US researchers boost power output of thin silicon solar cells

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Researchers in the US have discovered a new way to trap light in thin film silicon solar cells which could reduce the thickness of the silicon used by more than 90%, while still maintaining high efficiency.

The team at MIT used a pattern of tiny 'inverted nanopyramids' etched into the surface of the silicon. These indentations, each less than a millionth of a metre across, are said to trap light as effectively as solid silicon surfaces that are 30 times thicker. "We see our method as enhancing the performance of thin film solar cells," commented MIT mechanical engineering postdoc Anastassios Mavrokefalos, but it could work for any silicon cells. "It would enhance the efficiency, no matter what the thickness," he claimed. Highly purified silicon can account for up to 40% of the overall costs of conventional solar cell arrays. The new method could not only reduce the amount of silicon needed to make the solar cells, but also cut the weight of the cells, in turn reducing the material needed for frames and support. According to Mavrokefalos, the potential cost savings are "not only in the cell material, but also in the installation costs." To create the indentations, the researchers used two sets of overlapping laser beams to produce tiny holes in a layer of material — called a photoresist — that is deposited on top of the silicon. Potassium hydroxide was then used to dissolve away parts of the surface that were not covered by the photoresist. The crystal structure of silicon caused this etching process to produce the desired pyramidal shapes in the surface. The researchers say the technique uses equipment and materials that are already standard parts of silicon chip processing, so no new manufacturing machinery or chemicals would be required. So far the team has only produced the surface on a silicon wafer and demonstrated its ability to trap light. The next step will be to add components to produce an actual photovoltaic cell and then prove its efficiency. The expectation is that the new approach will produce energy conversion efficiencies of about 20% - compared to the 24% of the best commercially available silicon solar cells. Gang Chen, the Carl Richard Soderberg professor of power engineering and director of MIT's Pappalardo micro and nano engineering laboratories, said that if all goes well, the system could lead to commercial products in the near future.