Self-repairing solar cell copies plants

Nature’s been doing it for millions of years, but humans are still trying to unlock the secret: how to effectively and continuously harness the power of the sun’s energy.

While photovoltaic devices have dramatically improved — and dropped in price — over recent years, no man-made solar cell yet comes close to being able to achieve the energy conversion efficiency of the humblest green plant. New alternatives to the conventional silicon-based photovoltaics show promise, but also come with a new challenge to conquer: the fact that they degrade significantly over time, victim of the same energetic solar radiation they were designed to capture.

Now researchers at the Massachusetts Institute of Technology (MIT) have turned to plants once again to try and solve that problem. What if, they reasoned, we could develop a solar cell that — just as plants do — breaks down light-capturing molecules before they become too degraded and then reassembles them like new to keep solar energy conversion rates (that is, photosynthesis) high?

“I was really impressed by how plant cells have this extremely efficient repair mechanism,” said chemical engineering professor Michael Strano, noting that, in full summer sunlight, “a leaf on a tree is recycling its proteins about every 45 minutes, even though you might think of it as a static photocell.”

Strano and his research team have developed a system of seven different components that effectively create a self-healing photovoltaic device. Their solution includes microscopic disks, made of synthetic molecules called phospholipids, which provide support for other light-sensitive molecules in structures called reaction centres. Added to a mix of carbon nanotubes — hollow, electricity-conducting tubes of carbon atoms that are just a few billionths of a metre thick — these reaction centres come together in a uniform structure in solution that can both capture solar energy and conduct it as electricity.

The “magic” begins when researchers added a surfactant to the solution, which then breaks apart the seven components into an unstructured “soup.” After the surfactant is removed again using a membrane filter, the components spontaneously reassemble themselves back into an organised liquid solar cell that’s as good as new.

“We’re basically imitating tricks that nature has discovered over millions of years,” Strano said.

Strano’s research team ran the new cell through repeated cycles of assembly and disassembly over a 14-hour period, with no loss of efficiency.

“One of the remaining differences between man-made devices and biological systems is the ability to regenerate and self-repair,” said Philip Collins, a professor of experimental and condensed-matter physics at the University of California, Irvine, commenting on the MIT team’s development. “Closing this gap is one promise of nanotechnology, a promise that has been hyped for many years. Strano’s work is the first sign of progress in this area, and it suggests that ‘nanotechnology’ is finally preparing to advance beyond simple nanomaterials and composites into this new realm.”