An unexpected “traffic jam” for photo generated electrons in silicon

04.02.2026

An electron traffic jam. Generated with Gemini.

• Researchers at IMDEA Nanociencia reveal how photoexcited electrons get temporarily stuck in one of the world’s most important materials for solar energy harvesting.
• A phonon bottleneck mechanism is observed in silicon when charge carriers are excited with infrared photons.

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Madrid, 4th february, 2026. When light hits silicon, it excites electrons, allowing them to move freely and generate electrical current. In contrast to other materials, silicon requires not only photons to trigger this process, which has to be assisted by phonons (lattice vibrations). Understanding exactly how fast and efficiently this process happens is crucial for improving silicon based optoelectronic devices. Yet, even in a material as well studied as silicon, some of the ultrafast processes that control how energy flows at the microscopic level can remain obscure.

In a new study, researchers at IMDEA Nanociencia and MPIP institutes use time resolved terahertz spectroscopy to observe how electrons behave immediately after silicon is excited with light. To probe the photo generation of charge carriers across the bulk of the sample, they excited silicon using infrared photons close to the bandgap (i.e. absorption edge). Unexpectedly, instead of an immediate rise in electrical conductivity, as standard theory predicts, they observed a delayed response lasting a few picoseconds. Detailed analysis revealed that a large fraction of electrons were temporarily trapped in bulk shallow defect states near the band edge before being released into the conduction band via phonon absorption.

“What we observed was an accident,” says Enrique Cánovas. “We were expecting an instantaneous response, but instead we saw electrons taking a break” The researchers identified the cause as a phonon bottleneck: a temporary shortage of lattice vibrations —phonons— needed for electrons to escape from these shallow traps. While phonon bottlenecks are well known when exciting silicon with highly energetic “hot” electrons, this is the first experimental evidence that the effect can also occur for low-energy excitations generated near—or even below—the bandgap. This discovery provides new insights into how electrons are photogenerated in silicon and could have implications for how efficiently materials absorb light in real photovoltaic devices.

The study has been published in Physical Review B and is the result of a collaboration between researchers at the Madrid Institute for Advanced Studies in Nanoscience (Spain) and the Max Planck Institute for Polymer Research (Germany), and is partially funded by the accreditation Excellence Severo Ochoa awarded to IMDEA Nanociencia (CEX2020-001039-S). The findings open new questions about how low-energy infrared light interacts with silicon and whether this transient bottleneck could be harnessed—or mitigated—to further optimize solar cell performance.

Glossary:

• Bandgap: also called the forbidden band, determines the amount of energy needed for electrical conduction. When an electron possesses the bandgap energy, it is excited to a free state and can participate in electrical conduction.
• Phonon: is a quasiparticle, a collective excitation or vibration in a periodic, elastic arrangement at a single frequency.

Reference:

Contact:

Enrique Cánovas
This email address is being protected from spambots. You need JavaScript enabled to view it. Nanostructured Photovoltaics Group 
https://nanociencia.imdea.org/nanostructured-photovoltaics/group-home

IMDEA Nanociencia Dissemination and Communication Office / Oficina de Comunicación y Divulgación de IMDEA Nanociencia
  

Source: IMDEA Nanociencia.

IMDEA Nanociencia Institute is a young interdisciplinary research Centre in Madrid (Spain) dedicated to the exploration of nanoscience and the development of applications of nanotechnology in connection with innovative industries.