Harvesting hot electrons could break solar panel barrier
University of Groningen
image:
The visual abstract of the paper in ACS Energy Letters, showing how two synergetic effects create a delay in the loss of energy from hot electrons:
1) The Hot Phonon Bottleneck
2) Band-filling, resulting in the Burnstein-Moss effect
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Credit: ACS Energy Letters
Physics predicts that no more than 33 percent of the solar energy that falls on a solar panel can be converted into electricity. However, experiments and simulations by scientists at the University of Groningen (the Netherlands) revealed a way to harvest extra energy from ‘hot electrons’, which could break this barrier.
When light falls on a solar panel, the energy of the photons brings electrons in the solar cell material into an excited state, thus transferring the energy to them. This reaction can free an electron from the solar cell material and create a voltage.
However, very energetic photons give the electrons extra energy, producing ‘hot electrons’. In theory, the extra energy these hot electrons carry could increase the voltage. In practice, the extra energy is lost as heat in a matter of picoseconds (0,000000000001 second). ‘This means that the energy is lost before the hot electron exits the solar cell material’, says Jan Anton Koster, Professor of Physics of Novel Semiconductors and Devices at the University of Groningen.
Scepticism about the claims
However, in an experimental setup, his colleague Maria Antonietta Loi, professor of Photophysics and Optoelectronics, managed to produce a delay in heat loss by hot electrons. She created a solar cell material called tin-based perovskite, and observed that the loss of extra energy is slowed down to nanoseconds, roughly a factor of 1,000. ‘The measurements were clear, but we didn’t understand the physics behind this’, says Koster. This led others in the field to question this claim. ‘We even started to doubt the measurements ourselves’, he admits.
In order to solve this conundrum, Koster and his PhD student Tim Faber used simulations to study the physics of this energy loss. They found that in perovskite solar cells, two different mechanisms combine to extend the time it takes hot electrons to lose the extra energy.
Nanosecond range
When energy is lost as heat, the environment surrounding the electrons will become warmer. Koster and Faber realised that this lingering heat can be reabsorbed by the electrons. ‘When we added this well-known process called Hot Phonon Bottleneck to the simulations, it slowed the loss of energy, but not enough to explain our measurements.’ This required adding a second mechanism to the simulation.
The extra energy of hot electrons can bring them into a number of excited states. Energy loss means that the excited state is reduced to a lower energy level in discrete steps. However, when the different steps in this process are already occupied, the way down to the lowest energy level is more difficult and takes more time. This is called the Burstein-Moss effect. Koster: ‘When we added this process to the simulation as well, we saw that energy loss was now in the nanosecond range, as seen in the experiments by Maria Loi.’
As both of these processes are present in tin-based perovskite solar cells, Koster and his team finally understood why the energy loss of hot electrons was slowed down. There are many other questions that still need answers, but in theory, this discovery could allow the creation of more efficient solar cells, beyond the theoretical limit of 33 percent.
Reference: Tim Faber et al.: The Physics of Ultra-Long Cooling Times in Metal Halide Perovskites. ACS Energy Letters, 11 September 2026.
Journal
ACS Energy Letters
Method of Research
Experimental study
Subject of Research
Not applicable
Article Title
The Physics of Ultra-Long Cooling Times in Metal Halide Perovskites
Article Publication Date
17-Aug-2026
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