Tuesday, September 23, 2008

Efficient, Cheap Solar Cells

New materials for high-performance cells could make solar power affordable.

By Kevin Bullis

Further reflection: This new solar cell is extremely efficient but can be made cheaply, thanks to a new technique for affixing mirrors to silicon. Credit: Suniva

A cheap new way to attach mirrors to silicon yields very efficient solar cells that don't cost much to manufacture. The technique could lead to solar panels that produce electricity for the average price of electricity in the United States.

Suniva, a startup based in Atlanta, has made solar cells that convert about 20 percent of the energy in the sunlight that falls on them into electricity. That's up from 17 percent for its previous solar cells and close to the efficiency of the best solar cells on the market. But unlike other high-efficiency silicon solar cells, says Ajeet Rohatgi, the company's founder and chief technology officer, Suniva's are made using low-cost methods. One such method is screen printing, a relatively cheap process much like the silk-screen process used to print T-shirts.

So far, the high cost of solar cells has limited them to a marginal role in power production, accounting for less than 1 percent of electricity worldwide. Rohatgi calculates that the company's low-cost manufacturing techniques will make solar power competitive with conventional sources, producing electricity for about 8 to 10 cents per kilowatt-hour--the average cost of electricity in the United States and far less than prices in many markets.

Suniva's cells are efficient largely because they can trap light, keeping photons inside the active material of the solar cell until their energy can be used to free electrons and generate an electrical current. The basic concept of trapping light is not new. It relies on texturing the front surface of the layer of silicon that forms the active material of the solar cell. The texturing creates facets that redirect incoming light, refracting it so that, instead of passing directly through the silicon, it travels along the length of the silicon layer. The photons thus stay in the material longer and have a better chance of being absorbed by atoms in the material. When that happens, the energy in the photons can free electrons that are used to generate current.

Light trapping can be enhanced by pairing the textured surface with a reflective layer at the back of the silicon layer. The mirror keeps the light in the solar cell still longer, further increasing the number of freed electrons. As a consequence, the silicon can be half its ordinary thickness while absorbing the same amount of light. Using less of an expensive material reduces costs directly. But it also allows solar-cell makers to make do with cheaper, less pure forms of silicon. In a conventional solar cell, which can have a silicon layer 200 micrometers thick, impurities within the material can easily trap electrons before they reach the surface and escape to generate a current. In a layer of silicon just 100 micrometers thick, however, the electrons have a shorter distance to travel, so they're less likely to encounter an impurity before they escape. Lower-grade silicon is much cheaper and easier to make than the highly refined silicon ordinarily used in solar cells.