Near-infrared (NIR) covert illumination is becoming increasingly important for facial recognition, motion sensing and depth sensing functions in mobile devices. However, these NIR illumination sources, which are usually non–transparent LED chips, occupy valuable space and can result in unsightly black notches on an otherwise full-area colour display.
A research team led by Prof TAN Zhi Kuang from the Department of Chemistry and the Solar Energy Research Institute of Singapore (SERIS), NUS has developed efficient NIR LEDs that are highly-transparent, with an average transmittance of more than 55% across the visible spectrum.
These devices employ an ultra-thin layer of a perovskite-based semiconductor, which is capable of intense and efficient light emission. This perovskite semiconductor also benefits from low-cost processing and versatile integration into a variety of device substrates. By further replacing the traditional non-transparent metallic electrode with a new electrode comprising layers of ultra-thin metals and conductive oxide, the team was able to achieve an optimal balance of high optical transparency, low electrode resistance, and efficient charge injection that are required for the LED to generate light efficiently.
Mr XIE Chenchao, a Ph.D. student on the research team said, “We found that the implementation of a thin aluminium interlayer in our transparent electrode greatly reduced plasma damage to our device during the electrode deposition process, and allowed our devices to function efficiently.”
Prof Tan added, “NIR technologies have made significant inroads into wearable, mobile, gaming and augmented reality gadgets in recent years, and have seen uses ranging from security to health-tracking and 3D sensing.
"We believe that our transparent NIR LED concept could open up an exciting array of new advanced functionalities in small wearable devices which were previously unattainable with traditional III-V semiconductor LED chips. For instance, we may one day be able to apply this technology in smart-watches for facial recognition, or use this for high-security contactless payment.”
As a proof of concept, the team has demonstrated a transparent LED that is overlaid across a smart-watch display to provide intense NIR illumination.
