Efficient Thin-Film Solar Cells

The first prototype cell to use photonic crystals looks promising.

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. Credit: Lirong Zeng

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.

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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.