New research from North Carolina State University sheds light on the process occurring in organic solar cells during the conversion of light into electricity. Researchers have introduced a novel method to visualize the interfaces where sunlight transforms into electrical charges. Their findings also led to the creation of design principles aimed at enhancing the efficiency of organic solar cells.
Organic solar cells utilize carbon-based polymer materials that are potentially low-cost and derived from abundant natural resources. They possess appealing characteristics, such as the ability to be manufactured as semi-transparent or transparent windows. Moreover, as thin film solar cells, they are lightweight and flexible, making them suitable for roll-to-roll production, which could facilitate easier transport and installation.
Despite these advantages, organic solar cells currently lag behind silicon and perovskite technologies in terms of efficiency when it comes to the conversion of light into electricity.
According to Aram Amassian, co-corresponding author of the study and a professor in the materials science and engineering department at North Carolina State University, “Organic solar cells consist of a blend of two materials. Both materials capture electrons from sunlight; however, one is a polymer that collects electrons and must interact with a second material to transfer those electrons. This polymer is referred to as the donor material, while the other, usually a small molecule, is known as the acceptor material. We understood that the interfaces formed between these donor and acceptor materials could lead to voltage loss, which currently limits the effectiveness of organic solar cells. Our objective was to better comprehend the factors at play at these interfaces that cause voltage loss, so we could make improvements.”
To tackle this issue, researchers developed a scanning-probe microscopy method that allowed them to examine not just the topographical features of the donor and acceptor blend, but also the energy characteristics at their interfaces, like the energy gradient and the level of disorder present.
“This technique enabled us to assess how the disorder in donor and acceptor molecules at the interface influenced energy variations,” explains Daniel Dougherty, co-corresponding author and a physics professor at NC State. “After mapping the energetics across all interfaces, we could compare these results with traditional methods used to gauge the overall performance and voltage loss in organic solar cells.”
However, the team faced a significant obstacle. The scanning-probe microscopy method does not directly measure voltage loss, making it challenging to pinpoint which interface was primarily responsible.
“The blends of donor and acceptor materials produce numerous interface types simultaneously, making it unclear which ones contribute to voltage losses,” states Amassian.
Dougherty adds, “Our research uncovered that in high-performance organic solar cells like PM6:Y6, the critical interface is the well-defined donor-acceptor interface. Our results suggest that focusing on this interface is essential for further minimizing voltage losses.”
After identifying the key interface linked to voltage loss, the team embarked on a series of investigations to discover what factors affected these losses, according to Amassian.
He notes, “A long-standing debate exists within the organic solar cell community: some believe voltage loss stems from energy differences between donor and acceptor materials, while others argue it arises from energetic disorder at the interfaces. Our experiments revealed that both theories hold true, as voltage loss results from a combination of the two.”
The researchers successfully demonstrated that it is feasible to “adjust” the energy differential and manage the disorder at interfaces by modifying how the donor and acceptor materials are blended during manufacturing, thereby minimizing voltage loss as much as possible.
Amassian remarks, “By controlling one aspect of voltage loss, we could identify engineering strategies that would help the organic solar cell community to mitigate the other factor contributing to voltage loss.”
“Effectively, voltage losses can be minimized by opting for material pairs that exhibit minimal energy differences. Practitioners can then further decrease energy losses by selecting appropriate solvents and processing conditions that significantly lessen interfacial disorder. We are hopeful that the design principles we formulated through this method can guide future research and development in organic solar cells.”
The paper titled “Mapping Interfacial Energetic Landscape in Organic Solar Cells Reveals Pathways to Reducing Nonradiative Losses” has been published in the journal Matter. Co-first authors include Gaurab Thapa, a former Ph.D. student at NC State, and Mihirsinh Chauhan, a previous postdoctoral researcher at NC State. The research also included Jacob Mauthe, a Ph.D. student at NC State.
This study was supported by the Office of Naval Research under award number N00014-20-1-2183.
