Molecular ‘catapult’ fires electrons at the limits of physics

University of Cambridge

Electrons can be ‘kicked across’ solar materials at almost the fastest speed nature allows, scientists have discovered – challenging long-held theories about how solar energy systems work.

The finding could help researchers design more efficient ways of harvesting sunlight and converting it into electricity.

In experiments capturing events lasting just 18 femtoseconds – less than 20 quadrillionths of a second – researchers at the University of Cambridge observed charge separation happening within a single molecular vibration.

“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, Cambridge, 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.

“Instead of drifting randomly, the electron is launched in one coherent burst. The vibration acts like a molecular catapult. The vibrations don’t just accompany the process, they actively drive it.”

A femtosecond is one quadrillionth of a second – one second holds about eight times more femtoseconds than all the hours that have passed since the universe began. At that scale, atoms inside molecules are physically vibrating.

The team observed charge transfer unfolding just as fast as the pace set by the molecule’s own motion. “We’re effectively watching electrons migrate on the same clock as the atoms themselves.”

The research, published in Nature Communications, challenges decades of design rules in solar energy research. Until now, scientists believed ultrafast charge transfer required large energy differences between materials and strong electronic coupling, features that can reduce efficiency by limiting voltage and increasing energy loss.

When light strikes many carbon-based materials, it creates a tightly bound packet of energy called an exciton – a paired electron and hole. For solar cells, photodetectors and photocatalytic systems to work efficiently, that pair must rapidly split into free charges. The faster this separation happens, the less energy is lost. This ultrafast separation is one of the key steps that determines how efficiently solar panels and other light-harvesting devices can turn sunlight into usable energy.

To test whether that trade-off was unavoidable, the Cambridge team built a deliberately ‘weak’ system. A polymer donor and a non-fullerene acceptor were placed side by side with almost no energy offset and only minimal interaction – conditions that should have slowed charge transfer dramatically.

Instead, the electron crossed the interface in just 18 femtoseconds, which is much faster than many previously studied organic systems and occurring on the natural timescale of atomic motion. “Seeing it happen on this timescale within a single molecular vibration is extraordinary,” said Dr Ghosh.

Ultrafast laser measurements revealed why. After absorbing light, the polymer begins vibrating in specific high-frequency motions. These vibrations mix electronic states and effectively kick the electron across the boundary, producing directional, ballistic movement rather than slow, random diffusion.

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

“Our results show that the ultimate speed of charge separation isn’t determined only by static electronic structure,” said Dr Ghosh. “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.”

The discovery reveals a new pathway to designing more efficient light-harvesting technologies. Ultrafast charge separation underpins systems such as organic solar cells, photodetectors and photocatalytic devices used to produce clean hydrogen fuel and similar processes occur in natural photosynthesis.

Professor Akshay Rao, Professor of Physics at the Cavendish Laboratory and former St John’s College Research Associate, who was a co-author of the study, said: “Instead of trying to suppress molecular motion, we can now design materials that use it – turning vibrations from a limitation into a tool.”