Silicon and Germanium Nanostructures for Photovoltaic Applications: Ab-Initio Results

Actually, most of the electric energy is beingproduced by fossil fuels and great is the search for viablealternatives. The most appealing and promising technologyis photovoltaics. It will become truly mainstream when itscost will be comparable to other energy sources. One way isto significantly enhance device efficiencies, for example byincreasing the number of band gaps in multijunction solarcells or by favoring charge separation in the devices. Thiscan be done by using cells based on nanostructured semiconductors.In this paper, we will present ab-initio results ofthe structural, electronic and optical properties of (1) siliconand germanium nanoparticles embedded in wide band gapmaterials and (2) mixed silicon-germanium nanowires. Weshow that theory can help in understanding the microscopicprocesses important for devices performances. In particular,we calculated for embedded Si and Ge nanoparticles thedependence of the absorption threshold on size and oxidation,the role of crystallinity and, in some cases, the recombinationrates, and we demonstrated that in the case of mixednanowires, those with a clear interface between Si and Geshow not only a reduced quantum confinement effect butdisplay also a natural geometrical separation between electronand hole.