Unveiling the Hidden Rule of Organic Opto-Electronics

Researchers at Chiba University in Japan have made a significant discovery that could guide the development of organic solar cell materials. They have found that the exciton binding energy in a material is always a quarter of its transport bandgap, regardless of the specific material.

"A previously unpredicted nature of exciton binding energies in organic semiconductors was revealed," said engineering professor Hiroyuki Yoshida. "Our study contributes to the understanding of the mechanism of excitons in organic semiconductors. Moreover, these concepts are not limited to organic semiconductors, but can also be applied to a wide range of molecular-based materials, such as bio-related materials."

The team conducted experiments to measure exciton binding energies for 42 organic semiconductors, including 32 solar cell materials, seven organic LED materials, and three crystalline pentacene compounds.

To compute the exciton binding energies, the researchers calculated the energy difference between the bound exciton and its 'free carrier' state. The optical gap, which is linked to light absorption and emission, was used to determine the former, while the transport gap, denoting the energy needed to move an electron from the highest bound energy level to the lowest free energy level, was used to determine the latter.

Experimental photo-luminescence and photo-absorption techniques were employed to determine the optical gap, while ultra-violet photo-electron spectroscopy and low-energy inverse photo-electron spectroscopy, a technique pioneered by the research group, were used to find the transport gap.

The researchers were able to determine exciton binding energies with a precision level of 0.1eV. "The researchers believe that this precision level can help discuss the exciton nature of organic semiconductors with much higher confidence than previous studies," added the university. "The researchers believe that these findings are likely to be included in future textbooks."

Chiba University collaborated with the RIKEN Center for Emergent Matter Science and Hiroshima University for this research. The results of the study have been published in the Journal of Physical Chemistry Letters and are available in full without payment.

Image credit: Hiroyuki Yoshida, Chiba University