NUS researchers have developed transparent, near-infrared perovskite light-emitting diodes (LEDs) that could be integrated into the displays of smart watches, smart phones and augmented or virtual reality devices.

A transparent PeLED overlaid across a smart-watch display imagea A transparent PeLED overlaid across a smart-watch display to show high optical transparency and neutral color. b Near-infrared photo showing bright NIR electroluminescence from the transparent PeLED above the smart-watch display. Image from article

These transparent devices are constructed with an ITO/AZO/PEIE/FAPbI3/poly-TPD/MoO3/Al/ITO/Ag/ITO architecture, and offer a high average transmittance of more than 55% across the visible spectral region.

In particular, the Al/ITO/Ag/ITO top transparent electrode was designed to offer a combination low sheet resistance and low plasma damage upon electrode deposition.

The devices emit at 799 nm with a high total external quantum efficiency of 5.7% at a current density of 5.3 mA cm−2 and a high radiance of 1.5 W sr−1 m−2, and possess a large functional device area of 120 mm2.

The devices employ an ultra-thin layer of a novel 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.

The efficient performance is said to be ideal for battery-powered wearable devices, and could enable advanced security and sensing features on future smart-watches, phones, gaming consoles and augmented or virtual reality headsets.

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.