  Large-volume deployment of Si-based Seebeck generators can be foreseen only
if polycrystalline rather than single crystalline materials can be actually
used. The aim of this study was therefore to verify whether polycrystalline Si
films deposited on top of a SiO$_2$ insulating layer can develop interesting
thermoelectric power factors. We prepared 450-nm thick heavily boron doped
polysilicon layers, setting the initial boron content in the film to be in
excess of the boron solubility in polycrystalline silicon at 1000 {\deg}C.
Isochronal thermal annealings were then used to modify the B$_{Si}$ content by
precipitation. Quite unexpectedly, a concurrent increase of the thermoelectric
power and of the conductivity was observed for heat treatments at temperatures
above 800 {\deg}C. Upon annealing at 1000 {\deg}C we found a power factor $P$
of 13 mW K$^{-2}$ m$^{-1}$, more than three times higher than previously
reported $P$ for Si nanowires. These findings could be explained observing that
degenerate polysilicon displays a remarkable enhancement of its Seebeck
coefficient as an effect of the large amount of boron it can dissolve. Band gap
narrowing and band tailing modify the density of states around the Fermi energy
leading to a dramatic improvement of its log-derivative in the Mott equation.
These results apparently point out an interesting direction for the development
of Seebeck and Peltier devices sharing low cost and relatively high efficiency.
