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New technology could transform solar energy storage

21 June 2015

A new technology developed by chemists at the University of California, Los Angeles is inspired by the way that plants generate energy through photosynthesis.

Polymer donors and fullerene acceptors (illustration ourtesy of UCLA Department of Chemistry)

The materials in most of today's residential rooftop solar panels can store energy from the sun for only a few microseconds at a time. 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 scientists think about designing solar cells.

"Biology does a very good job of creating energy from sunlight," says UCLA's Professor Sarah Tolbert and one of the authors of a paper describing the research in the journal, Science. "Plants do this through photosynthesis with extremely high efficiency.

"In photosynthesis, plants that are exposed to sunlight use carefully organized nanoscale structures within their cells to rapidly 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."

There is currently a big push to make lower-cost solar cells using plastics, rather than silicon, but today's plastic solar cells are relatively inefficient, in 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," says Tolbert. "But this new 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."

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

The plastic materials (organic photovoltaics) are typically a disorganised mass of long, skinny polymer 'spaghetti' with random fullerene 'meatballs.' But this arrangement makes it difficult to get the cell to generate current because the electrons sometimes hop back to the polymer 'spaghetti' and are lost.

The UCLA technology arranges the elements more neatly. Some fullerenes are designed to sit inside the polymer bundles, but others are forced to stay on the outside. The fullerenes inside the structure take electrons from the polymers and transfer them to the outside fullerene, which can effectively keep the electrons away from the polymer for weeks.

"When the charges never come back together, the system works far better," says co-author, UCLA Professor Benjamin Schwartz. "This is the first time this has been shown using modern synthetic organic photovoltaic materials." In the new system, the materials self-assemble just by being placed in close proximity.

The new design is also more environmentally friendly than current technology, because the materials can assemble in water instead of more toxic organic solutions that are widely used today.

"Once you make the materials, you can dump them into water and they assemble into the appropriate structure because of the way the materials are designed," says Schwartz. "So there's no additional work."

The researchers are now working on ways to incorporate the technology into actual solar cells.

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