Aberration-corrected scanning transmission electron microscopy of nanostructures for photovoltaics

The introduction of semiconductor and metallic nanostructures in photovoltaic materials is an active and promising area of development. This is due to the existence of several characteristics of nanostructures that improve the functionality of photovoltaic materials, for example by an enhancement of efficiency in solar cells, an increase of collected sun light and by the use of up- and down-converters to change the energy of incident photons. Nanostructures for solar cells offer, in comparison with single junction solar cells, new photovoltaic mechanisms for transforming sun light energy into electrical energy. A critical issue to understand the behavior of nanostructures based solar cells is to know their structural characteristics (size, shape, strain, distribution and composition) at nanoscale. In addition to this, the knowledge of the processes of nucleation, segregation and intermixing in nanostructures at nanoscale, and in some cases to atomic scale, is another important point to improve their design. This communication reviews our contribution toward the understanding of these issues and present some prospective studies and experiments in this context, by the use and development of methods based in aberration-corrected scanning transmission electron microscopy, high resolution electron microscopy and other complementary techniques. These methods are applied to several semiconductor nanostructures for intermediate band solar cells and some prospective results are presented in relation to their potential use for nanostructured thin film solar cells. We also present a method to prepare special samples to correlate, in individual or some few nanostructures, high resolution structural properties with other functional properties for the better design of photovoltaic materials. The nanostructures studied by aberration-corrected scanning transmission electron microscopy that are reviewed in this paper consist of III-V quantum dots and wires, silicon wires and strain- compensated stacked nanostructures grown by epitaxial techniques.