Reliability Investigation on 265 nm UV-C LEDs from a Commercial Point of View

The reliability of ultraviolet light-emitting diodes (UV LEDs) has acquired relevant importance over the last few years, thanks to increasing requests for disinfection systems, particularly against the Sars-CoV-2 virus. For this reason, it is important to understand how these LEDs perform over time, to identify the criticalities that system designers had to consider.<br/>UV-C LEDs are the perfect solution as a UV light source because they are smaller, lighter, with a faster start-up, and environmentally friendly with respect to the classical mercury-based UV lamp. However, these LEDs do not have the same performance as their visible counterparts, showing a still limited lifetime and a low emitted optical power.<br/>In this work, we studied four interesting devices found on the market, with a nominal emitting wavelength of 265 nm and promising electrical-optical characteristics and lifetimes. We carried out accelerating lifetime tests on each sample, choosing as stress current their absolute maximum reported in the datasheet, and evaluating their electrical (J-V), optical (L-J) and spectral (PSD-J) characteristics.<br/>From the electrical characteristics, we found: (i) an increase in subthreshold leakage current in all devices, corresponding to an increase in parasitic conduction paths in the active region associated with an increase in defect generation. (ii) The high operating voltages, from 6 to 8 V, implied elevated power dissipation over time, which led to a fast decrease in optical power. (iii) To provide useful data for the design phase, we extrapolated the junction temperatures, obtaining values from 44 to 84 °C.<br/>Optical characteristics represented an interesting turning point: two devices reached an L90 higher than 140 h, and three of them presented an L70 of at least 650 h. Just one LED showed a limited L70 of 190 h, aligned with classical results reported in the literature. It is interesting to note that one LED does not show significant degradation after 330 h of stress test, caused by a conservative choice of the manufacturer, which indicated an absolute maximum current density much lower with respect to the other devices. For this reason, we decided to exclude this sample from the following analyzes. In terms of emitted optical power, a LED (called LED S) distinguished itself from the others by an OP an order of magnitude higher at the same current density, although it was not the best for lifetime duration.<br/>To conclude our analyses, we decided to compare the most promising LED with emitting wavelength of 265 nm with the most promising LED at 275 nm, studied in previous tests. From the J-V comparison, we noted similar behaviors, with a lower turn-on voltage for the 265 nm device; OP-J followed similar trends, with a higher optical recovery for the 275 nm LED, which implies a slightly longer lifetime. Instead, PSD-J characterization highlighted the same parasitic spectral peaks in both samples, suggesting similar issues in structure optimization.<br/>Other interesting results came when we compared the total number of doses to disinfect the Covid-19 virus that an LED can provide during its lifetime. Considering a dose of 3 mJ/cm2, the best 265 nm LED can provide double doses with respect to the 275 nm LED, but their cost is higher, due to the elevated cost of the device. In fact, normalizing these results for device expense, we found that 275 nm LED is the right choice for a disinfection system. But both results are outdated if we consider LED S: despite its lower lifetime, the higher total emitted optical power allowed it to provide 6.5 times the number of doses per LED with respect to the best 265 nm LED, and 3 times the number of doses per LED per Euro with respect to the best 275 nm LED.<br/>These results are important design considerations because they show the importance of achieving the best trade-off between wavelength, optical power, and reliability to build an efficient disinfection system.