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Slow 'hot electrons' could improve solar cell efficiency

29. 1. 2018 | AcademiaNet member Prof Maria Antonietta Loi and her team have described a new material which could be used to build solar cells that harvest more energy from sunlight.
Laser light exciting the hybrid perovskite material
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(© Arjen Kamp, University of Groningen)

Laser light exciting the hybrid perovskite material

University of Groningen Professor of Photophysics and Optoelectronics Maria Antonietta Loi has discovered that a new substance retains 'hot' electrons' high energy levels for much longer than other materials. This may make it possible harvest more of solar energy to convert it to electricity. Her results were published on 16 January in Nature Communications.

The efficiency of solar panels is hampered by a Goldilocks problem: photons need to have just the right amount of energy to be able to create free electrons which contribute to the voltage. Too little energy, and the photons pass right through the solar panel. Too much, and the excess energy disappears as heat. The latter is due to the creation of hot (high-energy) electrons. Before their energy can be extracted from the solar cells, these hot electrons first give off excess energy by causing vibrations in the crystalline material of the solar panel. "This energy loss puts a limit to the maximum efficiency of solar cells", explains Loi.

She is working on a special type of solar cell that is made of organic-inorganic hybrid materials called perovskites. Perovskites are minerals with the chemical formula ABX3. Many materials in the perovskite family adopt a distinctive crystal structure, called the perovskite structure, which gives them unique properties.

Most hybrid-perovskite solar cells contain lead, which is toxic. Loi's group used crystals containing tin instead of lead in their new study, and the researchers describe that their material reaches a record-breaking nine-percent efficiency in a hybrid-perovskite solar cell. "When we studied this material further, we observed something strange", she recounts. The results indicated that the hot electrons produced in the tin-based solar cells took about a thousand times longer than usual to dissipate their excess energy. "The hot electrons gave off their energy after several nanoseconds instead of some hundred femtoseconds. Finding such long-lived hot electrons is what everybody in this field is hoping for", says Loi. Their longer lifespan makes it possible to harvest these electrons' energy before it turns into heat. "This means we could harvest electrons with a higher energy and thus create a higher voltage in the solar cell." Theoretical calculations show that by harvesting the hot electrons, the maximum efficiency for hybrid-perovskite solar cells could increase from 33 to 66 percent.

The next step is to find out why the tin-based hybrid perovskite slows down the decay of hot electrons. Then new perovskite materials could be designed with even slower hot electrons. "These tin-based perovskites could be a game changer, and could ultimately make a big contribution to providing clean and sustainable energy in the future."

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