National University of Singapore Develops Large-Area, Flexible NIR LEDs

A research team led by Tan Zhi Kuang from the Department of Chemistry and the Solar Energy Research Institute of Singapore (SERIS) has developed high-efficiency, near-infrared LEDs that can cover an area of 900 mm² using low-cost solution-processing methods.

SERIS notes that infrared LEDs have applications in optical communications and covert illumination, and are commonly found in remote controls and security camera setups. Infrared LEDs are generally small point sources, and according to the institute this limits their efficacy if illumination is required in larger areas when in close proximity, such as those found on wearable devices.

Electroluminescence from the SERIS large-area flexible perovskite LED. Courtesy of: SERIS.

According to the report published in Nature Photonics, the SERIS devices use a perovskite-based semiconductor, which is a direct-bandgap semiconductor capable of strong light emission.

By using a new device architecture, the research team can tune the injection of electrons and holes into the perovskite with precision, allowing for a balanced number of opposite charges to meet and give rise to efficient light generation. The team also found that this improvement allowed large-area devices to be made with significantly higher reproducibility.

“We found that the hole-injection efficiency is a significant factor that affects the performance of the devices,” said Zhao Xiaofei, a Ph.D. student on the research team. “By using an organic semiconductor with a shallower ionization potential as part of the device structure, we were able to improve the hole injection and achieve charge balance. This allowed our devices to emit light at efficiencies (external quantum efficiency of 20%) close to their theoretical limit, and additionally reduced the device-to-device performance variation, hence enabling the realization of much larger devices.”

“Some of the technologies that our device could enable may include covert illumination in facial recognition or augmented reality [or] virtual reality eye-tracking technologies,” Tan said. “In particular, we have demonstrated that our LEDs could be suited for applications involving subcutaneous deep-tissue illumination, such as in wearable health-tracking devices. These materials could also be developed to emit light in the full range of visible colors. They could therefore be applied in newer generations of flat-panel electronic displays.”