Quantum solar cells could explain why plants are green

Quantum photocells could copy plants

Nathaniel Gabor QMO Lab

By Leah Crane

The next wave of solar cells might be green… literally. Quantum mechanics is helping to make better solar cells – and may give us another perspective on why plants are green in the process.

A big problem in solar power is that sunlight is not constant: because of seasonal changes, night-time and clouds, the amount of sunlight that reaches panels is constantly shifting. This means we must regulate the power from the cells so that the grid doesn’t fry on sunny days and the lights don’t flicker as clouds pass. Trouble is, this dents the efficiency of the panels.

This is a potential issue for plants, too. Unlike solar cells, plants can regulate light levels by dissipating some solar energy as heat. But it turns out that plants’ green colour may also play a role.

Nathan Gabor at the University of California, Riverside, stumbled on this idea by chance. “I was sitting at this seminar, and I thought to myself ‘physicists are often credited with explaining why the sky is blue’,” he says. “So I thought, ‘well, why are plants green?’ ”

Gabor found that, while there are many hypotheses, none have been definitively proven. “The evolutionary evidence has several missing links along the way,” he says. So, when he and his team designed a solar cell that would efficiently regulate its power intake and output, they were surprised to find a potential answer.

A green dilemma

When a molecule in a solar cell absorbs sunlight, some of its electrons jump to a higher energy level. The molecule can’t hold on to that energy, so it transfers an electron to a different molecule, setting up an electric current.

Different materials are sensitive to different wavelengths, so it’s possible to tune your cell to respond to different kinds of light.

If a solar cell or a plant wanted to simply take in the most possible energy, absorbing green light would be the obvious choice: the sun emits more green light than any other colour. Most plants reflect it, though, giving them their colour.

That matches up with Gabor’s calculations: he and his team found that the best way for the cells to maintain high efficiency is to take in two different colours of light, neither of which was green. Over the course of the day, when the amount of light in different wavelengths varies, the cell takes in more of one colour and less of the other to keep output steady.

Gabor thinks green light’s very abundance is what makes it undesirable for both plants and solar cells. The more light you get in a particular wavelength, the more that signal fluctuates, making it harder to absorb it efficiently.

“Green light is very noisy just because there’s a lot of it in our solar spectrum,” he says. “The plants don’t actually want that very noisy light because it’s harder to fine-tune it to create a steady flow of energy.”

Richard Cogdell at the University of Glasgow, UK, is sceptical of that explanation, however.

“Biology hasn’t had freedom to choose which pigments it uses,” he says. “I think it’s smashing science in its own right, but the link to photosynthesis and the natural system is not there.”

Gabor remains confident in his model. “It’s based on very simple quantum mechanical rules, and from that emerges this idea that maybe plants have a very very good reason to be green,” he says. And the self-regulating, extra-efficient solar cells could be pretty good too.

Journal reference: Nano Letters, DOI: 10.1021/acs.nanolett.6b03136