We just discovered that silicon has an invisible bottleneck, and that has a direct impact on our solar panels

You turn on a solar cell and wait for the electrons to flow. But there is a moment, invisible and very brief, in which a part of them simply stops. A new study published in Physical Review B just explained why.

The discovery. Researchers from the Madrid Institute for Advanced Studies in Nanoscience (IMDEA Nanoscience) and the Max Planck Institute for Polymer Research in Germany (MPIP) have discovered that, in silicon, photoexcited electrons do not activate immediately when they receive light. For a few picoseconds (millionths of a millionth of a second) they become stuck in small traps of the material before they can circulate and generate current. The person responsible has a name: a phonon bottleneck.

What are phonons and why do they matter? Silicon has a peculiarity compared to other materials: for an electron to be released when receiving light, photons are not enough. According to account IMDEA Nanoscience in its note also needs the collaboration of phonons, which are the vibrations of the crystalline lattice of the material itself. As has been discovered, when such timing vibrations are scarce, electrons become temporarily trapped in surface defects near the edge of the energy band.

What no one expected to find. Enrique Cánovas himself, one of the authors of the study, recognize that the discovery was accidental. “What we observed was an accident. We expected an instantaneous response, but instead we saw the electrons take a breather,” he says. Until now, the phonon bottleneck was known in high-energy situations, when silicon was excited with very energetic electrons.

This is the first experimental record of the phenomenon with low-energy excitations, which occur with near-infrared light, or even below, the absorption threshold of the material. Until now unexplored territory.

Why it has practical relevance. Silicon is the heart of the vast majority of solar panels of the world. Any inefficiency in how your electrons respond to light has direct consequences on the performance of those photovoltaic cells. Understanding that this transient delay exists, and that it has an identifiable cause, opens the door to two possible paths: designing materials or structures that minimize this jam, or even taking advantage of it in a controlled way to improve the behavior of the device.

It remains to be seen if the impact of this phenomenon is significant enough to justify redesigns in the manufacturing of solar cells and photovoltaic systems.

Cover image | yue chan

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