Lasers Spur Progress at German Solar Institute

An attentive observer of today’s photovoltaic sector will have noticed that saving material and obtaining higher efficiency values are of central importance to solar cell manufacturers. It is necessary to reach both goals simultaneously, due to the relatively high costs for the input material, crystalline silicon, which forms the basis for more than 90 percent of the solar cells that are produced around the world.

Freiburg, Germany – December, 12, 2003 [] In the laboratory of the Fraunhofer Institute for Solar Energy Systems ISE, thin crystalline solar cells have been produced with a thickness of 37 micrometres (ým) and an efficiency value of 20.2 percent. In comparison, the institute said current industrial cells are comparatively thick – 300 ým (= 0.3 mm) – and significantly less efficient, with an approximate value of 16 pecent. “The enormous potential that is still offered by the well-proven crystalline silicon technology is immediately obvious”, according to Gerhard Willeke, Head of the Solar Cell Department, “and we now have the technology to realise this potential in industrial production”. The solar cell scientists in Freiburg said they have developed an inexpensive process which allows highly efficient solar cells to be produced even from extremely thin silicon wafers. A decisive step leading towards the successful result is a process which has been developed and patented by Fraunhofer ISE for back-surface contacting of the solar cell. The so-called LFC technology (laser fired contacts) offers an opportunity to combine the potential for high efficiency with low production costs. The existing slow and expensive photolithographic processing steps for the back contact are no longer needed, according to the institute. In conventional, laboratory processes for highly efficient cells, considerable effort is needed to open small holes in the insulating layer, and then deposit the back-surface electrode of aluminium. “In LFC processing, we evaporate the aluminium layer directly onto the passivation layer and then fire through the metal with a laser to create the local contacts”, explained Stefan Glunz, the co-ordinator of work at the Institute on monocrystalline silicon solar cells. The institute said the process is inexpensive, requires only one second per solar cell, does not impose significant stress on the material, and functions for all wafer thicknesses and doping levels, making it ideal for industrial mass production. A niche market for extremely thin, high-efficiency cells already exists in aeronautic and space applications. However, to make the cells attractive for terrestrial applications, an appreciable amount of research and development is still needed to manufacture the extremely thin wafers economically.
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