Light-harvesting complexes do it themselves

New technique could yield self-assembling solar cells

A new technique may one day lead to solar cells that bring themselves together like a molecular flash mob and repair damage they sustain during the rough business of turning light into electricity.

COME TOGETHER | Future solar cells may build and repair themselves using a handful of ingredients: photosynthetic reaction centers (green) to capture light, carbon nanotubes (grey bar) to carry current and nanodiscs of lipids (yellow) and proteins (red and blue) for structural support. M-H. Ham et al./Nature Chemistry

The research lays the groundwork for cheap, self-repairing solar cells with an indefinite lifetime, a team reports September 5 in Nature Chemistry.

“It’s a manmade version of what nature does,” says nanocomposite expert Jaime Grunlan of Texas A&M University in College Station. “This really looks like ground-breaking seminal work; I’ve never seen anything remotely like it.”

The sun’s rays can be brutal, even for a leaf that’s harvesting them. When photosynthesis is going full blast, a leaf is constantly building new photosynthetic reaction centers to replace those damaged by harsh oxygen species and other destructive molecules generated by intense ultraviolet light.

So rather than trying to make solar cells that are extremely durable, the team decided to take a literal leaf from nature’s book and go the route of self-repair, says chemical engineer Michael Strano of MIT, who led the project. He and Stephen Sligar and Colin Wraight of the University of Illinois at Urbana-Champaign, along with other colleagues, designed a system where damaged parts could be easily replaced.

The researchers began with light-harvesting reaction centers from a purple bacterium. Then they added some proteins and lipids for structure, and carbon nanotubes to conduct the resulting electricity.

These ingredients were added to a water-filled dialysis bag — the kind used to filter the blood of someone whose kidneys don’t work — which has a membrane that only small molecules can pass through. The soupy solution also contained sodium cholate, a surfactant to keep all the ingredients from sticking together. 

When the team filtered the surfactant out of the mix, the ingredients self-assembled into a unit, capturing light and generating an electric current.

The spontaneous assembly occurs thanks to the chemical properties of the ingredients and their tendency to combine in the most energetically comfortable positions. The scaffolding protein wraps around the lipid, forming a little disc with the photosynthetic reaction center perched on top.  These discs line up along the carbon nanotube, which has pores that electrons from the reaction center can pass through.

Adding the sodium cholate back into the mix disassembles the complexes. But filtering it out again brings them right back together.

“The idea that it happens reversibly and at will is quite amazing,” says Strano. “It approaches what happens in biology — forming a huge amount of order with the flip of a switch. It’s kind of like taking puzzle pieces and throwing them up in the air and them coming down assembled.”

The complexes eventually lose power, but they are easily revived, says Strano. The research team disassembled the units and replenished the photosynthetic reaction centers. Four such replacements over the course of a week kept keeping the complexes humming along.

“This is very nice work — the procedure they’ve got, the control they have over the system,” says biochemist Mike Jones of the University of Bristol in England. “It’s simple, it’s very nice.”

The units can’t compete with silicon-based solar cells in use today. But silicon-based solar cells reached their current level of efficiency only after decades of research and development, says Jones. Similar investment in this new technology could yield a system that’s highly efficient, can self-repair and works well under low light conditions, he says.

What’s more, the main ingredients for these solar cells might one day be easily extracted from plant material, says Strano, perhaps even from garbage biomass. “We could turn waste into an organized product,” he says.   

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