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Quantum Dots and More Used to Beat Efficiency Limit of Solar Cells

New approaches, not yet ready for a rooftop near you, explore simple designs that are different from what's out there



Warren Gretz / NREL

Most photovoltaic solar cells have an inherent efficiency cap, limiting how much useful energy they can extract from the sun. But scientists are finding ways around this obstacle with new research that could make solar energy more efficient and more cost-effective.

At the National Renewable Energy Laboratory (NREL) in Golden, Colo., researchers are investigating how to get a unit of light to push more than one electron at a time. Meanwhile, a team at the Massachusetts Institute of Technology is working on getting the right type of light to hit solar cells to make sure its energy doesn't go to waste.

"One of the major limitations of solar energy conversion is that these high-energy photons are not efficiently converted. You lose a lot of energy to heat," said Matthew Beard, a senior scientist at NREL. He co-authored a paper last week in Science that demonstrated a device that, at the quantum level, peaked at 114 percent efficiency using a process called multiple exciton generation (MEG).

"It operates in some ways the same way a traditional solar cell would," said Beard. "Instead of bulk crystals, it uses quantum dots." Most solar cells are made of a sandwich of two crystal layers: one that's slightly negatively charged and one that's slightly positive. The negative crystal has extra electrons, and when a photon with enough energy strikes the material, it dislodges an electron on the positive side, increasing its energy and leaving behind a "hole." The electron-hole pairing is called an exciton.

MEG is one of the technologies at the vanguard of "third-generation" solar technology. Using these advances, solar panels can be thinner, lighter, cheaper, more flexible and fundamentally more efficient than current devices on the market. As a result, solar energy will be more cost-effective and will form a greater share of the world's energy portfolio.

But first these panels must bypass the Shockley-Quiessler limit, the bane of current-generation photovoltaic systems.

Saving wasted solar energy

The "SQ" limit describes the maximum efficiency of a solar cell using a conventional single-layer design with a single semiconductor junction. For most common solar cell materials, the efficiency limit is about 32 percent in ideal conditions. This means that at least two-thirds of the energy from sunlight that hits a solar panel is wasted, more if you account for losses from reflections, wiring and mounting hardware. The efficiency increases if you add layers to the cell, but this substantially raises the device's price and complexity. Currently, multi-junction solar cells are limited largely to satellites, where the need for efficiency, low weight and small space trumps cost concerns.

Now scientists are tweaking solar cell materials at nanometer scales to squeeze out better performance without increasing their prices or complexity, finding loopholes through the SQ limit.

In current photovoltaic cells, sunlight dislodges electrons, creating a moving charge that travels into the negative crystal, through a circuit, and then back to the positive side, where it fills the hole back in. If the photon doesn't have enough energy, the electron stays put. If the photon has too much energy, the charge flows using only the energy it needs, and the remainder warms up the device.

Beard's team found a way to make several holes with one photon using quantum dots—tiny chunks of semiconducting material between 2 and 10 nanometers in size. Their small size allows them to contain charges and more efficiently convert light to electricity. In this case, the dots are made out of lead and selenium. When a photon that has at least double the energy that is needed to move an electron strikes the lead selenide quantum dots, it can excite two or more electrons instead of letting the extra energy go to waste, generating more current than a conventional solar cell.

"The overall device efficiency here is about 4 percent. We need [the devices] to be about 10 to 15 percent before they attract significant commercial attention," said Beard. He expects that with further development, an MEG solar cell could operate with 44 percent efficiency.

Another way around the SQ limit is through singlet fission. Justin Johnson, also a senior scientist at NREL, said the process is similar to MEG, except that it uses organic molecules—compounds made from carbon—instead of semiconductors. "If you design your molecule in such a way that, instead of cooling to a lower state, you generate two electron-hole pairs, you don't lose as much energy as you would if you let the molecules relax and release the energy as heat," said Johnson.

Working with Arthur Nozik at NREL and Josef Michl at the University of Colorado, Boulder, Johnson demonstrated singlet fission in a compound known as 1,3-Diphenylisobenzofuran. When a photon strikes a molecule of this substance, its electrons enter a higher-energy, excited state. As it relaxes, it can transfer its energy to nearby molecules. In a study published last year in the Journal of the American Chemical Society, the team observed a 200 percent yield of these split energy states. In real device, Johnson expects an upper limit of 46 percent efficiency.

Keeping it simple

A singlet fission solar cell can also be more cost-effective than more common silicon-based solar cells. "The nice thing about organic materials is that almost all organic compounds can be mass-produced cheaply," said Johnson. "The primary limitation is how to optimize materials to make them [produce electricity] efficiently. Once we understand these design principles, we can make molecules just like solar cells."

Though both of these processes can circumvent the SQ limit, they still require high-energy light, which makes up only a small slice of the solar spectrum. At the Massachusetts Institute of Technology, research scientist Peter Bermel is seeking a solution using materials that absorb the sun's heat and emit light. "A selective emitter radiates high-energy photons, not low-energy photons," said Bermel. "You squeeze the spectrum into a very narrow range of wavelengths."

The absorber is made from a metamaterial, a substance engineered to have properties not found in nature. It heats up in sunlight like most other compounds, but it does not radiate its heat into its surroundings as much. This allows it to concentrate thermal energy and become extremely hot. The absorber transfers its energy to an emitter, which is then coupled to a photovoltaic panel, where it produces electricity. In a paper published in October in Nanoscale Research Letters, Bermel calculated that such a device could function with 37 percent efficiency.

The scientists acknowledge that this research still needs work before it comes to a rooftop near you. "We need to improve the primary conversion process even further," said Beard of MEG, referring to how light is converted into excitons. In addition, "we need to look at other quantum dot materials than lead selenide, since lead is toxic," he said.

Johnson noted that another issue is keeping the devices simple. This would allow new solar cells to be made easily and reduce their costs, making it more likely that they will gain widespread acceptance. Bermel agreed and added that he wanted to "explore designs that are very simple but different in some sense from what's out there. Rather than just trying to make very incremental improvements, we're trying to develop new concepts."

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500

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