, while ZT is an appropriate performance index when optimizing the heat conversion rate, it may mislead research in view of applications aiming at large electric power production. This is of special relevance when related to the surge of research in the area of low-dimensionality semiconductors where ZT is increased by lowering the thermal conductivity κ. It will be shown that, when operating between sources at fixed temperature, the highest power output can be obtained by increasing κ, not decreasing it, the larger electric power output economically enabling thermoelectric generators for massive electric power production.

Thermoelectric generators, which convert heat directly into electrical power, have great potentialities in the energy harvesting field. The exploitation of these potentialities is limited by the materials currently used, characterized by good thermoelectric properties, but also by several drawbacks. This work presents a silicon-based thermoelectric generator, made of a large collection of heavily $p$-doped silicon nanostructures. This macroscopic device (area of several mm$^2$) collects together the good thermoelectric features of silicon, in terms of high power factor, and a very reduced thermal conductivity, which resulted exceptionally low (1.8 W/(m K), close to the amorphous limit). The generated electrical power density is remarkably high for a Si-based thermoelectric generator, and it is suitable for scavenging applications which can exploit small temperature differences. A full characterization of the device (Seebeck coefficient, thermal conductivity, maximum power output) is reported and discussed.