The ultrafast spectroscopy system used in the study. Image: Maxim Pchenitchnikov, University of Groningen.
The ultrafast spectroscopy system used in the study. Image: Maxim Pchenitchnikov, University of Groningen.

Semiconductors can convert energy from photons (light) into an electric current, but some photons carry too much energy for the semiconductor to absorb. These photons produce so-called 'hot electrons', and the excess energy of these electrons is usually lost as heat.

Materials scientists have been looking for ways to harvest this excess energy, and now scientists from the University of Groningen in the Netherlands and Nanyang Technological University in Singapore have accomplished this by combining a perovskite semiconductor with an acceptor material for 'hot electrons'. They report their work in a paper in Science Advances.

In photovoltaic cells, semiconductors absorb photon energy, but only from photons that have the right amount of energy: too little and the photons pass right through the material; too much and the excess energy is lost as heat. The right amount is determined by the semiconductor’s bandgap: the difference in energy levels between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO).

“The excess energy of hot electrons, produced by the high-energy photons, is very rapidly absorbed by the material as heat,” explains Maxim Pshenichnikov, professor of ultrafast spectroscopy at the University of Groningen.

To fully capture the energy of hot electrons, materials with a larger bandgap must be used, but this requires transporting the hot electrons to this material before they lose their energy. The current approach to harvesting these electrons involves trying to slow down the loss of energy by, for example, using nanoparticles instead of bulk material. “In these nanoparticles, there are fewer options for the electrons to release the excess energy as heat,” says Pshenichnikov.

Together with colleagues at the Nanyang Technological University, where he was a visiting professor for three years, Pshenichnikov studied a system that combined an organic-inorganic hybrid perovskite semiconductor with the organic compound bathophenanthroline (bphen), a material with a large bandgap. The scientists used laser light to excite electrons in the perovskite and studied the behavior of the hot electrons that were generated.

“We used a method called pump-push probing to excite electrons in two steps and study them at femtosecond timescales,” explains Pshenichnikov. This allowed the scientists to produce electrons in the perovskites with energy levels just above the bandgap of bphen, without exciting electrons in the bphen. This means that any hot electrons in the bphen must have come from the perovskite.

The results showed that hot electrons from the perovskite semiconductor were readily absorbed by the bphen. “This happened without the need to slow down these electrons and, moreover, in bulk material. So, without any tricks, the hot electrons were harvested,” says Pshenichnikov. However, he and his colleagues noticed that the energy required was slightly higher than the bphen bandgap: “This was unexpected. Apparently, some extra energy is needed to overcome a barrier at the interface between the two materials.”

Nevertheless, the study provides a proof of principle for the harvesting of hot electrons in a bulk perovskite semiconductor material. “The experiments were performed with a realistic amount of energy, comparable to visible light,” says Pshenichnikov. “The next challenge is to construct a real device using this combination of materials.”

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.