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Are Silicon "Quantum Dots" a Key to High-Efficiency Solar Cells?

From the 2007 Research Review.

NREL's research shows new promise for "nanoscale" particles of silicon—particles on the scale of a billionth of a meter, or nanometer. These submicroscopic dots of material are called "quantum dots" because they are small enough to exhibit unexpected characteristics as a result of their quantum mechanical behavior. For instance, quantum dots made from the same photovoltaic solar cell material can capture different frequencies of light, depending on their size. This unique characteristic has led many researchers to try building quantum-dot solar cells.

It's tricky, though. When a normal photovoltaic material captures a photon of sunlight, it produces a negatively charged electron, which leaves behind a positively charged "hole" in the material. Under the solar cell's electrical field, the two charge carriers migrate to the terminals of the cell and produce a current. But quantum dots produce "excitons," consisting of electrons loosely bound to positive holes. Solar cells employing quantum dots must dissociate the excitons into electrons and holes that must then migrate to the cell terminals without recombining.

On the plus side, researchers have found that quantum dots of certain materials can produce more than one exciton per photon of light. That finding would hold promise for solar cells if it weren't for one problem: those quantum dots were made of materials not commonly used in solar cells.

That all changed in July 2007, when NREL researchers found the multiple-exciton effect in quantum dots made of silicon, the material used in most of today's solar cells. The finding, published in the August 8, 2007, edition of Nano Letters, suggests we are on a path toward solar cells that will produce more electrical current from the same amount of sunlight.

The NREL team discovered the effect using silicon quantum dots and exposing them to solar wavelengths from violet visible light into the ultraviolet. The research validated a prediction made by NREL Research Fellow Arthur Nozik in 1997. But despite the promise of the discovery, Nozik remains cautious about its practical applications.

"This is a significant scientific advance," says Nozik, "but for this discovery to be converted into a useful and technologically important solar cell, it is necessary to dissociate the excitons and collect the resulting free electrons and holes with high efficiency."

In other words, it's one thing to find a material that efficiently produces excitons from sunlight, but it's another thing altogether to build a device that can produce a current from those excitons. NREL and other laboratories are trying to tackle that challenge, knowing that the payoff could be great. Calculations by Nozik and NREL scientist Mark Hanna have shown that the maximum theoretical conversion efficiency of such quantum-dot solar cells would be about 44% under normal sunlight and about 68% under sunlight concentrated by a factor of 500. That's a big leg up over today's conventional solar cells, which have maximum theoretical efficiencies of 33% and 40%, respectively, under the same solar conditions.