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Efficient Thin-Film Solar Cells

The first prototype cell to use photonic crystals looks promising.
December 4, 2008

Researchers at MIT have unveiled a new type of silicon solar cell that could be much more efficient and cost less than currently used solar cells. Materials science and engineering professor Lionel Kimerling and his colleagues presented results of the first device prototype at a recent meeting of the Materials Research Society in Boston.

Light trapper: A transmission electron microscopy (TEM) image shows the back surface of a five-micrometer-thick silicon solar cell. The alternating layers of silicon and silicon dioxide form an excellent light reflector. The crests and troughs send the reflected light into the silicon at a low angle that keeps it trapped inside the silicon for a long time, increasing the efficiency of the cell.

The design combines a highly effective reflector on the back of a solar cell with an antireflective coating on the front. This helps trap red and near-infrared light, which can be used to make electricity, in the silicon. The research team is licensing similar technology to StarSolar, a startup in Cambridge, MA.

The researchers applied their light-trapping scheme on thin silicon cells that are about five micrometers thick. Their prototype solar cell is 15 percent more efficient at converting light into electricity than commercial thin-film solar cells. Project leader Peter Bermel, who is StarSolar’s chief technology officer, says that sophisticated computer simulations suggest that much greater gains in efficiency are possible.

Thin-film silicon solar cells could be cheaper than conventional devices because they use far less material. Conventional solar cells use silicon wafers that are over 100 micrometers thick, while thin-film devices have thicknesses of a few micrometers. But thin-film devices suffer from lower efficiencies. This is mainly because of the red and near-infrared photons, which don’t stay trapped inside the thin silicon long enough to get absorbed.

Today’s solar cells are backed with a metal layer, typically aluminum, to reflect light. But this scheme does not work very well, and of the light inside the silicon solar cell, thirty percent is lost every time it bounces off the metal.

Instead of using a metal backing, the MIT researchers engineer the back surface of a silicon solar cell to make it efficient at reflecting and trapping light. First they etch a series of ridges and troughs, called a grating. On top of this they deposit a photonic crystal–a periodic structure composed of multiple alternating layers of silicon and silicon dioxide.

The photonic crystal reflects light, while the grating sends this light back into the silicon at a low angle. This keeps the light bouncing around inside and prevents it from escaping. The longer the light stays in, the more likely it will be absorbed and converted into electricity.

“This work demonstrates the importance of improving the performance of thin-film technologies,” says Stephen Saylor, CEO of SiOnyx in Beverly, MA. SiOnyx is taking a different approach to increasing the absorption of red and infrared light in thin silicon devices. The company’s black silicon material has a surface with nanoscale roughness that helps it absorb all visible and infrared light. The material’s potential for solar cells has not been demonstrated yet.

Meanwhile, at the Ames Laboratory in Ames, IA, physicist Rana Biswas and his colleagues are using photonic crystals to make amorphous silicon solar cells more efficient. Their photonic crystal is composed of a lattice of tiny silicon cylinders inside an indium-tin-oxide layer. It could increase the efficiency of the solar cells by a maximum of 15 percent. But their amorphous silicon solar cells are only 0.5 micrometers thick, a tenth of the size of the MIT devices. “Generally, amorphous silicon-film solar cells need much less material, so cost goes down,” Biswas says. “Plus they could be deposited on plastics. That’s a big plus.”

The MIT researchers aim to make thin-film silicon solar cells that are good enough to compete with conventional solar cells, Bermel says. By optimizing the photonic-crystal and grating structures, the researchers could squeeze the maximum efficiency out of the solar cells, increasing it to 13 percent. That would be comparable with the 13- to 15-percent efficiencies of some conventional solar cells.

The solar cells are far from practical right now. The researchers use an expensive technique called interference lithography to make the grating. Furthermore, the alternating layers in the reflector are deposited one by one, which is time-consuming. The researchers need to find a manufacturing technique that allows them to make the solar cells on a large scale and at low cost. “The ultimate question that must be answered is scalability,” Saylor says. “To have a real impact, any solution must cost-effectively scale to mass production.”

Bermel says that his team is already considering other production methods. One promising option is nanoimprint lithography, but they haven’t tried it yet. “A 35 percent efficiency increase is clearly predicted in simulations,” he says, “but the challenge is, ‘Can you make it practically?’ That’s what we’re working on.”

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