Two for one: Quantum dot solar cells boost power

Researchers seek third-generation solar cells able to capture high-energy photons normally wasted as heat to boost solar efficiency and lower costs.

Breaking free: in the pursuit of third-generation solar cells, researchers have found materials that give off two electrons, rather than one, when struck with light, which could lead to a jump in solar cell efficiency. Science/AAAS.

Scientists designing the next generation of solar cells are trying to do away with waste heat.

Two research groups this week reported advances in a technique to capture a portion of the sunlight's energy that's normally lost as heat. The advances are aiming toward a breakthrough in how much light can be converted to electricity on a solar cell.

In a paper published in Science, researchers at the National Renewable Energy Laboratory (NREL) reported developing tiny crystals a few nanometers in size, called quantum dots, that are able to capture high-energy photons that today's solar cells don't.

Separately, a team from the University of Texas in Austin developed a plastic semiconductor material that was able to double the number of electrons produced from a photon on light. Capturing "hot electrons" normally lost as waste heat promises solar cells that work at 44 percent efficiency, far beyond today's theoretical limit of 31 percent, according to chemist Xiaoyang Zhu at The University of Texas at Austin.

In conventional solar cells, the photons in sunlight transfer their energy to electrons. Once "excited" by light, an electron jumps from one layer of semiconductor to another to form a current of electricity in a circuit.

Both groups of researchers are working to take advantage of a phenomenon called Multiple Exciton Generation (MEG), where a photon striking a solar cell can generate more than one electron.

Harnessing this process could allow scientists to create materials able to take draw usable energy from high-energy photons in the violet and ultraviolet part of the spectrum, according to an article in Science magazine. But in practice, building effective solar devices has proven difficult.

"Using quantum dots as your absorber material, you can better convert that high-photon electricity," Matt Beard, a senior scientist and author of a paper in Science on the topic, told Bloomberg. But the overall efficiency of the cell needs to be improved and it will take at least five years before quantum dots solar cells are commercial, he said.

The University of Texas team studied the conditions for the MEG multiple electron process in pentacene, a plastic semiconductor that could lead to cheap solar cells, said Zhu. "Combined with the vast capabilities for molecular design and synthesis, our discovery opens the door to an exciting new approach for solar energy conversion, leading to much higher efficiencies," he said in a statement.

 

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