Stacking molecules like plates boosts solar cell efficiency

Researchers from Osaka University have discovered how to improve the efficiency of optoelectronics devices, such as solar cells, by controlling how light-absorbing molecules stack together.

Organic optoelectronic devices, such as organic solar cells, are increasingly sought after for their flexibility and light weight; however, their performance depends on how well their light-absorbing organic molecules convert light energy into ‘free-charge carriers’, which carry electric currents. The energy needed to generate the 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 the better the device performance. However, researchers are still struggling to design molecules with low exciton-binding energy in a solid state. The research team from Osaka University found that the exciton-binding energy of solar materials is affected by how their molecules stack together, which is known as aggregation.

Lead author Hiroki Mori said the researchers synthesised two types of similar star-shaped molecules, one with a flexible centre and the other with a rigid centre. “The individual molecules behaved similarly when they were dispersed in a solution, but quite differently when they were stacked together in thin solid films,” Mori said.

This difference in behaviour is due to the rigid molecules stacking together well, like plates, whereas the flexible molecules do not. When in a solid state, the rigid molecule has a lower exciton-binding energy than the flexible molecule. To verify this, the team built a single-component organic solar cell and a photocatalyst using each molecule. The solar cell and photocatalyst made of the rigid molecule showed improved performance because their low exciton-binding energy led to a high generation of free-charge carriers.

“Our findings — that making molecules that aggregate well can decrease the exciton-binding energy — are really exciting. This could provide us with a new way to design more efficient optoelectronic devices,” said senior author Yutaka Ie.

The research findings show that the interaction between molecules is more important for device performance and that the design of molecules for high-performance optoelectronic devices should look beyond individual molecular properties. This new method of decreasing exciton-binding energy could support the development of the next generation of optoelectronic devices.

The research findings have been published in the journal Angewandte Chemie International Edition.

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