Organic solar cells (OSCs) are attractive due to their light weight, flexibility and high energy conversion efficiency. However, the lack of control over active layer morphology makes it difficult to develop OSCs with large active regions. Now, researchers from the Gwangju Institute of Science and Technology in Korea are taking it to the next level by using water treatment to control the morphology of active-layer thin-film fabrication, and improve the performance and stability of large-area OSCs.
Organic solar cells (OSCs), which use organic polymers to convert sunlight into electricity, have received significant attention recently for their desirable properties as next-generation energy sources. These features include light weight, flexibility, scalability, and high power conversion efficiency (>19%). There are currently several strategies to improve the performance and stability of OSCs. However, a problem that persists is the difficulty of controlling the morphology of the active layer in OSCs when scaling it to large areas. This makes it difficult to obtain high-quality thin films with an active layer, and thus adjust the efficiency of the device.
In a recent study, a team of researchers from the Gwangju Institute of Science and Technology in Korea set out to tackle this problem. In their work published in advanced functional materialsThey proposed a solution that appears counterintuitive at first glance: the use of water treatment to control the morphology of the active layer. “Water is known to impede the performance of organic electronic devices, since it remains in the ‘trap states’ of organic matter, impeding the flow of charge and degrading the performance of the device. However, we found that using water instead of an organic solvent- explains Professor Dong Yue Kim, who headed the study, said that the active solution based on the treatment method would enable the necessary physical changes to be made without causing chemical reactions.
The researchers selected PTB7-Th and PM6 polymers as donor materials and PC61BM, EH-IDTBR and Y6 as acceptor materials for the active layer. They observed that vortex stimulation to mix the donor and acceptor materials in the active solution could result in an effective, well-mixed solution, however it was not sufficient on its own. The active solution was hydrophobic, and accordingly, the researchers decided to use deionized (DI) water and vortexes to stir the solution. They allowed the donor and acceptor materials to sit in chlorobenzene (active host solution) overnight, then added DI water into the solution and stirred it, creating small swirls. Due to the hydrophobic nature of the solution, the water pushed out the donor and acceptor molecules, causing them to dissolve more precisely in the solution. Then they let the solution rest, causing the water to separate from the solution. This water was then removed and the hydrotreated active solution was used to prepare thin films of PTB7-Th:PC61BM (F, fullerene), PTB7-Th:EH-IDTBR (NF, fullerene), and PM6:Y6 (H-NF, high-efficiency non-fullerene).
The researchers next examined the photoelectric performance of these thin films in the inverted hole-coated OSC formation and compared it with that of OSCs without water treatment.
“We observed that the hydrotreated active solution resulted in a more regular active thin film, which showed higher energy conversion efficiency compared to the non-water-treated ones. Furthermore, we fabricated large-area OSCs with an active area of 10 cm.2which showed a high conversion efficiency of up to 11.92% for the hydrolyzed H-NF films,” Professor Kim highlights.
Overall, this study provides a guideline for the large-scale and efficient development of OSCs using an easy, economical and remarkably environmentally friendly method, which could open doors to their realization and commercialization.