Hot electrons harvested without tricks

November 15, 2019

Semiconductors convert energy from photons (light) into an electron current. However, some photons carry too much energy for the material to absorb. These photons produce 'hot electrons', and the excess energy of these electrons is converted into heat. Materials scientists have been looking for ways to harvest this excess energy. Scientists from the University of Groningen and Nanyang Technological University (Singapore) have now shown that this may be easier than expected by combining a perovskite with an acceptor material for 'hot electrons'. Their proof of principle was published in Science Advances on 15 November.

In photovoltaic cells, semiconductors will 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 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. However, this means that the hot electrons should be transported to this material before losing their energy. The current general approach to harvesting these electrons is to slow down the loss of energy, for example by using nanoparticles instead of bulk material. 'In these nanoparticles, there are fewer options for the electrons to release the excess energy as heat,' explains Pshenichnikov.

Together with colleagues from the Nanyang Technological University, where he was a visiting professor for the past three years, Pshenichnikov studied a system in which an organic-inorganic hybrid perovskite semiconductor was combined 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. Therefore, any hot electrons in this material would 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.' However, the scientists 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 bulk perovskite semiconductor material. Pshenichnikov: 'The experiments were performed with a realistic amount of energy, comparable to visible light. The next challenge is to construct a real device using this combination of materials.'
Reference: Swee Sien Lim, David Giovanni, Qiannan Zhang, Ankur Solanki, Nur Fadilah Jamaludin, Jia Wei Melvin Lim, Nripan Mathews, Subodh Mhaisalkar, Maxim S. Pshenichnikov, and Tze Chien Sum: Hot carrier extraction in CH3NH3PbI3 unveiled by pump-push-probe spectroscopy. Science Advances, 15 November 2019.

Simple Science Summary

The efficiency of solar panels is hampered by a 'Goldilocks problem': the light needs to have just the right amount of energy to be converted into a voltage. Too little energy and the photons (packages of light energy) pass right through the panel. Too much and the excess energy disappears as heat. Several tricks have been tried to harvest the high-energy photons. Scientists from the University of Groningen and Nanyang Technological University have now shown that by combining two materials, the excess energy is used rather than wasted as heat. This can potentially increase the energy efficiency of solar panels.

University of Groningen

Related Electrons Articles from Brightsurf:

One-way street for electrons
An international team of physicists, led by researchers of the Universities of Oldenburg and Bremen, Germany, has recorded an ultrafast film of the directed energy transport between neighbouring molecules in a nanomaterial.

Mystery solved: a 'New Kind of Electrons'
Why do certain materials emit electrons with a very specific energy?

Sticky electrons: When repulsion turns into attraction
Scientists in Vienna explain what happens at a strange 'border line' in materials science: Under certain conditions, materials change from well-known behaviour to different, partly unexplained phenomena.

Self-imaging of a molecule by its own electrons
Researchers at the Max Born Institute (MBI) have shown that high-resolution movies of molecular dynamics can be recorded using electrons ejected from the molecule by an intense laser field.

Electrons in the fast lane
Microscopic structures could further improve perovskite solar cells

Laser takes pictures of electrons in crystals
Microscopes of visible light allow to see tiny objects as living cells and their interior.

Plasma electrons can be used to produce metallic films
Computers, mobile phones and all other electronic devices contain thousands of transistors, linked together by thin films of metal.

Flatter graphene, faster electrons
Scientists from the Swiss Nanoscience Institute and the Department of Physics at the University of Basel developed a technique to flatten corrugations in graphene layers.

Researchers develop one-way street for electrons
The work has shown that these electron ratchets create geometric diodes that operate at room temperature and may unlock unprecedented abilities in the illusive terahertz regime.

Photons and electrons one on one
The dynamics of electrons changes ever so slightly on each interaction with a photon.

Read More: Electrons News and Electrons Current Events is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to