Silicon nanostructures for third generation photovoltaic solar cells

The concept of third generation photovoltaics is to significantly increase device efficiencies whilst still using thin film processes and abundant non-toxic materials. This can be achieved by circumventing the Shockley–Queisser limit for single band gap devices, using multiple energy threshold approaches. At the University of NSW, as part of our work on Third Generation devices, we are using the energy confinement of silicon based quantum dot nanostructures to engineer wide band gap materials to be used as upper cell elements in Si based tandem cells. HRTEM data shows Si nanocrystal formation in oxide and nitride matrixes with a controlled nanocrystal size, grown by layered reactive sputtering and layered PECVD. Photoluminescence evidence for quantum confinement in the Si quantum dots in oxide agrees with the calculated increase in PL energy with reduction in dot size. Resistivity measurements with temperature give tentative proof of conduction and we are investigating junction formation in these materials. We are also using similar Si quantum dot structures in double barrier resonant tunneling structures for use in hot carrier solar cell contacts. These must collect carriers over a limited energy range. Negative differential resistance has been observed in room temperature I–V on these samples, a necessary proof of concept for selective energy filter contacts.