Researchers have discovered a key reason organic solar cells degrade, a finding that should help promote the development of more stable materials for these type of photovoltaics, they said.

A team from the Cavendish Laboratory, University of Cambridge, examined the degradation pathways of two materials used in organic solar cells—the electron donor and electron acceptor, the researchers said. In their research, they identified an ultrafast deactivation process unique to the electron donor material that wasn't known before, they said.

Organic solar cells are photovoltaics that use carbon-based semiconductors and other earth-abundant materials, so they are more environmentally friendly than traditional silicon-based photovoltaics. They also have the potential to provide more inexpensive and renewable energy.

Organic solar cells also have properties that give them the potential to be used in a wide range of applications for which traditional silicon photovoltaics aren’t well-suited. These include electricity-generating windows for greenhouses that transmit the colours of light required for photosynthesis or even photovoltaics that could be rolled up for easy transportation and mobile electricity generation.

Despite these benefits, one drawback to photovoltaic modules made from organic semiconductors is that they don't maintain their efficiency in a comparable way to traditional solar cells. That also means that so far, they haven't been commercially viable for widespread use.

That could change with the new discovery, which potentially can provide scientists with new insight for the development of organic solar cells that degrade less quickly than current designs, making them perform on-par with silicon-based voltaics, the researchers said.

Making the Discovery About Organic Solar Cells

Researchers specifically studied the degradation mechanisms for the two components used in the light-absorbing layer of organic solar cells: the electron donor and electron acceptor materials. The cell uses these two materials to split the bound electron-hole pair formed after the absorption of a photon into the free electrons and holes that constitute electrical current, the researchers said.

To make their discovery, the research team—composed of scientists not only from the UK but also Belgium and Italy—combined photovoltaic device stability studies, subjecting an operational solar cell to intense light that closely matches sunlight and then using ultrafast laser spectroscopy performed in Cambridge.

The laser technique enabled them to identify a new degradation mechanism in the electron donor material that involves twisting in the polymer chain of the material. Specifically, when the twisted polymer absorbs a photon, it undergoes an extremely rapid deactivation pathway on femtosecond timescales, which are in the range of a millionth billionth of a second.

“It was interesting to find that something as seemingly minor as the twisting of a polymer chain could have such a large effect on the solar cell efficiency,” said Alex Gillett, a Cambridge professor and one of the researchers on the project.

Indeed, the deactivation process is so fast that it outcompetes the generation of free electrons and holes from a photon, the researchers found. The researchers were then able to correlate with the reduced efficiency of the organic solar cell after it had been exposed to simulated sunlight, they said.

Researchers published a paper on their research. They plan to continue to develop their findings by collaborating with chemistry groups to design new electron donor materials with more rigid polymer backbones, Gillett said.

"We hope that this will reduce the propensity of the polymer to twist and thus improve the stability of the organic solar cell device," he said.