Solar Cells Info

New materials for high-performance cells could make solar power affordable.
By Kevin Bullis, September 23, 2008
Source: MIT TechnologyReview.com
http://www.technologyreview.com/Biztech/21405/

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.

Some companies have already introduced products that pair a textured front with a mirrored back, and the technique has been demonstrated to work well in laboratories for years. But adding the reflective layer typically requires expensive processing and lithography. Rohatgi has developed proprietary materials that can be incorporated into the solar cells using screen printing. This, along with other advances that simplify the manufacturing process, allowed the company to produce highly efficient cells at a low cost.

Tonio Buonassisi, a professor of mechanical engineering at MIT, says that Suniva’s new solar cell is “exciting” because “it’s a demonstration that some of the high-efficiency technologies that have been worked on for years in the laboratory can be applicable in the marketplace.” He says that Suniva’s decision to use such technologies is a risk that most other solar-cell companies have been avoiding. Now that Suniva has developed a way to apply these techniques cheaply, he predicts that other solar-cell companies could be forced to do likewise to compete.

To be sure, significant work remains before the goal of 8 to 10 cents per kilowatt can be achieved. Suniva has demonstrated the crucial first step, which is to show that it can make solar cells that are more than 20 percent efficient using screen printing. The results have been confirmed by the National Renewable Energy Laboratory, in Golden, CO. But for those tests, Suniva used cells with 200-micrometer-thick silicon wafers, and reaching 8 cents a kilowatt will require 100-micrometer wafers. That this is technically possible has been established. The challenge lies in acquiring large amounts of such silicon, since wafers that thin aren’t commercially available, Rohatgi says. What’s more, factories will need to be retooled to handle 100-micrometer cells, which machines designed to handle thicker wafers could break.

The company’s priority now is to scale up production of its highly efficient 200-micrometer cells, which could still lower the cost of solar power. Once it has established high-volume manufacturing, the next step is to introduce thinner wafers, bringing down the costs yet further.