Bright Days for Solar

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Colorful Concentration
Vladimir Bulovic, an associate professor in the department of electrical engineering and computer science, inserts a copy of Pirates of the Caribbean into a Blu-ray disc player, and the screen in his office--an elegant device just a few millimeters thick--comes to life. The remarkably vivid and crisp colors look good from any angle. The images appear touchably real.

An expensive Sony showpiece, the next-generation display uses organic light-emitting diodes as pixels. Although Bulovic's research helped make the display possible, he's not just showing off his handiwork. The organic molecules at the heart of the device could be the key to making solar power as cheap as power from coal.

In traditional solar panels, a semiconductor is spread out over the entire surface to absorb sunlight. But for decades, engineers have known that curved mirrors or lenses can gather sunlight from a large area and direct it to a small solar cell, enabling a fraction as much semiconductor material to absorb the same amount of light. What's more, solar cells perform better under concentrated light, converting a higher percentage of photons into electricity.

This approach has its drawbacks, however. Bulky mirrors and lenses can't be installed on rooftops and are easily damaged by wind. They also require mechanical tracking systems that keep the sunlight focused on the small solar cell throughout the day. These systems add cost and tend to break down. In the 1970s, scientists proposed a simpler scheme that would use light-emitting organic molecules to concentrate sunlight. Such materials--like the ones in Bulovic's display--are now available for the first time.

MIT electrical-engineering professor Marc Baldo, with whom Bulovic is collaborating, has championed a way of using the organic molecules in solar technology. It goes like this: Coat a glass sheet with the molecules, then expose the glass to sunlight. The mole­cules will absorb the light and reëmit it at another wavelength. Because light moves at different speeds in glass and in the surrounding air, the reëmitted light reflects back into the glass at the boundary between the two. About 80 percent of the light the mole­cules emit will bounce around inside the glass until it reaches the edges of the sheet. (The same phenomenon allows light to travel through fiber-optic cables.) As a result, the sheet of glass gathers light and concentrates it at the edges. Solar cells just a couple of millimeters wide (in most solar panels, they're several centimeters across) can be laminated onto the edge of the glass to absorb the light and generate electricity. A sheet of plastic with the organic molecules embedded in it can concentrate light in the same way.

Of course, luminescent organic molecules--better known as luminescent dyes--have been around for a long time. But they haven't worked for concentrating sunlight because they tend to fade after prolonged exposure and to reabsorb much of the light they emit, preventing it from reaching the edges of a sheet. Recently, however, researchers have made luminescent display dyes durable enough to survive for years or even decades in direct sunlight, making them viable for use in solar applications. What's more, Baldo has demonstrated a way to use dyes that don't absorb the colors they emit, which allows more light to reach the edges.