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Solar cell efficiency.

One method of increasing the photovoltaic power of solar cells is to increase the surface area and/or reduce the amount of reflected light. Straightforward surface area enlargement increases the footprint of the system, while modifying the surface material composition to reduce reflective properties can affect the power output negatively. An alternative solution to achieve both increased surface area and reduced light reflection, is to have a surface that is not perfectly flat but has micro V-shaped grooves on the surface. Engineers at the Fraunhofer-Institut fur Produktionstechnologie (IPT) at Aachen, Germany, have determined that mechanical structuring would be the fastest and most cost effective method. In cooperation with Konstanz University, in Germany, and SKF Linearsysteme GmbH (who designed and provided key positioning components), IPT developed a solution that can be used in mass production. Fine structuring of the silicon wafer demands an unconventional machine concept, as the required accuracies of the final product are in the micron range. Specifications for their design included: the ability to grind the brittle material without damaging the cells; grinding four cells measuring 125 x 125 mm in a single operation for economical reasons; precise typical structural geometries ~160 [micro]m wide worked into the entire wafer surface; and a machine throughput rate up to 12[m.sup.2] cell surface per hour. In appearance the silicon wafer grinding machine is not too different from any other modern grinding machine, yet it is capable of achieving extraordinary machining accuracies. Based on the rigid granite foundation, the workpieces are positioned under the grinding roll on an X-Y crosstable. The slide on the X-axis is driven by a clearance-free ball-screw oscillating stroke and throughout its travel is guided on profile rails with extremely high running accuracy. The deviations in the ground area of a stroke of [+ or -]100 mm must be kept under 4 [micro]m. The Z axis is for workpiece feed and is driven by a linear motor with integral needle roller guides and the direct linear measuring system delivery accuracy higher than that of the X axis. The high-precision grinding spindle is driven by a 33 kW motor is guided in the vertical direction by ultra-precise needle roller bearings. Through this drive the structuring tool rotates at 10,000 rpm and can be made to "nibble" at the surface as it passes across the silicon wafer. The drive consists of a planetary roller screw with AC servo motor while the overall control of the machine is by a normal CNC control system.

Circle 106--SKF Linear Motion, or
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Title Annotation:News From The Green File
Author:Mandel, Richard; Gooch, Stephanie
Date:Jan 1, 2005
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