A breakthrough in photon up-conversion has been achieved by researchers at Kobe University, shedding light on a crucial mechanism that could revolutionize the development of more efficient photovoltaic (PV) cells, OLED displays, and even anti-cancer therapies.
Efficiently combining two low-energy photons into one high-energy photon requires a material where energy can freely hop between molecules, but not too quickly. This discovery by Kobe University scientists provides a vital design guideline for materials aimed at enhancing the performance of various technologies.
Different colors of light carry different energies, making them suitable for diverse applications. Researchers worldwide have been striving to develop materials capable of up-converting low-energy photons for purposes such as improving PV cells, OLED displays, and anti-cancer treatments.
During the up-conversion process, light is absorbed by the material, creating “triplet excitons,” where energy is passed among molecules. However, until now, it remained unclear how two triplet excitons could efficiently combine their energies to emit a high-energy photon, posing a significant obstacle in material development.
Kobe University photoscientist Kobori Yasuhiro and his team focused on the electron spin states of moving and interacting excited states, a property critical for understanding up-conversion. They applied their expertise to analyze a suitable material for their study.
Yasuhiro explains, “In solution systems, it is difficult to observe the magnetic properties of the electron spins due to the high-speed rotation of the molecules, and in conventional solid-state systems, the reaction efficiency is too low for electron spin resonance studies. The thin-film solid-state material used in our study, however, was suitable for observing the magnetic properties of electron spins and generating sufficient triplet exciton concentrations.”
Their findings, published in The Journal of Physical Chemistry Letters, reveal that aligning the electron spin states of two triplet excitons depends on the relative orientation of the molecules involved. For efficient energy transfer, triplet excitons must move between molecules of various orientations at an optimal rate to allow time for the conversion of excited states.
Yasuhiro elaborates, “We first directly observed the time evolution of the electron spin state inside up-conversion materials in solid-state systems, then modeled the observed electron spin motion, and finally proposed a new theoretical model for how the electron spin state relates to the up-conversion process.”
These results offer a guideline for designing highly efficient photon up-conversion materials based on a thorough understanding of the microscopic mechanism involved.
“I expect this knowledge to contribute to the development of high-efficiency solar cells to alleviate our energy problems, but also to expand into a wide range of fields such as photodynamic cancer therapy and diagnostics that utilize near-infrared light for optical up-conversion without harming the human body,” Yasuhiro concludes.