In experiments that captured events lasting just 18 femtoseconds, less than 20 quadrillionths of a second, researchers at the University of Cambridge observed charge separation occurring within a single molecular vibration. At this timescale, atoms inside molecules are physically vibrating, and the team found that charge transfer unfolds as fast as the motion of the molecule itself.
"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 across the interface between materials. "The vibration acts like a molecular catapult. The vibrations don't just accompany the process, they actively drive it," said Ghosh, describing how specific motions of the atoms give the electron a directional kick.
The research, published in Nature Communications, challenges decades of design rules in solar energy research that linked ultrafast charge transfer to large energy offsets and strong electronic coupling between materials. Those traditional design features can reduce efficiency by limiting the achievable voltage and increasing energy loss, creating a trade-off between speed and usable power.
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 operate efficiently, this pair must split into free charges extremely quickly. The faster this separation happens, the less energy is lost, making ultrafast separation one of the key steps that governs how efficiently light-harvesting devices turn sunlight into usable energy.
To test whether the conventional trade-off was unavoidable, the Cambridge team built a deliberately weak interface. They placed a polymer donor and a non-fullerene acceptor side by side with almost no energy offset and only minimal interaction, conditions that should have slowed charge transfer dramatically according to standard theory.
Instead, the electron crossed the interface in just 18 femtoseconds, much faster than in many previously studied organic systems and on the natural timescale of atomic motion. "Seeing it happen on this timescale within a single molecular vibration is extraordinary," said Ghosh, highlighting that the process keeps pace with the fastest internal motions of the material.
Ultrafast laser measurements revealed why the weakly coupled system could still move charge so quickly. After absorbing light, the polymer begins vibrating in specific high-frequency modes that mix electronic states and effectively kick the electron across the boundary. This produces directional, ballistic motion rather than slow, random diffusion, allowing the charge to separate before it can relax and lose energy.
Once the electron reaches the acceptor molecule, it triggers a new coherent vibration that serves as an unusual signature of such rapid transfer. This coherent vibrational response has only rarely been observed in organic materials and marks how fast and cleanly the electron hops between molecules without becoming trapped or scattered.
"Our results show that the ultimate speed of charge separation isn't determined only by static electronic structure," said 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 opens a new pathway for designing more efficient light-harvesting technologies that harness vibronic effects. Ultrafast charge separation underpins systems such as organic solar cells, photodetectors and photocatalytic devices used to produce clean hydrogen fuel, and similar vibronically assisted processes also operate in natural photosynthesis.
Professor Akshay Rao, Professor of Physics at the Cavendish Laboratory and former St John's College Research Associate, who co-authored 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." By engineering materials to support the right vibrational modes, device designers could route electrons more efficiently and reduce energy losses.
Research Report:Vibronically Assisted Sub-Cycle Charge Transfer at a Non-Fullerene Acceptor Heterojunction
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St John's College University of Cambridge
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