A new technology developed by chemists at the University of California Los Angeles (UCLA) is capable of storing solar energy for up to several weeks—an advance that could change the way engineers think about designing solar cells. The design is inspired by the way that plants generate energy through photosynthesis and the findings are published in the journal Science.

“Biology does a good job of creating energy from sunlight,” says Sarah Tolbert, a UCLA professor of chemistry and one of the senior authors of the research. “Plants do this through photosynthesis with high efficiency.”

The new arrangement of solar cell ingredients comprises bundles of polymer donors (green rods) and neatly organized fullerene acceptors (purple and brown rods). Source: UCLAIn photosynthesis, plants that are exposed to sunlight use nanoscale structures within their cells to separate charges, pulling electrons away from the positively charged molecule that is left behind and keeping positive and negative charges separated. That separation is the key to making the process so efficient, she says.

To capture energy from sunlight, conventional rooftop solar cells use silicon, a fairly expensive material. Currently, plastic solar cells are less expensive, but relatively inefficient, in a large part because the separated positive and negative electric charges often recombine before they can become electrical energy.

“Modern plastic solar cells don't have well-defined structures like plants do because we never knew how to make them before,” Tolbert says. The UCLA-devised system pulls charges apart and keeps them separated for days, or even weeks. “Once you make the right structure, you can vastly improve the retention of energy,” she says.

The two components that make the UCLA-developed system work are a polymer donor and a nanoscale fullerene acceptor. The polymer donor absorbs sunlight and passes electrons to the fullerene acceptor; the process generates electrical energy.

The plastic materials, called organic photovoltaics, are typically organized like a plate of cooked pasta. They resemble a disorganized mass of long, skinny polymer ‘spaghetti’ with random fullerene ‘meatballs.’ This arrangement makes it difficult to get current out of the cell because the electrons sometimes hop back to the polymer spaghetti and are lost.

The UCLA technology arranges the elements more neatly, like small bundles of uncooked spaghetti with precisely placed meatballs. Some fullerene meatballs are designed to sit inside the spaghetti bundles, but others are forced to stay on the outside. The fullerenes inside the structure take electrons from the polymers and toss them to the outside fullerene, which can effectively keep the electrons away from the polymer for weeks.

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