As the core component of products from computer chips to many solar cells, silicon’s the go-to semiconducting material of our age.

But like any such material, it has downsides, and its limited ability to absorb the sun’s rays is a big one. Its shiny surface, for example, reflects much of the sun’s ultraviolet radiation, just above the short wavelength end of the visible light spectrum. The element — especially in the comparatively thick form used in many photovoltaic systems — does fine with visible light. But as you move to longer wavelengths, it eventually loses its photon-capturing prowess.

“Silicon absorbs radiation with wavelengths up to about one micron, which covers some of the infrared,” explains MIT’s Vladimir Bulovic. (A micron is a millionth of a meter). “After that, the photons just pass on through.”

Can you grab enough of those photons so as to boost a photovoltaic system’s efficiency between 50 and 100 percent? Bulovic thinks so. His concept is to create a second solar cell, with all the functionality of the first, that can be literally printed right onto the silicon.

Bulovic’s photon-absorber of choice is a nifty item called the quantum dot. “Quantum dots are 3-D crystals that are reproducible from run to run,” he says, “and that are small in size.”

Small they are. You can think of the dots as being like chunks of rock candy that happen to range in width from two to 10 nanometers. (How tiny is that? If you put a 10 nm dot in the middle of a football field, and blew up the field to be as long as Earth is wide, that dot would still only be a millimeter across.)

A key feature of the dots is that you can in effect tune batches of them to absorb light of any frequency you want. If you make smaller dots, they can absorb, say, in the ultraviolet. If bigger, they can absorb well into the infrared.

Bulovic, an associate professor of electrical engineering, works with MIT chemist Moungi Bawendi. One way the researchers envision deploying the dots is to put layers of them on both sides of the silicon in a conventional solar cell. “On top, you’d put quantum dots that absorb the ultraviolet and short-wavelength visible photons,” notes Bulovic, “and on the bottom you’d put the ones that absorb in the far infrared.”

They’d also layer on two other materials: organic molecules like the dyes described earlier; and an oxidized metal such as zinc or indium oxide. Those layers would let you draw out the excited electrons that result when photons hit the quantum dots, thus allowing the layered-on coatings to create a current.

Bulovic, who got his Ph.D. at Princeton, has had a lot of experience with quantum dots. He won a Technology Review magazine “top innovator under 35” award for using them to create high-performance light-emitting devices — work that led to the founding of the firm OD Vision.

But creating photovoltaic systems with efficiencies in the 20 percent range, Bulovic’s current target, is more ambitious. One hurdle: making sure the quantum-dot, organic, and metal-oxide materials interface so as to maximize the number of electrons you capture from the “dot layer.” Such interfaces are critical in PV systems, notes Bulovic, “but here, we’re trying to mesh three different materials.” It’s challenging enough that his group for now is analyzing the three-layer system a layer at a time.

“It’s going to be five years before anyone is able to actually market a system like this,” Bulovic says. But he also notes that there’s been more progress than might have been predicted a few years ago.

“What’s exciting,” he says, “is that every day we’re learning more and more about how these systems can work.”

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