Photoinduced Polaron Formation In Conjugated Polyelectrolyte-fullerene

A new technology capable of storing solar energy for up to several weeks has been developed by chemists at UCLA. The materials used for current residential rooftop solar panels can store energy from the sun for only a few microseconds at a time.

The new approach was inspired by the way that plants generate energy through photosynthesis, said Sarah Tolbert, a UCLA professor of chemistry and one of the senior authors of the research:

“Biology does a very good job of creating energy from sunlight. 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,” Tolbert said. “That separation is the key to making the process so efficient.”

Plastic Solar Cells

Conventional rooftop solar cells use silicon, a relatively expensive material, for capturing energy from sunlight. There is now a large effort to make lower-cost solar cells using plastics, rather than silicon, but today’s plastic solar cells are 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,” Tolbert said. “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 parts that drive the new system 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.

Polymer Spaghetti

Known as organic photovoltaics, the plastic materials are typically organized like a plate of spaghetti in a disorganized mass of long, skinny polymer strings 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-developed technology arranges the elements more neatly, similar to 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.

Benjamin Schwartz, a UCLA professor of chemistry and senior co-author, said:

“When the charges never come back together, the system works far better. This is the first time this has been shown using modern synthetic organic photovoltaic materials.”

Self-assembled Design

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

“We worked really hard to design something so we don’t have to work very hard,” Tolbert said.

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,” Schwartz said. “So there’s no additional work.”

The researchers are now working on how to build the technology into actual solar cells.

Reference:

Rachel C. Huber, Amy S. Ferreira, Robert Thompson, Daniel Kilbride, Nicholas S. Knutson, Lekshmi Sudha Devi, Daniel B. Toso, J. Reddy Challa, Z. Hong Zhou, Yves Rubin, Benjamin J. Schwartz, Sarah H. Tolbert
Long-lived photoinduced polaron formation in conjugated polyelectrolyte-fullerene assemblies
Science 19 June 2015: Vol. 348 no. 6241 pp. 1340-1343 DOI: 10.1126/science.aaa6850

Illustration: The new arrangement of solar cell ingredients, with bundles of polymer donors (green rods) and neatly organized fullerene acceptors (purple, tan). Credit: UCLA Chemistry