Bright Days for Solar

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Cheap and Dirty Silicon
In spite of its predominance in the industry, in many ways silicon is a lousy material for solar cells. Because it doesn't absorb light as well as some other semiconductors, a relatively thick slab of silicon is needed to generate useful amounts of electricity. But using a thick material is problematic. When a silicon solar cell absorbs light, the light's energy frees electrons to move through the material. To generate electricity, the electrons have to escape the material and reach an outside circuit. If the silicon is thick, the electrons have far to go to escape. Along the way, they can be trapped by defects and impurities in the material. So the silicon must be made as defect free and pure as possible--an expensive proposition.

Yet if solar energy is ever going to be widespread, then silicon cells are a good choice, because silicon is extremely abundant--second only to oxygen in the earth's crust. The current leading alternatives to silicon-based solar cells use rare elements, significantly limiting their potential for large-scale use.

So Buonassisi and fellow mechanical-engineering professor Emanuel Sachs are developing techniques to overcome silicon's shortcomings. Sachs recently started 1366 Technologies, based in Lexington, MA, which will commercialize advances from his lab that make silicon solar cells more efficient. For example, much of the light that enters a conventional silicon solar cell reflects back out again. Sachs has found a way to manufacture solar cells so that more of the light that enters them bounces around until it's absorbed and converted into electricity.

Meanwhile, Buonassisi is figuring out how to keep contaminants and defects in silicon from trapping electrons. Today solar-cell manu­facturers use the same expensive high-grade silicon that goes into computer chips. But while the chip industry spends about $1,000 per gram on clean manufacturing processes to keep its silicon pure, solar-cell companies are limited to less than a dollar per gram, Buonassisi says; otherwise their products can't compete with conventional sources of electricity. And a dollar a gram isn't enough to keep silicon clean. At one stage in the manufacture of a solar cell, for example, the silicon is heated in a crucible to about 1,400 ºC, before being cooled again to make crystalline ingots. But iron from the crucibles and stainless-steel furnace parts dissolves in the silicon. "You take your ultrapure silicon," Buonassisi says, "and then you dump it into a very dirty production environment."

Buonassisi sees such impurities as unavoidable but manageable. He found that if the superheated silicon is cooled to about 500 ºC and held there, the dissolved iron will precipitate out, move through the silicon, and form clusters of iron silicide. Gathering the iron atoms together in this way makes it less likely that electrons will run into them, so more electrons can escape to produce an electric current. Holding the silicon at 500º for about 30 minutes can improve the efficiency of a solar cell by 3 to 7 percent. A solar-cell manufacturer could sell these more efficient cells at higher prices, generating enough profit in two years to build a new manufacturing plant. With more plants running, manufacturers could significantly speed up the production of solar cells.

By focusing on ways to minimize the impact of impurities, Buonassisi's research could also allow manufacturers to make solar cells out of cheaper, dirtier silicon. And cheaper raw materials combined with greater production capacity would put solar power in a better position to become a major source of electricity.