Nanoimprinted structures enhance green LED efficiency
Researchers from Wuhan University in China, in collaboration with industry partners, have achieved a breakthrough in enhancing the external quantum efficiency (EQE) of green flip-chip mini-LEDs by integrating double-sided hemisphere-shaped patterned sapphire substrates (PSS) using nanoimprint lithography (NIL).
This innovation boosted EQE by 22.46 percent compared to conventional single-sided designs at 5 mA, while simultaneously addressing the "green gap" challenge and advancing high-resolution full-colour displays.
GaN-based green mini-LEDs, critical for next-generation displays, wearable devices, and visible light communication, have long suffered from lower efficiency compared to blue counterparts. Traditional patterned sapphire substrates (PSS) improve light extraction efficiency but are limited by shape-dependent performance.
The team used NIL, a cost-effective, high-throughput technique, to fabricate precise hemisphere-shaped microstructures on warped sapphire substrates, which outperform frustum-shaped and cone-shaped PSS in light extraction.
"By optimising microstructure geometry and fabricating hemisphere-shaped microstructures on the backside of the mini-LED wafer, we achieved dual improvements: enhanced light extraction efficiency (LEE) and more uniform emission," explained Shengjun Zhou, a professor at Wuhan University who directed the research.
Hemisphere-shaped microstructures can better extract light than frustum or cone shapes. With backside hemisphere patterns, double-sided PSS boosted total LEE by 17.39 percent versus single-sided designs, as backside structures scatter photons generated in the multiple quantum wells (MQWs) into lateral escape cones, enhancing sidewall LEE and making top emission more uniform.
This advancement bridges efficiency and uniformity gaps in green mini-LEDs, pivotal for high-contrast displays. Future work will focus on scaling to micro-LEDs and integrating with advanced packaging for commercialisation.
Reference
Jiahao Song et al; Optics Letters 2025, 50(16): 5057-5060
