Plant inspired solar cells to revolutionise energy storage

New York: A new technology developed by scientists at University of California - Los Angeles (UCLA) can store solar energy for up to several weeks -- an advance that could change the way scientists think about designing solar cells.

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.

The new design is inspired by the way that plants generate energy through photosynthesis.

"In photosynthesis, plants that are exposed to sunlight use carefully organised 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," said senior study author Sarah Tolbert.

"That separation is the key to making the process so efficient," Tolbert said.

To capture energy from sunlight, conventional rooftop solar cells use silicon, a fairly expensive material.

On the other hand, plastic solar cells - which are cheaper - are relatively inefficient, 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. But this new system pulls charges apart and keeps them separated for days, or even weeks," Tolbert said.

"Once you make the right structure, you can vastly improve the retention of energy," she added.

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, called organic photovoltaics, are typically organised like a plate of cooked pasta -- a disorganised mass of long, skinny polymer 'spaghetti' with random fullerene 'meatballs.'

But 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.

"When the charges never come back together, the system works far better," another senior author Benjamin Schwartz said.

In the new system, the materials self-assemble just by being placed in close proximity.

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

The findings were published in the journal Science.