A Better Solar Collector - A more efficient way to concentrate sunlight could reduce the cost of producing solar power.

Looking to make solar panels cheaper, MIT researchers have created sheets of glass coated with advanced organic dyes that more efficiently concentrate sunlight. The researchers, whose results appear in this week's issue of Science, say that the coated glass sheets could eventually make solar power as cheap as electricity from fossil fuels.

The researchers show that the glass sheets can reduce the amount of expensive semiconducting material needed in solar panels and provide a cheap way to extract more energy from high-energy photons, such as those at the blue end of the spectrum. "This could be the cheapest solar technology," says Marc Baldo, a professor of electrical engineering at MIT. "And I think one day, it could be competitive with coal."

The simple, flat sheets of glass have a number of advantages over previous solar concentrators, devices that gather sunlight over a large area and focus it onto a small solar cell that converts the light into electricity. Solar power concentrators in use now employ mirrors or lenses to focus the light Because the new glass sheets are lighter and flat, they can easily be incorporated into solar panels on roofs or building facades. They could also be used as windows, which, connected to solar cells, could generate electricity. What's more, mirrors and lenses require mechanical systems for tracking the sun to keep the light focused on a small solar cell. These tracking systems add cost and can break down over the decades that solar panels are made to be in service. The flat glass concentrators don't require a tracking system.

Instead of using optics, the glass sheets concentrate light using combinations of organic dyes specially designed by Baldo and his coworkers. Light is absorbed by the organic dyes coating one side of the glass sheet. The dyes then emit the light into the glass. The glass channels the light emitted by the dye to the edges of the glass, in the same way that fiber-optic cables channel light over long distances. Narrow solar cells laminated to the edges of the glass collect the light and convert it into electricity. The amount of light concentration depends on the size of the sheet--specifically, the ratio between the size of the surface of the glass and the edges. To a point, the greater the concentration, the less semiconductor material is needed, and the cheaper the solar power.

The challenge of using organic dyes as solar concentrators has been that the dyes tend to reabsorb much of the light before it can reach the edges of the glass. Baldo overcame that problem by using dyes that don't absorb the light that they emit. For example, a dye might absorb a range of colors in the light spectrum, such as ultraviolet through green, but emit light in another color, such as orange, which the dye cannot absorb.