The corona pandemic is currently providing a boost to the development of new disinfection solutions. UV-C light from LEDs is a particularly promising approach – and a challenging one.
“Even under ‘normal’ conditions, we’ve had no lack of potential applications. The corona virus is now adding a new dimension to disinfection by means of UV-C radiation. We can now see the whole range of possible applications on a daily basis,” says Alexander Wilm, application engineer at OSRAM Opto Semiconductors in Regensburg. Together with development engineer Hans Lugauer he is working there on the development of UV-C LEDs to make them suitable for mass production.
“Germ killers to go”
Although short-wave low-pressure discharge lamps are already being used today to sterilize and disinfect water, air and surfaces, Lugauer thinks that the future of disinfection using UV-C radiation lies in LEDs: “They are much more flexible and smaller than conventional solutions. They can be switched on and off frequently without any problems and are resistant to vibration.” That gives rise to completely new applications such as mobile disinfection devices that fit in a jacket pocket. “Let’s call them ‘germ killers to go’” says Lugauer, referring to products everyone wishes they would now have. That this application does not yet exist has its reasons: “Apart from efficiency, safety represents the greatest challenge – because UV-C radiation must not pose a risk to skin or eyes,” says his colleague Wilm, and adds: “That applies to all UV-C products because they aren’t safe for people and the environment.” This is why employees whose protective clothing needs to be disinfected with UV-C light wear face masks and gloves that are impermeable to UV-C radiation. Special robots which move through rooms disinfecting the surfaces with UV-C are equipped with sensors and switch off immediately when a person comes close to them.
Germ-free taxis and buses
Basically, UV-C LEDs are a sensible choice wherever mercury lamps have disadvantages – if space is tight, vibrations and shocks are possible, or high ignition voltages are unacceptable. Preventing glass breakages and escaping mercury from posing a danger for people is particularly crucial. “UV-C LEDs can be used for different applications, for example for disinfecting surfaces in taxis, buses or shared cars. I’m convinced that as soon as UV-C LEDs have improved performance and their costs have fallen further, other exciting applications will open up that we don’t even see yet”, says Wilm.
Genetic information under attack
But what makes UV light so successful in the fight against germs? A major reason for the effectiveness of the UV-C solution is the fact that there is no natural UV-C radiation on the Earth’s surface. It is almost entirely absorbed and therefore captured by the Earth’s ozone layer. Bacteria and viruses therefore have not developed any defense mechanisms against it.
Short-wave UV-C radiation (200 to 280 nm) is the part of UV radiation with the most energy. Light with a wavelength below 280 nanometers has the effect in microorganisms of breaking up the chemical bonds in the RNA or DNA helix. This tears apart the genetic information. As a result, the virus or bacteria is no longer infectious, that is to say unable to reproduce. The “kill factor” of UV-C radiation – in other words the necessary dose – differs depending on the microorganism. The maximum disinfection effect is assumed to be at around 265 nanometers. Lugauer explains what is important: “Since the efficiency of UV-C LEDs decreases as the wavelength drops, we optimize the LED wavelength on the one hand to achieve high efficacy. On the other hand, we focus on reasonable energy efficiency so that at the same time we can produce high levels of effective radiation.”
Looking for new materials
The UV-C LED’s material system is what makes it so innovative. UV-C LEDs consist of aluminum gallium nitride (AlGaN). From a chemical point of view, it is very similar to the more familiar indium gallium nitride (InGaN) which is used in blue and green LEDs. Therefore, almost all of the manufacturing processes in chip production are applicable – only the epitaxy process, that is to say the growth of thin crystal layers, is fundamentally different. To produce AlGaN semiconductor crystals in epitaxy, much higher temperatures of up to 1400° C are required.
Adding to that, there are new processes that can only be carried out in specially designed epitaxy plants. What’s more, special encapsulation materials which increase the light output from the chip are needed for the LED packages. The silicone commonly used for this purpose in conventional LEDs would decompose very quickly when irradiated with high-energy UV-C photons. So new materials that are both stable and transparent enough for UV-C light need to be found and tested – not an easy task. The optics require special quartz glass as it is the only material that is UV-C permeable.
Better equipped in the future
“Once these basic production processes and material issues have been resolved, we need to address the problems of efficiency and reliability. Much development work is still needed before UV-C LEDs can be produced economically in high quality. The current situation is comparable to around ten years ago when LEDs for general lighting were still too inefficient and expensive. That has changed fundamentally – and will also change for UV-C LEDs,” says Lugauer. Definitely good news for the future fight against germs.