New electrode material helps UV diodes tackle Covid-19

09 Jun 2020

Penn State diodes using transparent strontium niobate could lead to disinfection techniques.

A project led by researchers at Pennsylvania State University believes that a breakthrough in UV-emitting diodes could now lead to personal handheld devices able to kill Covid-19 viruses.

The findings point towards high-performance diodes emitting the UV-C wavelengths that damage virus DNA and could be used to render Covid-19 inert, diodes potentially more portable and energy efficient than current alternatives. The work was published in Communications Physics.

"You have to ensure a sufficient UV light dose to kill all the viruses," said Roman Engel-Herbert of Penn State. "This means you need a high-performance UV LED emitting a high intensity of UV light, which is currently limited by the transparent electrode material being used."

According to the project's published paper, LEDs emitting at the required wavelengths deeper in the UV have commonly had significantly lower quantum efficiencies than visible emitters, thanks to a number of internal factors including defect density. But the lack of a transparent electrode material with high electrical conductivity and high optical transparency at the correct UV wavelengths, especially below 320 nanometers, while maintaining similar performance to existing transparent conductors in the visible range, has indeed been one particular roadblock.

"There is currently no good solution for a UV-transparent electrode," said Penn State's Joseph Roth. "Right now, the current material solution commonly employed for visible light application is used, despite it being too absorbing in the UV range. There is simply no good material choice for a UV-transparent conductor material that has been identified."

The project's solution was to look to a new class of transparent conductors and in particular strontium vanadate, a compound in which the physical properties were potentially suitable but the spectral absorption edge was located in the visible range - the wrong place.

Disinfection of surfaces

Modelling indicated that swapping vanadium for niobium in the same perovskite structure would push the absorption edge into the UV regime, and the project turned its attention to the strontium compound SrNbO₃ as its material of choice.

Growing films of this compound with the required quality posed some challenges, but the team developed a laser deposition technique in which films ranging from 10 to 60 nanometers thickness were formed from sintered pellets of Sr₂Nb₂O₇ irradiated by a KrF excimer laser.

Measurement of the films' properties, in particular assessment of a standard parameter for transparent electrode materials relating optical transmission and the electrical sheet resistance, found that values in the UV spectral range of 260 to 320 nanometers were on a par with those of the same parameter from conventional indium tin oxide in the visible range.

This suggests that strontium niobate is a suitable electrode material for high-performance UV light-emitting diodes for sanitation applications, and also potentially relevant for other applications in food packaging and biomolecule sensing.

The project's findings are part of a broader effort towards development of UV-C diodes, as a way to tackle the Covid-19 pandemic directly by killing the virus itself and disinfecting surfaces on which it is present. Recent research at the University of California, Irvine, has investigated modifications to UV diodes developed for Blu-ray players as a route to cost-effective UV-C sources, and the Penn State project has now indicated another possible line of attack.

"While our first motivation in developing UV transparent conductors was to build an economic solution for water disinfection, we now realize that this breakthrough discovery potentially offers a solution to deactivate COVID-19 in aerosols that might be distributed in HVAC systems of buildings," commented Joseph Roth. "Other areas of application for virus disinfection are densely and frequently populated areas, such as theaters, sports arenas and public transportation vehicles such as buses, subways and airplanes."