This illustration shows laser light exciting the hybrid perovskite material. Image: Arjen Kamp, University of Groningen.
This illustration shows laser light exciting the hybrid perovskite material. Image: Arjen Kamp, University of Groningen.

Photons with more energy than the 'band gap' of the semiconductor absorbing them in a solar cell give rise to what are known as hot electrons. Usually, this additional energy is lost very fast by being converted into heat and so does not contribute to the electric power generated by the solar cell.

Now, however, Maria Antonietta Loi, a professor of photophysics and optoelectronics at the University of Groningen in the Netherlands, has found a material in which these hot electrons retain their high energy levels for much longer. This might make it possible to use their additional energy to generate more electric power. She reports her findings in a paper in Nature Communications.

The efficiency of solar cells is hampered by a Goldilocks problem: photons need to have just the right amount of energy to be converted into the free electrons that provide electric power. Too little energy and the photons pass right through the solar panel; too much and the excess energy disappears as heat.

The latter outcome is due to the creation of hot (high-energy) electrons. Before they can be extracted from the solar cells, these hot electrons give off their excess energy by inducing vibrations in the crystalline material of the solar cell, which is felt as heat. “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 made of organic-inorganic hybrid perovskites. Perovskites are named after a mineral with the chemical formula ABX3, where X represents anions that form an octahedron, A represents cations that form a cube around them and B represents a central cation. Many materials in the perovskite family adopt this crystal structure; in hybrid perovskites, A is taken by organic cations.

Most hybrid-perovskite solar cells contain lead, which is toxic. Loi's group recently published a paper describing a record-breaking 9% efficiency at converting sunlight to electric power for a hybrid-perovskite solar cell containing harmless tin instead of lead. “When we studied this material further, we observed something strange,” she says. Their results could only be explained if the hot electrons produced in the tin-based solar cells took around 1000 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. The long lifespan of these hot electrons makes it possible to harvest their 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,” Loi explaines. Theoretical calculations show that by harvesting the hot electrons, the maximum efficiency for hybrid-perovskite solar cells could increase from 33% to 66%.

The next step is to find out why the tin-based hybrid perovskite slows down the decay of hot electrons, which could lead to the development of new perovskite materials 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,” suggests Loi.

This story is adapted from material from the University of Groningen, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.