Nano Four PV - Group IV based quantum structures for future Photovoltaics

Description
Photovoltaics (PV) is enormously rising in importance worldwide, both for the widespread economic growth and for the scientific challenges. Despite the world crisis, PV market currently grows at a rate which recalls the Moore’s law in microelectronics, since the PV modules production is doubling every 18-24 months. Photovoltaics drains a lot of interests also by material scientists. In fact, the efficiency in the conversion of sunlight into electricity can increase as a result of better understanding the conversion limiting factors and innovations in the cell design. Conventional materials are able to convert less than 20% of the incoming solar irradiation, while advanced solutions promise above 50% conversion. Among these new directions, nanotechnology, with its advanced maturation, allows new and smart opportunities, such as the chance of modulating the light absorption of the active material onto the solar spectrum, or the option to tailor the solar photons upon the conversion capability of the cell. Indeed, group IV nanostructured materials are attractive because of the easier integration with the present microelectronic technologies. In this talk, the photon absorption in Si or Ge quantum dots (QDs) embedded in SiO2 will be presented and compared for future PV application. Two fabrication techniques for the Si QDs will be compared (plasma-enhanced chemical vapor deposition and magnetron co-sputtering) in terms of light absorption. In any case, by increasing the Si content a reduction in the optical bandgap has been always recorded, pointing out that the density of Si-Si bonds has a crucial role in the absorption process. The structural phase of these Si-Si bonds also affects the photon absorption, since at higher temperatures the amorphous-crystalline transition of the Si QD is associated to an increase of the optical bandgap. At last, by adjusting the Si content and the annealing temperature, a full matching between the rainbow energy and the Si QD optical bandgap can be obtained. Still, the transport of the photo-generated carriers plays an important role, and only the Si richer systems, after a proper doping, exhibited a low enough resistivity to be implemented in PV devices. On the other hand, Ge QDs represent a hopeful route towards PV application too, because of the higher absorption in comparison to Si QDs. The confinement effect on the light absorption process induces an optical bandgap around 1.5 eV, well higher than that of not-confined Ge (~ 0.7 eV), but lower than that of Si QDs. Unfortunately, the thermal processes on Ge QDs affects their absorption ability, causing a reduction of the absorption coefficient (due to the amorphous-crystalline transition) and also a detrimental Ge out-diffusion due to surface oxidation. An overview of all these phenomena will be given.
Organised by Patrizia Strano