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Abb. 1: Nach der Kristallisation werden aus den Ingots einzelne Blöcke geschnitten.
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Solar cells from silicon wafers

Solar cells made of crystalline silicon utilise the second most common element in the earth’s crust. They are based on a relatively simple, robust and reliable technology with an ‘older sister’ – microelectronics. Presented for the first time more than 50 years ago, they have now become the absolute ‘classic’ among solar cells and have gained a firm foothold internationally with market shares between 70 and 100%.

The success story of silicon solar cells began in 1954 in the USA, with a 2 cm² laboratory sample that achieved efficiencies of between four and six per cent. In a short period of time it was developed for special applications where the costs were not the decisive factor. For example, the first satellite equipped with solar cells was launched just four years later. However, for many years photovoltaics remained a niche technology. It was the energy crises in the 1970s that first brought about a change in attitudes. In the ensuing period, a series of countries, including to a considerable extent Germany, began to initiate strategic research programmes. By achieving higher electricity yields with considerably smaller costs, the intention was to achieve solar cells that could be used as a mass-produced product for large-scale energy supplies. And indeed: in 2009, the worldwide installed output increased by 7.3 GWp to around 22 GWp. Whereas production was once based on decommissioned equipment from the microelectronics industry, it is now possible to purchase turnkey production lines with an annual production capacity of 120 MW and solar cell efficiency guarantees. Complete GW factories are now in the stage of development.

Acquired experience and mass production have positive effects on the prices, which can be described using an empirical learning curve: for more than 3 decades, the module prices have dropped by around 20% with each doubling of the worldwide installed peak capacity. This now happens roughly every two years. The costs correspondingly reduce each year by around 8 to 10 per cent. In the middle of 2010, the module prices were less than EUR 2/Wp and the system prices were in some cases less than EUR 3/Wp. This development has been made possible through the progress achieved in all process stages: for example the wafer thickness has more than halved from 400 μm in 1980 to 180 μm today while the cell size has more than doubled from 100 cm² to 240 cm². The maximum module efficiency has increased from 8% to almost 20%. The average module efficiency for all crystalline silicon modules reached 13.5% in 2010, compared with 12.0% in 2003.

If intensive research and development continues, industry experts expect the learning curve factor to continue for at least the next one or two decades. That would mean that the module manufacturing costs would fall in the next few years to around EUR 1/Wp, to EUR 0.75/Wp by 2020, and to even less in the following years. Researchers are working on this throughout all stages of the value added chain. The focus is on reducing the material consumption, increasing the efficiency and utilising innovative and automated mass-production processes. Further key areas include reducing the energy consumption in the production, the environmentally friendliness of the system, including the recycling, and the development of standard products.

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