Fleeting electron-hole pairs are giving scientists a new window into optimizing light-emitting devices (LEDs). Using quantum magnetic resonance, Osaka Metropolitan University researchers have discovered how shifting internal electric fields dictate whether these devices shine brightly or dimly.

Light-emitting electrochemical cells (LECs) are simple, flexible, and low-cost thin-film devices that generate light from an electric current. Unlike conventional organic LEDs, LECs contain just a single active layer — an organic semiconductor blended with mobile ions — sandwiched between two electrodes. This structural simplicity makes them promising tools for next-generation light-emitting technologies.

Inside that apparently simple structure, however, things aren’t so simple after all.

When voltage is applied, mobile ions assist with charge injection, allowing negatively charged electrons and positively charged holes (left by the removal of electrons) to enter the light-emitting material from the electrodes. Once inside, electrons and holes form electron-hole pairs. If these pairs recombine, they release energy as light. The efficiency of an LEC ultimately depends on how effectively this recombination occurs. Understanding such processes, however, has long been a challenge:

First, these electron-hole pairs are unstable and tricky to observe.

“Although optical techniques can track electrons and holes, they cannot clearly detect the short-lived electron-hole pairs that form just before light emission,” said Katsuichi Kanemoto, professor at Osaka Metropolitan University’s Graduate School of Science and lead author of the study.

Second, mobile ions are also present, adding further complications.

“As ions move, they partially shield and redistribute the internal electric field, creating a fluctuating and spatially complex environment,” Kanemoto said. “This makes it very difficult to understand how recombination is actually taking place.”

As a result, details of the behavior of electron-hole pairs and their interaction with internal electric fields have remained largely unknown.

To overcome this double hurdle, the team turned to electroluminescence-detected magnetic resonance (ELDMR), a technique that links magnetic resonance measurements to changes in light emission. By probing the spin properties of electron-hole pairs, which are highly sensitive to electric fields, the method enables selective detection of these intermediates while the device is operating.

“We successfully obtained highly sensitive ELDMR signals for the first time in a polymer-based LEC under operating conditions,” Kanemoto said. “Spectral analysis confirmed that the signals originate from electron spin resonance of electron-hole pairs.”

When the researchers swept the voltage forward and then backward, they observed a pronounced hysteresis in the ELDMR response: the signal depended strongly on the sweep direction.

“This told us that the internal electric field is not static,” Kanemoto said. “It evolves as ions rearrange, and the electron-hole pairs are directly sensing those changes.”

Detailed analysis showed that during the reverse voltage sweep, when the voltage was lowered after being raised, the internal electric field decreased. Under these conditions, electron-hole pairs were less likely to separate and more likely to recombine. This resulted in higher electroluminescence efficiency and a stronger magneto-electroluminescence effect.

“Our results show that there are optimal electric field conditions for efficient recombination,” Kanemoto said. “A stable, lower electric field can actually enhance light emission.”

Although the study focused on LECs, the fundamental recombination process is common to all organic electroluminescent devices, including organic LEDs. The findings highlight the broader importance of electric field management in device design.

Beyond the insights into the physics involved, the researchers’ work establishes ELDMR as a powerful quantum-sensing technique. By extracting information directly from light emission through electron spin detection, the method offers a new way to uncover the microscopic processes taking place inside working devices.

“This is a pioneering example of using quantum measurement techniques to understand light-emitting devices,” Kanemoto said. “We hope this approach will guide the development of more efficient organic optoelectronic technologies in the future.”

The study was published in Advanced Optical Materials on Dec. 16, 2025.

Conflict of Interest:
The authors declare no conflict of interest.

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