More Light for Tandem Cells

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- The tow individual cells currently are still stacked mechanically in the lab to form a tandem cell. Photo: HZB/M. Schmid
A solar cell always uses only a portion of the solar spectrum, that is it converts into power only a certain range of colours (frequencies). Therefore, what is more logical than to stack several kinds of solar cells on top of each other, in order to transform a larger portion of light into power? This approach is being pursued to assemble tandem solar cells consisting of a top and a bottom cell. In doing so, certain colours of the light are absorbed by the top cell and converted into electric power, while the bottom cell uses the remaining parts of the colour spectrum.
Yet in practice it appears that the top cell allows too little light to pass through to the bottom cell even at those frequencies, which cannot even be used for power production in the top cell itself. Now, Dr. Martina Schmid from the team of Professor Dr. Martha-Christina Lux-Steiner at the HZB achieved a significant improvement: In her PhD thesis on the optimal construction of chalcopyrite tandem solar cells, she developed an optical model of this layered cell and calculated which losses occur and where. This model enabled optimisation of the stacking structure and layer thicknesses of the tandem cell, so that the transparency of the top cell increased from 60 to 80 percent. As a result, the efficiency of the entire tandem cell could increase to above 10 percent. Before the optimisation only 8.5 percent was achieved. “If one now would optimise the electric properties of the top cell, for example by improving the material characteristics, then efficiencies of above 20 percent were feasible for the tandem”, says Schmid. The physicist is now working on strengthening the absorption of certain frequency ranges and their conversion into electric power by aid of specially applied nano-structures.










