Uppsala University engineers have managed to harvest hot electron holes, paving the way for improved solar cells and photosensors.
It is well known fact that certain metallic nanoparticles absorb light and in the proces generate positive and negative electrical charges. When these charges develop in light absorption, they are referred to as ‘hot’.
The negative charges are electrons and the positive ones are known as ‘electron holes’, where an electron in the valence band – the electrons in the outer shell of the atom – is missing.
Hot electrons – and how they accumulate in semiconductors – are a well-studied phenomenon. The same can’t be said about hot holes: There is very less literature on them.
The Uppsala study has now become the first to collect more than 80% of the hot holes in a semiconductor – three times as much as was previously thought possible.
According to the scientists, the process is astonishingly fast and it takes less than 200 femtoseconds (0.000000000002s) to accomplish the task.
The feasibility of collecting the charges in a semiconductor means that they can be used in solar cells and in artificial photosynthesis, for example, to reduce CO2 and produce hydrogen and oxygen from water.
The scientists have also come up with a theoretical prediction that the accumulation of the positive charges affects the dynamics of the negative charges as well.
This hypothesis is confirmed by observations included in the new study.
According to the scientists, when light is absorbed and electric charges are produced, the ‘electron temperature’ rises.
Harvesting the hot holes makes the electronic heat capacity increase, modifying how far the electron temperature rises.
This indicates that it is possible to manipulate the energy distribution of the electrons by controlling the degree to which the electron holes are removed.
According to the Uppsala team, this is significant as it could regularize the maximum voltage in a direct plasmonic solar cell, or control the reactive ‘window’ in a photocatalytic process.
Image and content: Matton/Uppsala University