MIT rsearchers have created tiny solar cells that can repair themselves after damage from the sunlight they're designed to process.
The team was inspired by the way plants constantly break down their light-capturing molecules and reassemble them from scratch, so that the basic structures that capture the sun's energy are regularly renewed.
In full summer sunlight, says Michael Strano of MIT's department of chemical engineering, "a leaf on a tree is recycling its proteins about every 45 minutes, even though you might think of it as a static photocell."
Inspired by this, Strano created a set of self-assembling molecules that can turn sunlight into electricity; the molecules can be repeatedly broken down and then reassembled quickly, just by adding or removing an additional solution.
Synthetic molecules called phospholipids form discs whichprovide structural support for other molecules that respond to light, in structures called reaction centers, releasing electrons when they're struck by photons.
The discs are held in a solution where they attach themselves spontaneously to carbon nanotubes. The nanotubes hold the phospholipid discs in a uniform alignment so that the reaction centers can all be exposed to sunlight at once, and they also act as wires to collect and channel the flow of electrons knocked loose by the reactive molecules.
The system is made up of seven different compounds, including the carbon nanotubes, the phospholipids, and the proteins that make up the reaction centers, which under the right conditions spontaneously assemble themselves into a light-harvesting structure that produces an electric current.
Strano says he believes this sets a record for the complexity of a self-assembling system.
When a surfactant is added to the mix, the seven components all come apart and form a soupy solution. Then, when the researchers remove the surfactant by pushing the solution through a membrane, the compounds spontaneously assemble once again into a perfectly formed, rejuvenated photocell.
The team came up with the system based on a theoretical analysis, but then decided to build a prototype cell to test it out. They ran the cell through repeated cycles of assembly and disassembly over a 14-hour period, with no loss of efficiency.
Strano says that with conventional silicon-based photovoltaic cells, there is little degradation - but that with many new systems being developed, the degradation can be very significant.
"Often people see, over 60 hours, the efficiency falling to 10 percent of what you initially saw," he says.
The individual reactions of these new molecular structures in converting sunlight are about 40 percent efficient, or about double the efficiency of today's best commercial solar cells. Theoretically, the efficiency of the structures could be close to 100 percent, he says.
But the concentration of the structures in the solution is currently low, reducing overall efficiency. The team is now working on increasing the concentration.