Intensifying the Sun

In his darkened lab at MIT, Marc Baldo shines an ultraviolet lamp on a 10-­centimeter square of glass. He has coated the surfaces of the glass with dyes that glow faintly orange under the light. Yet the uncoated edges of the glass are shining more brightly–four neat, thin strips of luminescent orange.

The sheet of glass is a new kind of solar concentrator, a device that gathers diffuse light and focuses it onto a relatively small solar cell. Solar cells, multilayered electronic devices made of highly refined silicon, are expensive to manufacture, and the bigger they are, the more they cost. Solar concentrators can lower the overall cost of solar power by making it possible to use much smaller cells. But the concentrators are typically made of curved mirrors or lenses, which are bulky and require costly mechanical systems that help them track the sun.

Unlike the mirrors and lenses in conventional solar concentrators, Baldo’s glass sheets act as waveguides, channeling light in the same way that fiber-optic cables transmit optical signals over long distances. The dyes coating the surfaces of the glass absorb sunlight; different dyes can be used to absorb different wavelengths of light. Then the dyes reëmit the light into the glass, which channels it to the edges. Solar-cell strips attached to the edges absorb the light and generate electricity. The larger the surface of the glass compared with the thickness of the edges, the more the light is concentrated and, to a point, the less the power costs.

Baldo, an associate professor of electrical engineering, published his findings recently in Science. On their basis, he projects that his solar concentrators could be made big enough for the electricity they help generate to compete with electricity from fossil fuels. Indeed, says Baldo, panels equipped with the concentrators “could be the cheapest solar technology.”

Secret Ingredient
The process for making Baldo’s solar concentrators begins down the hall in another lab. A postdoctoral researcher, Shalom Goffri, takes several bottles filled with colorful dye powders from a cabinet and measures the powders into small vials. Some of the dyes were developed for use in car paints; others have been used in organic light-emitting diodes. Both types of dyes can last for years in the sun, a quality essential for solar concentrators. Once he has measured out the powders, Goffri adds a solvent to each to make a liquid ink.

The next steps take place inside a sealed box, so that Goffri doesn’t inhale the solvents used to make the dye. He reaches into the box, using thick black gloves mounted in its glass front, and carefully mixes together different inks. Determining the right combination of inks solved a fundamental problem that researchers have encountered with this type of solar concentrator. If the glass sheet is coated with a dye that absorbs sunlight in, say, the green-to-blue range of the solar spectrum and emits light of the same wavelength, the emitted light will be quickly reabsorbed by the dye, and little of it will ever reach the edge of the glass. The problem has limited the size of these solar concentrators, since the further the light needs to travel to the edges, the less of the light will make it.

By using certain combinations of dyes interspersed with other light-absorbing molecules, Baldo makes coatings that absorb one color but emit another. The emitted light is not quickly reabsorbed by the coatings, so more of it reaches the edges of the glass sheet.

The coatings that Goffri is making absorb ultraviolet through green light and emit orange light. Once Goffri has prepared the final mixture, he pours a small amount on a 10-centimeter-wide glass square–the largest that can fit inside a device that spins the glass at 2,000 revolutions per minute to spread the ink evenly. Within a minute or two, the solvent has evaporated and the process is finished. The solar concentrator, with its coating of orange dye, is complete.

Baldo’s concentrators offer an inexpensive way to combine solar cells optimized for different wavelengths of light: different colored coatings can be paired with different types of solar cells in the same device. To make a prototype, Goffri takes a type of solar cell well suited to high-energy colors and glues it to the inside of a plastic frame; then he slides the concentrator into the frame so that its edges line up with the cells. The concentrator captures ultraviolet, blue, and green light and emits orange light that the cells convert into electricity. The lower-energy light, from the red and infrared end of the spectrum, passes through the solar concentrator to the next layer. In the prototype, the next layer is a full-size, conventional silicon solar cell that isn’t paired with a solar concentrator.

The prototype, Baldo says, can convert almost twice as much energy from sunlight into electricity as a conventional cell can, provided that the concentrator is roughly 30 centimeters square. This translates to a 30 percent decrease in the cost of solar electricity.

In the future, the cost savings can be much higher, Baldo believes. He doesn’t use a concentrator for the infrared light because, so far, no good dyes for capturing those wavelengths exist. But he is confident that such dyes can be developed. When that happens, he will be able to add a second concentrator, for little additional cost, and replace the full-size silicon solar cell with smaller, cheaper cells attached to the concentrators’ edges. If the cost of photovoltaics drops over the next several years, as expected, this setup could make solar power about as cheap as electricity from coal, he says.

There’s more work to be done in the lab, such as improving the range of colors the concentrators can absorb, which will make it possible to tailor them to specific slices of the spectrum. But Baldo says that it’s time to start moving the technology out of the lab and into the market. He and his colleagues have founded a company called Covalent Solar, which is starting to raise money. The company, based in Cambridge, MA, plans to have its first products–­probably tandem solar modules–available within three years.

Kevin Bullis is Technology Review’s energy editor.