Molecular vibrations launch electrons across solar materials in 18 femtoseconds

Eighteen femtoseconds. That is how long it took an electron to cross the boundary between two carbon-based materials in a Cambridge laboratory -- roughly the time it takes the atoms in a molecule to complete a single vibration. One second contains about eight times more femtoseconds than all the hours that have passed since the universe began.

What makes this measurement remarkable is not just its speed but that it should not have happened. The researchers deliberately built a system that, according to conventional theory, should have been slow. The polymer donor and non-fullerene acceptor they paired had almost no energy offset between them and minimal electronic coupling -- conditions that textbooks say should result in sluggish charge transfer.

Instead, the electron was launched across the interface in a single coherent burst.

The catapult mechanism

The study, published in Nature Communications, reveals that molecular vibrations are not passive bystanders in charge transfer. They actively drive it. When light strikes the polymer, it triggers specific high-frequency vibrations in the molecule. These vibrations mix electronic states and effectively kick the electron across the material boundary -- a mechanism the researchers describe as a "molecular catapult."

"Instead of drifting randomly, the electron is launched in one coherent burst," said Dr. Pratyush Ghosh, Research Fellow at St John's College, Cambridge, and first author. "The vibrations don't just accompany the process, they actively drive it."

The researchers used ultrafast laser measurements to track the process in real time. After absorbing light, the polymer begins specific vibrational modes that mix its electronic states with those of the neighboring acceptor molecule. The electron transfer that follows is directional and ballistic rather than the slow, random diffusion predicted by standard models.

Once the electron arrives at the acceptor molecule, it triggers a new coherent vibration -- an unusual signature that has rarely been observed in organic materials. "That coherent vibration is a clear fingerprint of how fast and how cleanly the transfer occurs," Ghosh said.

Why this overturns the old rulebook

For decades, solar energy researchers have operated under a set of design principles: achieving ultrafast charge separation requires large energy differences between materials and strong electronic coupling. These features help split the electron-hole pairs (called excitons) that form when light strikes organic materials. But they come at a cost -- reducing voltage and increasing energy loss, which limits the overall efficiency of the solar device.

The Cambridge result shows that this trade-off may not be necessary. A system with minimal energy offset and weak electronic coupling achieved charge transfer at or near the physical speed limit -- the timescale set by the molecule's own atomic motion.

"Our results show that the ultimate speed of charge separation isn't determined only by static electronic structure," Ghosh said. "It depends on how molecules vibrate. That gives us a new design principle. In a way, this gives us a new rulebook. Instead of fighting molecular vibrations, we can learn how to use the right ones."

Implications for solar technology

Rapid charge separation is one of the key steps determining how efficiently solar panels and other light-harvesting devices convert sunlight into usable energy. When an exciton forms, the faster its electron and hole separate into free charges, the less energy is lost to recombination. If materials can achieve this separation through vibrational coupling rather than large energy offsets, it opens a path to devices that are both fast and efficient.

The principle extends beyond solar cells. Photodetectors, photocatalytic systems for producing clean hydrogen fuel, and even aspects of natural photosynthesis rely on ultrafast charge separation. Understanding that vibrations can drive this process provides a new lever for engineering better materials across all these applications.

"Instead of trying to suppress molecular motion, we can now design materials that use it -- turning vibrations from a limitation into a tool," said Prof. Akshay Rao, Professor of Physics at the Cavendish Laboratory and co-author.

A proof of concept, not a product

The study demonstrates a mechanism, not a device. Translating this understanding into practical solar cells or other technologies will require identifying which specific molecular vibrations are most effective at driving charge transfer, and designing materials that reliably produce them. The particular polymer-acceptor system used in this study was chosen to test a principle, not to optimize efficiency.

The researchers also note that their measurements capture events at extremely short timescales under controlled laboratory conditions. How vibrational-driven charge transfer behaves under real-world operating conditions -- varying temperatures, light intensities, material degradation -- remains to be explored.

The study involved researchers at the Cavendish Laboratory and the Yusuf Hamied Department of Chemistry at Cambridge, alongside collaborators in Italy, Sweden, the United States, Poland, and Belgium.