A working organic photovoltaic device. Photo: Dr. Alex Gillet, Cavendish Laboratory, University of Cambridge.
A working organic photovoltaic device. Photo: Dr. Alex Gillet, Cavendish Laboratory, University of Cambridge.

Due to recent improvements in the efficiency with which solar cells made from organic (carbon-based) semiconductors can convert sunlight into electricity, improving the long-term stability of these photovoltaic devices is becoming an increasingly important topic. Real-world applications of the technology demand that the efficiency of the photovoltaic devices can be maintained for many years.

To address this issue, researchers have been studying the degradation mechanisms for the two components used in the light-absorbing layer of organic solar cells: the ‘electron donor’ and ‘electron acceptor’ materials. These two components are needed to split the bound electron-hole pair formed after the absorption of a photon into the free electrons and holes that constitute electrical current.

Now, for the first time, an international team led by researchers from the Cavendish Laboratory at the University of Cambridge in the UK has considered the degradation pathways of both the electron-donor and electron-acceptor materials. They report their findings in a paper in Joule.

The detailed investigation of the electron-donor material sets the current research work apart from previous studies and provides important new insights for the field. Specifically, the identification of an ultrafast deactivation process unique to the electron-donor material has not been observed before and provides a new angle on material degradation in organic solar cells.

To understand how these materials degrade, the Cavendish researchers worked as part of an international team with scientists in the UK, Belgium and Italy. Together, they combined photovoltaic device stability studies, where the operational solar cell is subject to intense light that closely matches sunlight, with ultrafast laser spectroscopy performed in Cambridge.

Using this laser technique, the researchers were able to identify a new degradation mechanism in the electron-donor material involving twisting in the polymer chain. As a result, when the twisted polymer absorbs a photon, it undergoes an extremely rapid deactivation pathway on femtosecond timescales (a millionth billionth of a second). This undesirable process is fast enough to outcompete the generation of free electrons and holes from a photon, and the scientists were able to correlate this process with the reduced efficiency of the organic solar cell after it had been exposed to simulated sunlight.

“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 from the Cavendish Laboratory, who is the lead author of the paper. “In the future, we plan to build on our findings by collaborating with chemistry groups to design new electron-donor materials with more rigid polymer backbones. We hope that this will reduce the propensity of the polymer to twist and thus improve the stability of the organic solar cell device.”

Due to their unique properties, organic solar cells can be used in a wide range of applications for which traditional silicon photovoltaics aren’t suitable. These could include electricity-generating windows for greenhouses that still transmit the colors of light required for photosynthesis, or even photovoltaics that can be rolled up for easy transportation and mobile electricity generation. Thus, by identifying a new degradation mechanism that needs to be resolved, the current research directly brings the next generation of photovoltaic materials and applications closer to reality.

This story is adapted from material from the University of Cambridge, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.