Grain Boundary Metallization of Solar Cells II

Today, multi crystalline silicon is the most common material for solar cells. It is expected, that its share will even increase in the future, because it can be procured with less energy input than mono crystalline silicon. But solar cells of multi crystalline silicon do not achieve the same conversion efficiency of sun light to electricity than do solar cells of single crystalline silicon. (Currently 19.8% versus 24.8%.) In order to approach the maximum conversion efficiency of which solar cells from multi crystalline silicon are capable in principle, it seems to be unavoidable to process each cell according to its individual characteristics, because the paths of the grain boundaries between the crystallites, and thus the areas of high concentration of traps, differ from cell to cell. This becomes possible with a method based on pattern recognition, which was developed at the Atominstitut of the Austrian Universities. There, the metallic contact lines, which are necessary for the collection of current at the face of the solar cell illuminated by sun light, are mainly placed along grain boundaries. This shades the photovoltaically inefficient regions in favor of the more efficient regions, and reduces the average recombination rate of excited electrons and holes. An increase of the average power output by as much as 8% could be observed. In the present proposal the detection of grain boundaries shall not only be done by an optimized optical method, but also by a physical method which effectively maps the local lifetimes of minority carriers. This should lead to a further improvement of the power output of the cells. Particular emphasis shall be placed on achieving that degree of improvement of power output of multi crystalline silicon solar cells, which can be obtained in principle by the grain boundary contacting method.