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. The team found that the exciton binding energy in a material is always a quarter of its transport bandgap, regardless of the specific material. This finding sheds light on the mechanism of excitons in organic semiconductors and has implications beyond solar cell materials.

"A previously unpredicted nature of exciton binding energies in organic semiconductors was revealed," said Professor Hiroyuki Yoshida, an engineering professor at Chiba University. "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 researchers conducted experiments on 42 organic semiconductors, including 32 solar cell materials, seven organic LED materials, and three crystalline pentacene compounds. To calculate the exciton binding energies, the team measured the energy difference between the bound exciton and its "free carrier" state.

"The optical gap, linked to light absorption and emission, determines the former, while the transport gap, denoting the energy required to move an electron from the highest bound energy level to the lowest free energy level, determines the latter," explained the university.

The team used experimental photo-luminescence and photo-absorption techniques to determine the optical gap, while the transport gap was found using ultra-violet photo-electron spectroscopy and low-energy inverse photo-electron spectroscopy, a technique pioneered by the research group at Chiba University.

The researchers were able to determine the exciton binding energies with a precision level of 0.1eV. "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 on this research. The results have been published in the Journal of Physical Chemistry Letters and are available in full without payment.

Image credit: Hiroyuki Yoshida, Chiba University