UV LEDs grown on graphene: ten times more efficient, cheaper and flexible
The company CTO and co-founder and also professor in nano-electronics & nano-photonics at the Norwegian University of Science and Technology, Dr. Helge Weman co-authored a paper comforting all these claims.
The paper “GaN/AlGaN Nanocolumn Ultraviolet Light-Emitting Diode Using Double-Layer Graphene as Substrate and Transparent Electrode” published in the Nano Letters describes how a multi-disciplinary team of Norwegian and Japanese researchers leveraged the unique properties of atomic-thin layers of graphene as a hexagonal lattice template for the nucleation and growth of perfectly crystalline GaN/AlGaN nanowires (with less than 2% of lattice mismatch). Standing vertically on their graphene substrate (on top of amorphous silica), the 220nm diameter nanocolumns exhibit near-perfect hexagonal cross-sections and their growth is so perfectly controlled by radio frequency plasma-assisted molecular beam epitaxy (RF-PAMBE) that they all have the same average height, making it easy to cap them with a planar electrode. The growth process is fast too, with nanowires reaching a 1µm height within minutes.
Being transparent in all parts of the electromagnetic spectrum including the whole UV region, graphene offers an excellent alternative to indium tin oxide as the top-emitting transparent conductive electrode (TCE) for GaN and InGaN LEDs, the authors write. Another benefit of graphene is its extreme thinness and flexibility.
Here, the original graphene substrate serves as the top conductive electrode in what becomes a vertical “flip chip” GaN/AlGaN LED configuration. The GaN nucleation droplets coalesce at the top of the nanowires which are then capped with a gold electrode, while a metal contact on the continuous graphene electrode allows for efficient vertical current injection through the nanocolumns, the paper reports. Based on the measured temperature-dependent photoluminescence of their device, the authors estimate its internal quantum efficiency (IQE) to around 46%.
device (before flip-chip).
This technology is patented by NTNU and licensed by CrayoNano AS who claims on its website a 10-fold wall plug efficiency (WPE) increase over other state-of-the-art UV LEDs. Weman and his co-workers aim to further increase the devices’ IQE by optimizing the nanowire’s geometry through various growth parameters. With this new development, the company aims to bring significant efficiency improvements and cost reductions to UV lighting applications such as water and food processing disinfection, air purification and environmental monitoring and life science measurements.
Questioned about its business model, Weman said it was still an open question whether CrayoNano would become a UV LED chip producer or if the company would license the technology to established UV LED manufacturers. The efficiency is still behind today’s traditional UV LEDs, the CTO admitted, adding that CrayoNano is an early stage company and expects to have commercial devices ready in three to four years. CrayoNano is backed by Norwegian investors with the largest one being “Norsk Innovasjonskapital” fund NIK III.
Norwegian University of Science and Technology – https://www.ntnu.no
CrayoNano AS – www.crayonano.com
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