Efficient Solar Cells from Cheaper Materials  

Technology Review has an article on "efficient cells made using abundant elements" from IBM, which may be an alternative to thin film solar cells - Efficient Solar Cells from Cheaper Materials.

Researchers at IBM have increased the efficiency of a novel type of solar cell made largely from cheap and abundant materials by over 40 percent. According to an article published this week in the journal Advanced Materials, the new efficiency is 9.6 percent, up from the previous record of 6.7 percent for this type of solar cell, and near the level needed for commercial solar panels. The IBM solar cells also have the advantage of being made with an inexpensive ink-based process.

The new solar cells convert light into electricity using a semiconductor material made of copper, zinc, tin, and sulfur--all abundant elements--as well as the relatively rare element selenium (CZTS). Reaching near-commercial efficiency levels is a "breakthrough for this technology," says Matthew Beard, a senior scientist at the National Renewable Energy Laboratory, who was not involved with the work.

The IBM solar cells could be an alternative to existing "thin film" solar cells. Thin film solar cells use materials that are particularly good at absorbing light. The leading thin film manufacturer uses a material that includes the rare element tellurium. Daniel Kammen, director of the Renewable and Appropriate Energy Laboratory at the University of California, Berkeley, says the presence of tellurium could limit the total electricity such cells could produce because of its rarity. While total worldwide electricity demand will likely reach dozens of terawatts (trillions of watts) in the coming decades, thin film solar cells will likely be limited to producing about 0.3 terawatts, according to a study he published last year. In contrast, the new cells from IBM could produce an order of magnitude more power.

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.