Stacking particles like tiles improves the efficiency of organic solar devices

August 6, 2024

(Nanowerk News) Harnessing the power of the sun is key to a clean, green future. To do that, we need optoelectronic devices, such as solar cells, that can efficiently convert light into electricity. Now a team led by Osaka University has discovered how to make the devices even more efficient: by controlling the way light-absorbing molecules stack.

The results of their research were published in Angewandte Chemie, international edition (“Organic semiconductor based on dibenzo(g,p)chrysene with low exciton binding energy via molecular aggregation”). Overview of stacked organic semiconductors and their applications in this study. (Photo: Osaka University)

Organic optoelectronic devices, such as organic solar cells, are becoming increasingly desirable due to their inherent advantages, such as flexibility or light weight. Their performance depends on how well their light-absorbing organic molecules convert light energy into “free charge carriers” that carry electric current. The energy required to generate free charge carriers is referred to as “exciton binding energy.”

The lower the exciton binding energy, the easier it is to generate free charge carriers and therefore the better the device performance. However, we still have difficulty designing molecules with low exciton binding energy in the solid state.

Upon closer examination, the research team discovered that the binding energy of excitons in solid materials depends on how their molecules arrange themselves, a phenomenon called aggregation.

“We synthesized two types of similar star-shaped molecules, one with a flexible center and the other with a rigid center,” explains lead author Hiroki Mori. “The individual molecules behaved similarly when dispersed in solution, but completely differently when stacked together in thin, solid layers.”

The difference in behavior is due to the fact that rigid molecules stack well, like plates, while flexible molecules do not stack. In other words, in the solid state, a rigid molecule has a much lower exciton binding energy than a flexible molecule. To verify this, the team built a single-component organic solar cell and photocatalyst using each molecule. The solar cell and photocatalyst made of the rigid molecule showed impressive performance, as their low exciton binding energy led to a high generation of free charge carriers. Molecular structures of organic semiconductors (top), performance of a single-component organic solar cell using a stacked molecule (left), and performance of both heterogeneous organic photocatalysts (right) (Photo: Osaka University)

“Our findings that creating molecules that aggregate well can reduce the binding energy of excitons are really exciting,” says senior author Yutaka Ie. “This could provide us with a new way to design more efficient optoelectronic devices.”

The team’s findings show that the interaction between molecules in a solid state is important for device performance and that the design of molecules for high-performance optoelectronic devices should go beyond individual molecular properties. This new way of reducing exciton binding energy could form the basis of driving mechanisms and architectures for the next generation of optoelectronic devices.