Micro Solar Cells Handle More Intense Sunlight
A startup company hopes to bring down the cost of generating power with concentrated sunlight by using microscale solar cells that can utilize twice as much light as other panels, without the need for expensive optics or cooling systems. Panels made from the tiny cells, which the Durham, NC-based company Semprius developed using a novel microprinting technology, also offer significant savings on materials costs. In late January, the company announced a joint agreement with Siemens to develop demonstration systems based on its technology. Semprius plans to begin volume production of the modules in 2013.
Adding concentrating lenses to solar panels increases the amount of electricity they can produce. But photovoltaic concentrators add a great deal of expense to a solar installation. The optical systems themselves are expensive and bulky–the larger a cell, the larger its paired lens must be. More intense light also means that more performance-degrading heat must be dissipated using heat sinks or fans. Although the cost is partly offset by the efficiency of high-concentration photovoltaics, it limits the potential power of such concentrator systems. The two major suppliers of concentrated solar modules, Amonix and Emcore, both sell systems based on conventional-size cells that operate under 500 times concentration sunlight with costly cooling systems.
Semprius’s solar modules contain arrays of square cells that measure just 600 micrometers on each side. These cells have three semiconducting layers–each of which is based on gallium arsenide and absorbs a different band of sunlight–and they are made using a combination of chemical etching and printing, which means fewer raw materials are wasted. They can operate under sunlight concentrated 1,000 times using cheap optical systems. According to the National Renewable Energy Laboratories, the efficiency of the resulting modules ranges from 25 to 35 percent and they can provide electricity for about 10 cents a kilowatt hour. The company expects the final costs, including installation, to be $2 to $3 per watt.
Last year, a study by researchers at Sandia National Laboratories in Albuquerque, NM, suggested that microscale solar cells might offer various cost and design advantages. “You reduce the amount of semiconductor you need, so there can be a big cost savings,” says Gregory Nielson, head scientist on the Sandia project. “And you can do things with the optics that you can’t do with larger cells.”
Smaller solar cells are more efficient at dissipating heat. “When the cells are below a millimeter, they reject the heat so efficiently they’ll be just as cool as a one-sun panel,” without the need for any cooling systems, says Nielson. This is because the tiny cells have a much greater percentage of total area given up to heat-diffusing edges.
The key to making Semprius’s cells is a printing process developed by researchers led by John Rogers, professor of materials science and engineering at the University of Illinois at Urbana-Champaign.
Solar cells are typically made by building up active layers on the surface of a semiconductor wafer, then sawing the wafer into pieces. Semprius’s printing process begins by treating wafers in much the same way. But instead of sawing, the company uses chemical etching to score the surface of a wafer into microscale cells, leaving them attached to the wafer’s surface by a small tab. The key to the etching step is adding a sacrificial layer when the wafers are treated. The chemical etchant eats away at just this layer, cleaving the cells from the surface. A robot bearing a polymer stamp then moves over the wafer, picking up the cells and placing them on top of an array of ceramic backings printed with electrical contacts. The process uses only a thin layer of the surface of the wafer, which can be sent back to the foundry to be reused. Each four-inch wafer can be used to produce 36,000 cells.
Each cell is then topped with a tiny spherical ball lens. “Normally there’s a huge hot spot at the center of the cell, but the ball lens uniformly distributes the light,” says Joseph Carr, Semprius’s CEO. These lenses capture sunlight from a wide angle. Finally, the lens-topped cells are grouped into 14-inch arrays, which are topped with silicone lenses that direct sunlight onto the smaller ball lenses. Together, the optical system concentrates the sun’s light 1,000 times. These arrays are stacked on a light tracker to make an 18-by-8-foot solar module.
Semprius plans to license its printing technology to enable volume production of the modules by 2013. The company plans to develop sun-tracking control systems with Siemens, and to further develop its microprinting technology, which is compatible with a range of semiconducting materials, including silicon.
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