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Unleaded. New solar cells are made with crystalline materials called perovskites that replace lead with tin (Sn).

Unleaded. New solar cells are made with crystalline materials called perovskites that replace lead with tin (Sn).

N. K. Noel et al., Energy and Environmental Science, The Royal Society of Chemistry (2013)

Solar Cells Capture Lost Energy

Researchers have created a new type of solar cell that captures some of the excess energy in sunlight normally lost as heat. So far, the new solar cells still convert sunlight to electricity at an efficiency well below commercial solar cells. But if the process can be improved, it could help pave the way to a new generation of solar cells with higher efficiencies.

For most materials, the conversion of photons of sunlight to electricity is well understood. Photons of different colors have different amounts of energy. In the visible spectrum, reds and oranges have less energy, while blues, violets, and ultraviolet photons carry progressively more. When high-energy photons hit a semiconducting material in a solar cell, they give up this energy to the semiconductor's electrons, exciting them from a static position so that they are able to conduct. In many cases, high-energy photons—violets and ultraviolets—carry far more energy than is needed to give electrons the nudge to conduct. But this excess energy is lost as heat.

Several years ago, researchers from a number of teams reported that high-energy photons in sunlight could actually excite more than one electron if the semiconductor they hit consisted of nanometer-size particles called quantum dots. This process, known as multiple exciton generation (MEG), held out hope that researchers could improve the efficiency of solar cells by collecting these extra charges. But making working MEG solar cells proved a bear. Even when extra charges were being generated, researchers struggled to get them to jump out of individual dots and make their way to an electrode that was connected to a wire, where they could be sent on their way to do work.

Last year, researchers led by Bruce Parkinson, a chemist at the University of Wyoming, Laramie, reported in Science that they'd made a device with a single layer of lead sulfide quantum dots atop another semiconductor, which excited more electrons than it received photons, thus generating a larger electrical current -- the MEG signature. But the apparatus was more a proof of concept than a working solar cell: The single layer of dots made it easy to transfer electrons out, but they captured only a tiny fraction of the light, and thus the overall device wasn't efficient.

Now, researchers led by Arthur Nozik, a chemist at the National Renewable Energy Laboratory in Golden, Colorado, report that they've created the first working MEG solar cell. The key in making the device, Nozik says, was coming up with a recipe for chemically synthesizing and then processing quantum dots. When synthesized, the dots—which are clusters of lead and selenium about 5 nanometers in diameter—end up decorated with long organic molecules that prevent separate dots from clumping together. But past studies showed that these long organic chains act like a plastic insulator around a wire. They prevent excited electrons generated inside one dot from skipping through its neighbors on to an electrode.

So Nozik's team treated their dots with a one-two chemical punch of two colorless liquids, hydrazine and 1,2-ethanedithiol, which left the dots coated with short-chain organics. That allowed charges easier mobility. And it resulted in solar cells with about a 5% overall efficiency at converting light to electricity, the team reports online today in Science. That's still well below conventional silicon solar cells, which still do a better job making use of the full solar spectrum and are about 20% efficient. But more importantly, the devices collected 30% more electrical charges than the number of photons that struck the quantum dots, making these the first true MEG solar cells.

"They've taken it to a real device and demonstrated that they were able to collect real power," Parkinson says. And that, he adds, "shows promise for next generation [solar cell] designs." Parkinson notes that the new work also shows that excess electrons are generated when the amount of energy in the photons is about 2.5 to 3 times the amount needed to kick electrons into their energized conducting state, roughly the same level seen last year by Parkinson's group.

Both Parkinson and Nozik say the key for the field now will be to find ways of synthesizing dots to get this closer to 2 times the energy needed, which should markedly boost the efficiency of the solar cells. If that happens, the solar cell business could find itself with a new contender.