UVC LEDs up their game

More powerful, efficient and longer lasting UVC LEDs are making a compelling case for water purification.

BY RICHARD STEVENSON, EDITOR, CS MAGAZINE

In years gone by, the performance figures for nitride-based LEDs operating in the deep-UV fell far short of those emitting in the blue. But differences in the key characteristics of these two emitters are narrowing all the time, and the metrics for UVC LEDs now feature a number of impressive specifications.

Take the recent announcement from ams Osram, for its latest generation of UVC LEDs, slated to enter volume production in just over a year. Its best 265 nm-emitters now deliver an optical output of 200 mW at a wall-plug efficiency of just over 10 percent, while offering a lifetime of more than 20,000 hours.

When commenting on these figures, ams Osram’s Senior Key Expert for Applications, Alexander Wilm, refrains from singling out any one number. Instead, he claims that it’s the combination of the three that’s particularly impressive.

Wilm points out that there are UVC LEDs on the market that are more powerful, and also those with a higher wall-plug efficiency. “But having this efficiency at this output power and at more than 20,000 hours lifetime, that is something that is not that easy to do. That's why we think we are really unique.”

The latest device from ams Osram has almost double the efficiency of its most powerful commercially available 265 nm LED, which delivers 115 mW at a wall-plug efficiency of 5.3 percent. And if the European opto-electronics powerhouse wanted to grab the headlines with an even higher wall-plug efficiency number than the 10.2 percent it is quoting for its latest devices, it could boast a value of almost 15 percent, realised by dialling down the drive current of its flagship LED to 20 mA.

“But then you only have 15 milliwatts, which is very little, so it will not help you in applications,” warns Wilm, who is focusing on meaningful figures, and ultimately the practical deployment of ams Osram’s products.

Recent improvements to the performance of the company’s UVC LEDs have come through refinements to epitaxy, alongside improvements in chip and package design.

Brighter, more efficient devices can be attributed to an increase in the conversion of electrons to photons, and to better light extraction from the chip.

“The biggest improvement we made is the new package,” says Wilm. He explains that improving the interaction of the package and the chip has led to an increase in outcoupling of UV emission.

While there is much interest within the UV LED industry in AlN substrates that offer excellent thermal conductivity and lattice-matching with epilayers, engineers at ams Osram are using sapphire substrates for their devices.

“Sapphire is a standard material for LED production. It's available, widely used, and the base for our other LEDs,” remarks Wilm. “Right now, we believe that sapphire is the future for us.”

UVC LEDs have attracted much interest over many years for the disinfection of air and surfaces.

Covid’s legacy?
During the height of the Covid-19 pandemic, UVC LEDs were widely viewed as an attractive option for killing this coronavirus. But since then, interest has dwindled in using this class of LED for that application.

However, Wilm does not see this as a major impediment for the future of the UVC LED, as by far its biggest application is water treatment, a multi-billion-dollar sector that’s growing fast.

Point-of-use water treatment is an application where UVC LEDs are an ideal light source

For water treatment, UVC LEDs compete with mercury lamps, the incumbent solution. Lamps emit significant powers, but have a number of weaknesses: they are bulky; fragile; they take several minutes to warm up; have a lifetime that’s shorter than a UV LED (typically lasting 8,000 to 14,000 hours); and their disposal is far from trivial, due to the damaging effects of mercury on human health.

Another weakness of the mercury lamp is that its emission peak, fixed at 253.7 nm, is not ideal for water treatment.

“The peak of the germicidal effectiveness is at 265 nanometres,” says Wilm, who adds: “We steered our LEDs exactly to this sweet spot”

There are a number of opportunities for UVC LEDs within the water treatment sector.

One is point-of-use, with solid-state emitters fitted to a water tap. And there is also what’s referred to as point-of-entry, with water bombarded by UVC as it enters a home.

In both these cases, the UV source only needs to be on when water is flowing. As well as ruling out the mercury lamp for this application, due to its very slow warm-up time, this operating condition allows the driving conditions for the device to be adjusted, to get more out of the LED – and use fewer of them. As this solid-state emitter is off far more than its on, it doesn’t need a lifetime as long as 20,000 hours, so this can be traded for a higher output power, produced by cranking up the current.

Where the mercury lamp dominates is large-scale water treatment, associated with industrial and municipal applications. In these cases, the UVC source is on all the time, and high powers are the key yardstick.

“200 [milliwatts] is a good starting point,” says Wilm, who points out that even more powerful devices will follow. “If you compare it to five years back, the power that you got was around below 50.”

Among the customers already buying UVC LEDs from ams Osram is Hergy International, a Taiwanese producer of water sterilisation equipment. 265 nm LEDs manufactured by ams Osram are deployed in Hergy’s Perazim series of drinking water sterilisers, which include a model that processes 2 litres of water per minute while consuming just 3 W, and a variant that handles flows of up to 6 litres per minute.

When 200 mW UVC LEDs enter volume production towards the end of 2026 they may lead to an even more impressive range of products from Hergy – and they may be joined by more competitive rivals, given that ams Osram’s more powerful 265 nm emitters will be available to all.

By then, ams Osram – as well as its peers – may have even reported more impressive results. After all, UVC LEDs are getting ever more powerful, efficient, and long lasting, and these trends show no sign of abating.