Improving the long-term durability of these photovoltaic devices has become an important topic as organic semiconductor (carbon-based) solar cells have recently improved their efficiency in converting sunlight into electricity. For the practical application of this technology, the efficiency of photovoltaic devices must be maintained for many years. To address this important issue, the researchers studied the degradation mechanisms of two components used in the light-absorbing layer of organic solar cells: “electron-donor” and “electron-acceptor” materials. These two components are necessary to split the bound electron-hole pairs formed after photon absorption into free electrons and holes that form an electric current. In this new study, reported in Joule, an international team of researchers led by the University of Cambridge’s Cavendish Laboratory investigated the decay pathways of both electron-donating and electron-accepting materials for the first time.
A detailed study of electron-donating materials distinguishes the current study from previous studies and provides important new insights into the field. In particular, the confirmation of an ultrafast deactivation process specific to electron-donating materials, which has not been observed before, provides a new angle from which to consider material degradation in organic solar cells.
A functional organic photovoltaic device. Credit: PhD. Alex Gillett, Cavendish Institute, University of Cambridge
To understand how these substances break down, Cavendish’s researchers worked as part of an international team that included scientists from the UK, Belgium and Italy. Together, they combined studies of the stability of photovoltaic devices where operating solar cells are exposed to intense light comparable to sunlight with ultrafast laser spectroscopy carried out at Cambridge. Using this laser technique, they were able to identify a new degradation mechanism in electron-donating materials in which the polymer chains twist. As a result, when the twisted polymer absorbs a photon, it undergoes a very fast deactivation pathway on the femtosecond (billionth of a second) time scale. This unwanted process is fast enough to outpace the creation of electrons and holes without photons, which scientists have been able to link to the organic breakdown of solar cells after exposure to sunlight.
“In the future, we intend to expand on our findings by collaborating with chemistry groups to design new electron donor materials with more rigid polymer backbones. We expect that this may lessen the polymer’s proclivity to twist and so increase the durability of the organic solar cell system.”
Dr. Alex Gillett, the lead author of the paper.
“It was interesting to learn that something seemingly trivial, like twisting the polymer chains, can have a big impact on the efficiency of a solar cell,” said Dr. Alex Gillett, lead author of this article. “In the future, we hope to work with chemical groups to reduce the bending tendency of polymers to design new electron donor materials. This will improve the stability of organic materials. Improving Solar Cell Devices”.
Due to their unique properties, organic solar cells can be used in many applications where conventional silicon photovoltaic cells are not suitable. This could be an electricity-generating window for a greenhouse that transmits the color of light needed for photosynthesis, or a photovoltaic cell that can be easily transported and rolled for mobile electricity generation.
By identifying the degradation mechanisms that need to be addressed, the current study brings state-of-the-art photovoltaic materials and applications closer to reality.
More information: Alexander J. Gillett & collegaues, The critical role of the donor polymer in the stability of high-performance non-fullerene acceptor organic solar cells, Joule (2023). DOI: 10.1016/j.joule.2023.03.002
