Because they can be produced as large flexible foils, these infrared LEDs could be used in new wearable and medical monitoring devices, in close proximity to the wearer’s skin while ensuring homogeneous illumination. In a paper titled “Large-area near-infrared perovskite light-emitting diodes” published in Nature Photonics, the researchers reveal a 900mm2 device claimed to be several orders of magnitude larger than other perovskite-based NIR LEDs.
The novel perovskite-based semiconductor used for its fabrication is a direct-bandgap semiconductor capable of strong light emission. By using an ITO/ZnO/PEIE/FAPbI3/poly-TPD/MoO3/Al device structure, the research team was able to precisely tune the injection of electrons and holes into the perovskite, such that a balanced number of opposite charges could 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. By using an organic semiconductor with a shallower ionisation 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 realisation of much larger devices” explained Mr ZHAO Xiaofei, a Ph.D. student on the research team. For their device, the authors reported a 799?nm near-infrared emission at an external quantum efficiency of 20.2%, at a current density of 57?mA?cm-2 and a radiance of 57?W?sr-1m-2.
Potential use cases may include covert illumination in facial recognition or augmented reality/virtual reality eye-tracking. But an interesting aspect is medical monitoring, since the researchers demonstrated the LEDs to be suited for applications involving subcutaneous deep-tissue illumination, such as in wearable health-tracking devices. The researchers anticipate the development of such flexible LEDs in the full range of visible colours for use in lighting or in flat-panel displays.
National University of Singapore (NUS) – www.science.nus.edu.sg