One of the problems with harvesting sunlight and converting it into stored energy is that the sun’s rays can be highly destructive to many materials,
leading to a gradual degradation of many systems developed to do just that. Once again, researchers have turned to nature for a solution. Plants constantly break down their light-capturing molecules and reassemble them from scratch, so the basic structures that capture the sun’s energy are, in effect, always brand new. By imitating this strategy MIT scientists have created a novel set of self-assembling molecules and used them to create a photovoltaic (PV) cell that repairs itself.
MIT professor Michael Strano first hit upon the idea of mimicking the process used by plants when reading about plant biology. “I was really impressed by how plant cells have this extremely efficient repair mechanism,” he says. 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.”
One of Strano’s long-term research goals has been to find ways to imitate principles found in nature using nanocomponents. In the case of the molecules used for photosynthesis in plants, the reactive form of oxygen produced by sunlight causes the proteins to fail in a very precise way. As Strano describes it, the oxygen “unsnaps a tether that keeps the protein together,” but the same proteins are quickly reassembled to restart the process.
This action all takes place inside tiny capsules called chloroplasts that reside inside every plant cell – and which is where photosynthesis happens. The chloroplast is “an amazing machine,” Strano says. “They are remarkable engines that consume carbon dioxide and use light to produce glucose,” a chemical that provides energy for metabolism.
Imitation the sincerest form of flattery
To imitate that process, Strano and his team, supported by grants from the MIT Energy Initiative and Eni, produced synthetic molecules called phospholipids that form discs; these discs provide structural support for other molecules that actually respond to light, in structures called reaction centers, which release electrons when struck by particles of light. The discs, carrying the reaction centers, are 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.