A quantum leap for solar power

Lessons from plant photosynthesis suggest ways to boost the efficiency of thin plastic solar cells. James Urquhart reports.  

Plants are extremely efficient at using the Sun's energy. We are now learning the tricks they use to separate charge. – iStock

Power-generating windows and battery-charging mobile and laptop screens are just some of the potential applications for a new breed of inexpensive, flexible, transparent solar cell made from thin plastic film. The only hurdle is making the cells efficient enough to be useful, but nature could inspire a solution. New research into plants has revealed important details of the quantum tricks they play to maximise the efficiency of photosynthesis.

Plants excel at putting every ounce of solar energy they capture to good use.

When a plant chlorophyll molecule absorbs a photon of light the jolt of energy knocks free one of the molecule’s electrons leaving behind a positively charged “hole”. If the electron, which is negatively charged, were simply attracted back into the hole, the energy would be lost. But plants – unlike plastic solar cells – are exceptionally good at “charge separation”, quickly funnelling the excited electron away from the hole. Almost all light absorbed by photosynthetic pigments in plant cells is transformed to a stable charge-separated state, photosynthesis’s first step.

“If we understand the tricks that nature uses to separate charge, we can try to mimic these to improve the efficiency of artificial light-harvesting devices,” explains Jennifer Ogilvie at the University of Michigan, Ann Arbor, US. But the process happens so quickly that the precise mechanism underpinning it had remained a mystery.

Now, two independent studies – one led by Ogilvie, the other by a team in the Netherlands – have solved it. The mechanism comes down to “quantum coherence”, a precisely choreographed dance between the plant’s pigment molecules as they pass the electron between them, away from the hole. The research shows that the pigments vibrate in synchrony, swiftly transferring the electron from one to the next like well-drilled firefighters swinging their arms in time as they pass buckets of water up a line, Ogilvie says. “How much water they transport depends on each person getting the right timing and the right motion to maximise throughput.”

Growing evidence suggests that coherent molecular vibrations
are a key component of photosynthesis.

To reveal the details of the process, both groups turned to ultrafast lasers, using very brief and carefully timed laser pulses to capture snapshots of charge separation in action.

Elisabet Romero and Rienk van Grondelle at VU University, Amsterdam, and colleagues had previously discovered that there are two possible pathways for pigment proteins to achieve charge-separation, starting from different sets of pigments. The group’s latest work, in collaboration with researchers in Russia and Sweden, reveals that the two pathways actually work together and are intricately connected via certain vibrations that keep the whole system carefully ordered, maximising energy conversion efficiency. Their findings are published this week in Nature Physics.

Meanwhile, Ogilvie and her team, along with colleagues in Lithuania, concurrently published their results in Nature Chemistry. They report similar conclusions. Their experiments pinpointed specific vibrations in the pigments that appear to be associated with the transfer of electrons and that are important for charge separation.

Both studies add to the growing evidence that coherent molecular vibrations are a key component of photosynthesis, says Alexandra Olaya-Castro, who investigates quantum effects in biophysical processes at University College London, UK. “This offers extraordinary possibilities for emerging technologies,” she says. She envisages that the discovery could help scientists develop materials that act like photosynthetic pigments, which could be integrated into future photovoltaic devices to boost performance.

While there are many potential avenues for research to boost plastic solar cells’ efficiency, “at this point it is too early to speculate about what type of efficiency improvement could be achieved”, says Ogilvie.

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