The new findings pave the way for stable organic solar cells that will make cheap, renewable electricity possible

(Nanowerk News) Due to recent improvements in the efficiency of solar cells made of organic (carbon-based) semiconductors to convert sunlight into electricity, improving the long-term stability of these photovoltaic devices is becoming a topic of increasing importance. Real-world application of the technology demands that the efficiency of photovoltaic devices be maintained over the years.

To address this key issue, researchers have studied the degradation mechanisms for two components used in the light-absorbing coatings of organic solar cells: ‘electron donor’ and ‘electron acceptor’ materials. These two components are needed to separate the bonded electron-hole pairs that are formed after the absorption of photons into free electrons and holes that form an electric current.

In this new study reported in Joules (“The critical role of donor polymers in the stability of high-performance non-fullerene acceptor organic solar cells”), an international research team led by the Cavendish Laboratory, University of Cambridge, has for the first time considered the degradation pathways of electron donor and electron acceptor materials. Working organic photovoltaic devices. (Image: Dr. Alex Gillet, Cavendish Laboratory, University of Cambridge)

Detailed investigations of electron donor materials distinguish the current research from previous studies and provide important new insights for this field. In particular, the identification of a unique ultrafast deactivation process for electron donor materials has never been observed before and provides a new vantage point for considering material degradation in organic solar cells.

To understand how these materials are degraded, the Cavendish researchers worked as part of an international team with scientists in England, Belgium and Italy. Together, they combined stability studies of photovoltaic devices, in which operational solar cells are subjected to intense light that closely matches sunlight, with ultrafast laser spectroscopy conducted in Cambridge. Through this laser technique, they have been able to identify a novel degradation mechanism in electron donor materials involving twisting of the polymer chains.

As a result, when the bent polymer absorbs a photon, it undergoes a very fast deactivation path on the femtosecond (one millionth of a billionth of a second) time scale. This unwanted process is fast enough to outperform the generation of free electrons and holes from photons, which scientists can correlate with the reduced efficiency of organic solar cells after exposure to simulated sunlight.

“It was very exciting to find that something as seemingly small as the twisting of a polymer chain could have such a large effect on the efficiency of a solar cell,” said Dr. Alex Gillet, the paper’s lead author. “In the future, we plan to expand on our findings by collaborating with chemical groups to design new electron donor materials with more rigid polymer backbones. We hope this will reduce the tendency of polymers to twist and thereby increase the stability of organic solar cell devices.”

Because of their unique properties, organic solar cells can be used in a variety of applications that are incompatible with traditional silicon photovoltaics. These could include power-generating windows for greenhouses that transmit the colors of light needed for photosynthesis, or even photovoltaic coils for easy transport and mobile power generation. Thus, by identifying the degradation mechanisms that need to be solved, the current research directly brings the next generation of photovoltaic materials and applications closer to reality.