Research has enhanced our understanding of how organic solar cells transform light into electricity. Scientists have introduced a novel technique that visualizes the interfaces where sunlight energy converts into electrical charges, leading to a set of design principles aimed at boosting the efficiency of organic solar cells.
Recent findings from North Carolina State University shed light on the processes involved in organic solar cells’ conversion of light into electricity. The research team has created a method to visualize the interfaces where sunlight’s energy is transformed into electrical charges and developed design guidelines to enhance the efficiency of these solar cells.
Organic solar cells utilize carbon-based polymer materials, offering potential advantages like low production costs, reliance on abundant materials, and features such as the ability to be semi-transparent or fully transparent for use in windows. Moreover, as thin film solar cells, they can be lightweight and flexible, making them suitable for roll-to-roll manufacturing, which facilitates transportation and installation.
Despite their benefits, organic solar cells have not matched the efficiency of silicon or perovskite solar technologies in the conversion of light into electricity.
“Organic solar cells consist of a blend of two materials,” explains Aram Amassian, co-leading author of the study and a professor of materials science and engineering at North Carolina State University.
“Both materials capture electrons from sunlight. One is a polymer that collects electrons but must interact with a second material to transfer them. The polymer acts as the donor material, while the other, usually a small molecule, serves as the acceptor material. We recognized that the interfaces between these donor and acceptor materials caused voltage loss, which currently limits the efficiency of organic solar cells. Our research aimed to better understand the interface characteristics contributing to this voltage loss so we could enhance their performance.”
To tackle this issue, the researchers created a scanning-probe microscopy technique that allowed them to map not only the topographical features of the donor and acceptor mixture but also the energy attributes at their interfaces, such as the energy gradient and the level of disorder present between the two materials.
“Using this method, we were able to assess how the disorder of the donor and acceptor molecules at the interface affected energy disorder,” says Daniel Dougherty, co-leading author of the study and a physics professor at NC State. “We then compared our energetic mapping of these interfaces with results from traditional methods that gauge voltage loss in organic solar cells.”
However, the team faced another significant challenge. Since the scanning-probe microscopy technique does not directly measure voltage loss, it was unclear which interface was primarily responsible for it.
“The combination of donor and acceptor materials creates numerous types of interfaces, making it difficult to identify which are causing the voltage losses,” Amassian notes.
“Our study identified that the sharp donor-acceptor interface in high-performance organic solar cells like PM6:Y6 is the critical interface,” Dougherty reveals. “This suggests that targeting this specific interface could further minimize voltage losses.”
“After pinpointing the functional interface associated with voltage loss, we explored the various factors influencing it,” Amassian adds.
There has been ongoing debate within the organic solar cell community regarding whether voltage loss arises from the energy difference between the donor and acceptor materials or from energetic disorder along the interfaces. Our experiments indicate that both factors contribute to the issue,” Dougherty explains.
The research team successfully proved that adjusting the energy differential and tuning the disorder at the interfaces by altering how the donor and acceptor are mixed during fabrication can help minimize voltage loss effectively.
“By addressing one of the causes of voltage loss, we could identify engineering solutions to limit the other cause,” Amassian states.
“In essence, voltage losses can be decreased by choosing material pairs with minimal energy offsets. Researchers can also further minimize energy losses by selecting solvents and processing conditions that significantly lower interfacial disorder. We are hopeful that the design principles we developed through this method will aid in guiding future organic solar cell research and development,” he continues.
The research paper titled “Mapping Interfacial Energetic Landscape in Organic Solar Cells Reveals Pathways to Reducing Nonradiative Losses,” is published in the journal Matter. Gaurab Thapa, a former Ph.D. student at NC State, and Mihirsinh Chauhan, a former postdoctoral researcher at NC State, are co-first authors of the paper. Jacob Mauthe, a current Ph.D. student at NC State, also contributed as a co-author.
This research was supported by the Office of Naval Research under award number N00014-20-1-2183.
