Electrons in solar materials can cross molecular boundaries in just 18 femtoseconds — a single burst of movement driven by atomic vibrations rather than the energy gradients scientists previously believed were essential, according to research from the University of Cambridge.
The finding, published in Nature Communications on March 5, 2026, directly challenges decades of design assumptions in solar energy science. Researchers at St. John’s College, Cambridge deliberately built a system that conventional theory predicted would transfer charge slowly — pairing a polymer donor with a non-fullerene acceptor at almost no energy difference and with only weak electronic coupling. The electron crossed the interface anyway, in under 20 quadrillionths of a second.
“We deliberately designed a system that, according to conventional theory, should not have transferred charge this fast,” said Dr. Pratyush Ghosh, Research Fellow at St. John’s College and first author of the study. “By conventional design rules, this system should have been slow and that’s what makes the result so striking.”
The Molecular Catapult Mechanism
The speed clocked in these experiments matches the natural rhythm of atomic motion inside molecules — the same clock, in effect, that governs vibrations at the smallest physical scales. A femtosecond is one quadrillionth of a second; one second contains roughly eight times more femtoseconds than all the hours elapsed since the universe began.
Rather than drifting randomly across the material boundary, the electron rode the molecule’s own vibration in a single coherent burst. “The vibration acts like a molecular catapult,” Ghosh said. “The vibrations don’t just accompany the process, they actively drive it.”
Ultrafast laser experiments helped expose the mechanism. When the polymer absorbs light, it vibrates in specific high-frequency patterns. Those vibrations propel the electron across the interface — not despite the weak coupling and minimal energy difference, but through them.
Why Charge Separation Speed Matters
When light strikes carbon-based solar materials, it generates an exciton — a tightly bound electron-hole pair. Splitting that pair into free charges quickly is critical. The longer the split takes, the more energy dissipates as heat or recombination, reducing how efficiently a device converts sunlight to usable power. Faster separation means less waste.
Until now, achieving ultrafast charge transfer was thought to require large energy offsets between donor and acceptor materials, along with strong electronic coupling between them. Both conditions carry costs: they tend to reduce output voltage and introduce additional energy losses, creating an engineering tradeoff that has shaped organic solar cell design for years.
The Cambridge results suggest that tradeoff may not be unavoidable. If atomic vibrations can drive charge separation this quickly under weak-coupling, low-energy-offset conditions, then materials engineered around that mechanism could potentially capture more sunlight without sacrificing voltage — a combination the field has struggled to achieve simultaneously.
“We’re effectively watching electrons migrate on the same clock as the atoms themselves,” Ghosh said. “Seeing it happen on this timescale within a single molecular vibration is extraordinary.”
The researchers say the work could open new design paths for solar cells, photodetectors, and photocatalytic systems — though how readily the mechanism translates into manufacturable devices the team has not yet said.
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