Nanostructuring has opened new ways to increase the thermoelectric performance of a host of materials, mainly by decreasing their thermal conductivity κ while preserving the Seebeck coefficient S and electrical conductivity σ. The thermoelectric properties of degenerated polycrystalline silicon films with nanocavities (NCs) have been studied as a function of annealing temperature upon isochronous annealings in argon carried out every 50°C in the range 500 – 1000°C which were used to modify the shape of the NCs. We found that presence of the NCs had no negative effect on the electronic properties of the system. The measured values of S and σ were close to those previously reported for the blank polycrystalline silicon films with the same doping level. The thermal conductivity was also found to be close to the value measured on the blank sample, about half of the reported value in polycrystals. This led to a power factor of 15.2 mWm-1K-2 and a figure of merit of 0.18 at 300 K.

We report the study of the thermoelectric properties of degenerate, boron-doped polycrystalline silicon on insulator structures. The occurrence of a regime where both the Seebeck coefficient and the conductivity increase is confirmed. This results in a power factor P of 13 mW K -2 m -1 . We propose that such high values of P may be determined by adiabatic energy filtering occurring at grain boundaries decorated by segregated boron.

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.

A new semi-quantitative method providing the relative efficiency of three different organic functionalization reactions onto porous silicon has been set up, based on infrared absorption data. Compared to previously reported techniques, it enables a direct titration of the grafted molecules. We demonstrated that grafting of Si-styrenyl moieties by ethylaluminium dichloride mediated hydrosilylation of phenylacetylene leads to higher yields than organometallic addition onto either hydrogenated or brominated silicon.