Wavelength-Tunable OLED with High Area-Efficiency for Multifunctional Healthcare Device

Wearable healthcare devices collect real-time biosignals and lifestyle data of users, providing important insights for disease diagnosis and management. In this study, we propose an area-efficient and wavelength-tunable OLED (WTOLED) for multifunctional wearable healthcare devices. While conventional light sources use LEDs that emit only a single wavelength or lack flexibility, causing discomfort to wearers, WTOLED features a structure that can selectively emit red and blue light in a single area.

WTOLED is a structure that stacks two OLEDs, and can selectively emit blue and red light in a single area, and the wavelength can be controlled by changing the voltage application direction and alternating current (AC) driving conditions. The emission wavelength is selected by utilizing the difference in absorption coefficients of oxidized and reduced hemoglobin, and oxygen saturation can be measured. Red light is suitable for detecting photoplethysmography (PPG) signals because it has high absorbance of reduced hemoglobin in the absorption spectrum of hemoglobin, and at the same time, it can provide therapeutic effects such as cell regeneration, inflammation relief, and wound healing in photobiomodulation (PBM). Blue light also plays an important role in photomedical applications such as neonatal jaundice treatment and antibacterial action.

In the PPG signal measurement experiment using WTOLED, the signal was successfully detected while maintaining the output intensity of each red light and blue light below 0.6 mW/cm² under the conditions of 1 mA current, 30% duty cycle, and 50 Hz. WTOLED improved signal quality by emitting light uniformly over the entire area, increasing the amount of light received by OPD and effectively improving the SNR (signal-to-noise ratio). In addition, it was confirmed that heat generation was minimized as the temperature increase was below 3.5°C even under the high operating condition of 5 mW/cm².

The fabricated WTOLED can be driven on a thin and flexible PEN substrate, and maintains stable light emission under bending conditions, proving its potential as a patch-type wearable device. This study suggests the potential for the development of multifunctional healthcare devices that can be used not only for PPG-based diagnosis but also for therapeutic purposes such as phototherapy, and provides a new direction for future wearable technology.

Acknowledgements

This research was supported by Korean Institute for Advancement of Technology(KIAT) grant funded by the Korea Government(MOTIE) (RS-2024-00436216, Human Resource Development Program for Industrial Innovation(Global)).